In two conversations I had this week, the topic of dynamic geometry came up, and how trail changes as the motorcycle pitches and leans its way around the racetrack. In the first conversation, I was talking with Bernie Broderick about his and his son Javelin's adventures at the Barber Motorsports Park AMA round. They had changed to stiffer fork springs in their Yamaha R6, and after the change it was difficult for Javelin to change direction in Barber's transitions. What happened?
Most riders are familiar with the basic geometry characteristics of rake and trail. Rake is the angle of the steering head with respect to vertical, while trail is the distance the front tire's contact patch "trails" the imaginary point where the steering axis (drawn through the steering head) intersects the ground. In general, it's trail that gives a motorcycle its steering character and what we feel through the bars when we ride. Trail is dependent on rake, front tire diameter and the offset from the steering axis to the front axle. More trail usually gives better stability and heavier steering, while less trail tends to reduce stability but make steering lighter. Rake and trail measurements published in specification sheets are static numbers, measured or calculated with the motorcycle at rest and with a fixed load.
But a motorcycle is not a static piece of machinery. As the front and rear suspension compresses and extends, rake - and hence trail - can change significantly. On most sport bikes, an adjustment in front or rear ride height of 4mm will change trail by approximately 1mm, a change that an experienced rider can feel on the track. Ride height can be changed by raising or lowering the fork tubes in the triple clamps or changing the length of the rear shock or a rod in its linkage. Your front and rear suspension moves through much more than 4mm of travel, however; if the front suspension is fully compressed and the rear fully extended - as is the case under heavy braking - trail can decrease by as much as 30mm from static, a huge amount.
Note that even adjusting preload will change ride height, in turn changing trail on the track, and many riders attribute the result - lighter or heavier steering - to softer or stiffer suspension, when in fact it is the geometry that has changed and is affecting the steering. Going a step further, changing fork or shock springs also affects ride height when the bike is underway. But whereas a preload change affects trail uniformly through the suspension's travel, a spring change has an increasing affect. A stiffer fork spring may compress just a few millimetres less than a softer spring near the top of its travel, but the difference will be much more pronounced - with a correspondingly greater change in trail - further down in the stroke.
In the Brodericks' case, substantially stiffer fork springs made a significant change to ride height and trail on the track. The solution? A spring change is usually accompanied by an adjustment in preload or oil level, so that the desired geometry at a certain travel is the same as it was before the change. And this leads to the second conversation I had this week about dynamic geometry: Using suspension potentiometers and a data acquisition system, we can generate a math channel that plots dynamic rake and trail, and see exactly how they change when adjustments to the suspension are made. It's important to remember that changing one adjustment in your setup can affect may other settings, and these must be taken into account with each step in the process.