StoogeMoe

You know, I don't agree with what this implying. It's saying that sway bars have no effect on weight transfer at all. But they do, because the keep the center of gravity lower. Imagine a car without sway bars and real soft springs. It will roll quite a bit almost to the point of rolling over. Think of a Suzuki Samurai. All the weight transfers to the outside tires, because the center of gravity changes.

Van

"Center of gravity", by its definition, is the point which does NOT change. Assuming your car is symmetric (which they really aren't... the battery is on one side, the driver on the other, etc.) the center of gravity is at some point on the centerline of the car. (But really it's offset to one side a little bit.)

When the car experiences body roll, the outside of the car gets lower and closer to the pavement, and the inside gets farther or higher from the pavement. However, the center of gravity stays the same distance away from the pavement.

A Suzuki Samurai has a higher center of gravity than a 944 because of its higher ride height and body size/construction. But for no other reason. If you reduced its ride height and put some lead in the floor, it could have the same CG as a 944. But, would it have the same roll center? Probably not... would it have the same spring/wheel rates? probably not... would it have the same swaybar rates? probably not...

konakat

I think he might be right, though I suspect it isn't significant enough a difference to matter. As the car rotates around its roll center, the center of gravity moves in relation to the wheels. The closer the two centers are the less difference it would make, and I suspect that on race cars the two are close enough and the roll angle would be small enough that it is not relevant.

StoogeMoe

The center of gravity is above the roll center, which is why the car rolls in a corner. If the car rotates on the roll center, then the center of gravity moves, contributing the weight transfer.

Van

I see what you guys are saying, but the CG will move far less relative to the road surface than the roll center will. Roll centers move all over the place as suspension arms move up and down.

My kids get a great kick out of the notion that since they, too, have mass, when they jump up, the earth moves away from them a very little bit - then it moves back to meet them as they come back down to land on it. (For every action there's an equal and opposite reaction.)

But CG is like the earth's movement and the roll center is like the kids' movement.

L Cubed

The "center of gravity" is more accurately noted as the center of the vehicle mass. The majority of this is sprung mass (everything supported by the springs/torsion bars). This center does not move relative to the chassis so if the chassis moves than the center of mass moves.

The roll center is a misnomer, it does not mean that the vehicle rotates about this axis the point that it rolls about depends on the roll stiffness, suspension geometry, tire spring rates and etc.

Now for the center of mass to move far enough to the outside (loaded side) to make a significant change in grip levels is a lot. For example on some bias ply Hooiser Racing slicks (Formula Ford tires) the data I have worked with showed no significant (defined here as loss of 0.1 lateral G) loss in grip until about 15% track width shift of center of mass. So for a 944 that would mean that the chassis would have to shift over 8" towards the outside, which doesn't happen. So its as Van said, the center of mass doesn't move enough to make an effect especially when considering all the other parameters that vary grip.

Hopefully this helped clear up the "center of gravity" movement issue without a whole bunch of boring details.

Onto the original issue, would raising the rear ride height help a little bit? My thought is since he's heading uphill he could be over cambering the outside tire reducing thrust capacity from the tire? I haven't raced a 944 so this is just a guess.

Dougs951S

Smart kids! good they understand concepts like that at a young age.