Chromoly vs Mild Steel

[customcreationsllc]

This discussion cam up during the meet and greet yesterday.

Which is lighter? Chromoly or Mild steel

And I was trying to explain they both weigh the same and both are just as stiff.
Chromoly can bend further without permanently deforming or breaking so the rules ALLOW you to use THINNER tubes therefore the chassis is lighter not because the Chromoly is lighter.
Most racecars design the chassis for stiffness so using Chromoly with thinner wall tubing would not be as stiff so they would end up using the same size tubes with Chromoly as Mild steel so end up with no extra benefit why waste the money. Plus Chromoly should be welded by TIG and normalized after welding, both Big $$$.
And there accidents are usually a little harder hitting where chromoly wouldn't hold up so you would be replacing tubes with either why use the stuff that cost more than 3 times the price.

Most circle track chassis that I know of are mild steel they might be converting but last I knew nascar still uses mild steel. But that was a few years ago.
Last I talked to my friend who is a nascar engineer about this it was 5 years ago when I visited him at DEI, they had just started removing tubes to allow the chassis to flex a little in certain spots giving them better performance. The thought used to be the stiffer the better.

So here are the material specs

4130 (Chromoly)
Density 0.284 lb/in³
Modulus of Elasticity 29700 ksi
Yeild 63100 psi lowest to 161000 psi depending on heat treat.

1018 (Mild)
Density 0.284 lb/in³
Modulus of Elasticity 29000 ksi
Yeild 300000 psi to 70000 psi depending on heat treat.


So which one do you think is lighter? They look the same to me, densities are identical.
Modulus of Elasticity are also almost Identical the 4130 is only 2% stiffer.

Now the 4130 is a lot stronger meaning it can take more stress before permanently deforming or cracking. So I'll give you can can build a chassis lighter and stronger with 4130 than 1018 but it will be flexing more.
With off road racing frame stiffness is probably not that important. But is it worth the money?

So after all that I'm going to make my chassis from mild steel, save a couple bucks.
I might consider making my suspension and steering with chromoly but I'll probably make them out of mild first.


This is where I got the information from.

4130
http://www.matweb.com/search/DataShe...5c2a4745f63593

1018
http://www.matweb.com/search/DataShe...0ecec2fa892787

[Zullock Holmes]

There are other ways you could save weight aside from the chassis as well.

Be it aluminum links (like the kind adam runs on the clayton buggy), aluminum knuckles, lightweight brakes, only thing is some of that stuff gets pretty expensive.

Also, if you design the chassis with as minimal waste as possible, no needless triangulation in certain points you could deff save some weight without jepordizing strength. You used to build race car chassis, didn't you?? I'm sure a lot of that caries over to here.

[HeavyMetal]

ive been told by more than a few people that the step up to a chromoly chassis isnt much worth it without heat treating the whole thing, which is MASSIVE BUCKS

[Zullock Holmes]

Si..... too much money

[customcreationsllc]

This was just a discussion that came up during the meet and greet.
A couple people were trying to tell me that Chromoly is lighter than Mild steel.
I was trying to tell them they weigh the same so I figure I would post some hard facts to back me up.

That is the only point of this post.
I know there is other ways to save weight.


As far as aluminum links great Idea until you want to mount a coil over on it.
Then you need to go with steel, bending and welding to aluminum link isn't easy first 7075 is not weldable and 6061 is not that strong until you heat treat it.

I want to put the coils on the links for 2 reasons. First longer travel with shorter coilover. Second mounting the coilover on the link allows the link to self center at the mount so less binding in the coilover mounts.
When you mount a coilover on the link it needs to be lower than the center line of the link so the coil keeps it straight if it is above the center line it will want to twist the link and you joints will have to counter all that force. Just like the problem with the inverted T steering design. I've also seen an extra set of links to keep the link centered.

[Toreadorranger]

You have a extra 0 added to the lower yield of mild.

[Buffalo Phil]

I'll bet you money those who thought CRMO was heavier than mild steel never took into account that the wall thicknesses would be different between different types of tubing.

[#DEZERTDAD]

http://www.rorty-design.com/content/tube_work.htm

[#DEZERTDAD]

Justin, you Jay and I were discussing this at the M&G

[customcreationsllc]

http://www.rorty-design.com/content/tube_work.htm

Nice article

[customcreationsllc]

FYI
This is straight from my friend who is an engineer for the Toyota Nascar Team

we build our chassis out of mild steel, no chromoly, reason being is that NASCAR mandates in the rule book that the chassis must be made out of " magnetic seamless DOM steel tubing 1 3/4" OD by .090" minimum wall thickness, alloys other than mild steel will not be permitted." We have tried to argue the use of an alloy of like strength but they claim they dont want to have to police people using thinner wall alloys so we are stuck with what we have.