Roll Centers

[260DET]

From this topic http://forums.hybridz.org/showthread.php?t=118964&highlight=Car+Pitch+Quiz and particularly from post #8 by tube80z a cut and paste -

 

Droop limiting is a means of swiftly changing the actual force-based roll center towards the inside tire at a pre-determined amount of lateral loading. The roll center shift occurs when the inside shock tops out in rebound, sometime before full lateral g loading occurs.

The reason for the use of any of these is to combat a push that cannot otherwise be cured with the springs that are being used. The need for it usually goes hand-in-hand with either the car being sprung too softly, and/or above ground geometric roll centers.

 

Above-ground geometric roll centers produce a high degree of Jacking Effect - simply put, a self-stiffening of the suspension from the lateral loads that are fed thru the a-arms. This self-stiffening can result is an overloading of the outside front tire, and therefore a bad push.

 

So this indicates that the traditional 'up and out' relocation of the front LCA pickup point to keep the front geometric roll center above ground is not always best for a competition S30? Or any other strut suspended car for that matter?

 

It would be interesting to know who uses what setup on their Z, the suspension details and results.

[boodlefoof]

Having the roll center above ground isn't necessarily bad and in fact it can be good to some extent.

 

Raising the roll center will help the car to resist roll. However, when you raise the RCH too much, you will experience jacking. From what I have read and from conversations with people much more knowledgeable on the topic than I, this typically doesn't occur (at least in to any noticable extent in a street car) until RCH is on the order of 10'' above ground or more.

 

When you lower RCH, you will need to increase sway bar rate to maintain roll resistance.

 

In my setup (which probably will not hit the street for 2-3 years yet), I'm running an SLA front suspension with an RCH of about 2.5 inches.

[johnc]

The reason for the use of any of these is to combat a push that cannot otherwise be cured with the springs that are being used. The need for it usually goes hand-in-hand with either the car being sprung too softly, and/or above ground geometric roll center

 

Too much droop causes the car to wallow around because the rate of weight transfer is slowed. It makes the car sluggish in response. Too little droop causes sudden tire unloading. Excluding high downforce applications - droop limiting in some cases is a band-aid for handling issues that cannot be corrected with more traditional methods. Sudden limits on suspension movement is generally a bad thing.

[JMortensen]

The other thing about RC heights, at least on a Z, is that you really can't get them very high above ground in the front anyway. With my pivots moved up 7/16" from stock and my sectioned struts and camber plates installed I set my car on the ground with no engine in the car, and the control arms were just barely pointing up (spring perches all the way up). By the time the engine gets installed I think the RC would be underground, but now I've modified things so that I can move the control arms up about an inch from the previous spot which may again give me a slightly above ground roll center if I'm lucky.

 

If you read the race car engineering books they all say that jacking is to be avoided, but if you look at a stock or mildly modded Z, the rear roll center is quite a bit higher than the front, but it's the front that doesn't grip.

 

I'm not sure how this relates to your ZX Richard, but I'd venture a guess to say that you too won't be able to raise the RC much above ground even if you max out your relocation possibilities with the stock crossmember.

[tube80z]

Raising the roll center will help the car to resist roll. However, when you raise the RCH too much, you will experience jacking. From what I have read and from conversations with people much more knowledgeable on the topic than I, this typically doesn't occur (at least in to any noticable extent in a street car) until RCH is on the order of 10'' above ground or more.

 

These people should have told you it depends on how much lateral force you generate. Above ground roll roll centers will generate jacking and this raises the car. If you have data logging and suspension sensors this can be seen. There's a relationship to the slope of the IC lines that's supposed to mathematically describe how this works.

 

And I agree it's not necessarily a bad thing. Above ground roll centers will generally heat the tires more. So of you're needing to get more heat into the tires it can be a good trick to use.

 

But what I think is far more important and what I really learned from Claude Roulle is that above and below ground RCs have different timings for lateral weight transfer and this is really the key thing we play with. To make the care respond quicker you can raise the RC. To make it respond slower you can lower the RC. In real terms this lets you use small ride height changes to balance the car and is a key tuning tool in my arsenal (albeit a very small one -- more like a gun closet).

 

In the case of a stock Z front end there's a limited range of what you can use to stay in the suspension's sweet spot. So this discussion can get academic real quick.

 

And back to Richard's point I use a slightly underground front roll center and stiffer springs than most people run. I had tried softer springs and a higher RC as well as stiffer springs and a smaller roll bar. For autox I found that the car was easier to drive when with an underground front RC. That may be related to the type of use but I can't say for sure.

 

My use of droop limiters was to constrain the stock geometry to a certain extent. And in my case the front droop limiter raises the RC a little after initial turn-in. Anyone who doubts any of this really works should try it. You can feel quite a lot of difference.

 

Cary

[260DET]

With the S30 I guess so called bump steer spacers could be used to adjust the front roll center. They may need to be dowelled though, other wise each time they were changed toe could be messed with as well.

 

The thing that prompted the topic is it appears that the S14 suspension being used on the project 280ZX is factory set up with low roll centers. With not much teaking a S13/14/15 will handle very well on the track so its worth looking at, they have struts at the front. Apparently items are available in Japan to change roll centers on them, or more likely preserve the existing centers when they are lowered.