Rollbars and cages

I am ready to put the 6 point cage in. The painter won't touch it til its in, even though it will be tig welded. My question is:

1. What size/thickness tubing is being used.

2. Which material is better/economical, soft steel or chomemoly?

3. What are the requirements of track built cars for bars/cages. Mine will be for road and track.

I know the requirements for drag cars. Single tube, uncut bends, no kinks in bends, etc. Their bars are inspected with electronic devises that detect defects in the pipes (say that 3 times fast) so that welds can't be hidden.
Any help wouold be appreciated. Thanks
Bill

A while back i did some research into this. All i can say is the regulations very quite a bit from one sanctioning body to the next. I found an SCCA website a while back with all the specs and attatchment specs but lost the link. I remember it has alot to do with weight, if i remember right, under 2500lbs is 1 3/4" .134 wall mild steel (it is actually spec at .120 wall but .120 wall is actually thinner than that). If you use a spaceframe, the plates that it should attach to need to be 6" x 6" i think, and 5/16" thick, this is all a guess off the top of my head, so dont quote me.
As for what material to use, i'm sure you will get alot of people telling you go chromoly but if you plan on actually driving this car on any street, stick with the mild steel, chromoly is hard, no flex. My buddies race car needs to be completely disassembled and rewelded every year cause of it. I think even the SCCA web site frowns on the use of chromoly.
As for the design of the cage, good luck, that was there too, it is complicated and it will eat alot of room in a car you plan on driving on the road. But it does give you good ideas.
The other thing i noticed is the specs are always changing, even faster than it takes to put a car together.

"1. What size/thickness tubing is being used."

Usually 1-5/8"OD x .134 wall mild steel, with .124 wall being the minimum requirement per the rules.

or 1-5/8"OD x .083" chromemoly

you can go with .124 wall mild but they test exactly to that and production tolerance and thinning from bends can cause it to be under .124".


"2. Which material is better/economical, soft steel or chomemoly?"

Chromemoly is about 40 percent lighter at equal strength but costs about 3 times as much.

You save weight with chromo, you save money with mild.



"3. What are the requirements of track built cars for bars/cages. Mine will be for road and track."

Varies widely and depends on sanctioning organization and type/class entered.



"I know the requirements for drag cars. Single tube, uncut bends, no kinks in bends, etc. Their bars are inspected with electronic devises that detect defects in the pipes (say that 3 times fast) so that welds can't be hidden.
Any help would be appreciated."


Art Morisson sells pre bent stuff and tubing but you can get tubing cheaper from your local metal supplier (depending where you live). He's mostly into drag and street cars.


http://www.artmorrison.com/sitemap.htm




" Thanks
Bill"


your welcome

You will need to define what you mean by "track". Cobra club like track days are NOTHING like entering a SCCA race. This is assuming you could find a class for your car. Most of the current 40's sold like the ERA,MDA,CAV,RF etc would be fine as bought for track days. One step up from that would be to order your car with a roll cage option included with you car. I think RF for example does this.

As far as a SCCA cage goes you are going to have to start talking to people who race mid engine cars at about your same power output and car weight to get some idea where you are going in relation to the rulebook. This is going to be tough.

Another way to go would be to build your roll cage without a rulebook in mind but with your use of the car and how much protection you want. Many dragrace sedans in the 11-12sec range are very safe and really don't have all that complicated roll cages in them. This is more or less what I did. I added a front hoop along the windshield with two bars down the middle to another hoop behind the seats. I attached this 4 point cage to the existing chassie hoop behind the drivers compartment. I may add two braces from the chassie hoop back to the rear shock towers later.

I had the guy who did my car use materials that would pass a SCCA materials requirement, tubing, OD wall thickness etc. He also welded in the mounting points so that they would also pass inspection for construction quality. But my system would not conform to any overall SCCA class inspection per say. I was mainly trying to protect myself from a front inpact into/under a car in front of me or a t- bone into the side of me at lower speeds like a minivan socker mom who doesnt see me and plows me in the door at an intersection.
These are more likely for me than a big one on the mulsanne.

"if you plan on actually driving this car on any street, stick with the mild steel, chromoly is hard, no flex. My buddies race car needs to be completely disassembled and rewelded every year cause of it. I think even the SCCA web site frowns on the use of chromoly."


Chromemoly has a somewhat undeserved reputation. For instance if you read Carroll Smith or Ron Fournier they really don't like chromemoly, but their opinion is dated. Normalized 4130 Chromo and advances in Tig welding methods have drastically reduced fatigue cracking problems.

That said though 4130 is harder to cut, grind, and bend, and is a bit trickier to weld, and you do have to stay at or below the .083 wall thickness. If it's not welded correctly it can crack much sooner than normal. I would recommend "performance welding" by Richard Finch. He's done lots of 4130 with aircraft and no problems with cracking. If it's good enough for aircraft it'll work for cars.

How old is your friends race car? Even mild steel will crack eventually depending on usage. And once it starts cracking it's keeps cracking.

Hi Bill

John Tiemann, Austin, Texas, has just received his GT40 Australia built, DRB GT40 race car, fitted with a 8 point roll cage that complies with the regulations of the Cannon Ball Run type of events that he competes in. (Average speeds of up to 180 mph over distances of 90 miles)

Generally chromolly tube is not acceptable for road registered cars, as the failure mode is splitting. On race cars that are regularly inspected chromolly is OK, but on road cars that may not be closley checked mild steel is best. (Some registration Authorities do not allow chromolly to be used)

The size of the tube used depends on the weight of the car. Generally for a GT40 the main hoop needs to be 1.75" dia, front hoop, front & rear hoop stays & hoop to hoop longitudinal members and diagonal braces 1.5" dia. The tube should be seamless C350 x 1/8" wall thickness and mount plates should be 1/8" MS plate. We used 48 mm dia x 3 mm main hoop and 42 mm dia x 3 mm for the rest of the cage.

With a full cage, including a hip bar, in a GT40 the longitudinal bar between the front and rear hoops at the top of the doors must be left out to allow occupants to get into and out of the cockpit, otherwise it would be like trying to get into the car through the window. The USA regulations that we have seen allow one member to be deleted to allow the speedy exit of the driver. We deleted this member and provided bending strength at the top corner of the front hoop by installing a stay brace from the top corner, forward to the chassis at the front of the inner sill. There is also a diagonal member in the roof that braces the top corner of the front hoop.

The whole roll cage needs to be installed prior to the cabin being fitted to the chassis as much of the cage must fit close against the cabin sides and roof (to provide maximum cockpit space) and it would not be possible to weld fully around the joints.

To assist entry and exit from the cockpit, we fitted a removeable steering wheel.

I hope this helps with your roll cage issues.

Peter Ransom
GT40 Australia

Peter, would you mind posting some pictures of your cage out of the car and then after being installed into the car. I would understand if you feel that you don't want to give your work away for free.

Thank you in advance, Howard

There is a picture of the usual DRB roll bar with struts and a picture of the full DRB cage in this thread.

They are not retrofit. They form an integral part of the chassis.

rollcages

I would like to see some pictures as well. How does the roll cage interact with the dash?

Thanks
Bill D
Los Angeles, CA

Thanks Lee, I remember those pictures now that I see them again. I plead guilty to not using the search engine. This is very much how I did my GTD except I located the front hoop feet farther forward at the intersection of the door mount vertical plate and the long 1.5 X1.5" side rails. This kinda ties in the "dashboard" framing. Interestingly the last picture on that thread is of a GTD with the engine bay braces. In a GTD you can use the exsisting roll hoop that forms the roof frame as the rear hoop in a 6 point cage like this one did or add a second hoop inside of the car and bolt it to the exsisting hoop sort of doubleing them up. This is what I did.

If I did it a second time I would build a front hoop that bolts to the roof frame and add bolt in engine bay braces. I think that the whole think could be removed from the car out thur the door opening which would make construction a lot easier. The hard part is welding in the mounting plates in a location that will add strenght to the chassis and not cause the cage to act as a lever to tear it appart in a wreck.

Those of you who are doing this for the first time talk to a race car builder who knows what he is doing in this area and not just a good welder who will do it the way it looks right.

After further research on Chromemoly roll cages I'm going to have to reverse my stance due to safety concerns.

The relatively new advanced welding book I was using as reference says it's OK but everything else I'm reading says it's not.

My best recent info comes from race car welders who unequivocally state that although Chromo comes normalized the weld process reverses that and it must be re-normalized after welding otherwise it is no stronger than mild steel. Combine this with the thinner wall typically used to make Chromo viable and you've got problems. In addition without normalizing the weld area is more brittle than mild steel.

Normalizing is problematic in the field. The only sure way I know of is with a vibration inducer like the METAL LAX which starts at 10K. If your roll bar builder has one of these then chromo is safe, otherwise it's not.

The welding book I was using as reference is dealing with aircraft that have never been crashed. Chromo in race cars only has problems when it's crashed which happens far more often with cars than with planes. The brittleness is a problem more so in a crash.