well, u are half right about the side wall ratio. larger rims
accomodates tires with thinner sidewalls. thinner sidewalls means
there is lesser room for flex during hard cornering, giving better
resistance to bodyroll and traction loss due to tire deformation.
on top of which, the stiffness of the sidewall also depends on how
the tire was constructed. 2 tires of the same size from different
manufacturers/ models will have different sidewall rigidity.
the other advantage of bigger rims is a wider profile. this means
the car leave a bigger footprint behind. as shutterbug pointed out,
its all about traction. sportscars tend to lay down more power and
to effectively transfer it into motion, the tires muz have enuff
surface area to grip.
the trade off for such advantages is a harsher ride and increase in
fuel consumption.
