Reasons for a Motorcycle Clutch Slippingby Chris Gilliland
A typical motorcycle clutch is composed of a series of friction-generating plates that are used to engage and disengage the engine flywheel from the transmission. A clutch slip occurs when the friction plates cannot engage completely, allowing the flywheel to spin at a different rate than the friction plates. Clutch slippage is usually a result of improper adjustments or caused by wear.
Improperly Adjusted Clutch Controls
Most motorcycles use a steel cable to engage and disengage the clutch from the transmission via an actuator arm built into the engine's clutch cover. A set of adjusters on both ends of the cable determine the actuator arm's range of motion. The clutch can be held in a partially engaged state if the clutch cable is adjusted too tightly, allowing the clutch to slip. Decreasing the cable tension, using the adjuster nearest the actuator arm, will allow the clutch to engage completely. On the other hand, some motorcycles use a hydraulically actuated clutch in place of a steel cable setup. Similar problems can occur if air or moisture has infiltrated by the clutch master cylinder's supply of brake fluid, reducing the hydraulic pressure needed to disengage the clutch from the transmission. The fluid must be bled and replaced with fresh brake fluid to restore the hydraulic pressure within the clutch circuit.
Incorrect Oil Types
With the exception of most Ducati motorcycles, which are fitted with a dry-type clutch, most motorcycle clutches are bathed in oil to reduce wear. Using an oil type that is different than what is specified by the manufacturer can have adverse effects on the clutch's operation. This is especially true if you are using a non-motorcycle specific oil. Most automotive oils contain friction-reducing additives that are beneficial to a car or truck's engine. However, these additives will interfere with the clutch's friction plates and cause the clutch to slip. Alternatively, clutch slippage can occur if there is too much oil in the engine or transmission. Drain the existing oil and replace it with the proper oil type and quantity, as directed by your motorcycle's owners manual, if you believe the clutch slip to be oil-related.
Weakened Clutch Springs
A set of springs housed within the motorcycle's clutch basket are used to push the clutch back into its fully-engaged state. Over time, the springs can weaken and prevent the clutch from returning completely after being disengaged. The springs must be removed and visually inspected to determine if they are within the serviceable specifications listed in the factory service manual published your motorcycle's manufacturer. If the springs are beyond the limit, you must replace them as a set. This condition is more common among motorcycles with high mileage, but can also occur through frequent high-performance usage.
Worn Clutch Plates
The clutch and friction plates used by a motorcycle's clutch are classified as consumable items with finite service life, similar to brake components. The plates will gradually thin as time goes on, until they can no longer provide the friction needed to adhere to the engine's flywheel. Clutch plates can provide 30,000 to 50,000 miles of reliable service if properly cared for. However, racing and other high-performance activities can cause premature clutch plate wear. Barring that the other possible causes have been ruled out (i.e. clutch control adjustments, weakened springs and proper oil types), you will need to remove and inspect the clutch plates for wear. Replace all of the clutch plates, if any of the plates are below the minimum thickness specifications listed within your motorcycle's factory service manual.