Motorcycle handling and chassis design: the art and science

By Tony Foale

Foreword The motorcycle is a complex system that has long defied full analysis. For a very long time, motorcycle handing was hardly even considered a subject. Engines, whose performance could be measured in "objective" terms, therefore received the lion's share of development. Engine development moved rapidly ahead of chassis, suspension, and tires, creating a succession of design crises that required new thought for their solution. Examples might be Rex McCandless's twin-loop swingarm chassis of 1950, Tony Mills's wide, belted Dunlop Daytona tire of 1974, and the present-day elaborations of Antonio Cobas's large-section aluminium twin-beam chassis of the early 1980s. In each case, motorcycle performance had ceased to advance because of specific problems that could not be solved by traditional means. In general, the innovations that have broken these deadlocks have been creations of practical persons, not of theorists. The role of theory in motorcycle design has, if anything, suffered at the hands of history, for the strange forkless creations of ELF, Fior, and Bimota have come and gone without solving any actual problem. Yet motorcycle performance is at present again deadlocked, with no

sunny uplands of easy progress in sight. As motorcycles lean over farther on their wonderful tires, their suspensions turn sideways, at a large angle to the bumps they are designed to absorb. As engine and brake torque is applied, motorcycles short enough to turn quickly, and tall enough for adequate cornering clearance suddenly lift the front or rear wheel, limiting maximum rates of acceleration and deceleration. While autos present 100% of the width of their tires to the pavement, the motorcycle offers only 1/3 of tread width at a time, severely limiting cornering grip. To make motorcycles steer well, front tires must be of modest section, while rears, to apply engine power, must be large. With the forward CG position necessary for rapid acceleration, a powerful motorcycle must therefore overload its small front tire in cornering, while under-using its larger rear. The result is that as a machine's power increases, its corner speed must decrease.