Calculated Roll Couple

[cyrus]

I have been drafting my 240Z's roll centers and still need some scales to measure my Center of Gravity. Here is what I have so far. At my present hight set by old saggy springs, my car's roll center moves 1.5 inches down for every 1.0 inches I lower the car. It is like chasing your tail. Maybe some experts can give a hand.

 

What height do I want my roll center to be?

 

How do I raise my roll center? (I assume that everyone's "bump Steer spacers" are acutually roll center raisers.)

 

http://www.geocities.com/cy_the_pie/index/240zrollcenter.doc

[johnc]

Your roll axis (the line drawn longitudinally between the front and rear roll centers) needs to be below the vehicle center of gravity and above ground with a downward slope towards the front of the car. Don't shoot for specific numbers because a lot depends on how a driver wants the car to feel.

[tube80z]

I have been drafting my 240Z's roll centers and still need some scales to measure my Center of Gravity. Here is what I have so far. At my present hight set by old saggy springs' date=' my car's roll center moves 1.5 inches down for every 1.0 inches I lower the car. It is like chasing your tail. Maybe some experts can give a hand.

 

What height do I want my roll center to be?[/quote']

 

Hate to rain on your parade but I don't think you can accurately measure your CG using scales. This really needs to be done by finding the tipping point and is not trivial by any means. If you're just looking for a rough estimate it's okay but that's about it.

 

The problem with attempting to look at geometric RCs and making some type of determination from them is that you need to take into account compliance of the tires, bushings, and suspension links. Ideally you also need to know torsional stiffness wheel to wheel as well.

 

A far better mehtod of looking at this has been described by Mark Ortiz. Basically you want to look at the slope of the line from the instant center to the tire contact patch, which he calls the N-line. The slope of this line and how it changes can help to determine how prone the car will be with certain behaviors.

 

In the case of a Z car, like John mentions you probably want to end up with RCs that are above ground in front and higher in the rear. Ideally these are close to the ground and you really want to minimze the laterall movement of the RC. This just happens to correspond to lateral movement of the tire contact patch and when this happens grip is lost. This also happens to be the point where the control arms are very close to level (why this keeps coming up).

 

The height of the RC controls the rate of force change from the contact patch into the chassis. This is a key tuning tool. The rear RC is generally higher than the front to quicken the rear of the cars generation of slip angle (or taking a set as some people call it). A car then generates equal slip angles front and rear is balanced.

 

I constantly adjust my coilovers to balance the car. You generally want to lower the end of the car that isn't sticking (typically two turns is a coarse adjustment for GC coilovers (not sure on others)). Once you find the appropriate point you will need less (less than one turn).

 

The process of modelling the car is a good one. It helps to know the limits of how far you can push things and it will help you learn a lot.

 

Hope this helps,

 

Cary