Tubing use in chassis fabrication

G'day Nigel and everyone.

Just wondering if you could give me a run down or if you know of any great articles/videos that can give me a better understanding of when to use each type of tubing (material/construction type) when it comes to chassis fabrication, roll over protection (roll cages) and suspension arms.

I know that basics which are easy to find but comparisons of each and the pros/cons and when to use one over the other is abit more vague. The main types ive seen thrown around are the following and what Ive managed to find out about them so far although some sources are conflicting.

ERW - Electric resistance welded (mild steel) starts as a sheet and cold formed to shape then welded. Dimensions can vary a little.

Pro's: Cheap, readily available

Con's: The welded seam is a weak point... How weak and if its every actually an issue I dont know?

DOM - Drawn over mandrel (mild steel) is ERW tube thats been drawn over a mandrel giving it a very accurate and consistent inside/outside dimensions and work hardens the material adding strength...

Pro's: Stronger? Smoother? More accurate dimensions? by how much I dont know

Con's: Harder to find, not in as many sizes, more expensive. Does the work hardening make it "stronger" but more brittle? ie does it mean it will resist bending compared to ERW but split/shear when it does fail?

CDS - Cold Drawn Seamless - Believe it is extruded from a billet?

Pro's: No seam so no weak point? Stronger?

Cons: Expensive?

4130 - Chromoly Pro's: Stronger then steel so a smaller diameter or thinner wall can be used and have the same strength but lighter

Con's: Expensive? More or less then CDS im not sure. Hard to find (locally for me anyway) I know there is welding issues for thicker stuff but I think thinwall (less then 0.120") is ok?

Stainless - Im not sure if Stainless steel is ever used in chassis/rollcage/suspension arms? why/why not? what grade?

Aluminum - Same as stainless...

So yeh if you could fill in the blanks or point out any info that would be appreciated! where would you use one over the other? any situations you would never use a certain type?

Thanks

Hi Clint. Great post and a good one to get answered as this information is difficult to come by and takes a little experience and understanding of tube to figure out the pros and cons.

ERW, you are correct in your comments, I wouldnt say the seam is the weak point its just the tube is not graded and not tested for any deformities making it a cheap product and one that can not be homologated for motorsports due to inaccuracies in its production. 9 times out of ten it would probably be fine for roll cages and other structures but the small risk is still a risk. I use ERW tube for the majority of light (semi non structural) applications and its a great budget material to work with.

DOM, I am not familiar with this stuff as its pretty rare to see this sold in Australia although I know its popular in other parts of the world. Yes more accurate dimensions being inside diamater and outside diameter meaning that it would be less prone to failure from irregularities in production.

CDS - I am aware of CDS as this is what I built my S13 and Hilux chassis from. CDS is (as you said) drawn from bar stock over a mandrel making the material a known substance, the ID a tight tolerance and the OD a nice smooth cold rolled scale free material that needs very little preperation for motorsport use. Its been tensile tested and when sold it is supplied with a specs sheet that proves its MPA which is well above most roll cage minimum standards.

4130 - Chromoly. I have also worked with this stuff and it has been the standard in motorsport chassis construction for a long time. It has a few downsides but its production method is the same as the CDS steel but chromoly has a higher percentage of carbon, chromium and molybdenum. This gives it a greater tensile strength and therefore we need less of it to meet roll cage and chassis requirements which means a thinner wall thickness and a reduction in weight.

ChroMo is expensive and it needs a lot of care when working with it, the elements that make it strong also make it adversely affected by heat, input too much heat into chromoly and you will create a brittle heat effected zone that will cause it to snap instead of bend. This has been known to kill drivers before in rally cars so its no joke and the TIG method combined with very little gap and mild steel filler rod is the only way to work with it.

If you want to use chromoly you have to ask yourself.... do I want to save around 10kg in a complete cage for a huge increase in cost? Here in Australia we need to get approval for all chromoly cages and chassis so the extra cost is large.

Stainless isn't used in chassis construction as its expensive, weak and also difficult to work with. I bet someone has but you just never see it motorsports and thats for good reason.

Aluminum is used in a few chassis and is a great material, most 6061 alloys are strong but very ductile so you would need a lot of it to create the strength we need and therefore you would be adding a lot of weight, hence alloy wouldnt be that great for motorsport roll over protection.

A few things I will add here is that there are a few other materials on the market that sit between CDS steel and Chromoly.

These are called T45 and Docol, T45 and Docol is a low carbon steel that has greater strength properties than Mild steel but can still be welded by the MIG welding method. The problem here is that most governing motorsport bodies haven't released sanctions for these materials to allow for thinner wall thickness roll cages so you would just be creating a more expensive and stronger chassis without the benefit of any weight reduction.

Its pretty hard to move away from good old CDS steel, it may be expensive but we only build a car once, it may be a little heavy but we just need to fit lighter components and engineer our cars a little better to remove material, we can still MIG weld it so it saves a lot of time. I dont see any point using chromoly or docol unless the chassis is designed with some form of FEA analysis that tells you that you are in-fact needing this strength and you can minimise weight in the process.

Hope this helps and thanks for the great questions

Most (all?) motorsport bodiesw ban aluminium anywhere in a roll cage/structure following some very nasty crashes back in the Group B era of rallying. Part of the reason, IIRC, was cracking and fatigue failures of the structure, especially around welds, and lack of energy absorbtion while maintaining shape. There may be a good case for using it for substructures that bolt to the main chassis, but when you consider some of the fatigue and other problems, the use of mild steel tubing is often a better option for a minor weight gain.

For suspension components, aluminium has some advantages and it's sometimes used in OEM applications, but the same problems can occur and a mild steel, fabricated part may be expected to be lighter and potentially stronger than a large casting.

Cheers Gord for that info on alloy.