[First Post]Camber Change in Roll

[Yunlong_Xu]

Hey guys, my first post here.

It’s a very basic question about camber change in roll but I just can’t get around it. I’ve looked through RCVD and Fundamentals of Vehicle Dynamics but can’t seem to find a detailed explanation about it.

Suppose we have a car with upper and lower suspension pick-up points on the chassis parallel to each other (hence no anti-dive? Well I’m just trying to make the simplest scenario in front view), then we will see a simple four bar mechanism in the front view. When the sprung mass rolls about the roll axis, the motion of the link between the upper and lower pick-up points is set. But I don’t see how the rest three bars are restrained so there’s only one way they can relocate themselves and therefore I cannot get the new suspension geometry in that roll condition and hence can’t get find the camber change. It seems to me there must be some restraint that I’ve missed.

But can anyone tell me what is it that I’m missing? My guesses are:

1) The pull/push rod. The amount of length change combined with the installation ratio gives the relative position change of the upper/lower ball joint on the upright, and this adds on one more restraint for the geometry to be settled.

2) The instantaneous centre/axis of rotation for the two suspension control arms, this gives restrains on the possible route of travel of the upper and lower ball joints and hence also gives an extra restraint on the geometry.

Any help will be really appreciated!

Cheers

Yunlong

[Chris Allbee]

Constrain the bottom link representing the lower a-arm. Use an angle driven approach and adjust the link representing the chassis by the amount of your roll angle. Solve your equations of motion to get the resultant angle of the link representing the upright.

[Yunlong Xu]

Hi Chris, thanks for the reply, but I'm sorry taht I still don't quite get it...

How do you contrain the bottom link (lower a-arm)? Shouldn't it be moving?

[woodsy96]

It seems like you are trying to rotate the chassis and calculate the coresponding rotation of the upright.

If you are rotating the chassis, then the wheel is going to stay on the ground. Therefore a point on your upright is going to be constrained in the z-direction (up and down). Your chassis is going to be at a set height above the ground as well. This restrains the lower control arm, and the rotation of the chassis defines the motion.

Perhaps an easier way to model it is to fix the chassis and move the upright. This turns the four bar linkage into a SDOF system. The variable is the bump/rebound of the upright relative to the chassis, and this defines the motion of all three links.

We use Susprog3D. It does all the work for you, and they offer a discount for student use http://fsae.com/groupee_common/emoticons/icon_smile.gif.

[Yunlong Xu]

Thanks,woodsy96. I think I kind of got it now

http://www.autozine.org/technical_school/handling/double_wishbones3.jpg

Is it also correct to think this way as shown in the pictures? The wheel rotates about the intersection of the centre plane (centre line in 2D) with the ground. And in the roll model, this intersection points doesn't leave the ground.

Is that right?