I've read that the reason for the differing tires sizes on bikes is to make they inherently oversteering for safety reasons - in steady state cornering you want the rear to slip first which is more controllable by the rider; understeering in a single-track vehicle where the front tire starts slipping first is inherently dangerous.
How this objective relates to why the rear is wider is a puzzle to me. I thought the Porsches were designed with wider tires in the rear to make them understeer rather than oversteer.
Porsches have wider rear tyres for 2 reasons:
- to get the power down in a straight line i.e. help with wheelspin
- to help prevent oversteer in the case of throttle lift mid-corner.
This case is not such a problem in the porsche 911 since the 993 variation, but the older cars had a problem with balance being off due to the engine being stuck out the back, and the suspension not being up to the job.
A quick description of the issue is:
imagine driving into a corner on the brakes, and then getting on the throttle just before the apex
( the car will go from loading the front on the brakes, to loading the rear on the throttle), and then imagine you overdid the corner and have to lift off throttle (the car will be turning and the front will load up due to being on a trailing throttle, and the rear light).
In extreme cases the car's engine's moment of inertia will cause the back of the car to come around and it will oversteer violently.
911s were famous for it.
Bikes luckily don't have the engine-out-the-back problem