Let's talk about chassis bracing

I've currently got the interior ripped out of my 2002 to repair a few rusty spots, and I've been thinking about doing a bit of chassis bracing while I've got the welder out. I'm considering three possibilities, all, some, or none of which necessarily need to be done, but it is a 44-year old car after all.

1) A brace tying the front subframe (in the area of the inner control arm mounting points) to the frame rail stubs that extend forward from the floor pan to support the front end. This would mimic the concept of the E36 X-brace.

2) Some sort of reinforcement between the inner rockers and the transmission tunnel, running across the body under the seats. The 2002 cabrios had something in this area, and while I have a nice solid roof, I figure more stiffness wouldn't be bad. Option 2b is to do the same thing, but along the line of the existing panel that runs along the base of the rear seat. This seems less valuable, as it's in an area with solid vertical panels rather than doors, but it wouldn't be hard to do.

3) A brace tying the rear subframe mounts to the floorpan. There are already short factory arms extending straight forward from the mounts, but I'm thinking of something that mimics the rear subframe mount brace I have in my E82.

1 & 3 would be relatively small tubing; 2 could be more substantial. I've got some stuff lying around that would more than likely end up being used here. 1 & 3 would be bolt-ons once fabricated, while 2 would have to be welded in.

Looking for input here. Which (if any) are worth doing? Anything I'm missing here?

I think they are all worth doing. However,  i know exactly Jack E36 M3 about that particular chassis. 

However,  im really familiar with the mopar unibody of the era. In those, torque boxed and subframe connectors make monumental improvements. May be similar to the 2002?

If doing exterior paint work, consider stitch welding the rockers and door openings. I have yet to fid a car that doesn't benefit from that. 

The chassis is very simple. Unibody, with the front subframe mounted to the frame rails at "A", and the rear at the outer points "B" and in the center by the diff hangar.

I'm not doing exterior paint, so welding the door openings and rockers will have to wait. Can you explain a little bit more about the Mopar mods you mention?

this is the  mopar stuff. A square tibe that ties front and read "framerail " together and adds the floor in. Torque boxes triangulation to rocker

In reply to Dusterbd13-michael (Forum Supporter) :

Ah, OK, I don't think that's going to work here, at least not as well as it might in its intended application. The rear subframe on the 2002 is mounted with rubber mounts bolted up through the floor, so it would be impossible to tie them together in this way.

Ok. Was a thought!

The other proposed bracing sounds good though. 

What about miata style frog arms? Again,  have no idea how a 2002 goes together,  but they helped tons on my miata and Dustin 280z

Here's a simple diagram of what I'm thinking about. The red lines are bolt-in braces, and the blue are welded in reinforcements.

I should point out that I already have bolt-in bars tying the front and rear towers together.

Monocoque racing chassis builder chiming in. These will all do a lot to stiffen this car. The front and rear as drawn are super. the center less so. It won't hurt, but won't gain much. Would it be possible to make them diagonals as well? Best from the rear end of the front leg to the anchor point of your rear brace near the center of the chassis. The interior pieces I would construct from folded sheet to use the existing floor as the bottom of a box.

In reply to TurnerX19 :

Thanks. To modify the center as you describe would be a lot of work because of the contour of the floor. Maybe better to leave that alone for now and concentrate on making the most of the front and rear braces. Here's an updated diagram of the bracing showing the existing braces in green and the proposed braces in red.

The front brace ties the towers to the firewall, and the rear ties the towers into the floor directly above the diff mount. With the addition of the lower braces I'm thinking about, would you consider this the way to proceed, or is there another area I should be concentrating on?

I know how to reinforce an MG chassis because racers have been building them for over 50 years. Is there not already a knowledge base for the 2002 you can lean on?

In reply to Keith Tanner :

Not that I've been able to find. The upper braces are pretty common (I bought mine 20+ years ago), but braces for the lower chassis don't seem to have been used. Later models have things in this area from the factory (usually on convertibles and performance models), and I'm sure those chassis are already far stiffer than the 2002 ever was, so I suspect there are improvements to be made, but I haven't found anyone who's experimented with them.

One suggestion that I would put forth.

Devise a before and after test to see if the change is meaningful.

While a real test would involve both static and dynamic response, a simple test for torsional could be to jack the car up from a front corner and see how high it has to go before the rear tire comes off the floor.  In theory, if you make it stiff enough, you should be able to lift three tires off the ground while jacking near the fourth.

This is relevant to my interest as well. My '02 could do with a little stiffening as well, and as 02Pilot said, the upper braces seem common, but beyond that I haven't come across much. 

In reply to NOHOME :

Good point. Since these will be bolt-on braces, it will be easy to do before and after (I'm assuming I will forget to do the "before" once I get involved with design and building).

Looking at that floor pan design, I would make some frame connectors with a bolt-in butterfly brace.

And I would test the before and after results to be sure it actually works. 

I love that "butterfly brace" is now a generic thing. That dates back to when we released a preproduction photo and I claimed it was a stainless steel butterfly... 

NOHOME said:

One suggestion that I would put forth.

 

Devise a before and after test to see if the change is meaningful.

 

While a real test would involve both static and dynamic response, a simple test for torsional could be to jack the car up from a front corner and see how high it has to go before the rear tire comes off the floor.  In theory, if you make it stiff enough, you should be able to lift three tires off the ground while jacking near the fourth.

Maybe only two other wheels. Unless you've got the car resting on the bumper. 

In reply to fanfoy :

A butterfly brace (I would probably try to design it so it was actually an X brace, just for simplicity) would probably end up almost 6ft long. It seems like it would require either very strong (and heavy) material or a lot of gusseting to make it stiff enough to do its job. Exhaust might be something of an issue as well.

How about some bracing from the cowl to the bottom front of the front crossmember (green line top picture).

I don't know how much room in in the engine compartment or if this possible.

Idea stolen from Herb Adams VSE for use on 2nd gen Camaros and Firebirds

Scott

In reply to noddaz :

The engine bay is awfully crowded for that type of bracing. Might be able to pull it off on the right side, but the left is a no-go.

With the engine out, some guys weld in a short vertical reinforcement from the bottom edge of the firewall down to rails, but I have no plan to pull the engine any time soon.

Another poster asked what other 2002 people do. The trouble with that is that serious chassis stiffening is usually accomplished with a roll cage. I see 02pilot's need here as maintaining street comfort, and while I suggested an internal "X" I also think it is overkill. Real need for this sort of reinforcement will only become apparent when the original structure starts to show stress cracks. If the car is not rusty that takes a whole lot of hard miles on a 2002.

In reply to TurnerX19 :

You are absolutely correct that this is a street car; many years ago it was autox'd frequently and did a few rallies up in New England, but those days are past. It's got a little rust here and there, but I'm going in now to address the stuff that might cause structural issues. In reality there is no need for the sort of bracing I'm considering, but I am curious about the effects. Having gotten good results from the upper braces installed many years ago, and seeing how BMW has developed bolt-on braces for later models, I figure there might be something to gain.

02Pilot said:

In reply to noddaz :

The engine bay is awfully crowded for that type of bracing. Might be able to pull it off on the right side, but the left is a no-go.

 

Maybe.  Could you run a tube from strut brace down to the left front crossmember?  It might have to have a curve in it  and have a tab welded on the strut brace.

  MS Paint at it's finest.

In reply to noddaz :

I hadn't considered that. There no room where you drew it (the relays and brake fluid reservoir are there, plus the inner fender tapers inward), but there might be further inboard. Tight fit in any case. I'll have a look.