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Rigid Forks

Rigid Forks have been around for a long long time. Anybody whose ever ridden an old bicycle is familiar with the concept. Over time most makers started to add some type of 'suspension' on their top-line models but for the most part the average store-bought bicycle still had rigid forks.

Very early on, years before the first chopper magazines were ever published, builders were running long Rigid forks on street bikes but once the rags began appearing on news stands rigid forks were immediate hits due to their 'wow' factor, especially among the show bike crowd. 40-over Rigid front ends were not scarce and 24 to 30 over stock were commonplace in southern California.

Freddie Hernandez and Denver Mullins both used to do over the counter Rigid forks that you could just buy and bolt on but there were dozens of custom shops building virtually anything a person could imagine.

The picture of Denver Mullins above shows a good example of the 'conservative' Rigid while the photo below of Freddie is good example of the more radical extremes.

I personally think Rigids are the best looking forks a person can run on a bike second only to a Harman Spirder but that's just my personal opinion.

Here are a few other web pictures of Freddie that have become web 'classics'.

The best thing about Rigids however is that they're very easy to build and if you get everything set up just right for a particular bike they perform wonderfully. In fact most folks I know who run a Rigid won't use any other type of forks on their builds.

I don't know who built this bike. I found the picture on the web but it's just so well executed that I had to include it.

I know that the first thing most readers will be thinking is the 'ride' just has to be horrible but in reality it's not to different what you'd experience running a long Springer.

 If you visit the various discussion boards you'll read all kinds of threads about Rigid forks with posts both pro and con about the concept but in almost all cases the posts that are negative are made by people who have never ridden a Rigid and in some cases by people who have never even seen one except in pictures.

Recently however one man got a lot of people interested in building Rigids when he decided to include one on a bike he was building for the Born Free event in 2017.

That man was Joel Hauenstein of Image One Art in Ada Ohio who started his build documentation in 2014 on the Jockey Journal Board. That thread got a lot of followers hooked on his project. You could almost see his brain at work as you followed every post. His final creation was the famous 'Jart' seen below.

Sadly the world lost Joel a few years ago in a car/bike accident. This incredibly talented man will be missed by all of us but he enriched the lives of everybody he touched either personally or by his works.

You can find his threads and various post on the Journal by searching for 'Image1'.

 My own contributions to the Rigid Revolution have been limited for the past few years since I've been spending most of my time getting some CNC machines up and running and learning Gcode.

My last set of forks was for a Sportster project, seen below that was sold before it was ever finished. 

I try to always build my bars so that they appear to be an integral part of the fork legs.

 

Design and Building

I've been building Rigid forks for about fifty years so I have a little experience with them that is based on reality and not theory so what follows can be considered to be fairly good information to be used by prospective builders, at least as starting points for a project.

First of all forget almost everything you've ever picked up on the discussion boards as to 'leg' design and material selection as most of the ideas presented on the boards have never proven to work very well in execution.

I'm sure you've read where filling the leg tubes with a mixture of fiberglass resin and strands or roving makes them stronger and more able to resist bending. Also forget about the idea of putting a sleeve of solid rod or secondary tubing inside the main legs as this won't help at all. In fact Joel built his first set of tubes from 1.25 x .125 wall DOM slugged with solid rod and these broke.

The only types of leg construction that have been proven to work over time is to use either solid bar stock or thick wall tubing.

When it comes to material selection you'll have probably also read that 4130 chromoly bar stock or tubing is the only way to go because it's so much 'stronger' than regular steel. This is not exactly correct as all steels including 4130 have a modulus of elasticity also called 'Young's Modulus' of very near 30,000,000 psi.

One other thing to take into consideration is that the same pothole that will permanently bend a rigid fork leg will probably be equally destructive for all other types of forks regardless of suspension type. I get a kick out of discussion board know-all's telling people a 12-inch deep pothole will destroy their rigid forks while they expect that they'll just glide through the same hole with their extended hydraulic front ends.

Steels

The various Grades of steel used in tubing are established by the Society of Automotive Engineers (SAE) and are expressed with a series of numbers that represent the chemical composition of the particular alloy. For the tubing that is typically used in racecars and bikes, mild steel carbon tubing, the first two digits (10) of the designation signify that the material is indeed ‘Carbon Steel’.

Table 3.1 - Composition of Common Tubing Steel

Grade

Carbon

Manganese

Sulfur

Phosphorus

Silicon

Nickel

Chromium

Molybdenum

 

1006

.08 max

.25-.40

.015

.025

.035

-

-

-

1010

.08-.13

.30-.60

.015

.025

.035

-

-

-

1018

.14-.20

.30-.60

.015

.025

.035

-

-

-

1020

.17-.23

.30-.60

.015

.025

.035

-

-

-

1026

.22-.28

.30-.60

.015

.025

.035

-

-

-

1030

.27-.34

.30-.60

.015

.025

.035

-

-

-

1040

.35-.44

.30-.60

.015

.025

.035

-

-

-

4130

.28-.33

.30-.60

.015

.025

.035

-

.80-1.10

.15-.25

If you look at the column labeled ‘Carbon’ you can see what the next two digits describe and that’s the percentage of carbon a particular alloy contains. As the amount of carbon in the alloy is increased the harder the steel becomes but it also gets more brittle and less ductile. Most mills manufacturer about sixty grades of steel but the ones contained in the table above are very common and readily available almost everywhere. We threw 4130 Chromo in there for reference.

Table 3.2 - Grade Properties

Grade

Tensile Strength

Yield Strength

Allowable Stress

Modulus of Elasticity

 

1006

55,000 psi

45,000 psi

22,000-31,000 psi

29,200ksi

1010

55,000 psi

45,000 psi

22,500-31,000 psi

29,300ksi

1018

60,000 psi

50,000 psi

25,000-33,000 psi

29,300ksi

1020

65,000 psi

55,000 psi

27,000-36,000 psi

29,300ksi

1026

75,000 psi

65,000 psi

32,500-43,550 psi

29,500ksi

4130

100,000 psi

90,000 psi

45,000-60,000 psi

30,600ksi

If we look at the data contained in Table 3.2 we can see that the various grades of steel have different structural properties. Tensile strength is merely a term that describes how much force it takes to pull a billet of steel apart just before it starts to completely fracture. Yield strength is a term that describes how much force is needed to pull a billet of steel until it stays permanently deformed and won’t spring back to its original shape once the load is removed. The modulus of elasticity, typically called Young’s modulus, is a term that describes the relative ‘stiffness’ of a material. Virtually all carbon steels fall within a very narrow band that ranges from 29.2 to 29,500Ksi. (29,500,000psi). Silicon bronze has a modulus of 15,000Ksi while pure Copper has a modulus of 16,000Ksi. Aluminum has a modulus of only 10,000Ksi. Most standard engineering formulas use a value of 30,000Ksi (30,000,000 psi) as the modulus of elasticity for all carbon steels including 4130 chromoly.

The column that tabulates the ‘Allowable Stress’ is one that most people don’t like to talk about since the numbers don’t look quite as impressive as those in the ‘Tensile’ column but these are the numbers you have to use when you finally get down to the mathematics of calculating tube sizes and materials. Most tubing manufacturers publish their own table of allowable stresses for their products but when this information isn’t exactly known engineers will generally set this value to a figure that ranges from 67 to 50 percent of the tensile strength. Basically this is the ‘Safety factor’ you often hear about. In addition, by using the Allowable stress figures in your calculations you’ll almost always end up with a steel tube selection that won’t have any significant long-term fatigue limitations.

Just so you know, for future reference, 1020 DOM is a very common and readily available variety of mechanical tubing and if you just ask for DOM at the supplier this is what they’ll probably sell you. The good stuff is 1026 DOM and it’s pretty expensive and much harder to find since there is little demand for it. If your builder says he’s using DOM ask him what grade and ask to see the receipts showing that it’s really 1026.

Another thing you often see quoted on the Internet is tensile strength used as a reference for the suitability of one type of tubing over another. In actuality the tensile strength has little to do with determining the proper size of tubing to be selected for a particular application.

The initial resistance to bending of all carbon steels is almost exactly the same. It makes little difference what the grade is or what type of process made the bar or tube. It all has nearly the same modulus of elasticity and in most applications is equally as stiff. Put another way all carbon steel bar or tubing, (of identical size) whether it is lowly 1010 CREW or top of the line 4130 DOM have almost identical initial strengths when they are used to build a frame or a set of forks. If it takes, say, 500 pounds of force to deflect a section of ERW tube, the same force will deflect a section of high quality DOM or 4130. And put even another way we can say that all carbon steel tubes (of identical size) are equally as strong (or weak) with respect to initially resisting an applied load. Once the load exceeds the yield point, for a particular cross-section, the tube or bar will start to bend. How far it bends; and how far it bends before fracturing; is determined by the physical dimensions of the tube or bar and the grade of steel used in the material and this is actually where the tensile and yield strength come into play.

All carbon steel will deflect the same amount under identical loads but steels with higher yield strengths will deflect to a greater extent before the reaching the elastic limit and become permanently deformed. This is the single most important fact you must come to grips with in order to make intelligent choices about fork designs.

In other words if two pieces of bar or tubing, one being 1026 the other 4130, are subjected to the same static load, they each will deflect an identical amount. It is only when they deflect enough to approach the elastic limit that the difference in tensile stress comes into play as shown in figure 3.28 below.

As steel is subjected to stress it will begin to deflect in a more or less linear fashion as the stress increases. As long as that steel is not stressed past the elastic limit point it will spring back to it’s original form once the load is removed. If the stress is so great that the steel moves past that point it will remain bent after the force is removed. The so–called high-tensile steels will simply bend further before becoming permanently deformed. High tensile steels offer no advantage to initial bending resistance in the normal range of applications for typical Chopper forks of any type.

Don’t let anybody tell you that a tube with 60,000 psi of tensile strength is structurally ‘stiffer’ than a tube with only 40,000 psi of tensile strength because it just isn’t so. Both grades of tubing are equally as ‘stiff’ with respect to initially resisting bending forces. In fact in some cases for both bike and car suspension construction the lesser grade of material with a lower tensile strength is actually a better selection for some applications. Any piece of steel bar or tubing becomes worthless, structurally speaking once it reaches the yield point, which as you can see in the tables, is significantly below the ultimate strength. If you crash a 1026 Dom fork and another identical fork made from 4130 they will both sustain the same amount of damage but the amount of deflection in the crushed and bent bars or tubing will be different. The members of both fork sets will begin to deflect under virtually identical loads.

What is misleading when we banter around these ‘structural’ terms is that they aren’t brought back to a comparison in the real world. For instance when we talk about a force of, let’s say 20,000 pounds per square inch, needed to bend steel we don’t mentally equate this to what it actually means. It sure sounds strong, but we have to remember that this reference number applies to a piece of steel that’s only one inch long by one wide by one inch thick and that’s why it takes such tremendous pressure to bend it. If you look at a piece of one-inch steel bar that’s 12 inches long it only takes a small fraction of this force to bend it completely in half.

Reality, and not Internet gossip, tells us that if 4130 is so much ‘stronger’ then other carbon steels why don’t we need ‘special’ high-powered benders to handle it. I bend all types of materials including 4130 in a manual bender and I can't tell the difference. All carbon steel tubing or bar stock has the same initial resistance to bending regardless of the grade or process used in making it. There is a difference in 'spring-back' and this gets back to the yield point mentioned earlier.

Raw materials in their own right really don’t have any ‘practical strength’ so to speak. It’s only when these materials are given ‘shape’ that we first begin to see practical strength become a reality. For instance raw steel is pretty worthless in its billet form from a structural application standpoint but once it’s formed into beams, bars or tubing it starts to become useful for building things and is given some basic intrinsic ‘strength’ purely due to the various forms it is shaped into.

As the designer of a set of forks we all have to arrive at a finely tuned compromise and balance between strength, stiffness, weight, practicality, workability (bending, welding or machining) and even costs and availability of materials. It’s a tough row to hoe in most instances, as there are dozens, if not scores, of variables to consider.

What makes the matter even more complicated is that, with respect to building a chopper, we’re trying to create a mechanical piece of rolling artwork that has to appeal to the visual and emotional senses and these factors can’t be explained mathematically.

 

Shape or Dimensional Strength

One of the easiest ways to add stiffness in any structural member is to simply make it bigger, but not necessarily heavier. We do this by moving the ‘surfaces’ of the member further away from the intersection of the x and y axis of the particular shape we’re working with but we don’t change the ‘thickness’ of these ‘surfaces’. For example we can increase the rigidity of a simple I-beam just by making it taller and wider but the thickness of the flanges and web can remain constant and in some cases can even become thinner. For tubing we can just go with a larger outside diameter while keeping the same wall thickness. For bars we just increase the diameter.

Thanks to science we have a very convenient ‘tool’ at our disposal that we can use to estimate the differences in relative stiffness between various types of ‘sections’ that have different dimensions and this tool is called the' Moment of Inertia', written with the capital ‘I’ in most equations. All that ‘I’ signifies is the cross-sectional area, in square inches, of the member in question, to the fourth power. So it’s actually just Area4. There is nothing mysterious or special about the moment of inertia, it’s just a mathematical convenience but for frame or fork builders it’s really pretty useful.

The moment of Inertia tables can be thought of as a fabricators ‘interchange’ book. Members that share similar Moments of Inertia also share similar structural characteristics if used in identical applications.

For instance we can see by looking at table 3.3 that 1.125x.134-inch tubing and 1.25x.083-inch tubing have nearly identical ‘I’ values so can be ‘substituted’ if need be. The advantage is that the larger diameter, thinner walled tube saves a bunch of weight.

For us Chopper builders the Moment of Inertia is extremely helpful since we can use it as a gauge against what has been successfully or unsuccessfully built in the past. We know from history that one-inch diameter (.120 wall) frames are marginal at best with power plants up to about 70 horsepower if ridden hard so we need to build above that baseline at least.

In a similar vein we know from history that very long Springers with up to 36-inch legs, built with 1.125” solid 1020 bar stock and 4130 have survived the ages with few failures. We can use this data as a benchmark and move forward without trying to reinvent the wheel.

 

Tubing and Bar data

The table below lists some of the more commonly used sizes of Mechanical Tubing that most large suppliers keep in inventory. Note the Moment of Inertia values in the right hand column. This is the value used to compare the relative stiffness of one size to another.

For instance we can see that 1.125-inch tubing with .156 wall is as stiff as 1.25-inch tubing with only a .095 wall and vise versa.

Table 3.3 - Tubing Dimensional Data

Outside

Diameter

Wall

Thickness

Inside

Diameter

Weight

Lbs./Ft.

Sectional Area

In Sq. Inches

Moment of

Inertia (I)

1-1/8” (1.125”)

.083

.834

0.813

0.272

0.037

.095

.810

0.918

0.307

0.041

.109

.782

1.037

0.348

0.045

.118

.764

1.112

0.373

0.048

.120

.885

1.288

0.379

0.049

.125

.875

1.335

0.393

0.050

.134

.857

1.418

0.417

0.052

.156

.813

1.614

0.475

0.057

.180

.765

1.817

0.534

0.062

.188

.750

1.881

0.553

0.063

.219

.687

2.119

0.623

0.068

.250

.625

2.336

0.687

0.071

.313

.500

2.710

0.798

0.076

1-1/4” (1.25”)

.083

1.084

1.034

0.304

0.052

.095

1.010

1.448

0.345

0.058

.120

1.010

1.448

0.426

0.069

.125

1.00

1.502

0.442

0.071

.134

.982

1.597

0.470

0.074

.156

.938

1.823

0.536

0.082

.180

.890

2.057

0.605

0.089

.188

.875

2.132

0.627

0.091

.219

.812

2.411

0.709

0.099

.250

.750

2.670

0.785

0.104

.313

.625

3.126

0.921

0.112

 

.375

.500

3.500

1.030

0.117

           

1-3/8” (1.375”)

.083

1.209

1.145

0.337

0.071

 

.095

1.185

1.299

0.382

0.079

 

.109

1.157

1.474

0.434

0.087

.120

1.135

1.608

0.473

0.094

.134

1.107

1.776

0.523

0.102

.156

1.063

2.031

0.598

0.113

.180

1.015

2.297

0.676

0.123

.188

1.000

2.383

0.701

0.127

.219

0.938

2.704

0.795

0.138

.250

0.875

3.004

0.884

0.147

.313

0.750

3.550

1.044

0.160

 

Solid Bars

 

5/8”

-

-

1.043

0.307

0.008

3/4”

-

-

1.502

0.442

0.012

7/8”

-

-

2.044

0.601

0.029

1”

-

-

2.670

0.785

0.049

1-1/8”

-

-

3.379

0.994

0.079

1-1/4”

-

-

4.173

1.227

0.120

Keep in mind that this isn’t a comprehensive list of tube sizes just the more commonly available size combinations that work well with most Chopper fabrication work.

Now that we're past the boring stuff it's pretty easy to see, based upon proven empirical data, that very long, sometimes 40-over Springers and Rigids have been built over the years using 1.125 and 1.25-inch solid bar stock without any failures in the legs themselves under normal road conditions. Using the Moment of Inertia values we can see that 1.25-inch tubing with a .375-inch wall is almost identical mathematically to solid 1.25-inch solid bar stock. Also that 1.125-inch tubing with a .313 wall is as stiff as solid 1.125-inch bar stock. Using tubing saves a few pounds of weight.

Notice that I said failures in the 'legs'. Over the decades there have been hundreds of documented cases, many I've personally seen, where both Springers and Rigids have had failures in the area of the welds where the legs penetrate the lower tree. The majority of such failures involve forks made using 4130 chromoly. Such failures are not because the fork legs bent to much but rather because the tubing fractured at this junction point. On the few front ends I've seen made from 4130 the failures were definitely due to stress fractures cause by work hardening or weld embrittlement, perhaps combinations of both. The failures on the mild steel forks looked like failures caused by deep weld penetration into the tube wall creating stress risers.

In an attempt to eliminate this problem many of us have been welding the legs to the lower tree only at the upper penetration point using very shallow welds. Some have switched over to brazing or silver-solder at this connection point. So far not enough time and not a large enough number of forks have been built to see if this made any difference. Joel suggested using a 'cinched' type lower tree but I don't know if anybody has tried that yet.

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