This part of the handbook is not about which forks to use for the salad but which forks to use on your motorcycle. To a huge extent this decision is usually based upon an economic limitation more than anything else. It is not at all unusual to spend more money for your forks than you did for your frame, which explains why so many people hold off buying forks until the bike is almost complete.
If you have an existing bike that you’re working on the decision about fork selection is a lot easier to make since you’ve at least got something to start from and in most cases the question is what length tubes are needed to get the frame sitting properly after you’ve raked the neck and/or stretched the down-tubes.
If you’re building a bike from scratch however things can get more complicated since you have control over more variables.
There are basically five different types of forks available today which are:
You don’t see to many rigid forks today except on show bikes but at one time they were pretty popular because they could be built quickly and cheaply. Outwardly they resemble a girder except there are no suspension links since the side rails or tubes are just bolted solidly to the yokes or trees. At one time we were building rigid forks over sixty inches long that actually functioned fairly well since they were very flexible but in order to provide effective cushioning rigid forks need to be on bikes having extreme amounts of rake with relatively low steering neck heights.
Telescopic, also called hydraulic, forks are mounted on about 95% of all modern motorcycles today and in operation function like a pair of giant oil-dampened Pogo sticks held together by the upper and lower fork trees. Harley first began using hydraulic forks in 1949 on the Hydra-Glide. A typical set of Hydraulic-telescopic forks looks like those mounted on our Old School Chopper project bike seen in Figure 1 below.
The market for aftermarket and custom hydraulic fork systems is huge and there are literally hundreds of different tree-fork combinations that you can buy ranging from low-end stock assemblies to complete custom fabrications costing about as much as an entire stock bike.
Figure 7.1 – Hydraulic/Telescopic Forks
Springers have been around for about eighty years and first appeared on bicycles but when we think of a Springer today we conjure up mental images of either the old stock Harley forks or the long narrow custom chopper forks that became popular in the late sixties.
Figure 7.2 – Classic Chopper Springer
Reproductions of the old original Big Twin Springer forks are being made today by several companies and can be ordered with almost any extension length needed for a chopper project. Some people just like the nostalgic look of the stocker as seen below or the more modern rendition of the classic 60’s chopper forks shown above.
Figure 7.3 – Stock Harley Springer
There are several derivatives of Springer’s where the springs are inverted or placed near the rockers and one style where the springs are concealed within the tube legs. There are also Springer’s that use flat leaf springs as seen on some Indians and British bikes. These Springer derivatives are called ‘Leafers’ and are becoming more and more popular especially on ‘Bobbers’.
Figure 7.4 – Indian Style Leafer
Girders have also been around about a hundred years and like Springers first appeared on bicycles but the style most typically adopted for chopper work is derived from the design first used on the old Indian motorcycles of the thirties/
Figure 7.5 - Indian Style Girder
Spurders, or Spirders as they are sometimes called, are a combination of an inverted springer and a girder with the springs concealed in the rear legs of the girder assembly. These forks are generally very clean and graceful in appearance and it is not unusual to see the handlebars built as a continuation of the forks themselves. Credit generally goes to John Harman for inventing the Spirder back in the seventies but.
Figure 7.6 – The Modernized Spirder
The modern example above was built by Jaxon Davis at Roadrat.biz and is typical of the outstanding design and fabrication work he does with his rendition of the classic concept.
There is no such thing as a ‘perfect’ front end for a chopper and no matter how radical the frame is configured or what type of forks are used there will be proponents and detractors for each configuration. What is ‘acceptable’ riding and handling characteristics for one rider will be totally unacceptable for another. To complicate matters a bikes handling characteristics are also affected by a variety of things that have nothing to do with the fork system being used. Some of these factors are tire and wheel size, neck height, wheelbase, total weight, and weight distribution just to name a few. An old Springer for instance may work wonderfully on one particular chassis configuration and not another. A certain amount of experimentation will always be needed and that’s why used front-ends are so easy to find.
I am no fork expert by any stretch of the imagination but over the years I’ve logged a lot of miles on various bikes having a wide variety of fork systems so I think that I’m at least a little qualified to make some general observations but I urge the readers to seek out the advice of professional fork builders before just going out and slapping any old eBay bargain to the front of their bikes.
There are some facts and then there are also some myths about front-end systems that just don’t ever seem to die. One myth is that Springer’s are sloppy and ill handling and this is simply not the case unless the rocker bushings are shot. Another myth is that hydraulic forks don’t work to well on radically raked front ends and this is true. Once you get beyond a 38-degree rake angle or so the effectiveness of hydraulic forks starts to diminish rapidly and they start behaving more like a ‘flexible-beam’ system where the fork tubes are taking the entire load by flexing.
Out of all the fork systems I’ve ever owned my personal favorite has been the Girder design and many racing frame engineers seem to agree, as you’ll usually see some type of girder design on cutting edge racing frames. Girder designs are adaptable to virtually any chassis configuration and they can be made to provide excellent handling characteristics over a broad range of steering neck rake angles and trail can be manipulated very easily to suit specific requirements. A Girder can be made very strong but also very light in weight and in general a properly designed Girder pound for pound will deflect less than any other type of fork system. The only reason that Girders are not more popular than they are is because they are more complicated and hence more expensive to manufacturer and they really need to be custom made for a specific frame configuration. Whenever you hear somebody bad-mouth a girder it’s usually because the particular set of forks in question were designed for a specific bike or frame and then adapted, unmodified, to a new frame that has slightly different geometry. You can’t just add a set of stock Indian forks and trees to a Harley chopper frame and expect the bike to handle well.
Unfortunately since forks in general are a relative high-dollar item and fairly easy to mass produce the field is populated by both good, and less than good, manufacturers so it pays to ask around and investigate a fork suppliers background and reputation before buying anything. Your life depends on your forks regardless of the style, type or design you finally select.
Always remember to take anything you see or read in a chopper magazine with a grain of salt. I am amazed at the number of times I’ve seen pictures in a chopper rag of some bike that was supposed to have 16 over forks when it was obvious that they could not possibly have been much over 6 inches at the most. Sensationalism seems to sell and builders love to feed the baloney to uninformed writers.
One of the most often asked questions on most cycle discussion boards is “ what length forks do I need for a bike having such and such rake.” Unfortunately the correct answer to this question is very complicated and depends upon a wide variety of factors and is almost always specific to one particular cycle so there is no ‘standard’ answer or mathematical formula that can be used over a broad range of chassis configurations.
Many people imagine that you can use a CAD program to compute the answer and I use such a program almost every day so yes, you can come close, to arriving at the correct answer but not as close as you can by doing a real mock-up of the bike. It is very hard to comprehend exactly how many variables are actually involved in trying to determine the ‘ideal’ fork length for any particular bike. To make matters a little more interesting doing a ‘good’ mockup is not as easy as many discussion board ‘experts’ make it sound.
The usual mockup recommendation asks the bike owner to set the frame on blocks of cribbing to get the frame level with the desired amount of ground clearance and then to extend an imaginary line through the steering head to the imaginary intersection point with the front wheel axle centerline. You measure along that line and add about 1.5” to the results for compression to arrive at the fork length you’ll need.
This suggestion is fine up to a point but even experts forget that a frame revolves about the rear axle centerline and not the point on the lower rails where the cribbing is usually placed to mock up the desired frame ground clearance in the rear. In addition the rear tire on a loaded bike at rest will be out of round by as much as an inch so we really need to take into account the rear axle centerline height of the tire at speed when it has grown to its full diameter when we put the mockup together. If we don’t take this into account the finished fork length measurement will usually be to short and the frame will sit lower in the front than it should when the bike’s actually on the road.
Another problem in trying to calculate fork length without doing a real mockup is that ‘published’ dimensions for stretch and neck rake are almost never exactly correct and the real dimension that is critical is a vertical measurement from the lowermost edge of the bottom-bearing cup to the ground. I can guarantee that if you go out today and take this measurement, called the neck height, on ten frames that are all supposed to be stretched by an identical amount that you’ll find at least 2 inches of difference from the lowest to the highest, sometimes a lot more.
Then there is the matter of fork offset. A set of forks with no offset, as seen on some Springer’s, on the average chopper, can be as much as 2 inches shorter than a pair of forks on the same bike that has 2.5 inches of offset and the frame will sit in exactly the same attitude. The effect of offset on fork length becomes more important as the neck rake angle increases. If you measure your mockup fork length it is imperative to take the offset dimension into account otherwise you’ll end up ordering forks that are to short for your frame. A good rule of thumb to use in factoring offset into the fork length determination process is to add the same amount of additional length as the amount of offset you plan to use. In other words if you want trees having 1.5 inches of offset add this same amount to the fork length if your original measurement was exactly on the centerline axis of the steering neck. Remember that this is a general guideline and is dependent of rake. For a stock bike you can add about three quarters of an inch in length for each inch of offset while on a wildly raked bike you may have to add as much as 1.5” of extra length for each inch of offset.
As an example, if you mocked up a stock 69 FL frame and measure the fork length along the steering neck axis you would arrive at a distance of very near 26.125 inches. However if you factor in the stock 2.5” of fork offset the actual length of the forks you would need to order is 27.625 inches. If you followed the ‘chat-room’ advice of many ‘experts’ you’d order forks that were actually 1.5” shorter than needed.
All front fork systems except a rigid design will ‘compress’ with load once they’re mounted on the finished bike and the amount of this 'compression’ will vary depending upon the spring rate used in the particular set of forks. In general most builders use a factor of from 1 to 1.5 inches for compression, for hydraulics, and .5 inches for Springer’s, Girders or Spirders. This amount needs to be added to any length measurements you take from your mockup or calculated dimensions.
If you’re ordering forks from a catalog or over a sales counter without consulting a real fork expert you’re pretty much left with buying a set that are ‘X’ number of inches over or under stock but what is that ‘stock’ length. Well in reality it varies widely from model to model and even from year to year and is different for Springer’s than it is for hydraulics. To make matters even more confusing there is a so-called ‘fork-length’ dimension and then there’s also a ‘tube-length’ dimension and to top it off some builders take all of their measurements from a different point on the steering head.
Again, this is just another reason why ‘used’ forks are so easy to find. It is hard as hell to buy a set of forks the ‘first time’ that will really fit any particular bike.
In general most builders measure Springer forks from the bottom of the lower bearing cup to the rocker pivot point. A stock Harley Springer for instance is 19.5” long measured this way so a ‘six over’ Springer would be 25.5” long. Some Springer manufacturers however measure to the bottom of the top tree so their advertised dimension for a stock Springer is 26.5 inches.
Hydraulic forks on the other hand are usually measured to the bottom of the upper triple tree so in general they’ll appear to be about 7 inches longer than they really are compared to Springers. A stock FXWG for instance is 24 to 25 inches long as measured by the ‘Springer’ method but is usually sold as being 31 to 32 inches long since it’s measured to the bottom of the top tree.
Another area that seems to confuse many people new to chopping is that the ‘published’ fork-length tables seen in manufacturer catalogs and on web sites are misleading in that they typically show a set of forks for chopped bikes with zero up-stretch as being anywhere from 2 to as much as 4-inches under stock. How can this be?
Well this gets back to the very foundational roots of choppers that started back in the days of rigid frames. All dimensions you see related to chopper terminology are based upon the old 1948 Harley rigid frame and not the newer 59 and up swing-arm frames which have a 2-inch higher neck height, 1” more ground clearance and two degrees more rake, hence longer stock forks.
If after reading all of this you are a little uncertain about fork length for you particular ride all I can say is that it is better to err on the side of buying forks that are to long than it is to be conservative and get a set that are to short. Buying a set of forks that are as much as 3” longer than what you think you need will only raise the forward edge of the bottom frame rails on a typical chopper by about an inch and it’s a whole lot easier to shorten forks than it is to lengthen them.
Many people have asked me how to extend an old stock Harley Springer using the ‘Ford radius rod’ technique and my answer is to forget it. First of all old original Springers are getting pretty rare and chopping one up destroys it value. Secondly the resulting modified forks are not nearly as strong as the new reproductions of original Springers that you can order in any length you happen to need. There are at least half a dozen companies today making very nice original style Springers in a variety of lengths. If the reader insists on doing this modification the best directions are contained in a book entitled “The Custom Chopper Cookbook” by Mike Geokan and Mike Arman.
The table below shows the length of various stock fork configurations that you can use for reference. All of these lengths are taken with stock tree offset already factored in and are measured parallel with fork leg from the bottom of the lower bearing cup to either the rocker pivot hole or the front wheel axle centerline on bikes equipped with stock wheels and tires in an unloaded condition.
Table 7.2 lists the tube length, not fork length, for the most popular bikes using hydraulic forks.
Use these figures for reference but please don’t rely on them for ordering forks. We’ve been doing this a long time and have yet to find any kind of published charts, graphs or tables that accurately provide correct fork length data for the average chopped cycle. Do a good complete accurate mockup of your bike, take into account the offsets and then consult with your supplier, manufacturer or fabricator. Buy a quality product if at all possible. Measure many times before you make a final decision and remember that it’s better to be over than under in your estimates.
You’ll notice the center column of Table 2 shows the diameter of the fork tubes used on the various bike models. Over the years the Harley factory has produced hydraulic forks in three different configurations typically referred to as being the ‘Wide-Glide’, Mid-Glide’ and ‘Narrow-Glide’. These designations refer to the center-to-center spacing of the forks legs as measured on the triple-tree. The narrower spaced forks had smaller diameter fork legs.
The ‘official’ factory spacing dimension for these various forks is:
Wide-Glide – 9.875” between fork tube centers.
Mid-Glide – 8.875” between fork tube centers.
Narrow-Glide – 7.000” between fork tube centers.
Be warned however that aftermarket tree makers have taken considerable latitude with respect to these dimensions and it’s not unusual to find differences as great as .5”, plus or minus, from one manufacturer to the next with respect to the bore hole centers on trees.
Today the de facto standard that most makers use as a baseline for their designs is the FXWG (Wide-Glide) fork assembly which is supposed to be made with 41mm tubes, 9.875” apart, on centers, with a length measured from the bottom of the upper tree to the axle hole of 31.75 inches (24.75” for Springer measurement from the lower bearing cup). These dimensions take into account the increased up-stretch of modern frames as opposed to the original rigid frames.
As mentioned previously Springer forks, in various design guises, have been around since the 1890’s on bicycles. As far as is known the first sprung forks that appeared on motorcycles was the Sager ‘Cushion-Ride’ design adopted by Indian back in 1905 as shown below.
Harley-Davidson copied it in 1906 but in 1907 the H-D factory developed a design of its own and called it the ‘Bottom-Link suspension’ but still advertised it as being a Sager Cushion Ride.
The 1907 Bottom-link concept (seen below) formed the mainstay of H-D front fork suspension systems for over forty years, finally to be discontinued in1948.
This was the birth of the Springer fork even though you couldn’t actually see the springs themselves since they were installed inside of long vertical spring tubes that ran up just ahead of the primary fork legs.
The design wasn’t especially successful and was modified and strengthened almost every model year until finally being extensively redesigned in 1916 when the first exposed-spring fork system was introduced. These external springs however were basically just ‘booster’ springs added in-between the internal spring tubes. By 1926 the factory had forks with six external springs to control rebound on the old original pogo-stick internal spring system.
The beautiful little Bobber shown above illustrates the state of the art Springer fork system, as it existed in late 1928. Some of us were still using these forks in the sixties on custom choppers.
The first real Springer that finally dropped the internal spring tubes altogether was introduced in 1929 (some say 1930) with the introduction of the ‘I-beam’, external-spring, front fork system that established the basic shape of the classic Harley Springer we all recognize today.
In 1936 the primary leg I-beams were dropped in favor of elliptical-tube legs that were used up until the end of the line in 1948.
In the early sixties if you wanted to have a ‘real’ chopper the first thing you usually did was to get rid of the hydraulic forks that just screamed ‘factory’. The alternative was to buy an old set of Springer forks, which were cheap and plentiful and extend them as far as possible by making legs from old Ford radius rods.
That’s exactly how I first got hooked, along with about a million other guys, on building Choppers. I built my first set of extended forks in 1961 but unfortunately I didn’t even own a bike to mount them on. It seemed at the time however like a good place to start since I already had the parts scrounged up for what it was that I wanted to ride someday.
By the mid-sixties building radically extended Springer’s was a big business, fueled largely by the little pamphlet Ed Roth was publishing in Southern California. The business was so huge in fact that some guys decided that they could actually make a living just from building Springer fronts-ends.
Many authorities credit Dick Allen as being the father of the custom built So-Cal narrow Springer front-ends but I distinctly remember a lot of guys doing completely custom Springer’s before Allen ever started to advertise his work in the mid-sixties. For instance Ron Finch was advertising custom forks as early as 67, about the same time as Allen’s work started to appear in magazines and many of us were already doing narrow front-ends by chopping old pre-1930 Harley straight-leg Springer’s very early on. In all fairness however it probably was Allen who opened the doors for the later so-called ‘production-custom’ Springer makers.
If you’ve only ridden bikes with hydraulic front-ends you’ll probably never really get used to the feel of a Springer and unless you’re willing to tweak around with a set of Springer forks you’ll never get them to behave very well. Some people swear by them and others swear at them. It’s largely a matter of the general characteristics of one’s particular bike and the specific forks that determine how well they handle.
Having grown up with having nothing but Springer’s most hydraulic forks feel weird to me while friends used to tell me that they can’t stand the way my bike ‘feels’ with the Springer forks since they tend to transmit every little road irregularity back up to the rider. It’s kind of like the difference between power steering and manual steering in a car where the hydraulics of power steering dampens out a lot of the feel for the road surface.
You can buy literally dozens of different repro Harley Springer’s with almost any extension imaginable from several different manufacturers. I’m personally partial to those sold by Paughco but there are other good ones as well. For the classic So-Cal look however the number of makers has dwindled over the years and while there were dozens of companies only a few short years ago the field has narrowed considerably leaving only a few sources for quality production/semi-custom front ends.
If you’re willing to wait a little longer for delivery however there are at least two independent completely-custom Springer builder’s still around who do 100% handmade custom work. Sugar Bear (Figure 7.11) is one. He’s been doing his thing for about thirty years and will be more than willing to talk with you about a specific project.
The other custom fork builder is Big Al at Bitter End Old School Choppers who, as the businesses name implies, builds traditional bikes and components on a custom basis.
We’ve included some very specific notes about how to set-up a Springer on the plans that are included in the appendix and in general they apply to tuning almost any Springer you might happen to acquire but be aware that a Springer is not the type of front-end that you just bolt-on and forget. They do require maintenance and periodic rebuilding but nothing that can’t be accomplished at home. Springer forks are also known to have some potential structural problems with the main legs bending and sometimes breaking at the point where the legs enter the holes in the lower tree and this area should be inspected on a regular basis for signs of stress like hairline cracks.
To aid in alleviating this potential problem some makers who build Springers with tube legs, as opposed to solid bar stock, insert another piece of tubing in each leg for a distance of 10 to 12 inches. Of course those who use solid steel bars for the fork legs generally claim that their forks don’t have this same type of problem to begin with which isn’t actually true as I’ve personally seen several solid-legged Springers bend and even fracture at this critical junction point.
Springers in general are not cheap and as such, in many cases, they are simply out of reach for many people. Fortunately however there are now several sources for obtaining Springer parts, components and even complete ‘kits’ that can reduce the cost about 50% over a store-bought set of forks. For the ambitious builders there are also plan sets available from several sources including our own which are available for free through the web site discussion board.
There can be no doubt that a nice set of Springer forks adds a truly custom look to almost any bike and about half of their popularity is based upon appearance alone.
As mentioned elsewhere Girder forks are probably the best front-end suspension system ever invented. In fact many experts believe that Girders, or their derivatives, will eventually be the ideal front suspension design used on all cycles in the future. For some insights into this future we encourage readers to examine the works of Foale and Britten but in the mean time we have to keep this section of the manual focused on the older or more traditional interpretation of Girder forks as found on most choppers.
Girder forks took their name from the classic structural shape of the Girder Truss or Girder Beam used primarily in bridge or roof construction since the fourteenth century but perfected during the late 1800’s. This shape represents the most fundamental engineering application of ‘triangulation’ as seen in Figure 7.12 below.
In such a structure the bending forces on one of the spans that puts it in compression are resisted by the spans on the opposite side that are then placed in tension. Such structural elements resist forces that can come from either side and can be arranged horizontally, as in a bridge, or vertically, as in a tower.
Basically the assembly is comprised of a single compression member and one or more tension members with struts connecting the members at midpoint. You can actually build a girder truss with a rigid compression member, represented by the ‘top chord’ in the diagram above, and wire cables as the tension members, represented by the ‘bottom chord’.
An excellent example of modern day girder trusses in action can be seen in sailboats where the mast is the compression member and the wire rope side stays are the tension members. The very same engineering principals that keep these masts from bending apply to motorcycle forks.
A truss or girder can be hundreds of times lighter than a solid structural member intended to resist the same forces. It was the invention and perfection of steel girders that made structures like the Eiffel tower possible back in 1889.
Not coincidental is the fact that almost all bicycle makers immediately adopted the ‘triangulation’ method for frame and fork construction at the same period in history.
The girder truss by itself however was only part of the solution for both early bicycle and motorcycle makers who initially used the design but only on a rigid set of forks. In 1909 the second part of the system was patented as shown below.
This invention was the Levedahl link suspension, which combined with the trussed fork, became the classic Girder style adopted by virtually all cycle makers except Harley-Davidson who held steadfast to their old bottom-link Springer concept.
Ironically Harley eventually did use a Girder on the 1948-S when they needed a good light weight high-performance set of forks for their little 170 pound sport bike.
The biggest advantage of a Girder, for chopper applications, as compared to Springers or telescopic forks is that Girders can be both extremely light and extremely strong at the same time. They are relatively inexpensive to build and can be constructed in a very wide variety of shapes and sizes to suit individual tastes. In fact there are very few if any design constraints so the limits of a Girders appearance is only restricted by the builders imagination.
While this is a blessing to many talented fabricators it is also a hindrance to the average home-based bike builder as it is this very flexibility of design that makes Girder layout relatively complicated compared to Springers for instance.
Where one can pretty much build a ‘universal’ Springer design that will work on a wide variety of frame types most good Girders are custom engineered and fabricated for particular frame geometry. This is the reason that almost all ‘mass-produced’ Girders that have been on the market over the past thirty years have had little appeal to the public since they usually didn’t work well on most bikes and many people had very bad experiences with poorly setup Girders that simply didn’t suit their particular bike. On the other hand those rare few who just happened to have frames that matched the geometry of the fork makers master-frame had nothing but praise for the handling quality of their front-ends. It was a hit or miss proposition and the vast majority of buyers were unwilling to take a gamble so Girders took a back seat to Springers for most custom builders. Even today with some new mass-produced Girders on the market the same situation still exists.
My first introduction to girders involved building rigid forks out of angle iron that could be used on ‘rollers’ in a shop that did a lot of custom work. We’d just hack together crude rigid girders and yokes to the appropriate lengths so the fabricators could move uncompleted bikes around the shop and the owners could at least have a better visual ideal of what their rides would look like. Some of these lash-ups actually saw the road under power before the final fork selections were made.
Girders appeal to many builders since than can take on so many different shapes and styles, just a few of which are illustrated below in figure 7.15.
As mentioned above Girder forks are inexpensive to build. It is entirely possible to buy all of the materials and components for a first class set of forks for under $250 in most parts of the country. Depending upon the techniques and methods of construction very few special tools or equipment are needed and improvisation can lead to many creative solutions to bypass the need for expensive machine work if you’re on a really tight budget. There is also tremendous opportunity for those out there who have access to aluminum casting equipment and some imagination. The sky is the limit as to what a person can come up with.
Figure 7.16 is a series of advertising scans sent in by site visitors taken from magazines of the sixties and seventies.
Many of these old Girders were made from bent solid stock and were pretty poorly welded with extremely crude link connections, which certainly didn’t do anything to help with user acceptance of the basic design. To make matters worse backyard builders just copied these old crappy ‘off the shelf’ designs to the point that almost all people came to believe that a Girder was just about the last type of front-end that you wanted on a bike.
What makes Girders complicated isn’t the design of the Girder Beams themselves but the geometry of the trees, links and attachment points which can be almost infinitely arranged to provide all types of trail and affect other elements of handling such as anti-dive and anti-squat to name only two. Some have said that Girder forks are an engineers ‘heaven’ but a builders ‘hell’.
People who don’t care for Girders to begin with are very quick to point out that on a well-designed Girder the trail will change by a whole 3/16 of an inch when the forks are cycled from maximum compression to maximum extension for about 3.5” or more of total suspension travel. Detractors cite this as a horribly dangerous characteristic of Girders.
What they forget however is that trail change, on a well-designed set of hydraulic-telescopic forks as they move through 3.5” of travel is a whopping 2”or more! This is why most serious road racing engineers are looking to girders instead of trying to improve hydraulic forks. This also shows that most people commenting about fork geometry don’t know what they’re talking about in the first place.
The above paragraphs of course refer to static changes in trail. The dynamic changes in trail as the frame dips and rises in response to suspension travel are of course more dramatic as the effective rake angle is dynamically changing but even then the Girder is far superior in performance. In reality however such changes in trail due to the longitudinal displace of the axle during suspension travel on almost any set of forks is virtually negligible but people still like to argue about it.
Plain and simple, Girders handle better than any other fork style you could possibly use; if properly designed and constructed. Poorly designed or poor built Girders however can be real nightmare.
Unfortunately this section of the manual can’t tell you how to build a ‘perfect’ set of Girder forks since a well-designed set is truly custom tailored to a particular bike but it can get you started in the right direction and save some wasted time in your development efforts.
All of the data and design information that follows concerns designing and building a set of forks that are intended to be used on frames having geometry very similar to our so-called ‘standard’ designs. In other words, for frames with a neck height of about 33” and a neck rake of about 40 degrees. The diagram below illustrates the key dimensions needed to start designing a typical Girder fork. If your bike is within plus or minus 2” or plus or minus 2 degrees of these specs our published Girder plans will work with only minor customization. If your bike is above or below these tolerances you’ll have to do some experimentation. Believe me it’s well worth it to get a good set of girders on your ride. Once you’ve had the experience you’ll never use any other type of forks again.
Before we go any further I’d like you to understand that it is impossible for me to know how much your particular bike weighs or how much you yourself weigh. I cannot know what the spring rates will be for the shock you eventually decide to purchase or what size wheel and tire you’ll eventually be running. For this reason I strongly suggest that you assemble your entire front end with only tack welds until you can actually mount it on your personal frame. Once the forks have the final weight on them you can adjust the location of the shock mounts and link lengths to optimize the geometry for your unique setup.
This ‘cop-out’ turns many away from building a Girder since they immediately think that the project will simply be to complex to be feasible but this is false reasoning. A lot of builders put together complete fork mockups from plumbing pipe, conduit and even wood or angle-iron before deciding on the final connection and attachment points for a good custom-designed front-end. Nothing ‘good’ is ‘easy’, and building a ‘good’ set of forks will always require a lot of work. Those looking for the quick fix can buy a set of $1500 hydraulics and be riding on crap all day long not having any idea of what good forks are really like. As they say ‘ignorance is bliss’.
Girder Link Attachment
There are at least three schools of thought about how to attach the links to the upper and lower yokes and forks on Girders. Old-school folks like to use the method where the yokes and forks each have a long ‘pin’ or ‘shaft running through the yokes and forks in bushings and the links attach at each end of this shaft. In this setup the shaft rotates in the bushings and the links are rigidly attached to the shafts.
The more modern approach, though not to say a better approach, is to replace the shaft with shoulder bolts or machined ‘pins’ and have the links themselves pivot in bushing on the rigidly mounted pins or bolts.
A low-cost way to fashion ‘pins’ or ‘studs’ is to plug weld a section of solid drill rod of a suitable diameter inside a piece of DOM tubing. You leave enough of the drill rod exposed on each end for the bushed links to slip on to and then thread the last half-inch or so for a nut and washer.
The third approach involves building the links and cross-members as an integral unit where the left and right fork legs are separate components and then the entire assembly is tied together with continuous shafts at the upper and lower fork pivot points. This hasn’t been a popular construction method in the past but I personally think it has the greatest potential for ‘modern’ chopper forks and is possibly the strongest assembly.
All three of these basic methods work just fine and it is largely a matter of what you personally prefer and what equipment you have on hand to do the machine work that makes the final determination on which way you decide to go.
Regardless of what style of attachment you decide to use it must be understood that in all girder designs the weak link in the system, structurally speaking, is in the attachment point of the suspension links with the pivot points. More specifically it is the diameter of the pivot shafts or the shoulder bolts that you have to be concerned with.
The weakest point of any threaded shaft is where the threaded portion meets the unthreaded area and most engineering manuals publish the shear strength of this junction for various types of material.
In the case of girders the shaft material is usually mild steel with a tensile strength of around 36,000 pounds per square inch and the shear strength for various shaft diameters is as follows:
3/8” shaft shear strength in the shaft proper = 10,280 psi
3/8” shaft shear strength at the thread neck = 3,970 psi
1/2” shaft shear strength in the shaft proper = 18,350 psi
1/2” shaft shear strength at the thread neck = 5,985 psi
5/8” shaft shear strength in the shaft proper = 28,785 psi
5/8” shaft shear strength at the thread neck = 11,140 psi
3/4” shaft shear strength in the shaft proper = 41,515 psi
3/4” shaft shear strength at the thread neck = 17,800 psi
It’s pretty obvious that the larger diameter shaft and thread is the way to go if you want to build strong front-ends but there is a ‘practical’ limitation where you run up against ‘looks’ vs. ‘build-ability’ and in the world of Choppers ‘looks’ pretty much overrides other engineering constraints.
You also run up against the wall of ‘engineering impossibility’ where an engineer will tell you a particular design idea will fail mathematically but the very same design has been in actual road use for decades without any problems. So who is right?
In the world of choppers we very often have to fall back upon empirical knowledge gleaned from years of trial and error experiments to find the correct answers to many questions. History has shown us that the typical maximum ‘normal’ load imposed on most motorcycles (catastrophic crashes excluded) is about ten times their dry weight with a ‘safety factor’ of three. Of course this an old handed-down ‘rule-of-thumb’ that is probably extremely conservative.
If you look at the loads on a girder, or even a springer, from an engineering standpoint based upon the rule-of-thumb above it appears as if the minimum bolt or shaft diameter needed is .75” for typically encountered maximum road impact (failure) loads but in the real world thousands of bikes have been riding the pavement with .5” shafts and bolts for decades with no problems.
From where I stand today, based upon the average weights of choppers being built in this decade I would be willing to say that a minimum solid shaft or shoulder bolt diameter should be .625” for either springer rockers or girder linkages which equates to a .5” threaded segment diameter at the minimum.
For a girder application this means using a shaft or shoulder bolt with a .625” diameter that terminates in a .5” diameter threaded portion at the connection point. This will give you a breaking strength of 5,985 psi at the intersection of the shaft and the thread neck, (which is the weak juncture) at each connection point, of which there are four.
Is this a ‘safe’ assumption? Well let me say that there are perhaps thousands of old girders out there riding around using .5” shoulder bolts or pivot shafts that only have .375” threaded studs and as far as I know none have failed to date in regular road use. Some of the old 640-pound Indian Chiefs had .5-inch shafts bend and the factory switched to 9/16” and they had no more problems. If you have a habit of hitting curbs or doing wheelies you’re probably at risk even using the larger diameter specifications. Each and ever builder has to fabricate his or her components to their own personal level of safety and sanity. What we specify herein works for our own level of what I consider ‘average’ road conditions which may be different than your own interpretation so in the end the fabricator has to be the final judge as to what is safe and what is not. To be very honest I personally have no problem riding rather long Girder forks using only .375” studs and half-inch shafts but I know my limitations. We have to remember that under ‘normal’ riding conditions, whatever those are, that the forks will seldom encounter much more than 700 pounds of load at any given point in time unless something gets in the way of the wheel or you’re doing wheelies.
There is another very important point we have to bring up however and that is exactly how the loads are to be placed on any particular shaft, shoulder bolt or regular bolt for that matter. The forgoing paragraphs assumed that the fasteners in question had loads applied in what’s sometimes called a ‘single shear’ scenario. Imagine for a moment that you screwed a big lag bolt into one of the wall studs in your garage so that half of the shaft was sticking out of the wall and then you gave a downward blow with a hammer right on top of one of the flats of the hex head. In effect you applied a ‘load’ at the head of that lag bolt and the ‘shearing’ point was where the screw met the stud. The wood stud supported half of the bolt and the other half of it was unsupported, just hanging out in the air. This is the ‘single-shear’ installation. Now imagine another situation where you screwed the bolt into two studs that are about 6” apart so that both the threaded tip and the hex head itself are completely embedded in the wood. Now in this instance if you whack the bolt right in the middle of it’s length, in the space between the two studs, it will resist the force of the blow at both ends and have twice the resistance to breaking. This is called a ‘double shear’ installation.
If you look at a typical clevis and pin you will see a good example of a ‘double-shear’ engineering application. The pin is supported at both ends and the load has to be applied right in the middle. This is exactly the same type of connection method you should endeavor to use in building the link connections for a pair of girder forks.
Girder Link Length
There really aren’t any limitations on how long Girder links can be. Some sport bikes with cutting edge technology use links that are almost 18” long. Old Indians used links that ranged from 4 to 4.75 inches. 60’s era ‘customs’ had links that were in the neighborhood or 5 to 5.5 inches long. I personally think that shorter links handle better and try to keep my designs in the range of 3.5 to 4.5 inches center to center of the attachment points.
Link length is closely related to not only the diameter of the shock you select but also the effective travel length of the shock. The links have to be long enough to provide clearance for the shock while at the same time permitting the forks to cycle completely through the shocks full range of motion, which is usually in the area of 3.5 to 4.5 inches of total travel from maximum compression to maximum extension.
Keep in mind that just like springer rockers the Girder links act as levers and have a significant impact on ‘perceived’ spring rates. Long links have a lot of leverage and can use much stiffer springs than shorter links. The relationship of the lower shock pivot point and link length is crucially interconnected from a design standpoint.
In an attempt to correct problems with trail encounterd by many riders who happen to have a set of improperly designed girders on their bikes some have resorted to using upper and lower links having different lengths. It is true that using unequal length links can be used to advantage on specially design fork geometries but it is a poor system to use on the vast majority of bikes. Some ‘unexpected’ and ‘unwanted’ handling characteristic may result if you don’t know what you’re doing. Just to give one simple example of a common situation, note that in the figure below, the lower links are 5” long and the uppers of 4” long. You’ll see this quite often in the real world. What you don’t see however is the rather radical changes in trail that occur as such links cycle through full shock compression and extension that can easily change the trail a full 3” every time you hit a bump.
In general almost all unequal length parallel link configurations will ‘bind’ at certain points in the travel arcs and even though the rider is unaware of it every time the links overcome one of these binding points the forks and links bend to compensate which eventually leads to stress fatigue of the components.
Using unequal length parallel links that don’t have the pivot points properly arranged can also lead to a catastrophic situation where one of the links can go ‘over-center’ and actually lockup the entire fork assembly.
Equal length parallel links will always give the best handling characteristic on typically driven street choppers and it’s my personal opinion that experimentation with unequal length links be left for your road racer where you really do want certain changes in geometry at specific points in the suspension travel.
This last statement may lead people to believe that the links themselves have to be straight and parallel to the ground but in reality it is only the link ‘pivot point’ that have to be equal and parallel to one another. The links can be almost any shape or configuration imaginable including ‘L’s’, ‘U’s’, swords, spears, lightening bolts, boomerangs etc., etc.
Girder Shock Absorbers and Springs
Unfortunately advertisers on Madison Avenue invented the phrase ‘shock-absorber’ and in reality the Brits have a far better term to describe what the modern day ‘shock’ really is and that’s a ‘damper’.
The only role that a shock plays in the world of suspensions is to ‘dampen’ spring oscillation and not to ‘absorb’ impacts from the pavement. You can easily drive a car or a bike without shocks but if you hit a bump you’ll have a roller-coast ride until the springs get through doing their thing. On the other hand you can’t drive a car or a bike without springs since you’ll just destroy the shocks when you hit the first bump in the road.
In the old days Girders had a central spring, usually progressively wound, and a pair of friction shocks mounted on either side of the lower pivot points. Even back then most builders realized that the friction shocks ‘sucked’ to say the least so when newer hydraulic shocks appeared on the scene they were quickly adapted. The Vincent builders placed a hydraulic shock up between the links and added long, narrow shielded springs to the forks as seen in figure 7.21 below. Their set up was called a ‘Girdraulic fork'.
Fortunately nowadays we’ve got dozens of coil-over shock combinations to choose from. Unfortunately most don’t fit in between the narrow constraints of chopper forks to well since the diameter of the coils are simply to large to clear the steering neck and the fork cross-members.
A full eighty percent of all of the effort you will put into building your Girder forks will be in selecting an appropriate coil-over shock (or shocks) and then deciding on the ideal mounting points for that particular shock between the top yoke and the lower fork cross-member.
You will have to experiment with various spring rates over time until you come up with the ideal solution for your particular bike. No two bikes are alike and depending upon the location of the center of gravity for your particular ride you might need a shock with a spring rate as low as 70 pounds to as high as 300 pounds.
Over the years I’ve become pretty suspicious about ‘published’ spring rates from various shock manufacturers so take anything you get from spec-sheets with a grain of salt. For example one popular mountain bike shock supplier has a unit with a so-called 180 pound spring rate but if you look at the little .25” diameter coils that are almost 2” on center vertically compared to the much beefier Koni with .375” diameter wire stock at 1 inch on center for the same ‘published’ rate you just have to wonder who’s closer to the fact of the matter.
We bought a very narrow shock from Italy that was rated at 150 pounds with 3.5” of travel all in a package that was only 1.75” in diameter but when we received it the real rate was more like 8 pounds since we could easily compress it to full bind with just body weight alone.
I’m sorry to say it but as to shock selection you’re completely on your own as we’ve bought and tried several dozens of units from various makers and to date I’m still not completely satisfied that we’ve found a starting point that we can recommend.
For those wanting to experiment we want to remind readers that spring ‘rate’ is expressed in the terms of ‘X’ pounds needed to compress the spring at it’s unloaded (free) height by one inch in length. For example if we have a spring with a rate of 100 pounds it will take a weight of 100 pounds to compress it one inch in length or height and a weight of 200 pounds to compress it two inches in length or height and so fourth. If we have a spring that is 10 inches long having a ‘rate’ of 100lbs./in. with no weight on it and we put it in a set of forks bearing 200 pounds of the bikes weight it will compress two inches under static load leaving only eight more inches to use for real suspension.
Unfortunately however very few springs have nearly this much effective range before they ‘bind’, or reach the point where the coils stack up against one another and in effect become a solid mass. In shock salesman terms this is the ‘coil bind’ position or point of maximum compression. Keep in mind however that springs ‘stretch’ as well as ‘compress’ so what we’re looking for, ideally, is a spring that will compress around 1” when the front weight of the bike is on it (about 2/5th the bike total weight actually) but still gives us around 2.5” more compression before binding. Such a spring will also stretch 2.5” (probably significantly more) at full extension. In other words we’re looking for a spring with fairly small diameter coils having a rate of around 200 pounds that has at least 2.5” of compression room before the bind point for a light chopper.
Don’t bother looking at car coil-overs for possible Girder springs since they are usually far too large in overall diameter to fit between the forks and the steering stem. Instead look at small coil-overs for ATV’s, golf-carts, motor scooters (Vespa), mountain-bikes and rear shock units for small displacement imported swing-arm bikes. Right now one of the old rear coil-over shocks on my Honda 750 is looking pretty good but it’s really about one inch to long for what I really want.
(Upon daily inspection however it seems to be shrinking in anticipation of the inevitable moment of experimentation).
For those who want specifics before they start a design project we’ve found that coil-over shocks having an overall (fully extended) length approaching 11.5” are a good starting point. At load condition these units typically are near 10” in length and shorten to around 8.0” at maximum compression giving you about 3.5” of ‘effective range of motion’. You can fit almost anything you find however by playing around with the location and angles of the shock mounting tabs so don’t be discouraged if you can’t find the ‘ultimate’ unit for your particular application right from the start. Sooner or later you’ll stumble across a unit that’ll be perfect.
Keep in mind that girder shocks can take of advantage of unequal valve rates and that you need more dampening on the extension stroke than on the compression stroke for a smoother ‘perceived’ ride. This is just the opposite of what you’d normally expect. Always keep in mind that the spring is really doing the suspension work and that the shock is just dampening the spring, which seems to be a hard fact for many Americans to grasp after fifty years of brainwashing about shocks.
Dual shocks look cool but they double your costs and believe me when I say you’ll never find two identically performing units unless you pay a premium for a so-called ‘balanced’ or ‘matched’ pair. Normally only one of the units is really doing the work and the other just follows along playing catch-up. If you just have to have multiple shocks why not try to run four ultra-mini coil-overs in some unconventional mounting position that nobody’s done yet'
Girder Fork Tube Size
The tubing you decide to use is based exclusively on how long the forks have to be. Whatever you do please don’t use solid rod in a set of girder forks. On the one hand it’s over-kill as to weight but under-kill as to strength.
Short forks, less than 26” in overall length can be constructed from stock as small as .75” O.D by .095” wall. Forks up to 30” in overall length can be safely constructed from 1” tubing with a .120 wall. Forks 31 to 32” in overall length need to be 1” O.D. by .134” wall. Forks 32 to 38” in overall length need to be 1” O.D by .156” wall material. For forks over 38” in length you really need to go to 1.125x.156” tubing at least for the front or primary legs. These suggestions are based upon not having any unsupported (non-gusseted) single spans over 28” in length. Of course these suggestions also assume that you’re building a lightweight chopper. If you’re building one of the new-age 680 pound sleds some folks call a chopper then I can’t possibly give you a starting point as to tube sizes. Short wheelbase heavy bikes need more beef in the forks as more weight is on the front end. Long chops with the motor well aft of the midpoint of course need less meat up front since the center of gravity is further to the rear.
If you keep the unsupported lengths of tubing short you can very easily build forks up to six feet long from stock as small as ¾ diameter x.083 wall and they will be incredibly stiff and strong. An example of such construction is shown below.
One of the weak points in girder forks is the inability to adequately resist torsion forces that are applied in a line perpendicular to the steering stem axis. If you can visualize your bike parked with the tire wedged in between two huge concrete blocks while somebody pulls on a twenty-foot cheater bar welded to the fork tree you can understand what I’m talking about. Under extreme road conditions the wheel axle acts like a long lever and wants to ‘twist’ the two fork legs in opposite directions. Of course such forces can be easily resisted if we had tension members that ran down the ‘outsides’ of the fork legs but we usually don’t. Instead we have to rely on having the main compression members (front fork tubes) large enough in diameter or thick enough to resist twisting as well as bending.
Common sense and experience has to come into play when you are building any type of fork system. The fabricator has to be responsible for putting together something that is both safe and effective at the same time.
In this era of the ‘lawyers’ many builders have fallen back on building massive over-kill solutions for chopper front ends which is why we’re seeing Springer and Girder forks with 1.5” diameter tubes which is simply ridiculous and we all know it. I personally think that if a person is afraid of his or her own capabilities and personal judgment then they shouldn’t be messing around with chops in the first place.
Can you possibly imagine some guy walking into an insurance company or a local law firm and saying “ I’m a custom chopper builder and I want to build cutting edge, balls-to-wall death machines but I don’t want to be liable for anything”.
Of course when he leaves the office his fully sanctioned, approved and safe frame specs for a chop is 2” diameter .25” wall frame tubing and 2” diameter .25” wall fork tubes. Do you want one of his bikes? If you want safety and sanity don’t mess around with choppers to begin with and don’t come crying to me for recommending something that broke down the line. You’re going to be the ‘builder’ so use your own judgment and build to you own personal levels of safety and sanity.
Choppers are pieces of mechanical art, not appliances that need the ‘Good Housekeeping Seal of Approval’.
Before starting to layout the design of your upper tree or yoke it’s a good idea to give some thought as to how you’re going to mount the handlebars with respect what type and size risers you’re going to use. Today there are basically two ways risers on mounted to trees. One method of course is to just drill a couple of nine sixteenth-inch holes, spaced 3.5” apart and just bolt the risers through the tree with ½-inch by 13 socket head cap screws. A refinement on this method involves milling a slight recess, having a diameter that matches the diameter of your riser, about 1/4-inch deep into the upper surface of the tree. This makes for a little tighter and cleaner installation. Beware that not all risers have the same base diameter and in fact some are elliptical, not circular, in cross section.
One problem with the methods outlined above and variants thereof is that the handlebars don’t have any type of vibration isolation and this bothers some riders. If you want isolated bars then you’ll have to use the big ugly rubber, or fancier polymer isolation bushing that set in big holes bored through the tree. The disadvantage of the isolators is that the risers have to be set further forward on the tree to allow clearance for the little dust covers and the steering stem dust shield so the whole tree is therefore larger than it would have been otherwise.
Of course the ultimate girder risers would be integral with the trees; that is welded into position, with nice graceful sweeps back to the handlebar collars.
We typically leave enough ‘meat’ in our standard tree drawings and patterns so you can build them for either solid or isolated risers. You can tighten-up the patterns a lot if you do away with the isolation mounts.
Girder Trees or Yokes
I personally prefer to call fork ‘Trees’ by the term ‘Yokes’ when speaking about Griders or Springers but I think everybody understands the particular pieces I’m talking about.
For Girder forks a person can build Yokes that range from the incredibly ‘crude’ up to incredibly ‘cool’ depending upon the amount of time and money they’re willing to invest.
Both extremes of design sophistication will get the job done so again it’s a matter of personal desire, machinery available and budget that determines what you’ll eventually come up with.
Stock Yokes on old bikes were pretty crude castings, many of which you can still buy from various restoration suppliers if you’re doing a ‘retro’ bike but the trend today is to use a ‘composite’ or ‘combination’ approach where the yokes are fabricated from both pieces of plate stock, and round or square tubing, or solid bar stock.
If you access to a lath and a milling machine you can build Yokes in ‘billet’ fashion but for the cost of materials and the amount of work involved most people prefer to fabricate the Yokes using the ‘built-up’ technique.
Simply stated the ‘composite’ or ‘built-up’ method involves using pieces of cold-rolled plate or wide strap stock to construct the main segments of the Yoke and then welding-on tube spacers for thickness, if needed, and sections of DOM tubing or solid bar stock for the link pivot points. You can easily see examples of the composite fabrication method in the pictures above.
There is no cut and dried system, formula, technique or pattern for building Yokes out of separate pieces. Every builder will invent their own unique design based largely upon available materials and the equipment on hand.
For instance you could simply weld a piece of thick plate stock, drilled for the steering stem, to a piece of DOM tubing or sold rod for the pivot carriers. Another method might involve building up plate thickness by using multiple pieces of thin sheet stock into a thick ‘stack’ cut every-other plate about one quarter inch undersized and you have a ‘ribbed’ looking Yoke to weld to the pivot shaft tube or rod. Another method uses a semi-billet approach, where the main flat portion of the yoke is milled from thick steel or aluminum plate and then welded to a pivot shaft or pivot tube. The possible fabrication combinations are numerous.
Keep in mind that Girder Yokes are very different than Springer or Hydraulic fork trees in that on Springer and Hydraulic systems it is the lower tree that takes the vast majority of the structural loads and the top tree is, in a way, just going along for the ride from a strength standpoint. On Girders both yokes are usually loaded very nearly to the same levels and have to be comparably identical structurally.
Copper or brass yokes and even forks are easy to do with Girders for those wanting something out of the ordinary.
Girder Yoke Geometry
Older bikes usually adopted yoke geometry where the lower link pivot tube or pivot shaft ran at right angles to the neck and backbone immediately below and on the centerline of the steering stem. The upper link pivot tube on the top yoke was placed about 1 to 1.5 inches forward of the stem nut. This configuration is about as close as one can get, with a girder, to the equivalent ‘zero offset’ found on Springers or hydraulic fork systems. In effect this configuration is usually considered the baseline for girder fork yoke geometry. It is possible with a little creative machining and welding to get both link pivot tubes centered exactly on the steering stem but from a handling standpoint nothing is to be gained by doing this although it looks pretty good.
Our current yoke design for use with standard H-D handlebar vibration isolators has 2.75” of offset in the upper and 1.5” in the lower and I personally can’t perceive any difference in the way this setup handles compared to an earlier design I used to run that had near zero offset.
One advantage of a girder over other fork systems is that you can run a significant amount of offset to make a short bike look longer and you can offset either of the link pivot points on the yokes to make changes in trail or to make a bike with a shallow rake look like it has a long front-end. In fact with a girder you can build trees that actually have the link pivot points ‘behind’ the steering stem axis and it will handle just fine.
Having said of all this however I do have to qualify my statement by saying that ‘offset’ as we are accustomed to thinking of it as applied to hydraulic or springer forks has no real comparable ‘function’ with respect to yoke design on a girder since the forks themselves are actually offset by the link length.
It is extremely hard to describe just how ‘open’ girder suspension really is to experimentation, artistic expression, easy of fabrication and low costs. Girder forks simply cannot be compared, at any level, with other suspension systems.
One of the ‘sticking’ areas however to building a true ‘low-cost’ girder system is that sooner or later you’re going to be faced with making a custom steering stem for your yokes and unless you have access to a lath you’re going to have to pay somebody to do the work on your behalf. This isn’t an item to cut corners on as your life depends upon the integrity of this small and seemingly insignificant piece of steel.
The actual dimensions of the stem will depend entirely upon the dimensions of your particular neck-bearing-yoke combination and it’s a very good idea to haul all of these pieces down to your local machine shop. You have to trust your machinist, not the Internet chopper ‘experts’ with respect to the material selected for the stem piece.
The Internet ‘experts’ will tell you to use threaded chromo tubing since it’ll save you 0.01 pounds of weight and it’s just the ‘trick’ thing to do on a custom chopper. Other experts will say that the stem has to be stainless steel or titanium with all kinds of special heat treating, etc, etc.
Nine chances out of ten your local machinist will have the right material on hand to make up a nice solid stem that has just the right amount of surface hardness and tensile strength to do the job without fracturing somewhere down the line. My guy likes 1040 mild steel and his partner likes 303 stainless. We haven’t tried the stainless yet. Your own guys will have their own ideas but if you don’t have 100% confidence in your machinists then shop around until you find a guy you trust more than your own doctor. Your life depends on it.
There are two ways to mount the stem on or into the lower yoke. One way is to simply drill a hole in yoke, cram the stem into it and then weld the stem in place from the bottom. The other way is to drill and tap the yoke to accept threads placed on the lower end of the stem, screw it all together and then tack weld the stem in place. It’s six one way and a half-dozen the other. The important thing is that when assembled, the stem is at exact right angles to the yoke, in other words perfectly perpendicular to the lower bearing surface of the yoke. There should not be any room for error whatsoever.
The overall assembly of the individual parts of a typical Girder suspension system can be quite varied. For example the links can be situated on the ‘inside’ or the ‘outside of the main fork legs. The links themselves can be fabricated from tubing or from flat bar stock or from castings. It’s not uncommon to see more than one link attachment method used on a single girder. You’ll sometime run across units that use continuous pivot shafts in the forks but rigid link pins on the yokes and vice versa. You’ll also see units where the upper links may be mounted inside the fork legs while the lower links are mounted outside. On some customs you’ll see forks with only one central upper link and two lowers or one central lower and two uppers. Occasionally you’ll find a fork where the shock or shocks are mounted below the lower yoke and attach to the rear legs in a modern adaptation of the old Vincent configurations.
Folks are finally catching on to the fact that a good mono-shock mounted lower in the forks reduces the height of the bikes center of gravity and permits one to use very short links while cleaning up that area ahead of the steering stem.
Hardware and Connections as to Appearance
Earlier we alluded to the fact that in the realm of chopper building there are often trade-offs, adjustments and compromises made that create a balance between strength, practicality, form, function and simple appearance considerations until just the right balance of critical factors, tempered with artistic judgment result in a final design concept that not only ‘works’ but also looks good at the same time.
All of us at one time, or another, have walked through a bike show and seen machines that defy the imagination. We’ve also made mental notes about which particular bikes were really ‘roadworthy’, or could actually be driven on a daily basis and which one’s were pure ‘eye-candy’.
If you decide you want to build Girders your design talents will be pushed to the limit since I cannot possibly imagine where artistic judgment and good engineering sense are put more to the test than in building one of these front ends that will not only handle the real road on a daily basis but look good while doing it.
From my personal perspective I rank fork system strength and functionality foremost and try to figure out a way to make it all look attractive and proportional with the rest of the bike secondarily. To be able to do this you have to have the raw materials and other components in hand to make valid decisions based upon not only sound engineering principals but also artistic visual comparisons of various potential assemblies. In other words you have to be able to actually see, feel and ‘experience’ the parts you want to experiment with. You just can’t pick parts from a catalog and expect anything to look proportional. You’ve got to have the stuff in your hands before you begin the design and fabrication process. As much as I like ‘plans’ this is the one area where real parts will far outweigh anything you see on paper.
Fortunately buying a lot of parts for a potential Girder project isn’t all that expensive and to start with you really only need a single ‘example’ of each item for each component assembly you’re planning to experiment with and you don’t need fancy top of the line parts to start with during the planning and mock-up stage.
To give you an example of what I’m talking about try this little experiment. Take two sections of 1” tubing about 36-inches long and polish one to a bright luster. Paint the other one with black paint, to simulate powder coating, and put them side-by-side. The polished tube will look considerably ‘narrower’ or smaller in diameter than its painted counterpart. If it were chromed it would look even thinner. In this instance if you were after a certain ‘look’ for a set of forks that were to be powder-coated you would have selected slightly smaller tubing, 15/16” DOM for instance, so it would have the same visual ‘weight’ or ‘size’ as a skinny looking piece of chromed 1” tubing.
The picture below illustrates a variety of common fasteners and fittings that you might be using on a Girder fork system. The steering neck and stock rocker bolt are to provide scale.
The larger bolts in the upper left-hand corner are 5/8” cap screws and in the next row are half-inch and then down further 3/8” fasteners. The bronze bushing range in size from half-inch to ¾-inch in inside diameter and the washers and spacers shown are all grade-8. Some of the cap nuts are nickel-plated and some are stainless steel.
As you can see there is a huge ‘visual’ difference in size and mass between a 5/8” cap nut (acorn nut), for instance, and a one-half inch cap nut even though ‘on paper’ there is only a one-eighth inch difference in diameter and a lot of people would opt for the larger fastener for safeties sake even though it would look pretty out of place on a girder made from 7/8” tubes for example.
DOM Tubing Schedule
The table below lists some of the more commonly used sizes of DOM tubing that you might need if you’re building Girder forks.
Keep in mind that this isn’t a comprehensive list of tube sizes just the more commonly available size combinations that work well with Girder fabrication.
The construction of girder forks will introduce some new challenges to the average frame builder who hasn’t had the opportunity to work with long lengths of small diameter tubing in the past. Welding distortion is extremely magnified and some of the tube miters, like the intersection of the lower primary leg and rear truss, are virtually impossible to make with a conventional tube notcher’s or even with some larger milling machines that don’t have at least 14 inches of horizontal travel.
Leaf springs used in suspension systems have been around for a long time but it was the Indian Motorcycle company that used them the most extensively from almost the outset of production but made them a standard part of all bike forks in 1906.
The 1907 V-twin model above has both a leaf-spring fork and a rear swing arm with leaf springs on each side of the frame.
Since leaf springs are by nature, a ‘progressive’ type of suspension they often provide for a better ride than coil spring forks. In addition, if the links are properly designed, they provide for a longer range of travel than coils and in general are more robust and trouble free.
In both Word Wars the Indians with leaf spring forks easily out handled the Harley Springers in rough terrain. Figure 7.25 illustrates a nice old WWII vintage Indian with a very massive front spring.
After the war supplies of old surplus bikes with Leafer forks were plentiful and they were seen on many ‘customs’ of the fifties and into the early sixties. When the trend started to move more towards radically raked and extended front-ends however Leafers fell out of favor.
The introduction of the nostalgia bike craze of recent years however has brought new life back into this old fork system and it’s been seen on many show-winning cycles.
The ‘Flying Pan’ is perhaps the most notable of these ‘retro’ rides as seen above.
One problem with Leafers that has held them back from being more popular is that they just don’t look very good when the fork legs are extended much more than three of four inches over stock. They are also relatively expensive to build and very expensive to buy ready-made from one of the few companies that still manufacture them; about twice as expensive as a high quality Springer.
In an attempt to hold costs down we’ve designed our Leafer plans to utilize very inexpensive utility trailer springs but there is still the cost of having the trees water-jet or plasma cut. In general these forks are about twice as complicated to build as compared to a Springer since the fork tubes have to be bent and/or rolled and there are more linkages.
Despite the cost and complexity disadvantage however these forks do look incredibly nice when combined with the right shape of bike frame.
Figure 7.27 illustrates a very nice Indian racer that has all of the right proportions to really pull this style of bike together.
Compare the stance of the bikes in previous pictures showing Bobber style frames with more or less short fork legs to the image below of another retro-custom having extended legs.
Note that as the legs get longer the distance to the spring starts to get out of proportion unless it’s length is significantly reduced.
I personally don’t like the ‘look’ of ‘flat’ leaf springs on any type of frame and prefer to use a spring that has an arch that compliments the curvature of the wheel or front fender. Use of a curved spring also helps in the appearance department if you are running extended fork legs.
Here’s another Bobber picture of a very lightweight little 1915 racer with good lines.
The reason I included Figure 7.29 was to point out the difference in the type of rocker used on these forks as compared to some of the others.
Also note that all Leafer rockers have a much larger ratio between the pivot points than seen on typical Springer rockers and this is because a leaf spring has to ‘move’ (flex) considerably more distance than an equivalent coil spring to achieve similar rates.
Copyright 2020, All Rights Reserved