I've only ever seen oversized 250GP width slick tyres used on NSR race bikes. Sure enough, the bike i bought a while ago had 120/600R17 and 165/600R17 written on its Bridgestone slicks. In both cases the tyre edges of the 'tread' are almost vertical so you'd never be able to lean over far enough to use that portion of the tyre's surface. Those sizes were on standard rims. I've been using Michelin Power Ones for a while now, they stick really well but the rears seem to wear out REAL fast. I think that's down to a suspension problem though.
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After years of moaning about immigrants now i am one...