mikegarrison

It's not lighter, just stronger. So if the rules allow using thinner walls or smaller diameters for the chromoly, the result is lighter. However, many of the rules have shifted to favor DOM mild steel rather than chromoly because it is less brittle, less sensitive to weld-induced heat tempering problems, and cheaper.

krazik

yeah exactly you used to be able to run thinner wall chromo and that is why it was lighter. But to stop people from having to build cages that cost 2-3x as much as DOM to be competive they changed the ruling so that the 2 metals had to be the same thickness.

-Ry

mxt_77

I hadn't heard anything about chromoly being more brittle. Do you have any supporting info for this? I assumed it was structurally superior to mild steel. I had heard it was slightly more difficult to weld, but any decent welder shouldn't have a problem with it.

So, does anybody on here actually use chromoly?

mikegarrison

"Structurally superior" is like "faster". Faster at what? Superior in what ways? Everything has its proper uses.

1020 mild steel has a Brinell hardness of 121, 4130 chromoly has a Brinell hardness of 197. Brittleness is approximately proportional to hardness.

But it's been a long time since I had a metalurgy class.

Different heat tempering and such can manipulate this. In fact, that's what makes chromoly "hard to weld". It is very sensitive to heat tempering and removing the heat tempering because of the heat of welding. With chromoly, it's better to weld it up and then heat temper the resulting welded part. Hard to do with a welded-in roll cage.

[edit. oops! I meant that harder metals are also more brittle, generally speaking, but I originally wrote exactly the opposite.]

mxt_77

In all the ways that save my bum in a wreck.

I can handle a cage that's "brittle", as long as the brittleness is only evident in a wreck that's so severe that I'd be dead anyway (due to extreme impact/G-forces/etc). However, I don't want a cage that's going to crumble, crack or fail in a scenario where a mild steel cage would simply "stretch" or deform, but still maintained enough integrity to protect me.

Edit... BTW, I didn't realize heat tempering was required for building a cage. What exactly does that mean, what does it do, and how does it work?

mikegarrison

[QUOTE=mxt_77,May 30 2007, 01:18 PM] I didn't realize heat tempering was required for building a cage.

krazik

If you want the safest, in recent FIA testing they've found smaller diamater solid mild steel tubing to be the strongest because it can stretch a lot farther before it tears. There was talk of moving to solid tubing for Rally cars.

-Ry

l8brakr

^If it's solid is it still considered tubing ?

Where would one find the FIA findings and what are the odds US sanctioning bodies would recognize it if/when it was built?

krazik

I read about it in a friends copy of Race Car Engineering.