Tuesday, December 18, 2012

Motorcycle Frame Making and Materials Information

[Frame] Tube work.

What's the difference between tube and pipe?"
The purpose of pipe (CHS - Circular Hollow Section), is to carry either gas or fluids. Its size refers to its Nominal Bore, as it is this dimension that is used to calculate the flow.
CHS is rolled and then welded from thick, mild (low-carbon) steel and withstands being pressurised OK, but CHS is not intended for structural use. It is identifiable by a visible internal weld ridge. It is not dimensionally accurate material either and is often visibly out-of-round with a scaly black appearance.

Structural or mechanical tube on the other hand, is (at least, externally) dimensionally accurate and has a bright finish. It is also made from mild steel, and sometimes rolled and welded too. The welded variety of tube (ERW - Electric Resistance Welded) begins as a bright strip of steel which is shaped between rollers and has a visible blue weld seam along its length. Some ERW tube ends up being Drawn Over a Mandrel (DOM, or sometimes, Cold Drawn Welded), which removes the internal weld ridge, and mechanically strengthens and perfects the whole tube to its finished dimension.
Note: In the U.S. ERW can also be called CREW (Cold Rolled Electric Welded).
Another type of mild steel tube is Cold Drawn Seamless, which begins as a solid bar of carbon steel, and, as the name implies, is drawn over a mandrelto form the tube section. This is a very strong form of structural tube and can be strengthened by further cold working.

There are other families of tube, but only one is of any interest to us for buggy building: 4130 chrome moly tube.
4130 is carbon steel alloyed with chromium and molybdenum, and has a justified reputation as a tough material for highly stressed structures such as aircraft frames, dragster chassis etc.

Its main advantage is its superior resistance to bending, therefore, it can be used in thinner wall sections, thus producing lighter structures than those made from similar sized mild steel tube. It is not (as many people believe)any lighter than other types of steel tube. Unless you have a set of laboratory scales, all steel tube weighs the same (size for size).
4130 does have its drawbacks though. Due to its complex chemistry, it isrecommended that it be either brazed, or TIG welded, and may require pre and post heat treatment to help relieve the stresses built up during the welding process. Failure to do this can cause catastrophic HAZ (heat affected zone) failure.

The difference in strength between CDS and 4130 tube, is not as great as some think. CDS is typically 470mpa, and 4130 is typically 650mpa. CHS is around 250-350mpa. Section for section, CroMo is no lighter than any othersteel, and 
all steel has the same modulus of elasticity anyway (Young’s Modulus)and are therefore equally stiff. It is only its ability to resist permanent set that sets CroMo apart.