Q. What is the difference between tube and pipe?

Tubing is measured on the outside diameter, while pipe is measured on the inside diameter.

Q. Should I be using chromoly or mild steel for a rollcage?

4130 chromoly tubing is a superior material for any rollcage or chassis structure. One of the greatest advantages is its strength to weight ratio. You can achieve an equal or in most cases stronger assembly when using an alloy tubing. The weight of an average rollcage can be reduced by almost 40 percent when using appropriately sized 4130 materials. The other great benefit to using alloy tubing is its ability to flex without breaking or bending immediately. The spring-back of 4130 tubing is almost double that of ordinary mild steel tubing, leaving the chassis to flex without breaking. Other than the additional cost, welding chromoly tubing is the only other disadvantage, as it has to be tig welded. Even so, with todays technology of filler materials, as well as square wave welding technology (pulse welding), it is much easier to learn to tig weld. There are also better blends of filler material to create a more desirable weld. All the options weighed, chromoly is the best way to go.

Q. What are some key tips to welding chromoly tubing or plate?

The first and most important factor is to use an appropriate tig (tungsten inert gas) welding machine. The next tip would be to clean the material as best as possible outside the weld area, as well as inside the weld area. Clean material welds better; we generally use cold-drawn material as it is cleaner, and welds better than hot-drawn material. Expansion holes must also be drilled to keep the tube from pressurizing, and blowing back out distorting the weld area at final closing of the weld. These must be drilled in a tube that is going to be closed off. If the tube is not to be closed off (welded shut or mounted at either end) it is also good practice to purge the tubing with argon from the inside. Tiny welds are not always good welds; it is imperitive that an appropriate size filler material be used when welding two sections together. Fusing two thin components together is not recommended: as material is drawn into the weld area, the component will thin out around the weld losing strength. A slightly raised weld seam is much better than a flush weld. A weld should never be concave.

Q. Do the bends in my rollcage tubing make it weaker?

Not necessarily, just because there are bent sections in a chassis structure does not make them weaker- given that the structural design is up to par. When material is bent it actually increases the tensile strength of the material in the bend area substantially, as long as the material has been cold formed. Cold forming is the process of bending or shaping material without heat. Cold forming is one of the only ways to strengthen some types of material. The tensile strength of some material can increase more than 50 percent when cold formed. Cold forming introduces a certain amount of work hardening. Work hardening occurs when material is being stressed but not strained. When applied correctly, work hardening can significantly add to the tensile strength of the material.

Q. So many companies claim to mandrel bend their rollbars and rollcages- whats best?

The phrase 'mandrel bending' is thrown around quite a bit in the aftermarket automotive world. Most rollcage materials like DOM mild steel and chromoly tubing are not suitable for mandrel bending. Even CREW tubing, which is fairly elastic, is not recommended to be mandrel bent for chassis components. When the material is formed with an internal mandrel, it takes on an enormous amount of stretching and compression. The inside wall has to compress and the outside wall has to stretch. When mandrel bending materials that are already hardened or that do not have enough elasticity, such as DOM and 4130, the work hardening that occurs in the bend area will make the component subject to failure from being pushed past its useable limits. When bending round materials for chassis and roll cage structures, the empty bending methods are much preferred over mandrel bending.

Empty bending is bending without a mandrel and with specially machined tooling that incorporates a multi-radius bending die to form the material instead of compressing and stretching it as much. Even the most sophisticated mandrel bending machines with pressure die assist and collet boosting will not have as positive of an effect on the formed area as empty bending. The best way to bend harder materials used for chassis and race car structures is to empty bend them with correctly machined bending and follower dies to form the material with the least amount of wall thinning. Generous radii should also be chosen to bend these materials to limit the amount of stress and work hardening.