Tire deflection impacts on determining roll center?

[ksw100]

When determining a car's roll center in suspension design, it is common practice to draw a line to the center of the tire contact patch from the Instant Center. However, since the contact patch shifts during cornering due to camber changes, lateral load transfer, and tire deflection, is using the geometric center of the contact patch a valid assumption, or should we account for its dynamic movement to improve accuracy? Or is the change so miniscule, that it doesn't matter?

Say the car is cornering at 1G for a track race car like a Miata.

Generic image determining roll center.

 

[GregLocock]

Yes you could do that. It should make your hand calculations more accurate. Tire deflection will directly effect your roll gain as well. That is total roll gain=suspension roll gain+direct contribution from tires. Both effects will increase roll gain in the example given.

 

[ksw100]

Is "roll gain" the change in roll center away from the center of mass as the suspension compress in a turn?

(I tried googling "roll gain suspension" and found some mentions of the term but not a definition.)

 

[GregLocock]

roll gain is how much the car body rolls in degrees per g of latacc. It is not directly important in a circuit car unless you have aero, but it also affects how much roll steer you get which changes your understeer.

In a road car a car that rolls a lot may feel uncomfortable when cornering hard, but a car that doesn't roll will tend to have a harsh ride, and in particular cases poor traction on rough roads. Typical values would lie between 3 (sports car) and 8 deg/g (light truck or SUV).