sagjohnson

I'm doing some planning for lowering my 87.5 and putting 16x7.5 wheels on it. My concern is that the chassis is not stiff enough to handle the greatly increased stress and vibration.

As far as what steps come first, if the chassis is not strong enough, I would definitely want to strengthen the chassis first, before doing an mods that would increase stress on it.

Recommendations on if chassis needs strengthening? How to strengthen it? Your wisdom please?

CS_CGT

I have a full cage incorporated, a front structure incorporated to the chassis to carry the skid plate and bla bla bla.

I don't think that's what you looking for, if yes let me know.

On the more common side:

1. I have a front strut brace (aluminum from Sparco). That I consider as a good investment. This car is very heavy nose, so there are a lot of stress on the fire wall, which is also known to crack. Take a look at yours, you might find that you need to weld couple of cracks.

2. Another easy and effective way to improve the rigidicity of the chassis is to weld a tube between the beans. Open your trunk and at each side you'll see a beam at the rear. I welded a tube right in the middle. The trade off is that you loose the ability of carrying something big, but it's cheap and well worth.

Not much related with what you asked, but more of the handling side. You might want to consider a rear anti sway bar and replace the front one with the one from a 4K quattro.

Another issue with these cars are the wheel and strut bearings, make sure there is no play on them. It sure can make you feel is the chassis, when in reality is the brearing moving.

Cheers,

CS

rocketaudi

and my oil pan looks like hell. do i need special bracing for this? i figured one day when i was bored id weld or bolt one on.

CS_CGT

The one that I use might be a little too much for you, but either way you need to create a support for it.

I have one tube coming from each side beam welded to the from face, and another two tubes welding both together.
The parallel tubes are curved from the bottom of the beams to the front panel.
The parallel tubes have captive 1/2" nuts welded inside(do it before attachment to the car).

You could use a 1/8" aluminum plate and bend it to the parallel tubes format(not easy).

Make a cardboard mock up for a template to be used to cut/drill the aluminum (I actually attached to the structure to make sure it fits)

That should do it for you.

I need more than that, so my plate continue all the way to the begining of the floor pan. I welded 2 1/2" nuts to the square structure under the floor pan(look under the front foot wells), and 2 more at the middle of the subframe.

I do have protection to the engine, tranny and the curve of the fuel lines(the rest is protected with UHMW sheets).

I actually raise the car at the center of the skid plate.

I hope it helps,

CS

Dwight V.

You shouldn't need a skid plate unless you lower the car. I've TSD rallied mine quite a bit and never bottomed.
I didn't know Sparco made (or used to make) a strut bar. Correction then to my post above.
I think this is a pretty sturdy chassis. Remember, this is a German car, not some Corolla or Civic. The rear end is plenty strong thanks to the big bulkhead behind the seat and the small trunk opening. Adding a strut brace isn't going to help the firewall, IMHO, it's going to add more stress to it because it's going to limit the flex up front. Additionally, the strut towers are so close to the firewall that I don't see much benefit in adding a strut bar, far different than say my wife's Impreza which has the towers farther forward and more 'independent'.
Weakest part to me is forward of the wheels, but then nothing of any substance is bolted there. The suspension crossmember does a lot of the work to keep everything where it belongs.

MikTip

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First, because its a "street" car!

You cannot strengthen its "Uni-body" construction enough with simple "bolt-on" mods enough for it to really make a difference.

Main reason the Race cars have all their seams welded &amp; full roll cages installed! ;O)

The "bolt-on" pieces you &amp; I can get only serve to transfer stresses to other parts of the car.
Sometimes this is good, sometimes its bad!

Yes the CGT's Uni-body flex's a great deal, and it was "engineered" to do so! Alter this, and it will crack in places it wasn't suppose to! BTDT!

Drive the car hard, like at SCCA events, &amp; you see what I'm talking about! ;O)

sagjohnson

Does anyone have the figures on it? I wonder where such figures come from?

For example, BMW says it's Mini Cooper is 50% more rigid than it's current 3 Series.

MikTip

Audi stated once my 1993 90(B4) is 23% more rigid than previous(B3) models. Due in large part to newer computer technology.

The numbers your looking for, Audi would have.

But since cars "evolve" the numbers are a mute point.

What do you have to compair the Coupe numbers to?
Newer Coupes? Audi stopped producing the Coupe chassis in 1987 here in the USA &amp; in 1992 in Europe. It remained fundementally unchanged from 1985-1992.

Plus your Coupe was "designed" in 1984. The level of technology used back then, compaired to now, its no wonder newer cars are better &amp; more rigid! ;O)