Many bikes today have both front and rear suspension systems. The suspension lets the wheels move up and down to absorb small bumps while keeping the tires in contact with the ground for better control. It also helps the rider and bike absorb large shocks when landing jumps.
Both the front and rear suspension systems contain two essential elements: a spring and a damper. Sometimes these components are collectively referred to as a shock absorber.
The spring allows the suspension to move up when the wheel encounters a bump, and to quickly move back down after the wheel passes the bump.
The spring can be a coil of steel, like most springs we're familiar with, or it could be a cylinder containing pressurized air. In either case, the further the spring it is compressed, the more force it takes to compress it, which is exactly what we need for a mountain-bike suspension. You don't want the spring bottoming out when you land a big jump.
Above, the damper in a mountain bike shock absorber. It pumps oil through small holes as the piston moves up and down.
If the suspension were equipped with just a spring, it would bounce up and down several times after each bump. When compressed by a bump, a suspension system needs a way to dissipate the energy that is stored in the spring. The damper is the device that dissipates the energy and keeps the suspension from bouncing out of control.
The most common type of damper is oil-filled. This type of damper is used in car suspensions as well as bike suspensions. When the shock absorber is compressed, a piston inside it forces oil to pass through a small hole, called an orifice. It takes energy to force the fluid to pass through the orifice, and this energy is converted to heat in the oil. The cool thing about oil-filled dampers is that they dissipate more energy and give more resistance to motion the faster you try to compress the shock absorber.
When the shock absorber compresses faster, a greater volume of fluid has to flow through the orifice, so more pressure is required to force the fluid through. This increased flow does two things: It increases the stiffness of the suspension (because the pressure resists the motion of the shock absorber), and it dissipates more energy.
Designing a good shock absorber is partially about finding the best balance between the spring rate (the stiffness of the spring) and the damping. For this reason, many shock absorbers have adjustable spring rates and damping. Some compressed-air springs can be adjusted by increasing or decreasing the air pressure.
Now let's see how the damper is incorporated into a mountain-bike suspension.
The predominant type of front suspension is the suspension fork. It works like the front suspension on a motorcycle.
The bottom part of the fork, which holds the wheel, fits over the tubes that connect the fork to the frame. Inside each tube on the fork is a shock similar to the one in the diagram we saw earlier. When the fork moves up (when the bike hits a bump), the spring gets compressed and the piston forces fluid through the orifice.
There are as many different rear-suspension designs as there are bicycle makers. Most of them use a shock absorber similar to the one in our diagram, sometimes with a larger coil spring. It is mostly the design of the frame and the linkage that makes one bike different from another.
The rear suspensions usually need bigger springs because the linkage gives the wheel a mechanical advantage over the spring. The rear wheel might have to move 3 inches (7.62 cm) for the spring and shock to be compressed 1 inch (2.54 cm). This means that the force on the shock is three times the force on the tire. In the front, there are two shock absorbers, and the force on each shock is half the force pushing on the tire.
Going hand-in-hand with developments in suspension are developments in frame design.