[Shazaam!]

Tire Sizes

by Kevin Cameron

Because tires become bigger every year, many riders assume that (a) this is progress and (b) bigger must be better in every way. This leads to the temptation to overwhelm motorcycles with the biggest tires the owner can find. Little 250 racers from the 1960s - initially so nimble and quick-responding on their original narrow-section 2.75 x 18F/3.00 x 18R tires - become ponderous when their vintage-racing owners re-equip them with the biggest tires that will physically fit into their fork legs or swingarms.

Look at a cross-section of a tire on a rim and you will see that, as a bigger tire is mounted on the same-sized rim, its sidewalls become more cantilevered, and therefore more flexible, which increases heat. Any tire is what it was designed to be, only when mounted on the intended rim width. If you must put bigger tires on your bike, and they call for wider rims, use them. Otherwise, either be happy with tires that are right for the bike, or trade up to a later model machine that already has the size tires you want.

In tires, everything is compromise, so you must consider the whole picture, not just a single variable like size. The bigger the tire, the greater its rolling resistance and the lower the top speed. But a larger section tire may increase cornering grip. Balance against that the fact that a narrow, sharp tire profile handles quicker than does a fatter, round-profile tire. On the other hand, braking can be more powerful and more stable on the round tire.

In the end, it comes down to crazy, unanswerable questions like is it hotter in the city than it is in the summer? Would you rather have in-corner grip with slow, heavy turn-in and two miles per hour less top speed? Or quick turn-in, less speed at the apex but a bit more on the straight?

History shows a never-ending cycle of tire and chassis develpment. As Dick O'Brien, for years head of Harley's race team, said back in 1969, “About the time we get the chassis handling really good, the damned tire people come up with more grip, the chassis starts to flex again, and we have to start over.''

Although great big tires may look bitchin' on that 20-year-old Kawasaki Z, go carefully. At the time, those chassis had to be braced to make them even marginally stable with the grip of race-sized tires. Just levering on bigger tires is not enough of an up-date. If you want a vintage Superbike, build the whole bike - rims, tires, braced chassis, updated suspension.

Sportbike Performance Handbook, 1998, p127