Originally posted by Z:
Under/oversteer can be thought of in terms of "static margin". Roughly speaking, this is the plan-view-longitudinal-distance between the car's CG and the (roughly lateral) Line-of-Action of the resultant force vector from all four wheels. If the CG is in front of wheel-force-LoA, then understeer. If CG behind LoA, then oversteer. If CG right on LoA, then neutral.
Not sure that's right. In a steady corner, the tyre resultant force is going to go through the CG isn't it?

My understanding of the static margin is that its the distance from the CG to the neutral steer point. The neutral steer point is the longitudinal location which will give a neutral response if you apply a lateral force there. I.e. the slip angle response will be the same front and rear.

I have just done a quick calc on a bicycle model with 50% mass dist and an understeer (measured as delta of front to rear slip angles) of 3deg/g which I think is probably closer to a road car but anyway... I've calculated the neutral steer point and found that the static margin is in the order of 400mm. Obviously, the more neutral the car is, the smaller the static margin is and then a higher percentage of the balance is coming from the tyre Mz. However, at this point you are really trying to pick very small changes in balance so it doesn't really make sense to be speaking in terms of percent.

So, my feeling then is that the Mz would be interesting if you are really trying to accurately represent your slip angles to within a few percent. But for sure I wouldn't go as far as saying its a first order effect. At least not in steady state.

I think it would be important when assessing the response of road cars because people are interested in small changes of balance which are giving tactile feedback to the driver. But in this case, you are talking very small values.