Front End Suspension Tuning
By Richard Stanboli   page 2

At the corner entrance the bike dives under braking and the rake angle is decreased (figure 4). The front end’s compression dampening, the rear ends rebound dampening, the center of gravity, and the mass of both the motorcycle and rider determine the rate of dive. When the bike reaches full dive two things happen. The trail drops off dramatically and all loads are transferred to the front end where the forces of deceleration act on it. At 100mm of fork compression a modern sport bike possessing 1385mm wheelbase, 24-degree initial rake and 96.28mm of trail will transition to 20.11 degrees of rake and 74.23mm of trail. This reduction of trail works in a positive manner giving the rider more leverage to combat the increased forces. The riders leverage is simply the proportion of handlebar width to trail length. Assuming that the handlebars are 610mm (24inches) wide the rider would have a 6.3 to 1 leverage advantage on the trail. At the compression levels outlined above the rider’s leverage would go up to 8.2 to 1 which in turn makes entering the corner and fighting the increased forces much easier.

The leverage gain at full dive is not all fun and games, however. The momentum of the chassis can cause the bike to dive further than the eventual resting point. The exaggerated dive causes a sudden reduction in trail. This coupled with the rider’s movement and an irregular road surface can make for a vague, nervous and unpredictable feeling. Compression dampening of the front forks has the greatest control of the rate of dive. Increasing the front end’s compression dampening can decrease the rate of dive. Spring rate and preload have little effect on the rate of dive but they do establish the height of the final resting position. The greater the spring rate and preload the taller the bike will be at maximum braking. Ultimately, the bike will reach it’s full braking height at nearly the same rate regardless of all other factors save compression dampening.

The ideal situation would be to allow enough controlled chassis dive to make turning in as easy as possible without sacrificing predictability and traction. Next time we will delve deeper into what affects trail and how it can be tuned. Shortly after that we will make the transition to mid corner dynamics and chassis balance.

You see the trail and rake dimensions specified in just about every magazine test and factory information release. This information is a helpful start at comparing different brands but does not show the entire picture with regards to handling. next page