Rolling about

An explanation of the limitations of a previous load transfer article, bringing jacking forces into the mix

CLAUDE ROUELLE

First things first. The suspended mass does not rotate around the kinematic roll axis. After reviewing the previous simplified explanation of how loadtransfer works, this month we'll explain why it first had to be presented this way, and give a more correct perspective.

In earlier articles we decomposed lateral load transfer into suspended and non-suspended situations. We also broke down the suspended mass load transfer due to lateral acceleration acting on the suspended mass cof g in a geometric g and elastic load transfer, the repartition of which depends on the geometric roll centre altitude vs the ground.

We did this by assuming the suspended mass rotates about the roll centre in 2D or roll axis in 3D. Figure 1 is a quick reminder.

What is wrong with this picture? The equilibrium of the moments is respected. No matter how we decompose it, the roll moment resulting for the centrifugal acceleration acting on the suspended and non-suspended mass cs of g is balanced by the variation of tyre vertical load.

But wait, there aren't any horizontal opposite lateral forces to the ones acting on the two cs of g.

In figure 2, we only look at the decomposition of the suspended mass centrifugal applied at the kinematic roll centre and the geometric load transfer (red) of the suspended mass. The nonsuspended mass load transfer (shown in green in figure 1) and the elastic part of the suspended mass load transfer (yellow in figure 1) are not represented here.

At least there is an equilibrium of the centrifugal force: F=M* V2/R and the reaction at the outside and inside tyres. Still two things are fundamentally wrong in this sketch though. Firstly, there is little chance the kinematic roll centre would stay in the same position once the car gets some tyre and suspension deflection and, secondly, the tyres' lateral forces cannot be equal as the outside tyre is more loaded than the inside one.

Figure 3 therefore represents a more realistic perspective, with a more pragmatic roll centre position and distribution of lateral forces between the tyres.

We can now observe that the outside and inside geometric load transfers are unequal. This is what imposes jacking force and subsequent ride height variation. Depending on the positive or negative difference between the outside and inside geometric load transfers, the car suspended mass could be dynamically lifted or pushed down.