Over the last few weeks, several people have asked about the tire's contact patch, and how it plays a role in trail - especially given that the tire is constantly deforming and the contact patch is very much a dynamic feature itself. As most riders know, tires play a huge part in how a motorcycle works; steering, comfort, stability, cornering…almost every aspect of performance is highly dependent on the tires used.
My last three blogs have shown how trail changes as the motorcycle moves on its suspension and leans into a corner. We know that trail is the distance from the steering axis to the contact patch, measured either perpendicular to the steering axis (normal trail) or parallel to the ground (mechanical trail). But the big assumption is that the contact patch can be represented by an easily determined single point; the equations I used in my last blog are based on the tire having a perfectly circular profile with no distortion from weight or other outside forces. In reality, the contact patch is a large area affected by an almost infinite number of variables, and from that it's difficult to equate to a single point – and hence define trail.
The picture from my previous blog shows how the contact patch changes with lean angle. When the bike is upright, the patch is a nicely shaped oval and it’s fairly obvious that an equivalent point can be placed in the center of the oval. When the motorcycle leans over, a tire with a round profile retains that contact patch shape right to the very edge of the tread. Most tire manufacturers, however, design front tires with at least some triangular shape. This promotes quick steering and puts more rubber on the ground when the bike is leaned over - the contact patch widens and begins to lose that nice oval shape. Where to place a single point to define the center is less obvious, and it correspondingly becomes more difficult to put an actual value on trail. At extreme lean angles, the patch becomes smaller and crescent shaped as the very edge of the tread is used.
The tire’s profile can significantly alter the shape of the contact patch over the range of lean angles, in turn affecting trail and how the motorcycle behaves. Even the subtle change in profile due to wear can make a difference; most riders can feel that their bike handles differently when the tires are replaced, even if the exact same brand and model of tire is used.
The distortion of the tire due to weight transfer, cornering forces and acceleration/braking forces must also be considered, as these forces can further contort the contact patch, making it even more difficult to identify a central point. This brings the compound and construction into play, as those properties determine how the tire distorts under load. The contact patch loses almost any resemblance to an oval, and it’s almost impossible to determine its shape mathematically based on the properties of the tire.
We normally think of trail as a very definable and easily measured dimension that helps define the handling characteristics of a motorcycle. But there are so many factors that depend on the tire itself that the measurement given in a specification sheet, and even the values for dynamic trail calculated using approximations and assumptions, tell only part of the story. This at least partly explains why simply changing tires can transform a bike’s handling (in either a good or bad way!), and why some riders and bikes have difficulty adapting to certain brands or models of tires. In the pre-spec-tire era, riders could pick and choose a profile, construction or compound of tire that worked more favourably with their style or masked particular handling deficiencies. With that option largely gone, it’s up to the rider or the bike to adapt to the tire – sometimes a difficult proposition.