EXPERT’S GUIDE TO... STEERING GEOMETRY
If you think rake belongs in the garden shed and offset is a lumpy custard, this guide will set you straight
When discussing steering geometry people often only think in terms of rake and trail, but these are related by other parameters as well. Specifically, tyre rolling radius and the offset of the wheel axle from the steering axis. Wheelbase should also be regarded as part of the equation, too.
Tyre radius and trail are the two most important factors which affect how we perceive the steering feel. Trail is determined by the rake, offset and tyre size.
I am often asked if there are optimum values for these parameters. The short answer is no; values have varied over time and each type of bike. Sports and racing bikes need quick steering bordering on instability, tourers are concerned with stability.
Increasing power levels and tyre technology have been the main factors which have driven steering geometry values. It used to be said that 27 degrees was a magic value for rake angle, but nowadays sports and racing bikes use angles closer to 22/23 degrees, and often custom bikes might be as high as 45 degrees with tourers closer to 30. Trail values generally vary either side of 100mm. In general, less rake and trail will lead to a quicker handling bike, and higher values give more stability. However, steering geometry is not fixed, it is constantly changing as we ride.
A few mm can make a big change
THE EXPERT Tony Foale is a successful racer, chassis builder and author of the seminal ‘Motorcycle Handling and Chassis Design’ book. The steering geometry of many of today’s bikes follow the theories he set out 30 years ago