In my previous blog, I talked about how rake and trail change as the motorcycle's suspension moves through its travel. But I also alluded to the fact that trail changes as the motorcycle leans from side to side. This is something non-intuitive, but a very real factor that must be considered when discussing handling. Vittore Cossalter's book "Motorcycle Dynamics" goes into comprehensive detail with several pages of equations required to calculate trail based on a long list of variables.
One important distinction that Cossalter makes is between mechanical trail and normal trail. In general, when we discuss trail we are referring to mechanical trail - the distance the front tire's contact patch "trails" the imaginary point where the steering axis (drawn through the steering head) intersects the ground, as mentioned in my last blog. But where mechanical trail measures that distance along the ground, normal trail measures perpendicular (or "normal") to the steering axis and is a better indicator of what the rider actually feels and experiences through the handlebars. The difference between the two measurements is subtle, but must be considered in conditions where the motorcycle is leaning.
Cossalter's formulae take into account the usual factors that determine trail - triple-clamp offset, rake and front tire diameter - but then add a number of other variables: front and rear tire width and profile, wheelbase, lean angle and steering angle. As the motorcycle leans and steers into a turn, the front tire's contact point moves in relation to the steering axis an amount calculated from those additional variables. Crunch the new contact point into more equations (15 in total) and you get results for mechanical and normal trail. The combinations of variables are almost endless, but there are some interesting trends that can be seen from graphs of the data.
In general, Cossalter summarizes, mechanical trail and normal trail decrease with more steering angle. As the motorcycle leans into a turn, mechanical trail increases, but normal trail decreases. From the rider's point of view, steering becomes sharper and lighter, with less stability, at increasing lean and steering angle. Consider a typical middleweight sportbike: mechanical trail is approximately 100mm, with a normal trail of approximately 110mm - slightly more than the mechanical trail. When the bike is leaned at 45 degrees in a turn that requires a few degrees of steering input, mechanical trail increases to roughly 120mm. But normal trail <i>decreases</i> to almost 80mm, a significant reduction.
One other aspect to consider is that the contact point of the front tire also moves as the bike goes over a bump or through a dip in the pavement. As an extreme example, imagine running your bike up a curb; the front tire's contact point goes from its usual spot (on the road) to a point far forward (on the curb) instantly.
A large bump or deep depression can reduce trail significantly, and a large dip at the entry of a turn can bring all these aspects together: the front suspension is compressed, reducing trail. The front tire's contact patch moves forward, further reducing trail. The bike is leaned in the turn, with the steering angled in - yet even more trail gone. It's small wonder that those scenic tracks with multiple rolling hills can quickly turn a sharp-handling machine into a wobbly mess, and almost all my setup notes over the years show correspondingly more relaxed settings with increased trail at hilly tracks such as Atlantic Motorsport Park or Mid-Ohio, compared with more aggressive settings at flatter tracks such as Shannonville.