autox/track 280z roll centers

[mustard-z]

We've been preparing to transplant a '85 C4 vette rear suspension into our autox/track 280z. We've leveled the car and measured all suspension pivots for the front and rear suspension links to determine what roll centers the car currently has based on the book "How to make your car handle" by Fred Puhn.

 

Currently, the car has an AZC type chromoly tubular/heim joint suspension along with ground control camber plates and coilovers with 300# springs and an 1-1/8 suspension techniques swaybar. The rear has the same camber plates and coilovers but with 325# springs and no swaybar installed, and ajustable eccentric camber bushings on the inner control arms. We run 23x10x15 hoosier slicks on 15" steel diamond racing wheels. Additionally, we've modded the steering rack to the subaru power steering rack and located it as far down as possible, but haven't yet relocated the inner control arm pivot up to help minimize bump steer.

 

We've got a few questions:

1: What is a realistic and optimal roll center for a car of our use. Currently, our font roll center is about at ground level or maybe 1/8" below ground. What effect does having a roll center below ground have on handling?

 

2: What is an optimal roll center for the rear suspension? Our roll center is approx. 1.75" above ground.

 

3: Is there any provisions in the stock 280z rear suspension for anti-squat? If so, how is this calculated?

 

4: The C4 rearend can be setup to have anti-squat and from what we've read 25% is a theoretical maximum. What would be a good number to shoot for when installing this rearend? I know that Doug Rippie Motorsports offers some brackets to reduce the anti-squat in a stock vette, but we are unsure how much anti-squat is present in the factory suspension to begin with.

 

 

Any help is appreciated.

[JMortensen]

1. Your front roll center will generally be about 0, maybe slightly underground. Underground increases body roll, makes the car slower to react. Above ground decreases body roll, makes it faster to react. Close to level is good and keeps the jacking to a minimum.

 

2. Rear roll center should be higher than the front. Yours is. It is very difficult to adjust on the Z. blueovalz adjusted his. He was running 17" wheels and had a below ground rear roll center, so he added an additional roughly 3" of metal between the spindle pin boss and the bottom of the strut. He sold his Z but he's still around from time to time and a super nice guy. You could PM him and ask what the difference was, but I think he was done racing by the time he made that change.

 

3 and 4. No antisquat on the Z suspension. Anti-squat is a lot like roll center. The more you have the more the suspension tends to lock up. I would use springs to counter the squat, not anti. In general the same applies to the roll centers.

Edited November 30, 2010 by JMortensen

[tube80z]

We've got a few questions:

1: What is a realistic and optimal roll center for a car of our use. Currently, our font roll center is about at ground level or maybe 1/8" below ground. What effect does having a roll center below ground have on handling?

 

Either above or below ground can be made to work. The are some subtle difference and some major ones. Geometry transfers forces quicker than springs/shocks/bars. This can be felt by the driver and a key tool in changing balance. Generally above ground RC will generate higher tire temps and raise the front of the car as it rolls. My advice would be to make this adjustable if you can because if you change tires or do more track days. If you can't I'd shoot for running the lower control arm level. If I had to be pinned down on a range I'd pick someting like -2 to +3.

 

2: What is an optimal roll center for the rear suspension? Our roll center is approx. 1.75" above ground.

 

The rear RC definitely needs to be adjustable in my book. Lower to put power down and higher to get the car to turn. You have to manage the loading of the rear tires so that they don't get overheated but still let the car turn and put power down. While this isn't adjustable in a stock configuration I would make sure you take this into account on the C4 rear end. It will also let you experimentally determine what works best with the front suspension, in case you don't have time to make it adjustable.

 

4: The C4 rearend can be setup to have anti-squat and from what we've read 25% is a theoretical maximum. What would be a good number to shoot for when installing this rearend? I know that Doug Rippie Motorsports offers some brackets to reduce the anti-squat in a stock vette, but we are unsure how much anti-squat is present in the factory suspension to begin with.

 

There's a good thread at corner carvers that shows a bunch of C4 geometry. It's listed under putting this into the rear of a Mustang. Dave and Sonny run this rear in different cars in Medford. From what I was able to gleam the C4 uses a ton of rear antisquat to keep the suspension from bottoming, which is a consequence of spring rates picked for a flat ride. Removing most of this makes them work better. On Dave's locost the difference between mounting stock and making the links adjustable made a large difference in how well the car put down power. Install with the halfshaft near level and build adjustable inner brackets (or buy the ones from vette brakes). This will let you change the rear RC. I'd also make sure you have some leeway in being able to raise or lower the batwing and the front mounts. We learned this the hard way. I'd run less than 20% antisquat in the back. Also get rid of the stock spring and go to coil overs. It's a lot easier to setup and work on.

 

Cary

[mustard-z]

Thanks for the info. Generally our car turns in just fine and is very responsive. So our thinking is that we would like to maintain the current handling characteristics but generally improve things where we can. We have the most problems putting power down so making the rearend adjustable would be a good plan. It sounds like moving the inner pivot of the front suspension up a bit will help both raise the roll center and help minimize the bump steer. On the rear of the car, I think the C4 rearend can be mounted to allow for roll center adjustment and definately the dog bones can be mounted to allow adjustment in anti-squat.

 

Cary, do you have contact information or can you put me in contact with Dave or Sonny so that I can talk more with them? there is some reservation that the DANA36 that we have won't be strong enough for the power we are making, something like 300-350tq. Although, our car is lighter than a vette so maybe we will be OK.

[tube80z]

there is some reservation that the DANA36 that we have won't be strong enough for the power we are making, something like 300-350tq. Although, our car is lighter than a vette so maybe we will be OK.

 

Dave's running 436 WHP and 430 WTQ on the 36 with no problems. Neither he nor I were very kind to it this summer. The real downside is ratio choices compared to the 44. On the plus side the price is a lot less. Send me a PM and I can get in contact with Dave/Sonny.

 

Cary