bob_dallas

Anyone have any good links or information on custom rollcage design? I'd like to learn a little more about this so I can make better decisions about my own car.

Also - related question. Anyone have experience with seats mounted to the rollcage (custom cradle attached to crossbar under the front of the seat and also to the harness crossbar in back)? I bought a racecar that has this type of design. I understand the principle behind it - that if you have a side impact and the rollcage starts moving then you move with it - but I'd like to learn/hear more. Definitely fixes the seat in one position so no car sharing and I have other possible concerns/questions as well.

Thanks in advance

RedlineMan

Hi Bob;

I've got a few projects on my website. Feel free to poke around and chew on them a bit. I'd be glad to consult with you, and I'm sure GEO will chime in.

Of course, I do this for a living, but perhaps more importantly, I like to see people make wise, "value-added" decisions on such matters even if I don't make any money on them. Nothing worse than paying money to have your car made into a piece of junk! I hate to see the disappointment, and I HAVE seen it.



Regarding your seat setup, that is indeed a good way to go. If you are lucky enough to have enough headroom, the seat-to-cage mounting is a superior design. You might also consider adding sliders if you have need for multiple drivers. This can be done safely and securely... if again you have the headroom to spare.

Bryan Watts

With a little bit of creativity, you should be able to mount a slider bracket.

Geo

Hehe. John's right. I definitely have an opinion on cages and there are a lot of "professionally built" cages that are junk.

Some things to consider when designing a cage:

Tubes should have as few bends as possible. Bends are nature's way of saying "fold here." Bends cannot be eliminated, but they should be well thought out and the bends should be braced by other load bearing tubes.

A rollcage not only provides you protection, but if it's designed properly it can provide you superior protection and superior chassis stiffening.

Tubes should not be attached near each other. They should be attached all at the same junction. Same place. This is for maximum stiffening and load bearing (sharing).

NASCAR bars have become popular. After discussing them with an engineer friend (and SCCA racer), I think they are not all they are cracked up to be. They give you working room, but they have bends in them that weeken the tubes. The strength comes mass (so many tubes). NASCAR bars can bend until they mirror their original shape inside the cockpit (mother nature again). OTOH, X braces would have to be either literally ripped out of the car or they would have to stretch along their length to intrude further into the cockpit. They also stiffen the chassis better (if done right).

Basically, tie into the suspension mounting points as much as possible (second choice being large load bearing sections of chassis like the front frame rails on a 944), triangulate as much as possible, and tie tubes to the same junction as much as possible and you'll have a better cage than most of them out there. It's not hard to do. Just think it through.

bob_dallas

Thanks guys - some good stuff to take a look at and ponder. I don't have much time to work on it tonight but I'll be back with more questions.

John - WRT mounting the seats to the rollcage - I think that's a great idea but shouldn't the harnesses also be mounted to the rollcage? If the point is that you move with the cage then I would think you'd want the harnesses to move also. The reason why I ask is because in this car the substrap and lapbelts are mounted to the floorboards and the shoulder straps are of course attached to the rollcage. Seems like you might be pulled in a couple of directions there worst case...? Is there a regulation that prevents mounting substrap and lapbelts to a rollcage that I've missed somewhere?

NicolasW

What kind of car? Here are some pics from my cage that was installed by Mitch Piper of Piper Motorsport (http://www.pipermotorsport.com - Manassas, VA - (703)530-0353).

For my 951 it is imperative to tie into the chassis. Mitch welded in custom boxes and the X-brace in the back is to protect the fuel tank. Door bars were within 1/4" of the door panels and the kneebar (MUST HAVE!) goes behind the center console and had about 1/4" clearance so the radio still fit.







As expected, the rigidity of the car post-cage install was unbelievable. The car actually drove better on the street (over bumps, etc.) than it did before the cage because the suspension was actually working instead of the body flexing.

Whoever you choose, it is usually best to choose someone who only does cages. You will probably get a higher quality product at a cheaper price.

Can't answer your question about seat mounting. Mitch made custom brackets for my seats so the Sparco's were about 1/2" off the floor to increase head clearance.

Good luck,

Mike Buck

Wes,
That looks a lot like the cage Mitch just built for me. Mine is slightly different in a couple of ways, but that is the nice thing about being "custom". Can tailor it to the customer. I have a couple of pics I can post again if wanted.

He's only done a handful of 944's. Now having seen your car, I think I've accounted for all of them. Great guy, good work, fair price!

NicolasW

Yes, my car was the first 944 he did (way back in 2000 or 2001). He and I were very proud of it. A friend of mine had the 2nd 944 that he did....and his Porsche business has increased ever since. His work speaks for itself. He can make anything.

I would love to see pics if you have them.

Mike Buck

Here are two pics I have on my work computer. You can get an idea of it. My door bars are a little different than yours. Upper bar on drivers side is lower and absent on passenger side(for easier ingress/egress). No kneebar at this time, but I may have it added later. It would also be easy to add to the passenger door bar so it matches the drivers side.






I may get some better pics this weekend at Summit

RedlineMan

Hey Bob;

I think the argument that the seat will move with the cage if welded to it is a bit esoteric and over simplified. It makes sense logically, but I don't think it is the main purpose of such a configuration.

Unibodies are designed to fold. If your seat is attached to a unibody, its mounting may become compromised. A floor is an area likely to buckle. The area where the seat belt pickups are located is less likely too. A structure welded inside a unibody, but not attached at median points (only stress points) will tend to be more independent of the unibody and will also spread the load farther through tube strength as opposed to sheetmetal which will have more localized deformation. For that reason, it may stay intact while the outer sheetmetal does the deforming. In that sense, the seat-to-cage idea is a good one. Solid mounting, not likely to move.

In any hit, there should be some thought given to how the taughtness of harnesses would be effected if the mount points were compromised. In a side hit, the lap belt may loosen because the mount point moves inward with the rocker panel. Some things can't be helped. Shoulder belts mounted somewhere to the rear deck of a 944 stand to loosen if that rear deck moves. In a large rear hit, the rear deck will deform, usually bending downward, and I fear the belts might loosen. For that reason I do not like mounting back there. Mounting to the cage in ANY case is preferable.

I would be MUCH more concerned with floor mounting. This is something that I UTTERLY REJECT in ANY form. I don't care how big the foundation, a house built on sand is still so. Sheetmetal by itself is not strong unless it is configured in some sort of beam structure like in a unibody. Flat floors aint strong!!!

I don't care how big the backing plates are, you cannot overcome the fact that floor mounting is done is tension, not shear, and a tension mount of this type will never outdo a shear load center properly constructed.

End of speech... for now. Gotta get back to the shop!!

addictionms

When my cage was being built, the guy doing (TRG did the cage and the guy was ex Jordan I believe) told me that when you build a cage, you need to think just like the unibody builders , and figure out where you want the energy to go. Your cage should have designed into it places where it will bend, absorbing the energy of the impact, ideally not in the driver space,

I think the strength in the "x" doors is for them to push in the inner structure of the cage needs to pull in, so, the halo and rear hoop and front legs all need to bend for the "X" to move towards the driver.

Also consider how much of the car you want to replace in a crash, in our RX7 we hit the front, and the cage failed at the front shock towers, so the main structure was intact, all we replaced was the front clip, much less expensive than the entire car.


Jim

Geo

Here is an example of an excellent cage.

http://forums.bimmerforums.com/forum...13#post2040213

I would change a few details personally:

1) The main hoop would set on the edge of where the floorpan rises to meet the rear seat. This is incredibly strong, much stronger than the floor pan.

2) Along with (1) above, I would absolutely NOT have a curved brace behind the driver. Remember what I wrote about nature saying "bend here." That brace is nowhere near as strong as it could/should be.

3) I understand why people use "NASCAR" bars, but I still prefer the X brace. I won't restate my case here, but if you are curious, look at my previous post above.

What I like, no, love about this cage is all of the bars/braces connect at the same point. So simple, yet so crucial. All load paths have multiple paths to dissipate along. This is how all cages should be built. Also note that the cage ties into the suspension mounting points.

This is a cage that I would be happy paying for (most are not). I would have the builder make the above changes.

Carrera51

Wes and Mike:
Has Mitch Piper done any 911s? I am thinking that when my car is down for the engine rebuild, I may remove the Safety Devices cage and have a custom cage done.

NicolasW

Mark -
Yes, he has done 911's including one of Bob Williams' cars and he just finished Justin M's car. He also put a roll bar in my Dad's 911 SC and did a custom seat mount. Mitch is the best in the area hands down....Ivan sends all his cars over there for cages also. Please tell him I sent you because my car was the first Porsche cage he did that I know of. Since then he's done many more and all have been extremely happy.

He will do whatever you want and tailor's the cage to your needs and meets or exceeds the racing class rules of course. SCCA and PCA have slightly different requirements so you need to tell him what you want.