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Well, if you were rally racing you'd need a roll-cage...you'd be silly to go without one!

Undercarriage brace shouldn't weight too much, maybe 50lbs tops as far as I see it. Welding the seams would help, but that's for the body, not the main frame. If the frame had small spot welds it'd fall apart. It'd be a huge improvement over stock though, and if rallying a car, def. something that should be looked into!

I have a chassis stiffening fetish and I've been obsessing over this for a while for my miata and the mx3.

I found this link to be pretty helpful
http://members.fortunecity.com/jasoncuadra/id51.htm" onclick="window.open(this.href);return false;

That's somewhat similar to what I was thinking, except I'll be welding the main frame rail reinforcements. I hadn't thought of extending them to the suspension/subframe, but I might just do that now...

That's exactly how I was thinking to build it. I want to bolt it on, because at some point, it's going to get dinged by a rock, speed bump, etc...
Thanks for the link!

Hmm, maybe make some out of aluminum?

Bingo!

Aluminum = expensive..

But, if you can afford it, why not? I priced out my components in hot rolled steel at < $100 shipped. The same in aluminum or stainless steel were well over $500.

Also, aluminum has about half the strength by volume, meaning you might have issues with either continued flex, or ground clearance.

there has to be some flex. When loads are applied if the chassis cannot deform it will break. Too much flex is bad, but too little is really bad.

Ultra stiff racing type stuff is manufactured to not have to flex. Standard unibody manufacturing has thousands of spot welds that will break if the car doesn't give a little.

wytbishop wrote:there has to be some flex. When loads are applied if the chassis cannot deform it will break. Too much flex is bad, but too little is really bad.

Ultra stiff racing type stuff is manufactured to not have to flex. Standard unibody manufacturing has thousands of spot welds that will break if the car doesn't give a little.

Where do you think the major stress points would be? Assuming the reinforcement is very rigid and won't fracture/deform, what's the next likely trouble spot?

http://www.miata.net/hakuna/0007/h0001.htm" onclick="window.open(this.href);return false;
I think that this is an answer to cost/material question. Aluminum tubing filled with sand, the sand helps to reduce the amount of twist in the tubes but also gives it some movement. What most of you are talking about is a big underbrace like Flyin'Miata's butterfly, but a bunch of these put together could do the same thing for less money.

It wont do anything for the framerails themselves though.

Also the weak points would be:
Where the bolts go through the metal, because all of the force that we are trying to transfer from side to side will eventually come to a couple of bolts in thin sheet metal.
or
Where the welds tie the brace to the frame, welds can be broken pretty easily when you're dealing with high stress points that are used each time that you drive the car

We have a good scrapyard here. They have alot of Aluminum tubing that seems to be unused odds, and ends, and is priced good. I wouldn't use "used" peices because I don't know what kind of metal fatigue I'd inherit. I'd look for easton, or Alcoa.

To make any real advancements with a product like this you would have to model the chassis and perform a finite element analysis. It's just too complex a question to make any assumptions at all. Things like strut bars and tie bars and cross members are just an application of general principles without any real engineering. Tying the frame rails together though is a whole different deal. You could really seriously mess up the cars handling if you put it in the wrong place. Before you start strengthening stuff you have to have at least an idea where the weakness is. If you put a reinforcing device in the wrong spot the weakest point becomes even weaker relatively speaking. So now the part of the frame that was flexing a bit before is flexing a lot because the other part of the frame that was working with it is tight as a drum all of a sudden.

Without some serious engineering I will go on record as suggesting that this is a bad idea.

That's why I recommended seam welding right away. You can improve on the engineering in place, and add almost no weight. Of course it would ruin the crumple ability, and possibly injure you much more in a wreck.
I would do it if I could, though.

^I would rather have my car be slightly solid than a complete crumpled mess. People that would be doing this should have 5point harnesses if theyre taking the time to reweld the body to the frame of the car!

wytbishop wrote:To make any real advancements with a product like this you would have to model the chassis and perform a finite element analysis. It's just too complex a question to make any assumptions at all. Things like strut bars and tie bars and cross members are just an application of general principles without any real engineering. Tying the frame rails together though is a whole different deal. You could really seriously mess up the cars handling if you put it in the wrong place. Before you start strengthening stuff you have to have at least an idea where the weakness is. If you put a reinforcing device in the wrong spot the weakest point becomes even weaker relatively speaking. So now the part of the frame that was flexing a bit before is flexing a lot because the other part of the frame that was working with it is tight as a drum all of a sudden.

Without some serious engineering I will go on record as suggesting that this is a bad idea.

Bah. We're all just hobbyists tinkering with our $2500 cars... not an F1 race team or aircraft designer.

We're doing this to have fun, right? So if it's a total flop or makes things worse, that's life. Get it right next car!

If on the off-chance it turns it into a more enjoyable car, however .. what a rush. That's what I live for. That moment I lit up my KLZE for the first time. That moment I tried my new suspension setup. The moment it all hit me that it was coming together..

That makes it worthwhile.