BP-51 stand for bypass 51mm diameter bore.
Bypass shock absorbers have been used in off-road racing for many years and are renowned for their ability to handle the toughest terrain and endure through the harshest abuse.
While most shock absorbers use velocity sensitive damping (meaning the faster the shock absorber compresses and expands, the higher the level of damping force is), bypass shock absorbers take this to the next level by also generating damping dependent on the position of the piston within the shock absorber body.
Bypass refers to the ability of the hydraulic fluid to bypass the piston by being routed externally to the shock bore; creating a high level of comfort initially but increasing the damping forces as the shock reaches the end of its travel in both compression and rebound. In Old Man Emu BP-51 shock absorbers, the amount of bypass is adjustable so that it can be tailored to suit different terrain, speed and loads.