roll cage weight "steel versus chromemoly"

[Geoffrey Dick]

if i had two cages of the same design and specifications. how much lighter is chromemoly. i want a 6 point cage for my rag top sierra and just want to know approx. how much weight i would be adding to the car?

CCDA rules

5.4 MATERIAL SPECIFICATIONS
All tubing used in roll bar protection structures shall be circular section cold drawn steel tube (CDS, CDW or CEW).
Minimum dimensions
Component. Outside Diameter Wall Thickness
Main Hoop 44.45mm
50.00mm
2.5mm
2.0mm
All other Components 38mm
40mm
2.5mm
2.0mm
MANUAL OF FWD MOTOR SPORT ROLLOVER PROTECTION
These figures represent the minimum permitted specification. In selecting the steel, attention shall
be paid to obtaining good elongation properties and adequate weldability.
The CCDA may other consider steel composition and dimensions on application.
has anyone used a chromemoly cage for a CCDA event?

cheers
geoff

[awill4x4]

On a size to size comparison with both CDW and Chrome/ Moly tubes both the same diameter and wall thickness there's essentially no weight difference.
The Moly cage however will be stronger. The reason Moly tubing is used is because it's stronger it allows a reduction in wall thickness and still have comparable strength to a CDW cage, that's where the weight reduction occurs.
If you wan't to use Moly, particularly if your motorsport association is affliated with CAMS (C#nts Against Motorsport) then a Moly cage has to be engineered. We had a guy in today picking up some Chrome/Moly tubing for a Commodore Cup car he's building a cage for and he had just come back from the engineers and it cost him $1000 to get it engineered.
Regards Andrew.

[jeep97tj]

Whats moly worth a meter compared to seam or seamless tube?
I hear that moly cracks easy or something so u cant use it for hot rod frames or buggy’s? This has always confused me because with your race cars the roll cage is pretty much the chassie of the car.

[awill4x4]

Whats moly worth a meter compared to seam or seamless tube?
I hear that moly cracks easy or something so u cant use it for hot rod frames or buggy’s? This has always confused me because with your race cars the roll cage is pretty much the chassie of the car.

It ranges in price from reasonable to "oh my God" depending on diameter, thickness and whether it's a commonly used size.
Some of the CDW sizes aren't much cheaper than moly.
There's a whole lot of hype sprouted about moly failing generally by those who haven't used it. All the sprintcar chassis' are built with Moly as are all the Supercars, most of the sports sedans etc.
If you can build something lighter with the same strength then that's a benefit to you that your competitor may not have. Even in categories where a minimum weight is required if there is no restriction to using Moly the top guys will use it because then it allows ballasting a car to its best advantage.
CAMS have placed restrictions on it's use (engineering certificates required) because they simply haven't bothered to specify a diameter and wall thickness for the main hoops and legs etc. Essentially they've put it in the "too hard basket". That being said, I know of quite a few circuit cars using Moly when a cage modification or repair is needed.
As for welding, Moly is a great material to weld and is best performed by Tig welding.
Regards Andrew.

[jeep97tj]

Thanks Andrew, sounds like u know your stuff. So when choosing moly u can just half the wall thickness that u would normally use with seamless.

I have no idea about molly, would 2mm work for a buggy frame? What thickness do the speedway cars use for ther frames?

Thanks Shane

[Tiny]

Remember moly cost a lot more to weld as it has to be done right by a specialist. I dont fully understand the procedures but I understand if done incorrectly the weld will be no stronger than steel an less structure to hold

[awill4x4]

Thanks Andrew, sounds like u know your stuff. So when choosing moly u can just half the wall thickness that u would normally use with seamless.
I have no idea about molly, would 2mm work for a buggy frame? What thickness do the speedway cars use for ther frames?
Thanks Shane

Shane the top tubes and main down tubes of the sprintcar chassis' are 1 1/2" (38mm) X .095" (2.4mm) wall thickness. The bottom rails are 1 3/8" (35mm) X .095" (2.4mm). All the rest of tubes range from .049" (1.2mm) thick to .065mm (1.6mm) thick. As an example the supercars use 2" diameter main hoops but only .065 (1.6mm) thick, but, they also use "lots" of .049" (1.2mm) tubing to maximise structural integrity and torsional (twist) stiffness.
With Buggy's, check what your association requires you to use, it may dictate tube sizes and materials. A custom Moly cage is almost certainly going to have to be engineered if it's for a CAMS spec cage and that can cost lots of money.
Luckily the smart classes of racing, speedway, jet boats, tractor pulling etc didn't go down the CAMS route and have no problems in using Moly, they simply specify a diameter and thickness. They don't go overboard in reducing thickness and still get a weight saving. They also don't compromise on safety with the sizes and thicknesses they use.
When you're getting down to 1.2mm tube and under tight fitups are essential for the best welding quality. I've welded tubes down to .028" (0.7mm) (heat exchangers) but it gets pretty tricky then.
Everything we do is Tig welding, we rarely fire up the Mig. The precision and quality of a skilled Tig welder really shows who can and who can't do the work. We see lots of work by others in the industry, some good, some not so good. It's easy to pick the difference of those who take pride in their work and it's on small diameter and thin wall tube it really shows.
Regards Andrew.

[awill4x4]

Remember moly cost a lot more to weld as it has to be done right by a specialist. I dont fully understand the procedures but I understand if done incorrectly the weld will be no stronger than steel an less structure to hold

Actually Tiny, there's no difference between welding the two materials (CDW or Moly) You can still Mig weld Moly, in fact the Commodore Cup car we saw this week had a full Moly cage and it was Mig welded and quite nicely done from what I saw. You just need to use an LW1 (ER70 S2 from memory) mig wire and it's pretty cheap at about $55-$60 per 15kg roll of 0.9mm. The guy with the Commodore Cup car used 0.9mm wire but was thinking of going smaller (0.8mm or 0.6mm) to get a better finish.
We weld both CDW and Moly but we only Tig weld, if I had to Mig weld all day it wouldn't interest me at all I like the precision you have to have by Tig welding.
Regards Andrew.

[Tiny]

Remember moly cost a lot more to weld as it has to be done right by a specialist. I dont fully understand the procedures but I understand if done incorrectly the weld will be no stronger than steel an less structure to hold

Actually Tiny, there's no difference between welding the two materials (CDW or Moly) You can still Mig weld Moly, in fact the Commodore Cup car we saw this week had a full Moly cage and it was Mig welded and quite nicely done from what I saw. You just need to use an LW1 (ER70 S2 from memory) mig wire and it's pretty cheap at about $55-$60 per 15kg roll of 0.9mm. The guy with the Commodore Cup car used 0.9mm wire but was thinking of going smaller (0.8mm or 0.6mm) to get a better finish.
We weld both CDW and Moly but we only Tig weld, if I had to Mig weld all day it wouldn't interest me at all I like the precision you have to have by Tig welding.
Regards Andrew.

hmm interesting, the welder that does most of my stuff told me different, and you needed tig to get the heat right, maybe time to research. I understand it will weld and look A1 but the strenght will be lost??

[Mousie]

my uncle buildes drag cars and workes with molly all day long he says to only use a tig with it for best outcome.

not werth spending all the money on the tube then have a shit weld ....

my 2 cents

[F'n_Rover]

Amature aircraft builders have been welding molly frames for years, when done right = no problems.

some / most use mild steel filler rods, while its not as strong as the molly tube the extra area of the weld brings the join up to the tube strength. From what i have read its the pre and post heating (cool down) that is the critical part. Google "homebuilt aircraft" the US sites will bring up heaps of info on welding molly.

There's a place near moorabbin airport that sells mil spec tube (normalised), $$$$$$$$$$$$$$$$ each length comes with certification paperwork and has a fully traceable history back to manufacture.


I can understand aircraft and race cars using it to keep weight down, but why would you bother in a 4B?

[awill4x4]

Amature aircraft builders have been welding molly frames for years, when done right = no problems.
There's a place near moorabbin airport that sells mil spec tube (normalised), $$$$$$$$$$$$$$$$ each length comes with certification paperwork and has a fully traceable history back to manufacture.

We use 4130 mil spec normalised tube every day and we would be the largest user of Moly in Vic outside of the defence industry and if we had to buy from the guy in Moorabbin we would go broke.
His prices are outrageous. As for certification, we get that from our supplier/s when it's required, it's no big deal.
Regards Andrew.

[bru21]

mig welding is from what i understand not ideal for moly as the area behind the weld is cooled too quickly by the mig gas and shrinks whilst the area being welded is hot. this zone of high heat/low heat causes a severe stress in the weld area as the hot section peels the cold one apart. thus a small fracture often results along the weld seam.

(this is why many people attempt to stress relieve moly with an oxy after welding (or preheat it prior to welding)

coupled to this is the stress from a mig in the other (welder down) plane. migs atomise and spray the metal onto what is essentially cold steel, this in itself causes stresses as the weld is spray hot and the parent metal is absorbing the heat but always colder. mils steel is far lesser affected as its coefficient of expansion is less. a good example is to get 1x3mm thick sheet of stainless or moley (which are similar in expansion) and 1x3mm sheet of mild. weld a line down both and see what warps more!!! or tack with a mig on a normal weld setting and see how little penetration if any is present, where as a tig cannot leave a little penetration tack (providing enough metal is present and both halves are worked. (for this reason i always tack on full current and drop it down for long runs)

tig is far more progressive in its heat distribution, makeing for a more relaxed weld also the gas flow rates are lower with tig esp with big cups, ensuring the weld is allowed to stay hotter longer. filler metal is worked with the arc until uniform temps are realised. - a mig is not!

[awill4x4]

Bru, it's all fine in theory but in a "real world analysis" Mig is fine for welding Moly. We see lots of damaged chassis' from various classes of racing, some Moly frames some CDW and even some (what we call junk) ERW. Regardless of whether it Mig'd or Tig'd they all bend or break in roughly the same places.
As for heat treatment, in a nutshell "we don't" the only time we preheat is on some of the front axle eyes which are 4140 and about a 10mm wall thickness. We find it more of a problem with porosity on those thick sections where the weld pool is cooling down too rapidly and Hydrogen doesn't have a chance to be released from the weld pool.
Your comments about Mig "spray transfer" aren't really relevant because most of any chassis tube welding is done in the "short circuit" or dip transfer anyway as spray transfer is way to hot for thin wall tube welding.
We Tig weld not only because all the oppositions chassis are also Tig'd but that the precise bead appearance is 2nd to none.
There's a lot of myths sprouted about Moly and it's perceived welding difficulties when in actual fact it's easier to weld than mild steel mainly because it is so highly refined and has very little oxygen content which causes weld "bubbling" particularly when Tig welding.
Bru, you're dead right about the larger gas cups. I use "gas lenses" exclusively and my favourite is the biggest baddest gas lens designed for welding Titanium it's about 20mm diameter at the orifice and allows a Tungsten "stickout" of 20-25mm and still have gas coverage.
Regards Andrew.

[85lux]

Cromo has great benifits if you know how to work with it.
It can be dangerous if you don't know what you're doing.
Do a test yourself.
1) take a small piece of cromoly(4140, 4130, 4340)
2) heat it up to cherry red(simulates welding it.)
3) let it cool in air (as if you would when welding)
4) see how hard it now is with a file.

you'll find it is as hard as your file.
your cromo has air hardened. while it is hard it is brittle. thats the danger with cromo.
there are ways of combatting this as others have mentioned(controlled weld heat, pre/post heat treating) tig is really the go as it lets you controll the heat well. if you do not know about these things then steer away. working with cromo is for those trained to do so. usually when an engineer is looking at a moly cage he will ask to see it unpainted. this is so he can assess the heat input and distribution. you're not making your kid a rocking horse here, it's a roll cage or chassis, you owe it to yourself and your passenger to ensure that the welds will hold up and not crack
moly is not for the backyard fabricator.

[lux kid]

sorry to go off the topic but where can i get a copy of the latest CCDA standing regs. i need to get a cage built that will compliy to CCDA and CAMS regs. its for a patrol.
can any one recomend some to talk to?

thanks and sorry for the hijacking pls pm replys.

[awill4x4]

Cromo has great benifits if you know how to work with it.
It can be dangerous if you don't know what you're doing.
Do a test yourself.
1) take a small piece of cromoly(4140, 4130, 4340)
2) heat it up to cherry red(simulates welding it.)
3) let it cool in air (as if you would when welding)
4) see how hard it now is with a file.

you'll find it is as hard as your file.
your cromo has air hardened. while it is hard it is brittle. thats the danger with cromo.
there are ways of combatting this as others have mentioned(controlled weld heat, pre/post heat treating) tig is really the go as it lets you controll the heat well. if you do not know about these things then steer away. working with cromo is for those trained to do so. usually when an engineer is looking at a moly cage he will ask to see it unpainted. this is so he can assess the heat input and distribution. you're not making your kid a rocking horse here, it's a roll cage or chassis, you owe it to yourself and your passenger to ensure that the welds will hold up and not crack
moly is not for the backyard fabricator.

85lux, perhaps you'd like to tell us all just how much experience you have had with 4130 Moly. I hate to sound like a broken record but some of the info sprouted on this whole thread is just theoretical bulltish.
We work with this stuff EVERY DAY, we heat it up, we bend it, we drill it, we weld it and we cut the damn stuff. The only time we've ever had a hardness problem is on "some" laser cut hole edges which may need to be redrilled and then if we use a die grinder to chamfer the hole we can redrill with no problems.
We have far more problems with "work hardening" on the stainless steel stuff we build.
I'm telling peoply here if you have a need for Chrome/Moly and are willing to pay the premium price it costs, "don't be afraid of it". It's just a high grade alloy steel after all.
Regards Andrew.