Torsional frame flex reduction

[exwestracer]

That just seems like a lot of work when I think a simple 4 point will do the trick. If that is not enough for you add the two bars to make it a six point. The 4 point only affects my battery storage routine a little bit.

You are absolutely right. Silvermonte DID say he didn't want to put a cage in the car, so that puts the thinking in this other direction.

[silvermonte]

My problem is that i dont want the down bars to be going into the bed of my truck, would making loading and unloading larger items a pain in my truck. It seems also that having the main hoop sitting further back in the extended part of my cab with down bars going forward in the cab is not a safe option so im told. Not sure what the difference is between that and some of the other cages in other cars but rules are rules.

My main plan was to have the main hoop as far back in the cab as i could with the down bar at the 45 degree angle and it comes to about right below shoulder on me and then put another main hoop closer so that its within the 6" of my helmet like alot of rules require. The closer main hoop would be tied into the down bar, and then i would tie the 2 main hoops together the forward and aft direction and then the back hoop would have lots of diaginal brace and the front hoop would have left to right bars for the harness and what not. My problem is nobody seems to want to say that would be a safe way to have the cage and im just wanting something safe, im not really worried about the rules alot, i will just be doing mainly auto-x and a few tracks but in the rare case i put my truck on its lid i know it would crush without some sort of bar so thats what im wanting to do, but if its not safe i dont want to make things worse, so im just sticking with making the frame as stiff as possible till i can talk to someone in person that knows a thing or two that can tell me what i need. I dont do a very good job of explaining how i want my cage over the phone or on the net.

[exwestracer]

Ive been wondering about this topic for a bit, for those who dont want to put a roll cage in their car how much is there to gain by putting in a spine down the center of the car around the drivetrain, and would it have to be made out of round tubing or would square tubing work? I could keep the spine level with the bottom of the frame but could then raise it up a few inches above the top of the frame and into the body, i lowered my car and i have to cut the trans tunnel out anyways and this would be an option for me

Sorry, I'm confused now...

Regarding your cage design, you can have an extra hoop, but the one right behind the seat must go all the way to the floor, and your down bars would have to start there. The one at the back of the cab would be seen as the "extra". Either way, I don't think it will get you out of having the "kickers" going back into the bed area if you need to get it teched for racing.

[silvermonte]

i can understand the confusion, i do a horrible job of explaining. i have been told that the kickers have to go towards the rear of the vehicle,they will not provide the support they need if they go forward, even tho the down bar would be the same length going towards the front or the back if the main hoop was in the rear of the cab. I wanted the 2nd main hoop to be closer to the seat to be within the 6" of my helmet for that reg but as you just said it has to be the one that the down bar starts with.I then wanted the 2 hoops to be tied together however the cage builder said they would be needed to provide strength for a roll over. So since i cant get any roll bar shops to at least consider if my design is safe im just going to go without a cage. One of these days im just going to go to the store and buy a bunch of the round foam that you put in the pool and mock up my idea so people can see it better then me doing a horrible job or explaining it.

[silvermonte]

hey Ray i sent you a PM

[j-c-c]

You are absolutely right. Silvermonte DID say he didn't want to put a cage in the car, so that puts the thinking in this other direction.

It doesn't seem to me that the one written in stone simplest solution to reduce torsional flex no one has mentioned, and that solution is a simple tube. It outperfoms all other shapes. Almost all other solutions are simply trying to recreate a simple tube, but always with compromises. Rule number two is the larger the tube the less flex, again all other shapes usually are improved by simply making them larger in cross section. IMO the OP adding "X" bracing does little to increase the size of the holy grail sought after tube shape. Obviously we can't ride around in 5' dia tubes, but at least we can understand the sought after design target to minimize unproductive detours and/or added unnecessary weight. My reply addresses only torsion rigidity the op mentioned.

[exwestracer]

It doesn't seem to me that the one written in stone simplest solution to reduce torsional flex no one has mentioned, and that solution is a simple tube. It outperfoms all other shapes. Almost all other solutions are simply trying to recreate a simple tube, but always with compromises. Rule number two is the larger the tube the less flex, again all other shapes usually are improved by simply making them larger in cross section. IMO the OP adding "X" bracing does little to increase the size of the holy grail sought after tube shape. Obviously we can't ride around in 5' dia tubes, but at least we can understand the sought after design target to minimize unproductive detours and/or added unnecessary weight. My reply addresses only torsion rigidity the op mentioned.

Good point. Now how would you apply that thinking to a factory chassis without cutting the floor pan or adding a 3 dimensional structure like a roll cage?

[T.K.]

there is another option very similar to a x that i like better. some people call it an inner frame but its effectively a larger x and a cross-member. below is a pic borrowed from one of our sponsors art morrison. welded in it is probably the best you can do without an aftermarket frame or cage. http://www.artmorrison.com/images/53...0vette0232.jpg

[j-c-c]

Not to beat a dead horse, but I visualize a typical ladder frame to a sheet of PW, it twists just like an empty picture frame, and its completely solid, ie adding all the internal bracing you want up and until its completely solid gives little improvement in torsional rigidity. However going from a 3/8" sheet of PW to say 3/4" shows results, and if possible, going to 6" and beyond is the real deal for improvement. I know many want the magic bullet undiscovered brace that gets it done, but its hard to overcome physic, try as we might. Of course if we are talking the new OEM offset crash test requirements, or single wheel rear wheel drive, that big X looks pretty good.

[Norm Peterson]

Torsional stiffness is a 3D problem and all about getting the material resisting it spread as far apart as possible, within any other limits that may apply.

A thinwall round tube is particularly efficient, as all of the material is the same distance away from the center of rotation. Rectangular tubing is slightly less efficient, and the closest that a car frame can come to that is the backbone sort of frame pictured earlier in the thread. A limitation of the backbone and even the rectangular tube is that local distortions tend to concentrate at the corners of the cross section that compromises away some amount of the torsional stiffness that you'd calculate based on the cross section maintaining its 90° (or whatever angle) corner shapes.

An X-brace works essentially by converting the general chassis torsion into bending of the X-brace elements. Spreading the material further apart here means deeper X-bracing beams. This is a reasonably effective means of stiffening a flat plate, but has the obvious disadvantage when the X-bracing becomes several inches deep and running right where most everything else has to live (powertrain, exhaust, seat tracks, feet, etc.).

Another take is to beef up the closed cross-section sills in both width and depth. If you've ever climbed into an XKE Jaguar, you know exactly where this approach can go. You probably have better side intrusion properties than with a backbone, but the transitions to where the frame must narrow to go between the wheels (and permit steering up front) are apt to be more difficult. You still want to tie the sills together so that they act together rather than separately, and you could get into elastic buckling considerations if the ratio of width or depth to metal gauge gets too large. There is always a compromise to unravel.

3-D is why most coupes are much more rigid than their convertible versions - Corvette excepted since they're all based on a separate frame that gets little structural help from the bodywork. Even though there are large openings, there's a lot of 3-D help going on, and when the glass is bonded to the sheetmetal even that stuff adds to the torsional stiffness and has some structural function.




Norm