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How to do a Good Mock-up

A few weeks ago I was communicating with a site visitor who was doing a mock-up on his bike while he was trying to figure out what length Springer he needed. The individual in question was not a novice but the numbers he provided me just did not work out on paper so we had a series of back and forth conversations via email and it suddenly dawned on me that he was working with a mockup in his garage that was seriously flawed through no fault of his own as he was following instructions found on the net that had some serious problems.

As a result I decided to add this little page to the site in order to clarify some misconceptions and make it easier for people to do an accurate mockup of their projects and I am a firm believer in mocking up projects at various stages of construction. Drawings are one thing but a real world mockup is worth its weight in gold.

Many articles about mockups found on the web are completely wrong and saying that is being kind to say the least as whoever wrote the articles obviously has never built a bike before.

The whole point of doing mockups is to make sure that parts fit properly before you commit to welding or buying major components like front forks for instance and the object of this article is doing mockups specifically aimed at determining fork lengths and establishing the 'ride attitude' or 'stance' the finished project will exhibit.

The ride attitude can make or break a project and it is not uncommon for builders to spend a significant amount of time in determining just what the bike needs to look like when viewed in profile. Some people like the long low look and others like the old school 'up high' look while others shoot for something inbetween.

The key to success is knowing where to place your cribbing so that the frame mockups actually represent what it is you are trying to acheive.

Apparantly, and this was a surprise to me, many people don't know where to place cribbing under their frames when doing a mockup.

Rigid frames revolve around the rear axle when they are lifted or raised by the front forks. Swingarm or Softail frames revolve around the swingarm bearing pivot point but this is modified by the length of shocks being installed before the rotation moves to the rear axle. Softails are especially difficult to mockup unless the owner has the shocks and any lowering kits installed and the rear wheel and tire mounted to the frame. On swingarm and softail bikes it is almost essential to have the complete rear suspension system and wheel/tire combo installed before even starting to think about frame attitude. Rigids are much easier.

The frame of a rigid bike always revolves around the rear axle when being lowered or raised by the length of the front forks.

In order to do a good mockup on a rigid frame the rear wheel/tire combo needs to be installed in the frame but this is typically not the case in many instances due to budget reasons so according to internet wisdom the prospective builder is told to install blocking or cribbing under the rear of the frame at the desired ground clearance dimension the builders wants to maintain and then crib up the front of the frame to the anticipated ride attitude as shown in the following diagram. Note that the front cribbing is not show here.

The problem with this 'wisdom from experts' is that you end up with a horribly distorted geometry and if you follow these directions you will end up with a frame that is several inches higher in the rear than you intended once you install the wheel and a set of forks that can be several inches in error. Although this sketch is exaggerated you can see what might happen if the rear wheel diameter is ignored.

The correct way to mockup the frame if you do not have a rear wheel is to run some all-thread rod or an old axle through the rear axle slots and then crib up from underneath the rod, between the frame rails, to the height of the prospective wheel/tire combo you plan on using. Installing a rear wheel avoids this potential problem.

Mocking up the frame in an attitude where the front is higher than the rear is typically standard practice to take into account spring sag once the forks are under load. Sag occurs on all types of forks whether they are hydraulics, springers, girders, leafers, or spirders. When determining the stance also keep in mind that the suspension movement is dynamic once the bike is on the road and front forks will compress over bumps and potholes sometimes as much as another 2-inches.

There are also road hazard issues that effect frame stance. The old rule of thumb is that if your run over debris on the road or high-center on a driveway approach apron you want to take the impact close to the bikes longitudinal center of gravity which is generally close to the base of the seatpost crossmember.

Another reason for sitting the frame higher in front is artistic as it counteracts the visual affect of entasis. A frame that is perfectly level will sometimes appear visually to be 'going downhill' and this really ruins the appearance of most bikes. Cars look good when they're lower in front than in the rear but bikes look terrible and this isn't just a personal opinion but a belief shared by most builders.

My drag bike has 1.5" of ground clearance in the rear and a little over 2" in front but most people, including myself think it looks like its going downhill so do not underestimate the effects of entasis.

See our article on "How to Measure for a Springer' for charts tabulating spring sag. It is written for springers but the tables apply to all fork types.

Mockups for Forks

Lets say you've mocked up the frame exactly the way you want it to sit when you're on the bike and the suspension is fully loaded. You have a front wheel/tire or at least know the overall diameter of what you intend to run so the best and most accurate method of determining the front fork length is to run a piece of dowel rod, EMT tubing or whtever will fit through the steering neck and reach the ground.

Roll your wheel up to where the axle intersects the rod or tubing and make a mark. This gives you a point to work with. Make another mark on the dowel or tube where it intersects the lowermost edge of the lower bearing cup. If you don't have the cups installed make a mark .75" down from the point where the dowel intersects the cast steering neck at the rear edge. This mark represents the top surface of the bottom tree on all types of forks and this is the point that all forks are measured to no matter what you might have read on the web.

Now the fun begins. You have two known and measured reference points for the front end length. The frame is mocked up to perfection and you have added in a value for potential spring sag when cribbing the front of the frame so you won't have any surprises when you sit in the saddle after the forks are installed.

At this point if you are going to be using a hydraulic front end the distance measured between the two reference points are the length of your forks, plus the length the extends from the top of the lower tree to the top tree connection point. Fork tube makers can supply the correct tubes based upon the measurment your just took.

If you're measurig for a Springer however things get a lot more complicated and the reason is that few if any fork builders or aftermarket suppliers share the same 'standards' with respect to how Springers are measured and this includes the Harley factory.

I sat down at the computer today and looked at a random sampling of eight different companys building and/or selling Springers and came away with eight different 'lengths' for a so-called 'stock' length Springer on their order page that ranged from 19.0 to 24.625". That's a pretty big spread for a so-called 'stock' set of forks. To complicate matters even more some suppliers build 'in-line' forks and some build forks with 3-degrees of rake in the trees. This makes a difference in length of around an inch. I called two different 'popular' sites that sell the same set of 'popular' forks and got two completely different answers to my questions about fork length.

Some outfits measure from the top surface of the top tree, some measure from the lower surface of the top tree. Most did measure from the upper surface of the bottom tree which is a defacto measurement point so at least most folks agree on that point. Unfortunately, from that point of agreement there was wide deviation as to where on the lower end of the forks the final measurement was taken. Some measure to the axle hole in the rocker, some measure to the rear leg rocker pivot hole center. There is about an inch of difference in overall length depending on what 'hole' you measure to.

I called two companies just out of curiosity and found nobody on the other end of the line who could do much more than sell me a 'canned' product and had no knowledge of the 'technicalities' of their product line. The first thing you need to establish in a phone call to the maker is what vintage fork they use as the baseline for what they consider 'stock'. Most don't know but the few that do have different baselines. Paughco for instance uses the old FXWG for some strange reason and their measurements are overall length from top of top tree to rocker axle hole so their forks actually run about 3-inches longer than most.

The owners of a large site that specializes in selling DNA springers wrote on the order page that he ecommended adding 2-inches to the measured length needed to insure having forks that met the buyers requirements. That's one outfits 'technical wisdom'.

I imagine most people have had the same experience and it is extremely frustrating since we're supposed to be dealing with professionals and standards.

This is the reason that you see dozens and dozens of new but used springer forks being sold at swap meets and in the classifieds every day because they were the wrong length for the sellers bike. It has come to point of being a crap-shoot when you order a Springer but we'll shed a little more light on how to get what you want if you stay with us for a while longer as we get to the boring stuff.

Back in the old days Harley made several different versions of Springers for different models and vintages of bikes. There was the so-called 'standard Springer that was 19.5" long, made from 1936 to 1948, the XA that was 21" long, the VL and 45 forks that were 19" long, the WL, RL and others but the one that set the 'standard' for future fork comparisons, especially for chopper builders, was the Big Twin Springer that measured 19.5". Some call this the FL Springer introduced in 1941 and in most cases this established the base line for not only length, but rocker dimensions, rocker angle, spring size and rate, perch location and stem lengths that custom builders started to use around 1967.

The figure below illustrates the measurements taken from the FL forks.

 

 

Since then however fork makers have deviated widely from those old 'standards' and many fork makers have adopted 22" as the new 'standard' for the original 19.5" dimension shown in the diagram primarily because it is better to sell forks that are to long since they are still useable rather a set of forks that are to short which are definately unusable.

Potential fork buyers should also take note of the attitude shown for the rockers in the diagram. Notice that the pivot holes in the rockers are on a line parallel with the floor line and the factory standard angle between the rocker pivot points and the fork legs was set at 120-degrees. There was a reason for this angle to be selected and it has great bearing on custom made forks that we'll get in to later but it is the one area where mass market builders have taken the greatest latitude in the products they sell. Changing this angle sigificantly changes the overall effective length of the forks and impacts your ride height.

Some fork makers use different angles for their 'standard' setups and these range all over the board from 105 to 130 in the no-load condition, that's the static condition when you take the assembled forks out of the box.

The sketch below illustrates a set of forks where the rocker angle is set to 125- degrees in the static condition. Doing this makes the effective length of the forks a little over a half inch over stock.

 

There is nothing wrong with setting the rockers at such an angle if it is done intelligently to compensate for spring sag once the forks are under the weight of the cycle but some makers take a little beyond the spring sag values.

The next sketch represents the opposite situation where the fork makers reduce the angle to, say 115-degrees. This creates a set of forks that are around a half-inch shorter than stock and will be even shorter once installed and spring sag takes effect.

 

Most people think that a half-inch plus or minus won't make or break their project but in the examples we used here the angle changes are just 5-degrees, that's only a tenth of an inch difference in the length of the front legs and believe me when I tell you that a whole lot of fork makers deviate way more than 5-degrees when setting the rocker angles which change the effective length of the forks.

Fork buyers need to be aware of these differences to make better decisions when selecting where they want to buy their forks from and after reading this article you probably know 100% more than the guys selling forks at some shop or web site.

 

Spring Sag

The last thing that pertains to Springer forks is spring sag. All springs, whether leaf or coils will deflect or sag under any specific weight load. How much they sag given a specifc weight determines the Spring Rate and this varies by the physical properties of the springs in question.

I know that this a boring subject but Springer forks are all about Spring Rates as this determines how well they perform and if you don't understand this issue you should not be running a Springer on your bike.

When I used to build Sprint cars I picked up a 'Spring-Rater'. This is simple tool resembling a small hydraulic press with a strain gauge attached. It allows you to test the spring rate of coil springs which was essentail when build a race car where suspension tuning can be critical. I used it to test Springer springs as well and as a result we trashed a lot of springs we would have otherwise used on our forks. Almost all performance auto fabricators have such Raters if you care to have your springs tested but in most cases that won't be necessary today since spring manufacturing has improved considerably in the last fifty years.

Today most makers of springs for Springer forks use two basic 'models' for their spring products. One is based upon the old original Harley springs used prior to 1946 which are 6.25 long (free length) using 0.25" diameter wire with 10 active coils having an outside diameter of 2-inches and a rate of 104-pounds per inch (each).

In 1947 the factory switched to a heavier spring having the same basic dimensions except they were only 1.875-inch in diameter and 6.5-inches in free height which increased the rate to 130-pounds per inch.

Imported springs we have measured come in two configurations. One is identical to the 0.25" American spec (rate unknown) the other is a metric standard 0.236" (6mm) wire and these we did test at between 100.12 and 102.34 pounds per inch each. This pair was removed and replaced by the bikes owner who knew that something was wrong with his forks.

Both American versions already chromed are available from Paughco and in a parkerized finish from 45-Restoration Company.

Springs are expensive, perhaps the most expensive componet of Springer forks and for this reason makers are temped to try cost shortcuts and this usually means changing the wire material or wire size since the external dimensions are pretty much chiseled in stone.

99% of the time a person has an issue with the performance of their Springer forks the problem is usually due to their springs, improper spring installation or weight distribution.

Unfortunately imported Springers are noted for their poor springs which should not be to surprising. One set we actually tested because the owner had major problems that should have had a rate of around 130 pounds came in at only 90 and the surface finish under the chrome was the dead giveaway that they were made from regular old CRS that had been poorly tempered.

Most American made coil springs are made from what is called Oil Tempered MB (ASTM A229) because it is made with extremely close dimensional tolerances as well as rigid quailty controls with respect to tensile strength. Imported springs from Asian countries are made to no quality controls whatsoever unless they come from Japan.

It is the high quality spring material that guarantees uniform and long lasting spring rates before fatique sets in and for most American coils having the specifications used for springes sag is within a small range.

We have desribed how to install and set the preload on Springer springs in several articles on the site but in general on correctly built Springer forks the compression springs are installed with about 81-pounds of preload each (162# total), about the same as the rear shocks on a light swingarm bike. For each 2-pounds of additional load beyond this weight on the forks your springs combined will sag another .004-inches changing your ride height based upon front end weight bias including rider weight.

Foe example for a theoretical 600-pound bike with a 50/50 weight distribution (stockers are around 48 front 52 rear) we have a total weight on the front end of 300-pounds minus the 160-pound preload for an additional fork load of 140-pounds times 0.004= sag of .56".

That doesn't sound to bad except this 0.56-inches of spring compression equals a change in axle height of almost a full inch. Spring sag must be taken into account when you order your forks or do your mockup and the little formula above will help you make an accurate estimate.

If you don't have a set of shop scale handy you will need to just estimate the weight of your bike based on comparisons to other bikes of similar design and type. For the percentage of weight on the front wheel we use the following table based upon averages for choppers we have actually had in the shop over the years for fork work and I think it is accurate but still should be used as a 'guideline' since we have no way of knowing how your particular bike is configured.

Rake Angle in Degrees Percentage of Weight on Front Wheel
30 42-48
35 40-45
40 38
45 36
50 34

The reason for the weight spreads in the bikes with 30 and 35-degree rakes is because many otherwise stock looking aftermarket frames actually have some rear-stretch in the frames and sometimes the motor/tranny are set back to give the chassis a more streamlined or rakish look. There are also a number of Goose-Neck type and Digger frames in this class for some reason and this distorts the weight bias.

You can interpolate between the angles shown to find the values for you particular project.

The Rocker Angle

We talked about the 120-degree rocker angle earlier and the reason Harley decided to keep the front and rear rocker pivots parallel with the ground line was because they determined it gave the best spring action back in the old days when rakes were in the 26 to 30-degree range and the sprung legs (fronts) were at a relatively shallow angle.

As time progressed and neck rakes started to increase builders found the the old parallel rockers did not produce good spring action so the rocker angle started to change by canting the front leg pivot hole upwards relative to the rear pivot point.

Sugar Bear was blasted in the media when he told a reporter that it was best to keep the rocker pivots parallel to the ground but his remark was taken out of context because what he was actually saying was that the starting point for rocker angle was parallel to the ground for a stock fork and this was how the jig should be set up. If you build every set of forks using the 120-degree angle the rocker pivot points will automatically be canted to the best position when the forks are installed on bikes having any conceivable rake angle as shown below.

Note that although the angle between the rear rocker pivot and the legs of the forks is always 120-degrees, the angle of the axle moves upwards as rake increases which is the way it should be for best suspension action. This also changes your axle height so the difference needs to be added to the length of the forks you order.

 

Dropped Rockers

Doing a mockup where you might want to investigate using Dropped Rockers to reduce trail is more complicated and unless your trail figure is much beyond 4-inches and your rake is 37-degrees or less is probably not worth wasting the time on. If you can hold trail to between 3.5 to 4.5-inches you will have a well behaved cycle regardless of rake.

 

 

 

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