Front End Suspension Tuning
By Richard Stanboli
(The information below was originally published as a two-part article in SportRider Magazine almost a decade ago. Several figures and illustrations are missing but will be updated soon, so please hold the phone!)
One area of mystery for most Sport Bike owners is the chassis. Unlike engine performance, suspension performance is difficult to quantify without the use of data acquisition. Even with the use of data acquisition, however, top teams struggle to find optimum chassis performance. What works for one rider may not work for another.
Manufacturers have to make many compromises when developing a motorcycle. They have to take into account the range of riders’ weights, physical sizes, and cost effectiveness of chassis components. Therefore, the motorcycle you buy from the show room may not possess the ideal chassis setup.
This article will focus on chassis geometry not only at ride height, or fully extended as most manufacturers specify, but dynamically as the motorcycle drives into and out of the corners. Geometric changes to the motorcycle at differing loads and the subsequent forces that act on the chassis determine the direction and speed the chassis will travel. By breaking down these geometric changes into their basic component parts, riders may then comprehend the forces acting on the bike. As a result riders will improve their ability to affect positive suspension changes.
Corners have three basic sections. These sections are the entrance, mid, and exit. We will take a look at each of these sections and analyze how they affect the motorcycle. Before we can do that, however, we must have an understanding of some of the basic geometric and physical components of the motorcycle. These basic components are Rake, Triple clamp offset, Trail, Center of gravity, Swing arm angle, Spring rate, Rising rate, Compression and Rebound dampening just to identify the most important.
Rake, triple clamp offset, front tire diameter and trail are interrelated. Trail is the most important part of the equation and it is defined by the other three components. Trail is the component that is responsible for providing both steering feedback and steering difficulty. But before we jump right into the dynamics of trail we should first look at the components that define it. Tire diameter is self-explanatory. The distance between the steering neck and the fork tube centerline defines triple clamp offset (Figure 1). Rake is defined by the steering neck angle (not fork angle) relative to vertical and is variable with changes in ride height (figure 2).
Trial is defined by the amount of distance between the front end’s point of rotation and the contact patch of the tire. The point of rotation or point of load as it is sometimes referred to, can be found by passing a line through the steering stem until it meets the ground (figure 3). As stated previously, rake is variable. The rake angle from vertical decreases when the front end compresses or is lowered. Conversely, the rake angle increases when the front end is raised or under the force of acceleration. Since Trail is dependent on rake it is also a variable dimension that changes proportionally with the variation of rake during suspension action. next page