FOX Racing Shox forks use hydrodynamic lubrication. This system force feeds oil into the tall, slotted bushings during the compression stroke. When the fork cycles up and down, the oil is trapped between bushings, upper tubes and seals.
Thermal expansion rates can cause the bushings to close in on the upper tubes, causing high friction and binding during normal operation. Correct bushing clearance is critical to prevent binding of the fork during normal operation.
Geometric dimensioning and tolerance is a design practice used to ensure parts will work/fit during the manufacturing process. Bushings are sized before installation and re-checked for size after installation. Correct bushing tolerance is a diametric clearance of 0.0015”– 0.0090”.
As you rock the fork back and forth with the front brake applied, the bushings only have a small amount of lubricant separating the bushing/upper tube. At this time you may notice a small amount of bushing play. Fork bushings must have clearance to perform correctly. Too little clearance will cause high friction, binding, or bushing seizure when hot.
During normal riding conditions, hydrodynamic lubrication occurs when there is a complete separation of the upper tube from the bushing by a thin film of oil. Hydrodynamic lubrication is characterized by very low friction and no wearing of the bushings or shaft since there is no metal-to-bushing contact. During hydrodynamic lubrication, normal bushing clearance will not be noticeable.
Bushings should be checked annually for excessive wear. If excessive fore and aft movement is detected between the upper tubes and lower legs, contact an Authorized Service Center or FOX Racing Shox for further instructions.
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