Cold drawn mild steel vs. longitudinally welded.

[RollingCamel]

Hello everyone.

We have made the choice to buy seamless mild steel however I'm not sure that it was the right decision.

Welded steel is much cheaper and all the sizes we need is available in Egypt. The chassis was about 32.5 kgs based on the sizes we want. However, based on PatClarke's column and some recommendations from others, we have made the choice to go for seamless tubes which is much heavier....58Kgs the chassis should weigh.

Without testing the tubes in the lab, thanks to our stupid bureaucracy, I'm not sure that we have taken the right decision. It is important to me to know what you think because if seamless tubes are needed then our future teams must consider some serious replacement.

[RollingCamel]

Hello everyone.

We have made the choice to buy seamless mild steel however I'm not sure that it was the right decision.

Welded steel is much cheaper and all the sizes we need is available in Egypt. The chassis was about 32.5 kgs based on the sizes we want. However, based on PatClarke's column and some recommendations from others, we have made the choice to go for seamless tubes which is much heavier....58Kgs the chassis should weigh.

Without testing the tubes in the lab, thanks to our stupid bureaucracy, I'm not sure that we have taken the right decision. It is important to me to know what you think because if seamless tubes are needed then our future teams must consider some serious replacement.

[exFSAE]

58kg, wow that's a lot.

It really comes down to availability and need. Personally I would have gone with the option that gives you the tube choices you want and results in a 30-ish kg frame. However, if there's no other option and you're pressed for money.. big heavy tube it is.

I'd be amazed to see a FSAE frame fail tubes in overload (but things surprise me every day!), so I'd go with the welded tube.

[Lutter]

If the welded steel would fail, it isn't because of its properties compared to seamless, it is because of its poor design. If it is still possible to switch choices, it sounds like welded would be overall better. Source, price, etc.

[Hector]

Sounds like you've made an error in your chassis stiffness calculations. Perhaps you set your target too high. Maybe you are underestimating the stiffness of your design. I'd put the weight of a "very heavy" frame around 40 kg. 58 kg is without a doubt much heavier than necessary.

We have run seamless tubes each year. With the our target stiffness set by our suspension and chassis guys, we were able to achieve a 25 kg frame. Stiffness was calculated in FEA and validated through physical testing. We've gotten very good at getting our FEA and physical results to match.

Maybe so we can help you a bit more:
1) A picture or screenshot of current frame design with labeled tube sizes/thickness.
2) What is your target stiffness? Why is it that stiff?
3) What is your basic engine choice, suspension choice, etc...
4) How are you calculating the stiffness of the frame? What program are you using?

[RollingCamel]

Hector, what makes the chassis heavy is our assumption that we must use seamless tubes. Our initial weight was 32.5 kg using profiles same as the rules with very small deviation in some parts.

The only available seamless size is 26.7x2.87 mm or 3/4". That what makes it heavy, if we can change tubes sizes after sending the SEF, then we would replace the steel if welded tubes will work good.

[Hector]

Ah, I understand now. In that case I agree with the above statements - welded tubes are probably your best bet. A 25 kg increase in frame weight is definitely not in your best interest.

In the future I'd see if I could track down a seamless tube manufacture, but I have no idea what your availabilities, costs, or shipping times are, so use your best engineering judgment http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Tim.Wright]

bending, notching and welding an entire chassis worth of 2.8mm steel sounds like a death sentance too.

tim

[The_Man]

We have always used longitudinally welded tubes on our chassis exactly for the same reason as you. If we use seamless then it narrows down our available cross section options that we have and makes our chassis slightly heavier.

However, 35kg to 58kg just because you are using seamless tubes of the same size is surprising. Is it that you are simply replacing the thinner tubes with the seamless 2.8mm tubes in your design? If you use a thicker tube may be you can get rid of a few tubes here and there while not loosing too much rigidity.

On a side note: The Baja competitions in India have a rule that makes use of seamless tubes compulsory. This had sparked an argument focused on the real marginal utility of having the seamless tubes. One of our suppliers who seems to be an expect also, was surprised by the rule, he said it was not worth paying that much extra for the seamless. The seamless tubes started being manufactured because tubes(rather pipes) used with internal pressure of some sort started failing at their seams specially when the internal pressure was fluctuating. The seamless tubes are specially designed and drawn in a manner to withstand internal pressure rather than be more effective in bending or torsion.

I guess the answer is the testing rig. There is a chance that the seamless tubes are really not all that better(worth the extra weight).

[RollingCamel]

Could the pipes be changed after submitting the SEF?

I have sent the question to the rules committee and waiting for the answer.

[Bazanaius]

Waiting for the rules committee to reply is the only way to be sure, but I imagine it will depend on whether you've deviated from any of the standard tubes/dimensions as outlined in the rules.
If you've not had to justify any material choices/designs then as long as you still didnt require any for your new material I can't see how you SEF would be any different. therefore you should be ok - but definitely wait to see what they say!!

[RollingCamel]

They are taking too long and we should go tomorrow to start manufacturing the chassis. I know its our mistake to be so late.

[exFSAE]

Is there anything in the rules that says the tubes have to be seamless? Or does it say, just steel? If not.. fuck it. Go with some light weight welded steel tube and kick it.

[RollingCamel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
Is there anything in the rules that says the tubes have to be seamless? Or does it say, just steel? If not.. fuck it. Go with some light weight welded steel tube and kick it. </div></BLOCKQUOTE>

This is talking too long, i emailed since Monday and got no reply till now....My team mates had several questions before emailed to the rules committee and got no answer till now. The SEF should be the same since it is only change in geometry.

I just used section modulus, area moment of inertia and polar moment of inertia to compare stresses under same load and with same length.

Tomorrow we'll go and buy the tubes if we didn't get a reply.

[exFSAE]

What's there to even ask the rules committee? If there's nothing in the rule book that makes a differentiation between welded and seamless tube, then don't worry about it.

[RollingCamel]

Got the answer.

Since the SEF has been submitted the mandated parts are can't be changed.


Okay we'll now have to play on the non mandated parts and use less tubes.

[exFSAE]

I'm not sure there was a need for you to submit a SEF. Shouldn't have had to. The rulebook makes no differentiation between seamless and welded tube. Steel is steel.

[RollingCamel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
I'm not sure there was a need for you to submit a SEF. Shouldn't have had to. The rulebook makes no differentiation between seamless and welded tube. Steel is steel. </div></BLOCKQUOTE>

Shouldn't different tube size requires SEF?

[exFSAE]

Nah. SEF is if you're using alternate materials like a composite structure or aluminum tubes other than the roll hoops.

With steel frames... you've got a minimum size and thickness requirement. No SEF below that since you can't break that rule, period. If you're higher on wall thickness or diameter, you're automatically higher on tensile, bending, and buckling strength.

You could make the frame out of 2" diameter, .188" wall steel pipe and you wouldn't need a SEF.

[RollingCamel]

I thought that maintaining either the thickness or outer diameter wouldn't require the SEF.

[exFSAE]

Going up either way you're making the thing stronger. Combined.. even more so. Solid mechanics, broski.

SEF is for when you're swapping materials and in that nebulous realm of playing with modulus and moment of inertia and cross sectional area, etc.