Roll centre movement

[pete m]

I have modelled the suspension of a single seater and found the roll centres at varying suspension bounce and droops. My results show the front as being lower than the rear. My understanding is this allows the front to handle more of the lateral load transfer therefore leaving the rear to handle traction (its rear wheel drive) - is that correct?

The roll center is low at full bounce and rises up linearly as the suspension extends (rises slower than the chassis). Looking at engineering text books all i can find is information about the absolute position at any one time and nothing about how it changes.

Could any one tell me what is good and whats bad in terms of dynamic roll centre movement?

thank you
Pete

[Goran Malmberg]

Originally Posted by pete m

1
My results show the front as being lower than the rear. My understanding is this allows the front to handle more of the lateral load transfer therefore leaving the rear to handle traction (its rear wheel drive) - is that correct?
2
The roll center is low at full bounce and rises up linearly as the suspension extends (rises slower than the chassis). Looking at engineering text books all i can find is information about the absolute position at any one time and nothing about how it changes.

Could any one tell me what is good and whats bad in terms of dynamic roll centre movement?

thank you
Pete

1
The geometric load transfer is direct to the tire grip while the rest is distributed over the spring system. So, with low rollcentre it is moostley the spring stiffness for the respective axle that governs the load transfer precentage.
2
It seem to me like you are looking at Rc during heave motion of the chassis while the load transfer in question take place during roll motion.
Regards
Goran

[pete m]

yes sorry the first question is for chassis roll and the second for front-rear weight transfer. Does the relative front / rear relative roll centres not affect grip levels then? the front/rear springs are the same with front 0.56 and rear 0.59 motion ratio.

Also is the roll centre movement alone enough to tell us what is happening with the camber of the tyres under roll? Im under the impression that it isnt however the lap simulation software im using only requires roll centres to simulate the car suspension.

thank you
Pete

[Goran Malmberg]

Originally Posted by pete m

1
yes sorry the first question is for chassis roll and the second for front-rear weight transfer. Does the relative front / rear relative roll centres not affect grip levels then? the front/rear springs are the same with front 0.56 and rear 0.59 motion ratio.
2
Also is the roll centre movement alone enough to tell us what is happening with the camber of the tyres under roll? Im under the impression that it isnt however the lap simulation software im using only requires roll centres to simulate the car suspension.

thank you
Pete

1
Well, it takes a little more than I will tell here to explain the "Rc - grip" relation ship. In short, we are having a % of sideway weight distribution each axle. That precentage is then split up i two parts, geometric and sprung weight transfer. The geometric part is very harsh and could be looked at as to some extent destroy grip, but at the same time produce good "initial grip" caracteristics. Other than that, it is the sum of the load transfer per axle comparted to the weight distribution of the car that governs over-under steer.
The Wr=Mr squared, therfore we got Wr=Sr*0,31 front and Wr=Sr*0,35 rear.
And the car might have swaybars in roll to add.
2
We must agree about the definition of Rc. Myself I use the term as for what it says, namely a centre about what the car actually roll. This is a centre that very much stays in the middle centre line of the car but will alter its height. Then we have the intersection of the force lines that may greatley move sideways during roll. To get back to your question then, the shorter the instant centre, Ic, the greater the camber effect. And the shorter the Ic the greater will also the scrub alter the total Mr during wheel travel.

I maybe mess things up to be hard to understand, but these are rather complex situations to give an easy explanation. There are so many side effects besides what you are asking fore.

I have been anounced this before, but I have a simple counter for Wr taking the whole A-arm geometry in to calculation. Give me a mail and I will return the counter for you.
hemipanter@hemipanter.se
Regards
Goran