Cage Mild Steel vs. Crome Molly

[Shawn Anderson]

Looking at ordering a cage kit for my Thunderbird and am wondering if crome molly is worth the exstra cost. I am going to use a 10 point cage. I have heard that mild steel welds easer, and that cm needs to be tig welded and can harden when welded. If I go with mild steel I may get it in D.O.M. Any thoughts? It will most likely be a S&W kit.

[JeffMcKC]

Re: Cage Mild Steel vs. Crome Molly

Get moly I cant even list all the reasons on one page.

lets see there is:

WEIGHT
Strength (Protection)
Resale
Value
Stepping up your ready to add on.
Rule changes
It will not work harden (they are only justifing their mild steel frame or trying to.)
Weight
It welds better (tig welding undercuts mild steel)
Rigidity
Smaller Diameter
Weight
Weight Did I list Weight?

[urwurznitmahre]

Re: Cage Mild Steel vs. Crome Molly

Get moly I cant even list all the reasons on one page.

lets see there is:

WEIGHT
Strength (Protection)
Resale
Value
Stepping up your ready to add on.
Rule changes
It will not work harden (they are only justifing their mild steel frame or trying to.)
Weight
It welds better (tig welding undercuts mild steel)
Rigidity
Smaller Diameter
Weight
Weight Did I list Weight?

x2
plus it's cheaper in the long run to only install it once..
cutting it out 2 install a better cage, makes chrome moly .cheaper..
if you need a 10 point cage. won't it need to be cert, anyways..
you'll be dollar ahead , to go moly now

[Ron Ward]

Re: Cage Mild Steel vs. Crome Molly

Jeff,

I am pretty sure chromoly weighs less than mild steel, too.


;D


Ron

[DanStokes]

Re: Cage Mild Steel vs. Crome Molly

Joe Timney of Delaware Chassis works STRONGLY recommended mild steel - and he's the safety guru at Maxton. Yes, Chrome Moly WILL weigh less, but at what price? The only real structural advantage is that it will come thru an accident in better shape. So What? You're not going to reuse the cage anyway. Now - in order to get similar strength from a mild steel cage it will have to be thicker (heavier gage) but how important is weight for you? In LSR it's no big deal.

The requirement to TIG weld CM is no big deal - unless you don't have a TIG. MIG is just fine on mild steel as long as you have good penetration. Now - if you're going to do CM properly (it almost never is) it not only does it have to be TIG'd - it also has to have all the welds passivated. That's where the weld area is heated to a certain temp (I don't know that value) and SLOWLY allowed to return to room temp. This keeps the welds from being brittle. Mild steel just isn't fussy as long as the weld quality is decent.

Hey - it's your money. I'd rather spend the difference on good engine parts and more than make up for the weight difference. I'm admitting I'm looking at this from an LSR perspective, but few of us are made of money.

Dan

[squirrel]

Re: Cage Mild Steel vs. Crome Molly

this is one of those aspects where drag racing and land speed racing are way different things

[Shawn Anderson]

Re: Cage Mild Steel vs. Crome Molly

It looks like $250 for mild steel and $500 for crome molly, but I think shipping is like $100 either way (may just pick it up). So a extra $250 not a total deal breaker, more woried about the welding. I will farm out the welding (I suck at it) but will tack it in place.

[JeffMcKC]

Re: Cage Mild Steel vs. Crome Molly

I dont think they can be that Different, ( Keith has explained the weiht thing,,,sort of LOL) granted I have not built a LSR Yet (am getting a itch) but I would think as in any racing you should build it light, so the weight you carry is placed where you want it, not limiting you to where it is already, and rigid is just a huge plus, its not about the money you spend, its about where you chose to spend it, and recover it when your done ,and want to unload ,,,, I mean,,,, let someone else own your pride and joy ;)

I think from what I have learned from Keith the 250 would not go in my motor. Frame first to hold the structure, then Aero.

The 250 difference is not worth saving in your pocket, when your talking about the base that everything else works from. I would think a chassis guy would have more to it than just the "The only real structural advantage is that it will come thru an accident in better shape. So What? " When the base structure helps with so much more, like keeping the drive train out of bind (more smaller tubes to stiffen), keeping all four wheels heading in the same dirrection, making shock changes actualy make changes instead of flex because you tightend a shock or added spring rate,,,,,,, its worth so much more ,but hey we all have ideas as to how things should be built, this is why some are faster than others no matter what your doing.

[Darren N.]

Re: Cage Mild Steel vs. Crome Molly

Now - if you're going to do CM properly (it almost never is) it not only does it have to be TIG'd - it also has to have all the welds passivated. That's where the weld area is heated to a certain temp (I don't know that value) and SLOWLY allowed to return to room temp. This keeps the welds from being brittle.

My understanding is the same as Dan's. In fact, I was having this discussion with some of my colleagues last week. Our welder buddy (and mechanical engineering graduate) warned us of the specific procedure for welding chrome moly, 4130 in our case. Our FSAE team experienced driveshaft failure right through the welds because the base metal wasn't preheated properly (the process Dan describes) and possibly the wrong filler rod was used. He recommended that we purchase a good welding handbook to get the exact details. He suggested purchasing them from Lincoln Electric but I haven't been able to log onto their website (appears to be down).

[Shawn Anderson]

Re: Cage Mild Steel vs. Crome Molly

I will also add that this will be a drag car, thought about building a a LSR from it and may do Maxton with it (down the road) but the main goal will be to to run Drag Week 2010.

[hauen]

Re: Cage Mild Steel vs. Crome Molly

As mentioned above, CM welds will all need to be stress-relieved, of course you could get around that by gas welding them all. :P

I've never bought the argument that for a given weight/thickness CM is stiffer (and therefore better) than MS, using that logic you should be able to design and build an aluminum cage using modern finite element analysis software to keep everything in strictly tension or compression and run away with it all. :

Given the fact that CM is double the cost, but not half the weight, plus the particulars others have mentioned, I think you'd be better off with mild steel.

Just as a quick example: Let's say you are building your typical SCCA/NASA/NASCAR/ARCA/IMCA type car. Most specify 1.5 x .095 for CM and and 1.5 x .120 for steel (as always, check your rulebook). Checking the weight reveals that CM at that size is 1.426 #/' while steel is a whopping 1.769 #/' a stunning difference of .343 #/'. So lets say for easy math you use 100' of tube (with a small cage you'll use less, but again, it's because I'm not a mathematical genius) you'll have a difference of 34.3#... 34.3 pounds! Double the cost and complication for less than 35 pounds? Hardly seems worth it IMHO. What's that in the overall scheme of things? 3-4hp? You'd be better off figuring out where to drop some unsprung weight

[horsewidower]

Re: Cage Mild Steel vs. Crome Molly

An FAQ on tig welding chrome moly. Interesting that they don't think you need to preheat or heat treat the welds if you have a wall thickness of less than .120.

http://www.lincolnelectric.com/knowl...hrome-moly.asp

[Darren N.]

Re: Cage Mild Steel vs. Crome Molly

An FAQ on tig welding chrome moly. Interesting that they don't think you need to preheat or heat treat the welds if you have a wall thickness of less than .120.

http://www.lincolnelectric.com/knowl...hrome-moly.asp

off topic here but... I've been trying to access that website for a week with no success. Is the link working for you? Man, I hope Lincoln Electric hasn't been blocked in Canada, lol.

[SpiderGearsMan]

Re: Cage Mild Steel vs. Crome Molly

heck - fj smith builds some of the fastest super stockers with mig welded mild steel
nascar sprint cup is mild steel
t/f anf fc is chrome moly - costs more fore sure for light weight and more labor for tig and stress relieving - if you don't stress relieve , it breaks

[squirrel]

Re: Cage Mild Steel vs. Crome Molly

the basic difference between CM and MS cages is that MS has a lower yield stress, so it will bend sooner, and absorb more energy as it bends.

LSR is a strange deal, because traction is often the limiting factor in how fast you can go, and a heavier car simply has more traction.