Lower control arm brace - good idea for my car?

I got thinking on this after the last thread on strut braces.

At last weekend's autocross I noticed that if I turn really quickly into a tight slalom you can feel the back starting to loosen up, even though I'm steady on the gas. This is the first tight course we've had in a while. I'd lowered my rear tire pressures to use a bit more tread width (was slightly overinflated before) and increased the preload on the progressive springs in the back (effectively making the rear springs a bit harder) before this, but rather than taking out the rear preload I'll add a rear top strut brace - I put one in the front, I was planning to add one to the back anyways. I'll just mock one up in cardboard and have the metal shop around the corner make it. At some point I'll probably remove it and get a roll cage that connects to the rear mounts which will also stiffen up the middle of the car.

But back to the recent thread, someone said a "lower brace" - the kind that connects the mounting points of the front lower control arms - made a big difference on their Sentra. My 'rolla has all the mounting points attached to a bone-shaped crossmember so I figure that the only way such a brace would make an improvement would be reducing twist slightly, there's no way the lower arm mount points could move apart or together. Worth it for this car or am I wasting my time? I don't want to add weight and decrease ground clearance for no reason. If the mod shifts the handling balance further to oversteer again I'll remove some of the rear preload to fix it, there's no more room for stiffness improvement on the rear once there's an upper brace.

Worth it more if you can triangulate or get a 4 point under there.

Like so: http://www.piercemotorsport.com/servlet/Detail?no=13

This guy said it made a difference on his and it's just a rod to tie the front mounts of the front LCAs together:

http://www.ae92gts.com/gze/autox_pics1.html

That's a good enough answer for me, gonna get a 4-point fabbed.

Before I started fabbing up stuff, I'd look at the basics:

What kind of shock absorbers are you running? And could that be the source of your instability?

What about tires--make and model?

Bushings? Are they healthy or totally gone?

Front bushings are new, rears are...not bad but far from new. You can't buy replacement bushings, only a whole new rear subframe from the stealership for $750. Might have to fabricate new rear bushings somehow at some point.

Shocks are fairly new, D2/Ksport, known for their "36-way adjustability" actually being more like "on/off" but they work.

Tires are Star Specs, 4 autocrosses and a little street driving on them.

Thing is that the only things I've changed since I last did a tight course are the increased rear preload and lower (as in not overinflated) rear pressures.

You seriously can't get rear bushings? It's an AE92, right? Let me dig, i can find them.

You'd have to ship 'em to me (or maybe some family up there)...none of the parts places had them last time I checked and the stealership only sells the whole subframe.

A lathe and a block of delrin should cure your problem.....

GameboyRMH wrote: You'd have to ship 'em to me (or maybe some family up there)...none of the parts places had them last time I checked and the stealership only sells the whole subframe.

Well i mean i'm pretty sure i could find you a source for them, but they won't be stock... Are you against poly for any reason?

Is the problem because it's an SR5 and not a GTS? Whiteline makes bushing kits for AE92s.

And Superpro. (You can order through 935performance if they're still around.)

SPF0823K SuperPro SPF0823K GMH, Toyota Tr/Arm Bushing $64.40
1 SPF1039K SuperPro SPF1039K Toyota Seca AE93 R/con arm out $64.40
1 SPF1040K SuperPro SPF1040K Toyota Seca AR93 Rear con/arm $60.20
1 SPF0996K SuperPro SPF0996K Toyota Corolla AE 95 Front Low $30.10
1 SPF1370AK SuperPro SPF1370AK Toyota Corolla AE 90,92,93,95 $44.10

The bolded one has this anecdote with it:

I just installed the rear bushings over the weekend and just a heads up. If you have drums, the inner bushing kit isn't correct. The bushing on my SR5 lateral link for the front links was smaller in diameter.

http://203.31.191.243/fulcrumCat/

There's a parts search catalog for SuperPro.

Thanks will give them a try. I'd actually prefer poly bushings. I'm swapping the subframe to disc brakes so I won't have the problem of finding the SR5-specific parts.

You lost me at increasing preload on the rear springs...

Unless static ride height is topped out against fully extended shocks or droop limiting straps (which would be insane), adding "preload" can only raise the rear of the car, can't it?

I guess with progressive springs, if you could make them run further into their travel, they actually would increase in rate, but I don't see how you can achieve that with preload...

I have a sneaking suspicion I'm doing that thing where I'm missing a specific piece of information that makes everything make sense. If so, I'm curious about what that is...

I think the thing you're forgetting is independently adjustable ride height. Without that you would just be extending the lower part of the coilover and raising the car (and reducing your downward shock travel).

Edit: Wait, now maybe I'm confused... gonna do some research.

Good discussion on preload here:

http://www.apexspeed.com/forums/showthread.php?t=32275

Increasing the preload and lowering the tire pressure would most likely increase the over steer.

As far as I can figure, increasing the preload reduces the amount os spring movement befor it goes into bind. Could cause some nasty surprizes. Also it does not change the spring rate unless it is progressive.

David S. Wallens wrote: Before I started fabbing up stuff, I'd look at the basics: What kind of shock absorbers are you running? And could that be the source of your instability? What about tires--make and model? Bushings? Are they healthy or totally gone?

Long before I started to look at replacing parts, I'd stiffen up that frame section. How do you know what you have, if the frame is flapping in the breeze?