In recent years the art of bicycle making has expanded greatly in the area of bicycles which are intended for use off paved roads. These bicycles have become known as "mountain bikes" or "all terrain bikes." Because these bicycles are widely used on rough surfaces, The use of suspension to absorb the bumps and impacts of riding has become increasingly popular. A very common type of suspension is built into the fork and comprises a pair of parallel telescoping legs secured together by a fork crown, with the legs extending downwardly on opposite sides of the front wheel. These legs are each typically comprised of two generally concentric and coaxial tubular members with one male member sliding axially within the female other member to allow compression of the leg in response to an increased axial load. Most commonly, the telescoping mechanism is oriented so that the outer or female leg member is the lower leg member and the inner or male leg member is the upper leg member. The sliding movement of each leg is typically controlled means of a sealed chamber of compressed air, coil springs or elastomer material positioned between the tubular members to resist relative axial movement of the members in compression. Damping of the movement may also be controlled by additional friction created by hydraulic or mechanical means to resist axial movement in either compression or extension.
An outer surface port, of the male member of each telescoping leg is typically polished to slide smoothly axially within the female of the leg and is often at least partially exposed as it slides into and out of the female leg portion. At the point where this male member enters the female tube, an annular seal typically surrounds the male member to prevent entry of contaminants into the interior of the female telescoping tube member. The effectiveness of the seal can be dependent on the uniformity or smoothness of the surface of the male portion. Particularly in the case of a suspension using air springs, hydraulic damping, or both, where containment of the air or hydraulic fluid is necessary, the function of the seals may be crucial to assure appropriate compressive resistance in the fork structure.
When unprotected, the polished surface portion of the male leg member is susceptible to physical damage by scratching, abrasion or erosion. This can occur if, for example, a rider loses his or her balance and the bicycle falls over, hitting the ground and allowing the polished surface to comes into contact with a rock. Also, in dusty or muddy conditions the exposed outer surface of the male leg can attract dust or become covered with water, mud or a combination of the these three elements, all of which may have the effect of creating abrasion between the respective portions of the fork leg or the seal between the legs or which may increase friction between the sliding portions of the two legs. The moisture from water and mud pose the additional threat of causing rust on the steel surface of the male fork leg with a resultant decrease in the smoothness of the polish of the surface.
It has long been known that the dependability and longevity of the function of suspension forks can be increased by covering the sliding portion of the legs with a rubber accordion-like "boot" to protect the polished surface of the male fork member from physical damage, such as scratching, and to help limit the exposure of the polished surface and any seals to solid and liquid contaminants such as dust, dirt, mud and water.
To function most effectively at keeping contaminants away from the sliding mechanism such a boot is fitted to form an airtight seal around the movable portion of the fork leg. However, having a boot which is sealed airtight, can create several additional problems. If air which is sealed inside the boot has a sufficient moisture content, the moisture may cause rusting of steel parts exposed to the moisture. Additionally, undesirable condensation may occur within the boot, creating the potential for rust or an increase in operating friction as described above, as well as increasing possibility of that trapped water may work its way past the annular seals and into the interior of the fork itself. Further, as the fork leg is moved through its range of motion in compression and extension, the internal volume within the boot is increased and decreased. The air contained within the boot tends to resist any change in volume and can cause the flexible boot to bulge outwardly or collapse inwardly during operation. In bulging outwardly the folds of the bellows may come into contact with a tire cause rolling friction or damage to the boot. In collapsing inwardly the folds of the bellows may contact the male leg member creating additional friction in the movement of the suspension itself. The resistance to a change in volume by the air within a boot may also create a sufficient increase or decrease in air pressure within the boot to hinder the movement of the telescoping assembly as the fork leg is extended or compressed. This would most commonly occur where the boot is imperfectly sealed. After expulsion of air as the fork and boot are compressed, a low pressure or semi-vacuum condition is created within the boot as the boot and telescoping assembly begin to re-extend. This internal low pressure condition may cause the boot to become more tightly sealed against the leg members, preventing any entry of air, and thereby preventing or severely hindering the extension of the boot and the telescoping of the leg.
A boot may be provided with breather openings to allow sufficient free air flow to prevent distortion or immobility of the boot. However, this creates the additional problem of allowing dust, water and other such contaminants to gain entry into the moving portions of the fork leg, thereby decreasing the effectiveness of the boot in shielding the moving parts from such contaminants.