Longitudinally welded tubing for suspension?

[Alex J.]

How bad of an idea is using ERW tube for pushrods and a-arms? The seam adds another path for failure, but assuming good quality tubes, are the stresses enough that we need to worry about failure in 5/8" x .049 or .065 ERW tubing? My gut says no. But, then again, this is the first suspension I've ever designed, so I don't feel confident going forward on FEA alone.

Thanks ahead of time for any advice you guys can give.

[Owen Thomas]

I'm not sure where you're finding ERW tubing at such a low diameter, but if you are getting significant cost savings it shouldn't really make a difference versus seamless. The largest contributing factor to the strength of your linkage will be material type anyways, assuming some good welds at the joints (ends). Speaking of welds, the biggest pain in the butt with longitudinally welded tubes will probably be trying to get a decent weld overtop of the seam, but I cannot confirm this having never used them myself.

You can do a quick tensile or buckling analysis of the tube fairly easily if you know suspension loads; that will tell you what wall thickness you can afford. Definitely no FEA required there, just give a small extra safety factor and call it good.

[Alex J.]

Thanks for the reply.

Based on FEA (assuming DOM tubing), it's safe to use the smallest thickness (5/8" x .049") that is offered by my supplier, Steel Supply, LP in Houston, TX. The next size up that they offer is 5/8" x 0.058, then 0.065. Not knowing what effect the weld will have, I thought I'd go with 0.065, which I feel is probably overkill but a reasonable sacrifice for the peace of mind.

Like you said, it mostly comes down to material. I'll concentrate my efforts there. I've designed for 1020 steel, but chromoly isn't out of the question. I don't seem to have need for the extra strength, but won't know anything for sure until the thing is built and it holds up (or doesn't). Your thoughts?

[BillCobb]

I wonder if the judges will give you any latitude with this one....

[mech5496]

Alex, for all your wishbones, pushrods etc you could get away with a length of 6m or so. If I were you I would go with seamless tubing; whatever the price difference, it shouldn't be unaffordable due to the small quantity. Probably won't make a huge difference, but I would prefer it for my car. Oh, and you don't need CrMo tubing; your WT should be enough to be able to weld threaded ends for rodends or spherical housings, you name it, so the limiting factor would probably be having a tube thick enough to be able to weld it.

[Alex J.]

Speaking of threaded ends, I can only find 4130 threaded ends that fit 3/4" x 0.058" tubing. My supplier will have to order that size tubing in either seamless or welded, so I'm going to go with seamless. Thanks for the help on this.

On another note, anyone know where I can find 1020 threaded ends? I'd like to avoid TIG if possible.

[MCoach]

I'd like to avoid TIG if possible.

What? I'm curious. Do you have access to a TIG welder?

[Owen Thomas]

McMaster Carr has a selection (http://www.mcmaster.com/#tube-end-weld-nuts/=o5xs02). They don't look all that nice to me, however.

These are probably one of the easiest parts to make yourself though, so if you can't find what you like it's not a stretch to turn out a batch from some hex or bar stock.

[Alex J.]

What? I'm curious. Do you have access to a TIG welder?I do. But, I'm experienced with MIG. I spent a week trying to get the same quality with TIG that I get with MIG, failed, and decided I was wasting my time.


McMaster Carr has a selection (http://www.mcmaster.com/#tube-end-weld-nuts/=o5xs02). They don't look all that nice to me, however.

These are probably one of the easiest parts to make yourself though, so if you can't find what you like it's not a stretch to turn out a batch from some hex or bar stock.Thanks for the link.

As for making them, I'd love for us to, but we're a small team (6 members). If it doesn't defeat the purpose of the project to do so, then we're buying it.

[mech5496]

Alex, I have been there myself (really small team with really tight budget) and trust me, if you have workshop access and the appropriate machinery, it is way easier to build stuff than buying (especially true in Greece) and heaps cheaper most of the time. If we are not talking things like differentials/brake calipers or the such which are really hard to build, or standardized items like bolts, then build it! You will be much happier because it would be you that specifies the part, not some generic manufacturer, plus you will save you some money (and maybe time). As far as TIG welding goes, I'd suggest to give it another try. I haven't touch a welder since I got into the team and it was not that hard to learn. When you get it going, the quality of the welds is so superior you will never use MIG again! :)