The job of any suspension is to keep the bumps in the road or trail from upsetting the motorcycle’s chassis, and to keep the tires in contact with the surface. Here are some terms you’re likely to hear.
TRAVEL This is the measurement of how much a wheel moves up and down. Dirtbikes require lots of travel—up to 12 inches (30 cm) at both the front and rear—to deal with jumps, rocks, and ruts. Cruisers often have the least travel—as little as 3 inches (8 cm).
SPRING Most modern motorcycles use coil springs. The rear spring (or springs) is usually easy to see; the front springs are almost always concealed inside the fork tubes. The spring's strength is called spring weight; switching to a lighter or heavier weight can make your ride softer or stiffer.
RISING RATE Some suspensions, especially those using a single rear shock, get stiffer and stiffer the more the suspension compresses. That's called rising rate. On dirtbikes especially, this helps prevent bottoming over large bumps while still allowing for a plush ride over smaller ones.
PRELOAD Most motorcycle rear suspensions (and some front as well) push on the spring when the motorcycle is at rest—preloading it, which affects the ride height of the motorcycle, allowing it to carry widely varying loads. Increasing preload can be useful if you're carrying the extra weight of a passenger; racers use it to fine-tune a bike's attitude on the track.
BOTTOMING When a suspension deflects to its maximum travel, it's said to be bottomed out. If your bike bottoms regularly, you need a stiffer spring, a new shock, or to get your head out of the pasta at your local all-you-can-eat buffet.
UNSPRUNG WEIGHT The mass of your bike that isn't directly supported by the suspension springs:
i.e., the wheels, tires, and brake discs. Less unsprung weight means your bike's wheels will better follow the contours of the road.