Found this on Rctech.net on roll centers courtesy of the member “ray_munday”:
|The important thing to know is what the roll centre actually means. When you are cornering, the side force from the tyre is directed to the chassis via the suspension links (the ‘geometry’). Basically, the roll centre is the point in space at which the tyre force ‘points to’. If the roll centre is high, when you turn, the side force from the outer wheel will point upwards and this actually pushes the chassis up, reducing roll angle. If the roll centre is below ground, the side force will push the chassis downwards, increasing roll angle. When the roll centre is at ground level, there is no up or down force applied to teh chassis from the geometry.
So in mid corner, a low roll centre acts like having a softer spring at that end of the car (ie more grip at that end) while a higher roll centre acts like a siffer spring at that end (ie less grip at that end). But thats only part of the story.
What makes roll centres so important is that they act immeduiately when you turn. When you first turn the wheel, the front tyres develop a slip angle and build up grip very quickly. This means that the roll centre force acts almost immediately. The chassis then starts to yaw, which creates a slip at the rear, so the rear tyre force builds up an instant later, meaning the front roll centre acts first then the rear roll centre. The chassis is starting to roll, so the dampers are starting to work, but dampers provide more force as they move faster, so they have a delayed action from the roll centres. Finally, the springs start to have an effect as their force is proportional to displacement, so it takes time for them to build up a significant force (you dont have instant displacement). As you exit the corner, these things happen somewhat in reverse. (this is obviously a simplification, but should give you the general idea).
So basically the roll centre allows you to tune the way that the vertical tyre force builds up through the turn. A high roll centre makes the force build up quickly, which can be good for low grip tracks where you need the tyre to bite; a low roll centre makes the force buildup more progressive, which is more useful for high grip tracks where you need the car to be a bit less responsive.
If we look at a high front roll centre, when we first turn the wheel, the outer front tyre will have a very fast build up of vertical force (it is trying to push the chassis up which in turn plants the tyre into the ground). This can make the front of the car feel very responsive, but once the car is in the middle of the corner, it will add some weight transfer at the front and take away a bit of front grip. A low front roll centre will tend to make the car less responsive to initial steering inputs (as the rate of force buildup is slower) but in mid corner provides more front grip.
Both the height of the ballstud and the length of the arm are used to adjust roll centre. If we raise the inner ballstud, we lower the roll centre (see diagrams below) and vice versa. The length of the upper arm affects how the roll centre changes as the suspension compresses / extends. (As the suspension moves, the roll centre moves up and down as well – it is an unavoidable fact of independent suspensions.) A longer upper arm keeps the roll centre lower as the suspension compresses, while a short upper arm keeps the roll centre higher as the suspension compresses. As Wild Cherry pointed out, the ballstud height is less sensitive than the length of the arm, but it varies a little from setup to setup. It also acts in a slightly different part of the corner.
To answer your question about the rear – adding a washer to the rear ballstud will lower the roll centre, which will give more rear traction. This is most noticable when applying power to the car as you are exiting the corner.
Sorry for the long post, but its not an easy subject to give a 5 line answer to!
actually, richards diagram is correct – it just shows a geometry that is not usually shown for full size cars (ie the upper links pointing upwards at the inner end of the car). Having this layout gives a very low roll centre as shown, but can also make the camber become positive in compression. This is a big no-no for full size cars, but our tyres are much less sensitive to camber and much more sensitive to roll centre. unning 2 or more washers on the front B4 ballstud gives you a similar geometry.