Chromoly vs DOM

[Rockwells607]

Always thinking of the next rig . Anyone know a rough idea of strength comparison on say .120 wall chromoly tube to DOM. What size dom is comparable . I'm asking to see or gain an idea of how much weight you could shave doing a chassis. I've consider a v8 moon buggy for fun. Also does the chromoly chassis hafta be heat treated after being tigg welded? Does it bend the same as DOM?

 

[zukimaster]

No expert here by any means, but back in the drag racing days, chromoly was lighter and the way to go, but on rails ( like top fuel cars) when you hauled them in a trailer, they made airbags to go under them to keep from bouncing around, because they would break in half! And I have seen this happen in person. So in my opinion, the chromoly would not be any good for a rock machine.

 

[TacomaJD]

No expert here by my means, but back in the drag racing days, chromoly was lighter and the way to go, but on rails ( like top fuel cars) when you hauled them in a trailer, they made airbags to go under them to keep from bouncing around, because they would break in half! And I have seen this happen in person. So in my opinion, the chromoly would not be any good for a rock machine.

Learn something new every day...

 

[stuftmunky2k]

Commonly available DOM tubing is usually 1020 or 1026 so you dont need to be specific on the type of DOM. In the normalized condition 4130 tubing comes in, it is not much sronger than DOM, you would probably never tell the difference if you built two identical chassis, one 4130, and one DOM and did some sort of testing. DOM will work just fine, The drawback to using DOM is that you won't be able to tell people that "my buggy is built out of chrome moly", its kind of like saying "ooh my axles are made out of 300M" when a properly designed 4340 or 4140 axle would work just fine and the is other drawback is that chrome moly is lighter and stiffer than DOM

Quoted from mr goat built himself. I do a decent bit of a chassis work at the race shop out of chromoly and he sends everthing out to get heat treated after welding. There is also the fact the person welding it has to be up to snuff on his heat input. Last thing you want is to have a weld joint fail from to much heat input.

 

[Rokcrler]

Re:

Chrome moly will never hold up in the rocks...

http://m.youtube.com/watch?feature=share&v=i4FGywfrFxQ

 

[Rockwells607]

I do believe the bombers are all chromo one of the reasons I ask. He is able to get them super light and strong... I think. No idea if he's heat treating them or not

 

[sledneck]

The benefits in racing is smaller O.D. and thinner wall thickness for cro-mo. (generic example) NHRA safety rules state 1.75 dom is to be .120 wall if its cro-mo in can be 1.625 x 0.95 it just comes down to weight savings for race car stuff. It bends the same. Some say there is no need for heat treating. If done wrong it will make the chassis and welded area more brittle and prone to cracking. I remember reading that only 1% of all off road racing chassis are treated. For anything we do I don't think its worth the extra cost.

 

[zukimaster]

But let me add to my input, the race cars are built out of tubing half the size of what those guys are probably using.

 

[Neal3000]

Chromoly is a bit more brittle than mild steel isn't it? You could have a lighter chassis that is equal in strength to a heavier DOM chassis but i think the lifespan might be shorter?

 

[Rokcrler]

Re: Re: Chromoly vs DOM

I do believe the bombers are all chromo one of the reasons I ask. He is able to get them super light and strong... I think. No idea if he's heat treating them or not

He said he doesn't heat treat the chassis.

 

[TBItoy]

Re: Re: Chromoly vs DOM

Chromoly is a bit more brittle than mild steel isn't it? You could have a lighter chassis that is equal in strength to a heavier DOM chassis but i think the lifespan might be shorter?

Chromo (4130) is actually more "flexible" than DOM (1026). Look at the % elongation specs for that.

"By the numbers", if you are welding a chassis out if 4130, and not getting it normalized (ht) afterward, you are basically wasting money.
In practice, chromo is a little stronger. So if you build it relatively comparable to a DOM chassis, it will ultimately be stronger.

If you are going for weight savings and reducing material size/thickness vs what you'd normally use, then heat treat becomes a LOT more important.


Building SAE mini Baja cars (fancy go kart engineering competition), weight is a huge deal. We used super thin tube on those (lighter than conduit). Granted it was a 250lb, 10hp kart.

 

[zukrx]

just remember chromo and mild weigh the same.

 

[4byfan]

^^^^^ This. The wight savings comes from using smaller od tube and thinner wall thickness. Same thick same od will not weigh less.

 

[Nuts]

Chromoly .095" (i might be off .01" or so, can't remember the exact decimal right now) or thinner doesn't need to be treated after welding. I researched after dynamite dumb ass was putting those huge weave beads on chromo tubing, and will dig up the info tomorrow if I remember to look for it lol

 

[sledneck]

Chromoly .095" (i might be off .01" or so, can't remember the exact decimal right now) or thinner doesn't need to be treated after welding. I researched after dynamite dumb ass was putting those huge weave beads on chromo tubing, and will dig up the info tomorrow if I remember to look for it lol

I have also read that HT is not needed on anything under .120. That being said there are still a few builders that treat top fuel and funny car chassis's but that's some next level ****..

 

[collinmaune]

I was just talking to my local steel supplier I can get DOM .120 wall for $2.28 per foot give or take. Chromoly .95 jumps up to $6.95 per foot. If a chassis has 200 foot of tubing your looking at almost $1000 in difference for roughly a 25% to 35% lighter chassis. To me I think that 1000 would be better spent else where in my opinion. I would like to see how much weight you could shave off of axles by retubing them with chromoly instead of DOM. If you could save 35% on both front and rear axle for less then $1000 in my mind that would be more beneficial and a lot easier then making a whole chassis out of chromoly. I could be way the hell out in left field with my idea but its just a thought.

 

[TBItoy]

Re:

When welding the thin wall chromo, just make sure to not overheat it, especially if you aren't going to HT it. There is a lot of weld surface on a tube cope, and it doesn't take much heat to make 1/8" or thinner metal stick together.

"Crank it up and burn it in" is why you see a lot of chassis fail in the HAZ of the tube rather than the actual weld...

 

[44BRAND]

Chromoly is a bit more brittle than mild steel isn't it? You could have a lighter chassis that is equal in strength to a heavier DOM chassis but i think the lifespan might be shorter?

This ^^^^^ is what I was always led to believe. Chromoly is lighter and as strong as mild steel, BUT, more brittle. Cant take as much abuse. Drag cars are built out of it because they only ever have to hold up to ONE crash.Usually ruins the whole car anyway, so they go lightweight. A Rock car, on the other hand, gets the **** kicked out of it on a regular basis. Needs the metal even if a little heavier.
My 2 cents worth.

 

[Beerj]

I vote titanium

 

[blacksheep10]

This ^^^^^ is what I was always led to believe. Chromoly is lighter and as strong as mild steel, BUT, more brittle. Cant take as much abuse. Drag cars are built out of it because they only ever have to hold up to ONE crash.Usually ruins the whole car anyway, so they go lightweight. A Rock car, on the other hand, gets the **** kicked out of it on a regular basis. Needs the metal even if a little heavier.
My 2 cents worth.

A trophy truck gets the **** beat out of it every big hit for ungodly amounts of hours of service. Too much heat/too big a HAZ and it can crack. A lot of it has to do with design of how the load expected is dispersed though the chassis. Cracks are generally a repeated abuse cycles thing.
It weighs the same. You can use thinner as it is stronger and more ductile per equal cross section.
You don't have to PWHT it if you don't fry it. You are only going to HT it post weld if you use homogenous filler. Use er70 or 80. We are likely talking about 4130N.
There is a reason top tier cars use it. It works. See the super simple/minimalist bombers falling apart after several KOH and LOTS of prerunning/wheeling/other rock races? Not so much