You twist the throttle and the expected forward acceleration doesn’t feel the same. Or the in an extreme case, you drop into first gear release the clutch and the motorcycle doesn’t move ahead, only the RPMs go up. These are symptoms of a slipping clutch.
The clutch is a mechanical device designed to temporarily disengage the driveline, mostly to allow you to change gears smoothly. Clutch engagement, allowing power to travel from the engine to the rear wheel through the transmission and final drive is basically due to spring pressure and friction of rotating plates of different materials. Slipping is caused by loss of the correct spring pressure of reduced friction in the plates. This can be due to worn parts, improper adjustment, excessive heat or incorrect oil.
Checks to Confirm Slipping
A slipping clutch feels like something is not connected in the motorcycle’s drive train. An if the clutch is actually slipping then the drive system is functionally be disconnected. There are several ways to confirm slipping.
From a standstill, put the bike in 2nd gear and try to pull away, if the RPMs build and the bike doesn’t move ahead in a direct relationship to the throttle position the clutch is slipping. In other words, the tachometer goes up higher without the bike moving ahead proportionally.
Another version of the above test is to start on a hill, this will put extra weight on the clutch and the symptoms will show up more clearly.
Still another version is to drive at low speed, shift up to a higher gear than usual, then twist the throttle. Again, if the clutch is slipping RPMs will jump up and the bike will not lurch forward, or it may speed up by slowly and with very little gusto.
At the extreme, you may be in gear and go nowhere regardless of throttle position. Check to see the chain hasn’t fallen off or broken. This is a pretty extreme situation, and you should have had a lot of warning before this happens.
If slipping symptom shows up after recent heavy and prolonged clutch use, such as constant stop-and-go traffic, it could be an overheating problem. Let things cool done, wait at least 30 minutes and try again. The clutch should still be inspected and all adjustments made. The overheating could have also damaged clutch components that were at the end of their useful life and may fail soon regardless. A complete inspection is needed.
The vast majority of motorcycle clutches are designed to run in oil thus the term wet clutch. The clutch is made up of interspersed metal and friction plates that are pressed together with sets of springs. All these plates are housed in a clutch basket. One set of plates is directly connected to the outer clutch basket and the other is directly connected to a central shaft. When engaged the springs are pressing the plates together, and power can be transmitted due to the direct connects of the plates to the central shaft and outer clutch basket. The clutch pushrod causes the plates to separate and spin independently, disconnecting the inner shaft and the outer clutch basket.
Clutches are simple and effective designs, they can, however, be burnt out quickly with neglect and clumsy operation. The majority of the parts are relatively inexpensive and replaceable. However, be very careful around the clutch basket, to replace this will be very expensive. If you can even find a used one it will be surprisingly costly.
The first thing to check is the clutch cable adjustment free-play gap. Refer to your service manual for the clutch lever gap width. The gap is measured between fully resting position to the point where pulling the lever just starts to pull the clutch cable tight. Some practice is required, but it’s a very easy skill to pick up. If there is no slack at all and the cable is tight the clutch may be partially disengaged all the time. This could produce the same symptoms of a slipping clutch.
If there is too much slack there will not be enough movement in the cable and the clutch will not be fully disengaged when needed. You may notice gear grinding, clunking or jerky bike movements when shifting gears.
Some motorcycles use a hydraulic clutch, where hand lever motion is translated into hydraulic pressure and sent to an actuator near the clutch. Some designs have no adjustment, others that do will use the same principle, where you adjust a gap between the lever and pin that transfers pressure through the system.
Adjustments should not be needed very often since hydraulic systems compensate for other components wearing or heating and expanding. Check your service manual for your bike’s design and if there is an adjustment gap to check. Also, check the hydraulic fluid level, and for any leakage indicating a component may not be operating correctly. Also, the hydraulic fluid should be changed on a regular basis also – check the manual.
The other adjustment point is under the clutch cover. This is where the movement of the cable or hydraulic actuator causes a pushrod to move. The clutch pushrod will separate and disengage the clutch plates. This is adjusted in theory similar to the hand lever adjustment. Check your service manual for exact steps and process, it can be very different depending on manufacturer and model. This adjustment can be set incorrectly and cause the clutch to be partially disengaged all the time, resulting in clutch slippage.
If you have had a recent oil change or used an oil additive? If so double-check the oil used meets the recommended specifications. And it does not contain “energy conserving” or “high gas mileage” additives. This usually indicates molybdenum disulfide (moly) is present, which is not compatible with wet motorcycle clutches. These friction fighting additives will act on the friction plates in the clutch, which of course is not how it was designed to operate.
If the oil is not the problem, and adjustments do not correct slipping you will have to inspect the internal clutch plates for wear and glazing. This will require disassembly.
A possible cause can be friction plate glazing. This is where a smooth surface is burnt onto the friction plates. It looks and feels glass-like and very smooth. The plates may even smell burnt and be discolored since this is caused by heat and movement of the plates over each other. You may notice the metal plates are heat discolored or even warped. All plates should lay on a flat surface with no noticeable gaps or distortions.
Some light glazing can be fixed by roughing up the friction plates with sandpaper, otherwise replace them. The metal plates can wear and be deformed by excessive heat if so, replacement is required.
Inspect the clutch springs for obvious physical damage like breakage or collapsing. Once removed they should be measured for correct height and all fall within specs.
Clutch Component Upgrades
Sometimes the original clutch design is undersized or limited and a replacement high-performance sets are used. They can be used with stronger “performance” springs also. Just be aware that upping the spring rates can cause the clutch lever pull to be increased, and it may be uncomfortable to use when shifting a lot. In most cases, the OEM, or quality 3rd party sets manufactured to stock specs will usually be best for most situations. Friction plates and springs are replaced in sets since they tend to wear evenly.
Slipping After Replacement
Most likely cause – improperly adjusted cable too tight, possibly wrong oil (if drained and refilled), or wrong oil used to soak clutch plates (containing additives not compatible with wet clutches, such as molybdenum disulfide, aka moly)
Noise from the clutch area is not normal if you hear any noises and inspection is needed. Most bikes have a cover that is fairly easy to remove, some even have an inspection plate for this purpose.
Continuous grinding or rough sound could indicate bad bearings, but this would usually require some high mileages to go bad. A broken clutch spring could cause this too. Gently see if you move each spring with a thin screwdriver, if you can move it there’s a problem. Check that all locking nuts are tight, if not readjust whatever it was supposed to be locking down. Missing teeth on the outer clutch basket could produce a very rhythmic sound (I had one of these gear teeth break off and get wedged in very rhythmic and unpleasant sound).
There is only some much you can inspect before you have to start taking the clutch apart. If you can’t immediately see the issue, get out the wrenches or call your favorite shop.
Care & Use of the Clutch
- Put the motorcycle in neutral at prolonged stops
- Avoid excessive stop and go driving if possible
- Check clutch cable adjustments routinely
- Do regular engine oil changes, use the correct oil and grade recommended
- Note: synthetic oils do not cause clutch slipping
- Be careful with any oil additives (make sure they are designed for wet clutches)
- Inspect clutch at recommended service intervals
- Replace plates at recommended service intervals, or when measured out of spec
- Check clutch cable routing, gentle bends no kinking or binding
- Clutch cable condition – not dented, outer casing broken, rust, dirt, moves freely
- Check service manual, some clutch cables are not designed to be lubricated (Teflon lined)
- On Harley Davidsons never use automatic transmission fluid for oil in the primary case
- Do not overload your motorcycle and put excessive stress on the clutch, for a post on how much weight your motorcycle can carry see the post – How much weight can my motorcycle carry?
Motorcycle clutches are simple and robust mechanical devices. They rarely give the owner grief, other than when they start to wear out or have been mistreated or overheated. The clutch cable is the highest wear item, keep it adjusted and lubricated as required. Follow proper maintenance schedule and use specified oils, and your clutch will perform well and last a long time.