Bicycle Suspension - Front Suspension

Front Suspension

Front suspension is often implemented with a set of shock absorbers in the front fork. The suspension travel and handling characteristics vary depending on the type of mountain biking the fork is designed for. For instance, manufacturers produce different forks for cross-country (XC), downhill (DH), and freeride riding.

Suspension fork design has advanced in recent years with suspension forks becoming increasingly sophisticated. The amount of travel available has typically increased. When suspension forks were introduced, 80–100 mm of travel was deemed sufficient for a downhill mountain bike. Typically this amount of travel is now more normal for cross-country disciplines. Downhill forks can now offer in the region of 170 to 203 mm of travel for handling the most extreme terrain.

Other advances in design include adjustable travel, allowing riders to adapt the fork's travel to the specific terrain profile (e.g. less travel for uphill sections, more travel for downhill sections). Advanced designs also often feature the ability to lock out the fork to completely eliminate or drastically reduce the fork's travel for more efficient riding over smooth sections of terrain. This lockout can sometime be activated remotely by a cable and lever on the handlebars.

The shock absorber usually consists of two parts: a spring and a damper or dashpot. The spring may be implemented with a steel or titanium coil, an elastomer, or even compressed air. The choice of spring material has a fundamental effect on the characteristics of the fork as a whole. Coil spring forks are often heavier than designs which use compressed air springs, however they are more easily designed to keep a linear spring rate throughout their travel. Substituting titanium coils in place of steel coils in a design can decrease the weight of the design but leads to an increase in expense.

Air springs work by using the characteristic of compressed air to resist further compression. As the "spring" is provided by the compressed air rather than a coil of metal they can often be made lighter; this makes their use more common in cross country designs. Another advantage of this type of fork design is that the spring rate can easily be adjusted by adjusting the pressure of the air in the spring. This allows a fork to be effectively tuned to a rider's weight. To achieve this in a coil sprung fork the spring constant of the coil would need to be changed. One disadvantage of this design is the difficulty in achieving a linear spring rate throughout the fork's action. As the fork compresses, the air held inside the air spring also compresses; towards the end of the fork's travel, further compression of the fork requires ever greater force. This results in an increase in spring rate (stiffness). Increasing the volume of the air inside the spring can reduce this effect but the volume of the spring is ultimately limited by the need to be contained within the dimension of the fork leg. The use of dual chambers within the air chamber system has allowed a more linear feel to air suspension, this is achieved by having a 'reserve' chamber that is activated when the main chamber reaches a certain amount of compression; once achieved a valve opens and effectively makes the chamber larger. By linking the two, this reduces the force needed to compress the air in the chamber and reduces the exponential spring rate feel traditionally experienced with air systems when approaching the end of the travel (movement).

A damper is usually implemented by forcing oil to pass through one or more small openings or shim stacks. On some models, the spring, the damper, or both may be adjusted for rider weight, riding style, terrain, or any combination of these or other factors. The two components may be separated with the spring mechanism in one leg and the damper in the other. Without a damper unit, the suspension system would rebound excesively and control would be lessened.

Some manufacturers, Cannondale for example, have tried other variations including a single shock built into the steerer tube above the crown (also called a "HeadShok"), and a fork with just a single leg (also called a Lefty). Both of these systems claim to offer greater rigidity and better feel, with lighter weight - by having only one leg, and using Needle Bearings instead of bushings, as well as special forging techniques. Others, namely Proflex (Girvin), Whyte and BMW, have made bikes that utilize suspension forks that employ linkages to provide the mechanical action instead of relying upon telescopic fork legs.

To prevent damage to the suspension mechanism, gaiters have been used to cover the spring cylinder (stanchions), however, it was quickly realised that these acted as a trap for grit and actually wore down the stanchions quicker.