Motorcycle Suspension Geometry
A motorcycle's handling characteristics are very sensitive to several factors. Of course, the overall size and weight of the machine have the most pronounced effect on its handling. The ponderous touring machines meet the need of the high-speed, long-distance highway traveler while the local commuter rider takes advantage of a smaller machine's nimbleness in traffic.
Front wheel size, as described earlier, can be chosen to meet one of three needs:
1) 21-inch or larger wheels roll over logs and obstacles easily.
2) 18- and 19-inch wheels compromise "rollability" with lower overall height, weight, and greater strength.
3) 16- and 17 -inch wheels are used generally on smaller motorcycles where height must be kept minimal: The smaller wheels found on scooters are really only good for riders with a well developed death wish.
A low center of gravity is essential to good handling. It's easy to see that a top-heavy machine would be difficult to straighten up if leaned over too far.
Of course, ground clearance becomes a critical factor in the quest for a low center of gravity. Avoid extremes when selecting a machine unless the machine is to be used for only one purpose where an extreme center of gravity or ground clearance is required.
Wheelbase affects handling from the standpoint of turning ability and reaction time. Long wheelbase machines are generally slower and more predictable in turns and more stable at high speeds, while short machines tend to react more quickly and are capable of tighter slow-speed manoeuvring. Wheelbase may be increased by engineers through lengthening the swing arm. Hyperbike riders also tend to lengthen their wheelbases in this manner, but don't get me started on them...
Rake angle is an angle described by an imaginary line through the center of the steering head and an imaginary line through the vertical. The rake angle contributes to the caster effect, or straight line stability, of a motorcycle. Increased rake angles tend to enhance high-speed, straight line stability while detracting from low-speed manoeuvrability. Conversely, lower rake angles tend to quicken low-speed handling while causing the machine to be skitterish at extreme high speeds.
Trail is similar in its effects to rake angle; however, it is a linear measurement rather than an angular one. Trail refers to the distance between that imaginary point where the rake-angle line intersects the road surface and the center of the front tire contact patch. Once again, as increased measurements are obtained, high-speed stability is improved and low-speed handling is sacrificed. Some choppers actually have negative trail to the point where the front tire is always trying to turn itself around backwards. That's why so many of them adorn their bikes with skulls.
Motorcycle engineers have experimented for decades with suspension geometry and they have made a great deal of progress. Don't look for radical suspension changes as a cure-all for handling problems. If you want to fine tune your bike's handling the best option is to work to perfect what has been designed into your machine.