The purpose of any type of mountain bike suspension system is to make the ride feel as smooth as possible over bumps, roots, rocks, and other obstacles. Looked at another way, you may also say that the purpose of suspension is to maximize your efficiency by keeping the wheels of the bike in contact with the ground as much as possible.
Not all suspension systems are created equal, and bicycle and component manufacturers are constantly coming up with new ways to improve their designs. All suspension systems get their shock-absorbing qualities from one or a combination of the following materials: metal coil springs, elastomers (squishy pieces of plastic), air, and oil. Controlling the actions and interactions of these materials is how designers fine-tune their systems.
As a rider, you want to find a balance between the suppleness and firmness of your suspension. If it's too firm, then it won't respond over small bumps, but if it's too soft, then it will feel too squishy over large bumps, and may even "bottom out," meaning that it compresses to its maximum level, thus effectively becoming completely rigid.
Another important characteristic of a suspension system is the rebound. This refers to how fast the system returns to its normal state after compressing to absorb a bump. If the rebound is too fast, your bike feels like a pogo stick. If the rebound is too slow, then it won't be able to react to several bumps in rapid succession.
The majority of mountain bikes sold in recent years are what insiders refer to as hardtails. A hardtail has a suspension system in the front fork, but has no suspension on the rear wheel. A full-suspension bike (sometimes called a "dualie") also has a suspension system for the rear wheel. A dualie provides more shock-absorbing benefits, but with the greater number of moving parts, it also (usually) gives you increased weight, cost, complexity, and maintenance.
On a full-suspension bike, the up-and-down motion of the rear wheel in relation to the rest of the bike can cause it to interact in negative ways with other components, which can lead to issues such as "pedal bob," "brake jack," or chain tension problems. Suspension designers vary the placement of the pivot points, shapes of the linkages, and other design factors to try to minimize or eliminate these issues.
Many high-end suspension systems are adjustable, so that you can fine-tune the system to your own weight, riding style, and terrain. Some even have a "lockout," or the ability to temporarily turn the system off if you want a fully rigid ride. With all of these knobs, dials, and levers, though, your bike can start to feel more like the cockpit of a helicopter. As part of a backlash against this "feature bloat," many hardcore mountain bikers have recently taken to riding fully-rigid (no suspension front or rear) bikes. Many of these bikes are also singlespeeds, i.e. one gear, no shifting.
Kevin Madzia is the information systems manager for Century Cycles, Peninsula. Kevin's cycling experience, which spans more than seven years, includes adventure racing, mountain biking, and road cycling. Century Cycles is a full-service bicycle shop with additional locations in Medina and Rocky River. For more information, visit www.centurycycles.com.