Alright, lets see how much of it I can explain without having to draw pictures. A common misconseption is that suspension controls handling, which is not entirely true. Suspension components control how the wheels move in relation to the frame. Lets keep this in regard to bikes, because it gets much more complicated with cars.
Wheels have to move in respect to the frame of the bike for three completely different reasons.
1. Comfort. bumpiness sucks. Since we are talking about sportbikes, we'll disregard this entirely.
2. Surface irregularities. There are bumps and things on the surface, the road can reach up in a bump, or drop away in a pothole.
3. Weigh transfer. The bikes weight shifts front or rear depending on wether the bike accelerates or brakes.
First thing first (And I'm may well be going over things you know full well) is the property of inertia and momentum. Basically, the heavier something is, the harder it is to get it going in whatever direction, and the harder it is to make it stop. This is pretty self evident. So if a bike had no suspension, and it hit a bump, the wheel has no choice but to travel up, taking the rest of the bike with it. Once the entire weight of the bike is traveling up, it would take it a great deal of time to come back down, a time during which your wheels will not have contact with the ground and you will have no control.
If, you let the wheel move independently of the bike, it weights much less, and thus takes much less time to come back down. This is one of the reason why it is so important to have very light wheels. It is much more important to save weight below the suspension components (wheels, tires, brake calipers, swing arm, fork castings) than it is above them (the rest of the bike)
This being the real world, there are limitations to moving the wheels independently of the bike, and the main one is travel distance. The wheels can only travel so far before the top of the wheel hits the body of the bike, of the bottom of the bike hits the ground. This distance is called the stroke. You can increase the stroke by moving the bike further away from the ground, like you see in dirtbikes. But doing this moves the center of gravity higher, making the bike harder to turn, and more likely to stoppie/wheelie. So like all things, this is a compromize. Sportbike have around 5"-6" of travel on their suspension.
Continued on next post