As I was looking at my car and my SFCs, I began to wonder how they could add tortional stiffness to the car. I mean, I understand how they prevent Mopar body wrinkle, and I understand how a roll cage will provide roll stiffness, but here is a piece of box tubing that is only 2" in cross section and a car that is maybe 60" in cross section. I can't find my dang ME Handbook, but I remember that tortional rigidity is proportional to the forth power of the diameter. Based on that, a puny 2" bar is not going to add mesurable tortional rigidity. Yet I read here where people install them and feel the difference. What are they feeling, if not tortional rigidity? Or is my math wrong?

 

Now Carl, you wouldn't be trying to pull somones chain with this one would you? I'll assume not (either way, I'll gladly toss my ideas out, why not?).


Which subframe connectors do you have? If it is a 2 point design like BMR and others, they leave a considerable amount to be desired (in my opinion). If you can find your book before I can (I'm moving this weekend and am about to take apart my network and drag it to the new place...where I'll be waiting for cable service until Tuesday), you can likley calculate the torsional stiffness of a subframe connector (made of round material) by considering it to be a Anti-roll bar that is "X" long and has arms on it that are 50+ inches. I suspect that math will not yield a very favorable number for torsional strength. Boxed tubing (or mild steel round stock) is a lousy torsion bar (unless it is tempered spring steel and being used as an actual torsen bar, such as Mopar, but we'll leave that for another discussion) as I'm sure you already have figured out. One of the best ways to add torsional strength to any rectangular object (like a chassis) is through diagonal bracing (or thicker material, etc). I prefer the SLP style of subframe connector, it is still not a perfect design, but by attempting to tie the chassis together diagonally, they are attempting to address the problem.

I suspect that the best design (if it would fit the chassis) would simply to take a standard 2 point design and add an "X" brace from the 4 points of the connectors to the opposite diagonal side of the car. The problem is going to be clearance. You may need to move the "X" down away from the chassis a considerable distance to clear the exhaust and the driveshaft at full droop, this makes the resulting trade off between chassis stiffness and ground clearance a losing battle.

However, GM designed the chassis to what they considered to be "an acceptable level of torsional rigidity". It passed crash testing and they sent it to market. By adding at least "some" extra stiffness to something that was considered by many to be "good enough" will likley have at least some degree of impact on the package as a whole. This, I suspect, is what everyone is "feeling". The only other possibility is the "my wallet is lighter, so the car must be faster/stiffer, etc" feeling....one of the two will have to get my vote for the answer.

Just my thoughts....(and opinions).

Kevin

P.S. I don't think your math is wrong. But I'll try to recheck it once I find that box.....

 

My roll cage adds more rigidity than the SFCs do (my opinion).

However, the boxed SFCs make a great place to jack the car up NASCAR style. Plenty of room to get jackstands underneath.

 

How about the Double Diamond SFC's? Don't they do well to stiffen the chassis since they attach/welded to more points?

 

The Double Diamonds are better than many of the designs that are out there (at least in my opinion). I am not a fan of Kenny Brown engineering, but that is also a personal opinion (that has been discussed on a couple other recent subframe connector posts). Even so, I still consider them to be better than many (or all) of the 2 point designs that I have seen on the market.

As a side note, they are cadmium plated. Welding Cad plated steel produces a gas (as a byproduct) that is poisionous. If you do a set of those, be careful, or have a professional that has welded cad plated steel do the work.

Just my thoughts....

 

From the "other" discussion forum ...

Just some more data ....

And to give you an opinion as to what people are feeling, is more than likely a weight reduction in their wallet.

 

"Other forum"?

Which one is that (if I may ask...)?

Good Morning Mitch....

 

Hey TB ...

It's on LS1.com ... in a regional forum ....

 

That has been my experience (er ... my opinion) too.

However, the boxed SFCs make a great place to jack the car up NASCAR style. Plenty of room to get jackstands underneath.

[/quote]
Hadn't thought of that. Great idea.

 

Good sleuthing! So we are not the only ones that feel that way. I hope this becomes part of the forum folklore.

 

No, I'm not. It goes back to the good vs. bad information issue. I really was looking at my SFCs getting ready to weld them in, and the posts I'd seen came to mind - mentioning how much they improve handling, recommending them to improve handling, etc. And, well, the rest of my thought train was in the post.
They are two point. I wanted double diamond (without actually reasoning it out, they just looked like they would be better) but I read that they interfer with some things. In fact, they would interfer with my new torque arm.
Yeah, the ME handbook had the formulas for tortional stiffness. It is a lot easier to look them up than to derive them. Obviously, I am not concerened enough to do that I wanted to compute the stiffness in degree/ftlb.

Oh, I thought temper moved the yield point out, but the stiffness is controlled by the modulus of elasticity, which is related to composition. But, as I said, I can't find my ME handbook. Actually, these days I write embedded software so don't use the handbook much. Recently for things like the two satellite receiver boxes, although I've done a lot of machine control too.
Right. Diagonally across the box. A diagonal in a plane does not add much tortional stiffness.
I went an looked at SLP (didn't remember what they looked like). First, they would have to have a solid connection in the middle. Even then, I am not convinced that "the additional cross bracing increases chassis stiffness dramatically" as they say. It is much more difficult to analyze, and some FEA software would be a big help. Intuitively, the triangle on each individual bar would obviously increase the tortional stiffness of that bar, but since we have already decided that the bars themself don't contribute much, I suspect that the addition would still not help much. IMHO, mind you
I agree. That would certainly be better. I am not convinced that it would add signigicantly though. I think the right answer is two point SFC to get those loads off the body skin and then a diagonally braced cage.

I wonder if you may have hit it there
And mine too ...

 

I need to correct that statement. I was trying to say that it makes a good torsion bar...when you want a torsen bar. It makes a poor torson bar when you are looking for a bar of "infinite" spring rate. We are trying to achieve "zero flex" without using 500 lbs of very large diameter tubing. So, it is a good torsen bar, which is the problem with it as a material for subframe connectors.

I'm not sure that's any better.....


Kevin


PS I'll always take a cage for stiffness over connectors....that 3rd dimention helps alot....

 

I just read all the SFC threads and see that a lot of guys like SLP or LG with the crossbraces. I hope they will do a decent job keeping my T tops from leaking or getting loose. I wouldn't mind at least a 4 point roll bar. Who makes the best and easiest to install roll bars? Having easy access to the rear seats would be nice but if that would make it weaker than a different style bar I guess that won't matter much. I'd like to get decent SFC and a roll bar installed at the same time since I will need to remove the carpets to prevent fires from welding and to get the roll bar in.

 

I have some SFC's that are better than decent and can be bolted or welded.
Is my wallet lighter? Yes
Does my car handle better? Yes
Are they 3 point? Yes