I did my thesis on motorcycle dynamics and frame design so I'm not talking from my arse here. Also I'm talking about a racing/rideday context, there's other considerations for road going bikes as previously argued. A narrower rear tyre allows a faster roll response, but this is not the only performance consideration related the the rear tyre. A wider rear tyre reduces the contact pressure thereby allowing a higher coefficient of friction (ie slightly more grip), but also allows the power lost through the tyre a larger volume/area to absorb/dissipate heat. Think about the amount of time you spend on the throttle compared to the amount of time spent on the brakes. The rear tyre has to cope with that much more energy being transferred through it. Once the rear tyre is overheated it loses both cornering and acceleration performance, so by making the rear tyre larger you can prevent it overheating as quickly, thereby making back the time lost through the slightly slower steering. Also look at the different tyres used in different classes of racing. In 125s the front and rear tyres are narrow and there's not that much difference between front and rear. These bikes don't have the power to rely on point-and-squirt riding, so corner speed is more important. They rely more on getting into and through a corner quickly and cleanly than on out-powering each other in inter-corner drag races. Cornering creates a more even load distribution than accelerating or braking, so the tyres are optimised more for this condition. In MotoGP the tyres have to cope with more mass and more power, so the tyres have to be physically larger than the 125s to cope with the extra energy transfer. The MotoGP bikes have more power than 125s (oh really?) so the rear tyres have to cope with the increased mass AND the increased power, and they can make more time on each other through straight line racing than 125s can. So the rear tyre grows relative to the front. They can make good lap times by using their excess power as much as possible, even with their fat rear tyres they still over heat them but I guess at this stage it's not worth the compromises to go with an even fatter rear. It depends on how you the thing is intended to be ridden. For more point-and-squirt riding style a wider rear tyre give better tyre life. For higher corner speeds a narrower tyre allows faster turn in which allows more accurate steering and more efficient cornering. As John Littler said there's a heap of other factors to be considered, bikes are complicated systems and there's no point fixating on one particular aspect and neglecting the rest. If the suspension isn't set up properly, changing you tyres probably isn't going to help you. Mark PS: Yes Dave, wider tyres and a lower CofG do require a higher lean angle to balance cornering forces but the differences aren't huge, read Tony Foale's "Motorcycle Handling and Chassis Design" for more.
