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Suspension fluid is a critical component of shock performance and protection. When riding over road imperfections, navigating a rough trail or taking a jump, the force depresses a piston inside the shock that pushes fork oil through calibrated valves. The suspension fluid’s rate of flow (its viscosity) influences the amount of rebound and dampening you feel. A lighter suspension fluid leads to quicker rebounds while a heavier fluid leads to slower rebounds. In addition, suspension fluid must protect the shock tube, seals and valves against wear as they constantly rub together. The suspension fluid must also form a protective layer on parts to guard against corrosion, which can shorten component life if allowed to form. Look for a suspension fluid with a high viscosity index, which indicates its resistance to viscosity changes throughout broad temperature swings. This promotes consistent shock performance, helping you ride more effectively.
Like transmission fluid, brake fluid, power steering fluid or any other lubricant, suspension fluid degrades over time and requires periodic changing. The precise steps differ depending on your make and model, so consult your owner’s manual or a service manual. In general, loosen the bleeder screw to relieve pressure, then retighten. Depending on model, you may need to remove the wheel and handlebars. Loosen the pinch bolts on the fork clamps and loosen the fork cap with a fork-cap wrench. Once the cap is loose, the upper fork will slide down the lower fork. It’s full of oil, so be careful. Tip the fork upside down into a drain pan until drained. Add the amount of suspension fluid dictated in the owner’s manual and reattach the fork.
Generally speaking, a dirt bike or motorcycle rear shock takes more abuse than the front forks. Many applications use only one shock under the seat while the front wheel uses two forks, helping spread the load. As such, shock oil sometimes gets hotter and undergoes increased shearing forces, which degrades the oil and accelerates wear. A shock oil must be formulated to withstand the increased heat and stress. AMSOIL Shock Therapy® Suspension Fluid is formulated for the worst-case scenario, meaning it’s excellent for use in both front forks and rear shocks.