Mx - Also not a joke

On the note of overlooked forces and moments: overturning moment (Mx). Previously we saw how Mz was important as the tire moments can be translated to an understeer moment on the car, which effectively "robs" the front axle of lateral force.

What is overturning moment? Moment about the SAE x-axis (forward/back) produced by the tire. This is NOT just the moment created by the Fy vector crossed with the loaded radius. Explanation later...

Generally though, it would appear that if you assume your track widths are fixed, or only dependent on lateral "scrub" of the suspension... your predicted lateral load transfer will be low. Mx is the culprit. Check it.



We start on the left, in a cornering scenario. There's the normal load transfer we're familiar with, but then there's also a non-zero overturning moment at each tire. Since everything's all connected anyway, we could also sum all the Mx acting on the chassis and dump it in with the rolling and non-rolling moments we're already familiar with, generated by inertial chassis forces. Since things are in equilibrium, that moment has to be reacted by a change in Fz at the four wheels. In this example, it increases load on the outside tires. What I'm not sure of, and I'm open to comments, is how the tire Mx and that increase in LLT is distributed among the axles... ie if its a function of roll center height relative to the CG or what.

The other way of thinking about this is as follows: Your track width isn't constant! Certainly not if defining track width to be the distance between the effective center of each tire footprint. As the tire deflects from camber and lateral force the footprint does move around relative to the chassis, and it can shorten up or shift the track at an axle and destabilize the car a bit.

Think about that for aggressive camber curves and compliant tires...