tkerrmd

Hi all could some of you guys give me some opinions on a full cage. I am gutting my turbo and getting it ready to race in PBOC and NASA mostly.
I am having a full cage in of course and am weight concerned (also money concerned)
how does weight and price compare between steel and chro-moly?

would like to keep the comments to the cage not about why am I doing this with the turbo because the answer to that up front is .... I just am

thanks for any and all help, the cage is going to start soon


tom

Bill Lehman

chrome-moly is steel that has been alloyed with chromium and molybedenum to give higher strength. Mild steel and chrome-moly would have very similar weights per foot for a given diameter and wall thickness.

Bryan Watts

Weight is essentially the same. Chro-mo cages are only "lighter" if an organization allows you to use a smaller or thinner walled Chro-mo tube compared to steel as some have done in the past. NASA does not allow that (not sure about PCA), so I'd say you're much better off going with mild steel. It's cheaper and easier to work with.

tkerrmd

WOW that's why I love this forum!! fast good answers from guys who know and are willing to take the time to help out!! Bill and Bryan thanks!!

I was told the cage would be more expensive but 40% lighter!?

also I am using NASA as my baseline cause their criteria seems the strictest and I do alot of races with them. so if NASA doesnt allow it then the conversation would be over, and great help as I didnt see that in the rules!

thanks again,

any other opinions welcome

tom

onefastviking

Mild steel is not as brittle either

stownsen914

40% sounds optimistic. From what I recall, SCCA (as an example) used to allow 1.375" diameter .095" wall thickness main cage out of chromoly, compared to 1.5" .095" for mild steel, in cars up to 2500 lbs. Not sure what they currently allow. Considering that moly weighs essentially the same as an equal amount of mild steel, I don't see how you'd get 40% savings.

Moly is definitely stronger stuff though. If it's used correctly, I'm told it can we worth the effort to make suspension pieces, etc. from it. There seem to be lots of opinions about how it's to be welded though. Some will tell you that moly should be stress-relieved (in an oven) after welding, or you get brittle joints. Others say that proper welding technique gets you around that. Not sure how you'd get a car in an oven after doing a welded in moly cage ... : )

You don't have to worry about any of the above when working with mild steel. Just make sure you use 1020 DOM (seamless) material -- any respectable cage builder should know that.

Scott

ajcjr

how can you say that? Have you ever weighed one against the other, i have to bet chrome moly is much lighter and stronger and in my opinion safer. I know from drag racing we could save at least 100lbs or more by going with chrome moly, and i have seen some walk away from 300mph crashes. Nascar does use mild steel though. Just my .02!

Chrome-moly vs. Mild Steel Tubing
If you're going fast, you're going to need a rollbar or rollcage. Not only is it required by most sanctioning bodies at a certain e.t., but it's a safety issue. There are two materials you can use to build your cage: mild steel, or chrome-moly. The advantage of chrome-moly is that it's stronger, so a thinner wall thickness can be used, thus saving weight.

Design: Material: Weight: Cost:
10-point mild steel 157 lbs $295
10-point chrome-moly 118 lbs $604


again i come from drag racing and i am not up to date with all scca or nasa rules

disasterman

Confirm the builder's volume of work/experience with Chromoly. It is essential that they know how to weld it.

va122

I found a 40 lb difference respectively with my M3's cages

MarkM

Tom,

I have a factory matter cage in my race car and it came with FIA certification. PCA and NASA accept this as is. We did re-enforce the cage with door bars and other add-ons, and used matching tubing throughout. The entire cage weighs less than 100 lbs.

I did this because the car came with the cage, I did not buy it. They are expensive and may not be economically feesible if starting from scratch.

85Gold

I don't know about NASA & PBOC but SCCA does not allow Chrome Moly.

Peter

tkerrmd

Bryan does this mean Chro-moly is ok....

11.4.7 Roll Bars

All open cars should have a roll bar installed to help protect the occupant(s) from injury

during a roll-over. The roll bar should be able to withstand the compressional forces

involved in supporting the full weight of the car. The roll bar’s main hoop should extend

the full width of the car (except certain cars that have been approved by NASA). The

main hoop shall be one continuous piece with smooth bends and no evidence of

crimping or wall failure shall be present (i.e. should be Mandrel bends). All welds should

be of the highest possible quality, with full penetration [Ref15.6.15)]. All cars with roll

bars are required to have adequate roll bar padding per CCR section #15.6.4. In cases

where the driver’s head may come in contact with the roll bar should the seatback fail, a

seatback brace is required in conformance with section #15.6.22. The material and

minimums are as follows: (All cars with full roll cages should conform to the applicable

sections found in section #15.0.)

Vehicle weight DOM or ERW

Under 2000 lbs. 1.50" x .120”

2001 - 3500 lbs. 1.75" x .120"

Over 3500 lbs. 2.00" x .120"

Vehicle weight Alloy (CM)

Under 1500 lbs. 1.375" x .095”

1501 - 2500 lbs. 1.625" x .095"

Over 2500 lbs. 2.000" x .095"

kurt M

It is ether lighter OR stronger, not both. Safer comes from design and fabrication. Build in mild size but use 4130 might result is a stronger cage. Use the 4130 format and build to the low edge of the minimums might not be a safe as a 1020 mild steel cage as the 4130 tubes are smaller in dia, and have smaller footprints and weld presentations. 4130 Cr/Mo is not real world measurably lighter per volume of material, only lighter per foot of tubing when made in smaller dia or thinner wall. To be lighter it needs to be thinner. The thinner wall or dia tube is calculated to have the same strength as its thicker walled or larger dia 1020 mild steel tubing. It needs to be worked and welded right. One misconception is that it has to be heat treated after welding. This is only the case in wall thickness over 0.250 1/4 inch. Check carefully as some organizations require any alloy weld work to be done only by certified welders. Even if the cage is properly made, no signed cert paperwork and the cage is void.

4130 DOM normalized tensile strength 85,000 to 110,000 psi Rockwell of 90 -96

1020 DOM. 75,000 to 90,000 psi Rockwell. Of 80 -86

There are many alloy and mild metals that are made into tubing. You cannot make certain generalizations as much depends on the metal and methods used in making the tubing.

Money not an issue but weight is AND you have a cert welder/ cage builder 4130 or better. Weight less an issue then money local good guy cage builder? mild steel.

stownsen914

In his engineering books, Carroll Smith harps on the fact that stress relieving is required after welding moly tube in order to avoid brittleness. Over the years I've heard/read arguments on both sides (stress relieving needed vs. not needed). Do you think he's just being conservative?

Scott

Jason Judd

"Mild steel is not as brittle either "

+1