Roll bar Q

[CoryM]

Just wondering if anyone has done some engineering comparing a regular 4-point roll bar to a twin hoop one (see pic below). I cant see the 4-point being that much stronger, but it would be more likely to keep your head from hitting pavement in a roll.
Any input?
Cheers.

[GBodyGMachine]

4-Point is stronger. See the trianglation. You dont have any with the double hoop.

Jeff

[vanzuuk1]

I agree with jeff. You could add diagonals to the two hoops and tie them together, it would help.

[CoryM]

Well the triangulated 4bar is certainly stiffer but thats not what I am asking. I am after strength comparison . The tube lengths of the hoop are a lot shorter, making them stronger than a longer tube. I would say the hoops are somewhat triangulated but not effectively. The curves make it easier for them to buckle. I would guess the 4bar is still stronger, but not by much.
So since I am not that all smart, I am looking for a more informed opinion.
Thanks.

[Damn True]

If the load imparted to the assembly is completely along the z axis your twin loop arrangement would be stronger. However, I have yet to see a crash in which the car came off the ground and was dropped in a completely vertical plane directly on its roof. (Hollywood "Bad guy drives off cliff" scenario not withstanding)

Simple fact is than in 99.99999999% of crashes there will be load imparted into the cage assembly from all manner of angles. Therefore it is incumbent in the interest of safety to maximize the distance between the points at which input is transfered from cage to chassis. This distributes loads across a wider area which absorbs the energy and mitigates the chance of structural failure. The traditional arrangement you show on the right achieves that. There is a reason why nearly every roll bar / cage uses that as its structural core.

https://www.pro-touring.com/forum/sh...2&page=1&pp=20

[CoryM]

Thanks DT.
Thats sort of where I ended up. However the car is going to be roughly 1700lbs so I think I still would feel fairly safe with the classic roadster hoops. Still not decided yet though.

I have yet to see a crash in which the car came off the ground and was dropped in a completely vertical plane directly on its roof.

Heh, Kind of funny you should say that. I agree with you about the odds, but I managed to do exactly that in my poor truck last summer.

http://www3.telus.net/public/corym/crash/
Slid off the road backwards/sideways at about 50mph and when the tires hit the soft shoulder it sent me up in the air and landed roof/tailgate first on the far peak of the ditch. Truck stopped instantly, other than the slight decel of the roof collapsing into the seat. Oops.

Thanks guys.

[Speed Raycer]

Interesting that the double hoop would have resulted in about the same damage to the truck, while the traditional hoop may have saved the cab!

There are some bars being built for Maita's that use the double hoop look, but are one continous tube and has some cross bracing in it as well. Pretty good design. Hard Dog is the name of the company.

[CoryM]

Ya in the truck a standard 4-point would have been the way to go.
I considered something similar to the Hard Dog "Extreme" rollbar but with rear down tubes but the welded center bothers me. Im sure any of the 3 options will be acceptable. I think the rest of the car would fail before any of those tubes would collapse in an crash. They arent safe cars by design and the rear down tubes are too tight of an angle to the main hoop for any roll bar to be effective. They race caged locosts pretty hard in europe and seem to survive fairly well though.
Heres a neat pic

http://www.locost-racing.com
Thanks for the input Speed.