Cornering behaviour

[Erunanethiel]

If the front wheel has a higher cornering stiffness than the rear (so the vehicle tends to oversteer?) but has a lower coefficient of friction, under steady state limit turning codition, will the front or the rear break away first?

[Greg Locock]

Cornering stiffness is linear range. It doesn't have a whole lot to do with limit handling.

[Tim.Wright]

It will have linear range OS and limit US so the front will break away first at the limit. There is a brief discussion about this in Milliken. I've never seen a car like this before though - linear range OS has nothing but disadvantages and I've yet to see a tyre with such a big mismatch between cornering stiffness and grip.

You can also consider an instantaneous cornering stiffness as the derivative of the force vs sideslip curve from which you can calculate instantaneous understeer (among other things) thoughout the complete cornering range.

[Greg Locock]

Perhaps you could do it with different tire pressures front to back, or even totally different constructions.

[Tim.Wright]

It would have to be quite an extreme combination.

Perhaps some bargain basement tyres on the front that have been left out to harden in the sun might have a high cornering stiffness with a high enough tyre pressure but very low limit grip. Then a Rallycross tyre on the rear axle which has a very low cornering stiffness but high grip.

However you do it, large changes in balance between the linear and limit ranges will always result in a four-letter subjective rating.

[Erunanethiel]

I am actually trying to replicate motorcycle tire behaviour with camber forces substituting for some part of the lateral forces generated by slip angle to sort of understand front and rear tire's behaviors during limit handling.

[Greg Locock]

Tim - I was looking at some flattrac results as I wrote that. I've got a (nice production) tire tested at 30,35 and 40 psi. I'm trying to see why the modelling program gets the performance wrong. However on the real results psi makes no odds to max mu at a given Fz (to within 1%), but as you would expect the cornering stiffness at a given Fz varies substantially. So you could easily set up a notional car to be any combination of linear range understeer/oversteer, and limit understeer/oversteer, by messing with cgx and tire pressures.

[Tim.Wright]

Strange the pressure has no effect on grip on that tyre...

For production cars with a significant K&C component in the front and rear axle sideslips, the tyres are only responsible for about half the slip angle you see in the linear range. In other words doubling the cornering stiffness of the tyre only increases the complete axle cornering by about 25%. So I doubt you could realistically flip from US to OS just playing with pressures.

[Erunanethiel]

Tim - I was looking at some flattrac results as I wrote that. I've got a (nice production) tire tested at 30,35 and 40 psi. I'm trying to see why the modelling program gets the performance wrong. However on the real results psi makes no odds to max mu at a given Fz (to within 1%), but as you would expect the cornering stiffness at a given Fz varies substantially. So you could easily set up a notional car to be any combination of linear range understeer/oversteer, and limit understeer/oversteer, by messing with cgx and tire pressures.

Strange the pressure has no effect on grip on that tyre...

For production cars with a significant K&C component in the front and rear axle sideslips, the tyres are only responsible for about half the slip angle you see in the linear range. In other words doubling the cornering stiffness of the tyre only increases the complete axle cornering by about 25%. So I doubt you could realistically flip from US to OS just playing with pressures.

So if the front tire produces most of it's cornering force due to camber thrust with less of a need for slip angles than the rear wheel, will the rear slide out first? If they have the same coefficient of friction or the front has a larger coefficient.

[Tim.Wright]

If camber angle is the only difference then yes. But that's rarely the case. Even if you have thesame tyre on the front and rear you will still have a differnt coefficient of friction due to tyre load sensitivity, load transfer effects, camber.