Frame materials

[DaveC]

I thought this warranted a new discussion, as there seems to be some disagreement, and its no good to continue this in the other thread, which is on another topic.

Why do you use alloy or mild steel tubing to make the frame? 4130 is only .001 lb / cu in lighter than mild steel (1020 or 1050), the difference in stiffness of the welded cage is unknown, but I'd guess less that %2, as the stiffness of the two base materials isnt much different.

If youre not stress relieving your welds on alloy its debatable (and depends on the individual weld) whether theres even any stregnth improvement vs. mild steel. I believe this is of minimal importance as we're not risking crashing the car into anything really hard in fsae anyway.

Why spend more $$$ on crmo? We all can look up material properties, and the welding properties have kinda been beat to death elsewhere as well... so lets focus on what effect it will have on actual car performance or in the design contest.

[DaveC]

I thought this warranted a new discussion, as there seems to be some disagreement, and its no good to continue this in the other thread, which is on another topic.

Why do you use alloy or mild steel tubing to make the frame? 4130 is only .001 lb / cu in lighter than mild steel (1020 or 1050), the difference in stiffness of the welded cage is unknown, but I'd guess less that %2, as the stiffness of the two base materials isnt much different.

If youre not stress relieving your welds on alloy its debatable (and depends on the individual weld) whether theres even any stregnth improvement vs. mild steel. I believe this is of minimal importance as we're not risking crashing the car into anything really hard in fsae anyway.

Why spend more $$$ on crmo? We all can look up material properties, and the welding properties have kinda been beat to death elsewhere as well... so lets focus on what effect it will have on actual car performance or in the design contest.

[K. Werkley]

I was asked this very same question by a design judge, and now I'm wondering what are the big advantages of CrMo?

[Frank]

reason 1
more thin wall choices, and thinner wall available (at least in AUS anyhow)

reason 2
even in the supplied condition, its stronger than mild, less likely to bend, try and file it, its hard(ish)

[K. Werkley]

Frank
Reason 1: good point

Reason 2: I always thought it was stiffer (harder), but the judges didn't like that response very much. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[raska]

Our resident chassis designer will hopefully pipe in, but this has been a discussion and change in this years cars over previous as well. One concern switching to mild was the quality control. I think one of the design judges said that was a valid concern, but isn't really an issue these days. The mild is way, way way cheaper. Don't spend all your money on the engine stand, use it for making more power or something. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Colin]

Doesn't CrMo have the same Young's modulus as mild steel? Same Modulus of elasticity means same stiffness in the chassis, and mild steel is soooooooooo much easier to fishmouth.

[PPAM20]

With reguard to the file reference above...... Although moly is commonly available in several conditions, its toughness properties are what make is harder to machine - ie it doesn't want to shear.

With reguard to its use......Although I'm not big on it in every case, if the part is highly stressed and subject to cyclical forces, 4130/4140 properly fabricated/ designed/machined/heat treated is the way to go. Sway/torsion bars, axles, lightweight suspension arms that see some flexing in severe bumps are examples that come to mind. Also, any part that gets hardened even to RC35/40 is a canidate for 4140. Hardened moly's properties are far better than hardened mild steel's especially in fatique/crack propagation - axles and drive flanges come to mind - my 2.

[PPAM20]

More 2 cents. You are absolutely correct Colin as far as stiffness. However, 4130's fatigue properties are what MIGHT make a 4130 frame stiffer over time. When they use it in racecars for the frame, that is what they are after - better fatigue properties means I can use thinner material and thus a 4130 frame can be lighter. That being said, I like what I'm hearing in this thread - I dont think its a great material in every application from a cost/benefit point of veiw.

[rjwoods77]

http://archive.metalformingmagazine.com/2001/01/Lincoln.pdf

Also I have done chassis in both mild and 4130-N. The chromo ones always deflect less with loads. Unless you are building a car that doesnt deflect, I would think that it is an advantage you would like to build into your chassis. While they are the same stiffness based on an I x E basis, they are really different in the Sy x I x C. Chromo is 30% stronger. As far as cost I dont know where you guyys get your stuff from. I can get 4130 cheaper than i can get 1020dom. Smart companies that buy huge amounts of chromoly early are the ones with the lowest prices. You just have to look.

[Frank]

if i could get 0.9mm mild in a range of diameters, i'd use it

[Denny Trimble]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rob Woods:
The chromo ones always deflect less with loads. </div></BLOCKQUOTE>

Rob,
Stiffness does not equal strength. Sy has nothing to do with stiffness, unless you are in the plastic region during your stiffness test.

Mild steel and 4130 structures, otherwise identical, will deflect the same under loads that don't put any part of the structures above their Sy.

Sorry, don't mean to be militant about this, but it's one of the most fundamental concepts that is most commonly misunderstood.

[rjwoods77]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Denny Trimble:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Rob Woods:
The chromo ones always deflect less with loads. </div></BLOCKQUOTE>

Rob,
Stiffness does not equal strength. Sy has nothing to do with stiffness, unless you are in the plastic region during your stiffness test.

Mild steel and 4130 structures, otherwise identical, will deflect the same under loads that don't put any part of the structures above their Sy.

Sorry, don't mean to be militant about this, but it's one of the most fundamental concepts that is most commonly misunderstood. </div></BLOCKQUOTE>

Absolutely correct. EI/PL. I feel like a dummy.http://fsae.com/groupee_common/emoticons/icon_frown.gif Keep it under the yeild strength point and the fatigue ya,ya,ya. Its all coming back to me. So the real difference is if you go past the yield strength where 4130 is greater than 1018. I was basing my comments off of the deflection but misquoting half remembered material made me wrong none the same. Made me open my mechanics book again and review some stuff. Thanks again Denny. I think I still would play with chromo if I could get it at the same price which I already do. Is it just me or does mild steel "move" around more in welding.