Quote:
Originally Posted by Bill Verburg
Gonna have to disagree, the roll center can never be raised to the height of the cg because of the geometry of the components, the best you can do is keep it as high as you can while lowering the cg as much as you can. This will minimize the length of the roll couple. I haven't seen 911 mapped out but on 964/993 the rollcenter goes under the pavement unless the RS suspension pieces are used.
|
True in the case of the 911. I was speaking in general terms. I've got some suspension analysis software if anyone has the measurements. I'd be happy to make a few models. I could picture a car which has had the spindles raised as far as is possible, and then having the T-bars indexed so that A-arms are angled up on the inside having a very small roll-couple. In addition to the "wooden" feel, the there are two other problems with this:
1) Roll centers are not static. As was mentioned earlier, they can move around (in 2 dimensions) quite a bit depending on where the car is in its roll and bump travel. Having a roll couple that suddenly doubles or triples in size can cause an awfully weird handling car, especially if the roll-couple were to suddenly reverse so that the Cg was below the roll center!
2) Suspensions with high roll centers are prone to jacking. The classic example is the rear suspension of a early Triumph Spitfire
Basically, because the roll center is relatively high in relation to the contact patch, there is a tendency for the chassis to be jacked up and over the contact patch. Not a big deal on 50's sports cars with non-sticky 50 series tires, but this can be a very big deal with the high loads induced by today's super-sticky rubber. If the roll center goes over the Cg, the chassis will actually "jack down" and start to roll to the inside of the turn.
So the secret is that...
1) you want your roll couples to be consistent front-to-rear. In situations where the front roll-couple gets longer, you want the rear to get proportionally longer too. It's possible to accommodate changes in the roll couple relationship by using a stiffer roll-bar or springs, but these will then increase the risks of "unintended consequences". This is where the art of suspension design and set-up comes into play.
2) You want your roll centers to be high enough so as to not have terribly large roll couples, thus containing the roll to a reasonable amount (in conjunction with the rest of the suspension pieces), but not so high that it induces severe jacking forces.
3) You don't want your roll centers so high that the roll couples ever reverse (where the roll center is above the Cg).