Tubing and Materials
Structural and Mechanical Tubing
This section of the Handbooks isn’t about frame or fork engineering but is intended to provide some background, using engineering principals, that have guided the development of Choppers over the decades and to establish some basic structural fundamentals that the reader can build upon with further study and research. There are scores of technical books available that should be consulted and used as reference material when you set about the process of designing a Chopper frame or a set of forks. The summary that follows can be used as a rough outline that describes the general application of engineering fundamentals to Chopper fabrication.
First and foremost don’t be mislead by the dozens of Internet chat room ‘cycle engineers’ that seem to find a place to roost in many of the Chopper discussion boards. There are some knowledgeable people posting at these sites but the intelligent threads tend to get pushed to the bottom of the stack while the more outrageous hype keeps springing up to the top. The largest problem seems to be that many people think a Chopper is a motorcycle and try to apply motorcycle engineering to something that in reality is a pretty rude and crude machine by comparison. Unlike ‘Road-Bikes’, ‘Race-Bikes’, ‘Cruisers’ and mass-produced ‘Street/Sport-Bikes’ Choppers generally aren’t built to be ‘expendable’ after being used for a few years. In fact you’ll seldom if ever see a well-built Chopper being scraped or salvaged because of age or mileage. Most stay around for decades and are ridden pretty hard. For this reason alone they have to be built a little stiffer and tougher than you’d build a more conventional motorcycle.
My biggest complaint about the misinformation bantered about on the Internet is that it’s usually never challenged so the same old myths get reposted over and over again and moved from one site to the next.
The main area that causes a lot of confusion seems to be that the various posters usually mix and match terms, descriptions, nomenclature, specifications and steel grades to the point that nobody can figure out what’s going on. To clarify our position on the subject here are the pertinent facts, as opposed to myths, you need to know about tubing and tube specifications:
1. Tubing is not pipe and pipe is certainly not tubing. For cars, bikes, motorcycles and other types of equipment and machinery you should be using what manufacturers and suppliers call ‘Mechanical Tubing’.
2. All carbon steels have very nearly the same modulus of elasticity (Young’s modulus), even the highly touted DOM.
3. DOM and ERW are acronyms for the ‘process’ used to make the tubing and don’t have anything whatsoever to with the grade of steel used in the tube. The ERW stands for Electrical Resistance Welded and the DOM stands for Drawn Over Mandrel. CREW signifies Cold-Rolled Electrical Welded while HREW designates Hot-Rolled material. Both DOM and ERW start life as flat ribbons of steel that are rolled into a long cylinder and then welded along the seam.
4. You can buy DOM or CREW with exactly the same grades of steel such as 1020 or 1018 (both very common) and many other grades as well so the process by itself doesn’t determine the stiffness or quality of the tubing being used.
5. Terms such as ASTM 513A refer to the specifications for a particular type of tubing (or Pipe) that manufacturers have to follow. There are hundreds of ASTM specifications for both pipe and tube but a few you see referenced quite often are:
ASTM-A-120 is the specification for commercial grade plumbing pipe.
ASTM-A-501 is the specification for structural grade of pipe.
ASTM-A-513 is the specification for Electrical resistance welded tubing.
Within this specification are subdivisions for various welded tubing.
Type I is Hot Rolled ERW
Type II is Cold Rolled ERW
Type III is Sink Drawn hot rolled ERW
Type IV is Sink Drawn cold rolled ERW
Type V is DOM (drawn over mandrel) CREW
Type VI is DOM Special Smooth (SSID-DOM)
Remember that even DOM tubing is electrically welded and has a seam running along its length. This weld bead is pushed flush with the inner and outer tube surfaces by drawing the tubing through a series of mandrels. The seam is still there, just flattened.
People often use the terms ‘Strength’ and ‘Stiffness’ interchangeably but actually the words describe two completely different things that should not be confused. In the design of a frame or fork assembly the objective is to create ‘Stiff’ structures.
You’ll often see chat room posts that go something along the lines of:
“According to the specs I saw that 1026 DOM had a tensile strength of 80,000 pounds and 1010 ERW was only 50,000 so I built my frame from 1.25” DOM so it would be almost twice as strong as the ones built from 1.25” ERW”.
Notice the word‘s strength’, and ‘strong’ were used but nowhere was a mention of ‘stiffness’. In fact the statement isn’t true at all to begin with since a DOM frame isn’t any ‘stronger’ than an ERW frame when it comes to bending resistance so long as the same grade of tubing steel is being used. Comparing 1010 ERW to 1026 DOM is like comparing apples to oranges. Comparing 1020 DOM to 1018 ERW is much more akin to comparing apples to apples. Both of these grades of tubing are very commonly available and very widely used in the peformance frame building industry.
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
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 stiffer 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
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 two grades highlighted in red are what we recommend as the minimum tubing specifications for the average Chopper frame.
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.
After reading all of the above you should be better prepared to properly describe a particular type of tubing to be used for a project such as:
ASTM-A-513-, Type 5, grade 1020 DOM or ASTM-A-513, Type II, grade 1020 CREW, (As Welded, or Normalized) mechanical tubing.
You’ll often hear people tossing around phrases like “I make all my frames from DOM tubing”. Now that you’re armed with information you know that such boasts don’t really mean very much on their own. If the guy’s using 1020 DOM for instance it’s actually no better than regular old 1020 CREW.
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 tube. It all has nearly the same modulus of elasticity and in most applications is equally as stiff. Put another way all carbon steel tubing, (of identical size) whether it is lowly 1010 CREW or top of the line 1026 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. 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 will start to bend. How far it bends; and how far it bends before fracturing; is determined by the physical dimensions of the tube and the grade of steel used in the tube 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 frame or fork designs.
In other words if two pieces of tubing, one being 1018 and the other 1026, 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 which is no big deal to begin with since the objective in frame design is to make a structure that has ‘ZERO’ deflection to begin with. High tensile steels offer no advantage to initial bending resistance in the normal range of applications for typical Chopper frames.
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 frame construction the lesser grade of material with a lower tensile strength is actually a better selection for some applications. Any piece of steel 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 Frame and another identical frame made from 1018 ERW they will both sustain the same amount of damage but the amount of deflection in the crushed and bent tubing will be different. The members of both frames 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 DOM is so much ‘stronger’ then CREW why don’t we need ‘special’ high-powered tube benders to handle it. If you’re bending DOM, it will take slightly more effort in a hand powdered bender for sure, but that extra resistance is due to the more uniform wall thickness of Dom as opposed to ERW and not any ‘special’ strength DOM has over ERW. All carbon steel tubing has the same initial resistance to bending regardless of the grade or process used in making it.
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 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.
This is all well and good but we need to remember that ‘strength’ and ‘stiffness’ are two different things entirely. Raw strength comes from the material and shape of the structural members themselves but ‘stiffness’ comes about from the arrangement and design of those members. For a motorcycle or chopper frame (or forks) what we want is stiffness in the assembled component. What materials we use really doesn’t make much difference as long as each individual segment has enough basic cohesiveness not to fail in its individual role of being one part in the many.
As the designer of a frame or 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.
The Weight Issue
Part of the tubing selection process involves an attempt to build the lightest frame (or forks) possible that can withstand the rigors of the riding environment the bike in question is likely to encounter. For the sake of argument let’s just say that a typical chopper frame has 25 lineal feet of tubing in its structure.
If we build a frame from solid 1” diameter steel bar stock that frame will weigh 72.09 pounds. That’s not really all that bad for a chopper since stock softail frames weight about 69 pounds. But what if we decide we’d rather use cheaper 1” diameter ERW tubing for that particular project. If we used 1” diameter, .065-inch wall tubing that frame would weigh 17.55 pounds. We’d save about 54 pounds. Unfortunately a chopper frame made from thin walled tube of that dimension wouldn’t last very long. A more realistic selection would be 1” diameter, .125” wall tubing which comes in at around 32 pounds. This is still a savings of 40 pounds, which is worth about 5 horsepower on the average bike. From a structural standpoint there is no difference between the solid steel frame and the tube frame except that the solid steel frame can be bent into a pretzel without breaking where the tube frame will eventually fracture if bent to far. How far? Several inches. If your frame were bent that badly it wouldn’t make any difference what it was made of.
Let’s say that you’re sold on the idea of using tubing instead of solid steel bar stock for your frame. You know that old Harleys were made from 1” diameter by .120” wall tubes (ERW by the way) and you want to use this material instead of the .125” walled tube. Well you’d save 1.01 pounds. Logic tells us that it’s not worth it since we do know that many old Harley 1” frames didn’t hold up to well over time. You want a bigger margin of safety so you decide to move up to 1” diameter by .134 inch walled material. That frame would only weigh 2.87 pounds more than an old stock model and be significantly stiffer yet still 38.68 pounds lighter than a solid steel frame, or a stock softail for that matter.
For hard-ridden choppers it is always better to err on the side of conservatism and move up one or two notches in wall thickness if you’re uncertain about what it is that you’re trying to achieve. Adding five or six pounds to the average chopper means you lose about one horsepower at the rear wheel but that extra weight may save your life someday.
Having said all of this, most of you know from reading other sections of the handbook that I am an advocate for lightweight choppers. Ounces add up to pounds and pounds equal horsepower and so on. I do not however advocate building so-called lightweight frames for choppers and in fact I would never build a frame with anything less than 1.25 by .120-inch tubing for anything including old 45’s. Time is the great educator and time has shown that lesser frame specs just can’t endure without the rare, but eventual, failure and none of us want to be riding that particular machine when it finally goes south even if it’s forty years down the road. I personally would rather go five or six pounds heavier on the frame and switch to aluminum on the tank and fenders. It’s just a way more logical approach to weight reduction.
While we’re on the subject of weight it might be a good idea to take a brief look at the loads that are placed on bikes with respect to component weight/stiffness. Loads on the typical bike can be external, such as loads imposed by road conditions such as pothole encounters and even the load imposed by the weight of the rider. Loads can also be internal meaning they come about from the components themselves. Most notable of course is the load imposed on the frame by the torque of the power plant/drive system combination. If you, as the designer can ‘control’ these loads, or at least limit them to very specific ranges, you can build incredibly light bikes. Drag bikes are a good example since the condition of the strip surface is a ‘known’ variable and in this case external loads (impact) from the pavement itself are almost negligible. All you really need to do in this situation is to design a frame that’s stiff enough so that the motor won’t twist it out of shape.
If you’re building a Bar-hopper that’s only driven on smooth city streets the structural criteria is completely different that what you’d use to design a cross-country bike. A show bike only has to be stiff enough to go from the trailer to the showroom without collapsing from the weight of the rider! The point being that stress and ‘load’ numbers are extremely variable and have to be established at the outset of the design/build process.
You as the designer or owner or fabricator have to make a determination about how you expect the assembled frame, forks and components to behave under the conditions you anticipate the bike in question will be likely to encounter during its lifetime. I can’t make those decisions and neither can anybody else and that’s why Choppers are truly unique creations.
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. Again, in some cases we can even use a thinner wall.
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 40-inch legs, built with 1.125” solid 1018 bar stock have survived the ages with few failures. We can use this data as a benchmark and move forward without trying to reinvent the wheel.
Springers and Girder Forks
We’ve all seen the Internet chat room shop demonstration’s where a guy clamps a section of tubing in his bench vice and hangs weights on the end to show how much the tube bends under load and goes on to explain why you can’t possibly build what you really want to build because it’s ‘mathematically’ unsound. Oh yeah, sure enough dude, thanks for saving me from myself!
Well let’s look at the loads forks actually encounter when they’re mounted on a real bike and not in a bench vice.
First of all the loads imposed on a set of forks can come from a wide variety of directions. For instance you might run right straight ahead and smack head-on into a concrete building wall. On the other hand you might drop the front wheel into a deep pothole in the road while you’re out enjoying the wind. And a third option is that you’re a wheelie addict and like to slam the front-end down hard trying to impress young innocent girls. The stress transmitted up through the wheel-tire-axle combination; back into the fork legs themselves, come from very different directions in these typical examples. As a result the loads are distributed to the entire front-end assembly in completely different ways. If you’ve built even a half way decent set of forks the first thing that goes south is the wheel rim which will get dented or become elliptical and then the axle will bend. Lastly the forks will start to bend. The bike goes into ‘disaster’ mode the minute the wheel or axle fails, certainly long before the forks buckle. Ironically people will bitch about the forks bending but won’t say anything bad at all about the axle or wheel rim bending which lead to the problem to begin with.
Even a brief examination of Figure 3.29 gives one a pretty good idea of why forks can withstand the loads that are imposed on them since 90% of those loads involve curbs and potholes where the force is largely a compressive impact. Sure there are some bending loads depending upon the angle of the impact but by and large the major stress is one of tube compression. Nobody in their right mind expects that forks should withstand direct impacts against building walls or cars or even hard-hitting wheelies.
Before we build a strong set of forks we have to go back to the beginning of this section and decide on exactly what we expect those forks to do. Do we want them to withstand massive repeated wheelies? Do we want them to withstand 12-inch deep pothole encounters at 90 miles per hour? Do we want them to handle typical road conditions and bike-use that most people encounter on a day-to-day basis? Your particular expectations for the fork’s usage make a huge difference in how they are designed. In other words you get what you ask for, or in another manner, what you pay for. Cheap gets cheap, stock gets stock, custom gets custom.
I have no way of proving this, but I’m beyond being pretty sure, that no fork maker out there today has ever paid an engineer to actually review, structurally, what it is that they’re selling. The actual variables and dynamics are just too huge to be encompassed in a few calculations.
The closest structural analogy I can envision is trying to calculate the strength needed in medieval jousting poles. The poles have to be light enough for the Knights to handle yet strong enough to pierce steel plate armor; said armor having unknown deflection angles, density, approach direction and velocity and hundreds of other factors that have to be considered.
If we look at fork strength from a simple mathematical standpoint everything that results is massively heavy, virtually impractical and unusable. The old tube in a bench vice routine doesn’t begin to tell the real story.
The table below lists some of the more commonly used sizes of Mechanical Tubing that most large suppliers keep in inventory.
Table 3.3 - Tubing Dimensional Data
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.
Just for reference note that the solid 1.125” bar stock that many Springers are made from only has a moment of Inertia of 0.079, which is roughly comparable to 1.25x.156-inch, tubing which is considerably lighter.
If you’re wondering why manufacturers use what appear to be some ‘strange’ dimensions for wall thickness these values actually correspond to the comparable BWG (Birmingham Wire Gauge) sizes. Nobody knows exactly how or when these ‘gauges’ came to be adopted but it appears that they were in common usage by as early as 1735. Table 3.4 below lists the conversions and even today many fabricators specify both tubing and plate stock by the wire gauge and not the decimal equivalents
I would like to remind readers that at least 85% of all chopper work is an exercise in artistic expression as opposed to an exercise in engineering theory. As a result what we often build is done on the fly in back shops late at night where we have to rely on empirical knowledge gained through decades of experience rather than textbook fundamentals to guide us. If you are not 100% confident in your judgment based upon those same past experiences or in your skills as a fabricator I urge you to have the design and fabrication work done by qualified professionals. Building Choppers is a dangerous enterprise as the variables are largely unknown until the project is finished and those of us who do build choppers recognize that there are serious consequences associated with the work we do. If you build a bike you have to be personally responsible for that creation during your entire lifetime so you better be sure you know what you’re doing because somebody’s life will depend on it someday.
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