JohnnieMo

I have been getting a horrible bang out of the rear suspension for what seems like ever. At first I thought it was something out of place, but never found anything. Then I assumed it was wheel hop (due to its rapid succession). So I added the following:

Koni SA 4th gen shocks
Torque Arm
LCAs
Panhard
*also removed the rear isolator

Tried again and sure enough - the bump is still there. So I dug around and noticed that the bumpstops were the most likely problem. So I started stiffening the shocks. I got to full stiff and the bang was still there.

So now I don't know what to do. I guess that 1/2" lowering negated any other improvements I made. Is it possible to use a thinner bump stop? Is that dangerous? I am hesitant to change springs because everyone says weight transfer is a good thing!! So I just don't know what to do to fix this problem.

JimMueller

iTrader: (9)

The SA's only allow you to adjust rebound (rate of expansion), not bump (rate of compression) forces. If you have the rebound maxed out, I would think the orifices would flow much less fluid, causing it to return to the normal length more slowly, thus staying at the shorter length for a larger portion of the time once compressed. How were they at the lowest rebound setting (if you tried)?

I don't see it in your sig. I presume that since you are lowerd: 1) you have rear LCA relocation brackets installed, 2) the rear of the LCA is parallel or lower than the front of the LCA when the axle is loaded and 3) the LCA's were torqued while the axle was in the loaded position (not with it hanging)?

I've heard of some folks cutting down their bump stop, but I consider that to be a bandaid.

JohnnieMo

I was aware of the restriction of the SA, and I presumed it would not help any. But I tried it anyways. Stiff or full soft didn't make a difference (for the bang or for the 60 foot)

I only lowered 1/2" (by removing the isolator), but my car has always sat a bit lower than most. I did not get any relocation brackets, but after looking under the car with them installed, they would not be a bad idea. (they are on my list). It is a ways off parallel to be honest. I was surprised.

I don't see how relocation brackets will fix my problem. It sounds like I may need a stiffer rear spring, or a smaller bump stop. But cutting them down seems drastic as there are a lot of more expensive parts that might start banging if they are reduced.

I still have to test and see if it is the bump stop, but I'm fairly confident it is.

BTW - The suspension was loaded during install

DietCoke

iTrader: (18)

Relo brackets will make your car sit 1-1.5" higher in the back if you use the lowest hole, and will push the chassis UP instead of letting it go down when you launch the car.

JohnnieMo

Wow - I never even thought about that. Makes all kinds of sense. Can you maintain the current ride height and still get the chassis up advantage???

DietCoke

iTrader: (18)

I suppose you could if you had the means to lower the rear more, but it would be somewhat self-defeating.

JohnnieMo

That doesn't make any sense. If you install them and set them to be parallel, it should negate the effect if pushing up on the suspension without having to change ride height whatsoever...

DietCoke

iTrader: (18)

No, because when you launch your rear is going to sink. When it sinks is then they need to be at least parallel. If they're parallel at ride height, they wont be when weight transfers to the rear. Make sense?

JohnnieMo

YA - that makes sense, but I don't see how this would affect ride height. So long as the arms are installed while the suspension is "loaded", there can't be a change in ride height. But if the brackets are designed to be installed on an unloaded suspension, then that changes everything. It would also significantly reduce the effectiveness of the rear suspension.*

*now I understand why poly bushings are bad....

JasonWW

iTrader: (2)

I'm trying to figure out what is being said here. I don't think LCA relocation brackets are going to have any effect on the rear ride height. Why would it? The springs are what sets the ride height.

For dedicated drag racing, I thought most folks set the LCA angle so that the front is pointing slightly upward. When forward force is applied, the axle will get pushed down and the cars rear will go up. Since the weight transfers to the rear it kind of balances out the rear lift and you plant the tires hard.

This sounds all good and everything, but there is a down side. Under braking, the rear axle will want to lift. This causes the rear tires to lock up more easily, which can be very bad. So most folks set the LCA angle flat. It is the best compromise for most people.

Using poly bushings in the LCA is a whole story in itself. Basically the factory suspension design (3 link) requires the LCA's to twist under cornering loads. The factory acomplished this by making the arms out of U shaped sheet metal and also by using big rubber bushings in each end. When you replace the arms with a stiff tubular unit with stiff poly bushings, it is going to be very difficult to impossible to twist them as well as smoothly move up and down. What happens is that the bushings try to twist and end up binding, the arms then become very difficult to move up and down creating the same effect as if you were running much stiffer springs. This effect of having really stiff rear springs is what reduces the rear grip in corners.

Most of your more serious cornering guys will stick at least one rod end on the LCA because it will allow the arm to rotate freely, thereby avoiding all binding and hence, better cornering grip.

I hope that makes sense.

JohnnieMo

Well the LCA brackets are installed and set to the lowest hole. The arms angle up slightly towards the front. I tried a couple burnouts and launches and the car sits nice and flat now. No more banging. Who'd have guessed that a $60 part would have fixed this entire problem.

Now I have to see what effect there is on the road course.