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Chromoly usage in offroading

sled_dog

sled_dog

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Do people really use Chromoly in competition or trail useage? My uncle and I were sitting around discussing my thoughts on a tube buggy and he made some good points on Chromoly. His son races a Dirt Modified car and we were sitting around the car. I noted it would make a nice single seat buggy with some modification, too bad its chromoly. He informed me I was wrong on that. Its just DOM steel tubing. For a while chassises were produced from Chromo but they found it was far too stiff. From normal stresses and taking hits the frame wouldn't give but rather just break. Cousin gave an example of a friend of theirs leading a race by a good distance, tapped an inside corner tire and instead of just bending the axle, it sheered the damn thing right off at the knuckle. Well he was coming out of turn 4 on the last lap, he could have limped across the line with a bent axle, but the sheared off one, sent him into the infield spinning and cost him the race. Now wouldn't this same stuff apply to us?
 
Some guys are using Chromo. tubing in their comp rigs. I guess it really comes down to what you want to use.
The points you brought up are all of the reasons to use it and also not to use it. We built my chassis out of DOM and a few small sections of HREW.
I think the problem with Chromo is that it is already heat treated and when you weld, it weakens the Heat effected zone. So in reality to get back to the true strength the entire chassis would have to be annealed and re-heat treated. This would take a very big oven and a bunch of green stuff.
I think for our general use stuff DOM is the best way to go. The strength gains and material consistancy over HREW are very nice.
I belive that even in Nascar they are using HREW and DOM instaed of Chromo because of the bending factor instead of breaking. Those cars are designed to turn into a useless ball of parts with a driver in a pod that is somewhat unaffected by a crash.
I have heard of guys that had problems bending link arms or tie rods that went to a piece of Chromo of the same size and all their problems were solved.

I would definitly prefer that something bend and not break while in the trail. It seems that it would be easier to limp out or bend back than to weld back in place.
 
Considering that a lot of full on desert race and short course trucks are mostly chromoly tube, I don't see any problem with it, at least for chassis use. I think you just have to consider the application and the properties of the material to see if it's going to work for you. Some of us have seen the post on POR on Camo's busted chromo axle tube so axles may not be a great place for chromo but I know a lot of housings are built from chromoly tube that do work so maybe construction is the big deal. Trussing the housing more would go a long way.

I know that the chromo plate we're using for skidplates is great stuff for that application. It's a bit heavy but near impossible to tear up.
 
thanks guys, like mentioned I guess its all application and prefference. A dirt modified car crashes once every other week and takes a couple good shots a race. Not like Nascar, but the same principal of bending and giving a little.
 
I'll throw in my $.02 here. I used to drag race cars and being around htem for so long, I was very used to seeing chromo frames, suspension pieces, cages, etc. It was great because it was lighter than DOM BUT it came at a cost. Chromo is harder to weld (from what I hear) and it is so stiff that it has a tendacy to crack at joints when used on street vehicles over time. I would think that when used on a trail rig where it would see rocks and a lot of vibration and abuse it would be more of a headache than a blessing because your were constantly haveing to re-weld cracks that you found. Also coming at a higher cost than DOM I wouldnt see any reason to use it myself. Anyway, like I said, just my $.02
 
Anything that touches a rock on my buggy chassis is chromo because it will look good a be a safe rig for a longer period of time due to the fact it is way more dent and crush resistent. Anything that is a brace or doesn't touch rocks is DOM. Yes, it cost me a little more $$$ but being able to run a chassis for two or three seasons instead of rebuilding every year more than makes up for it.
 
Years ago I wrote a paper on Chro-Mo for my Engineering Materials class. Chro-Mo gets brittle in the HAZ unless you Oxy-Fuel weld it, which is how is was designed and intended to be welded. Chro-Mo was developed for building WWII aircraft subframes and TIG was far, far more exotic than it is now. They needed a high strength steel that could be welded with the common welding method of the day, Oxy-Fuel.
When you TIG or MIG weld Chro-Mo the heat is very concentrated as compared to the Oxy-Fuel process. This means that the room temp metal right next to the HAZ looks like an iceberg to the metal in the HAZ. That cold metal acts to quench the weld metal way too fast and from too high of a starting temp. You can build some Ductility back into the weld by Normalizing it. Most Pro Drag chassis builders do this with a Rosebud tip on a torch.
4130 work hardens, so unless you periodically Normalize the whole chassis it will have a finite life span. Once past it's life span it WILL start cracking, usually in the HAZ but not always.

Mild steel chassis' do not work harden so they are basically an infinte life fabrication (assuming you don't crunch it).
My simple rule is if it's a play machine then mild steel; if it's a comp machine and we expect to replace it in one to two seasons then it can be 4130 if the budget is there.
 
Here is my two cents about the whole life expectancy on CM. Many airplanes use CM in their wings and if you have ever been on a plane you have seen how they bounce around. Planes also put in thousands of hours or flying over many years. Big thing is CM must be welded by a professional or somebody who knows what they are doing. If done incorrectly it will fail.

-Nigel
 
The really scary thing is airplanes use alum for lots of structure and alum. has no fatigue stress limit so eventually, it will break, no matter how low the stress is kept. Yikes!
 
Sorry this is so far off-topic...

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The really scary thing is airplanes use alum for lots of structure and alum. has no fatigue stress limit so eventually, it will break, no matter how low the stress is kept.

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That is one of the biggest reasons why airplanes that carry paying passengers (which are registered under Part 135 of the Federal Aviation Regulations) are inspected regularly. A couple of these inspections require the aircraft to basically be disassembled to basic structure, i.e. interior totally removed, paint stripped off, all access panels removed. Here's a couple pic links:
cabin
wing
And here's the whole series, by a Southwest mechanic:
Southwest maintenance pics
 
Typical aluminum fatigue design calls for a life span of 500,000 cycles. I suspect that aircraft design is different but I couldn't guess which direction as I can make arguments for both.

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Sorry this is so far off-topic...

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The really scary thing is airplanes use alum for lots of structure and alum. has no fatigue stress limit so eventually, it will break, no matter how low the stress is kept.

[/ QUOTE ]

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