Motorcycle Multiplate Clutches
The most common motorcycle clutch works by utilising a series of steel and friction plates/ discs stacked in an alternate arrangement that sits inside a clutch basket/ housing. The clutch basket/ drum  is rotated by the crankshaft (the bit the pistons rotate) and the steel plates sit interlocked within the basket, the friction plates  are spaced inbetween the steel/ plain plates  and interlock/ mesh with the transmission gear shaft via the clutch centre , the set of clutch plates is often referred to as a clutch pack. When the plates are compressed together, usually by a set of four coil springs  and against a diaphragm plate/ pressure plate , the force applied and amount of friction generated between the plates allows the engine to transfer its torque power to the gearshaft and when the clutch lever is used it relieves this pressure to allow the plates to separate while allowing the engine to continue running and you to change gear.
- 1 : Clutch bolts – bolt the springs and pressure plate to the clutch center
- 2 : Washers
- 3 : Clutch springs – used to force the plates apart
- 4 : Clutch diaphragm/ pressure plate – compresses plates
- 5 : Bearing
- 6 : Pushrod tip
- 7 : Pushrod
- 8 : Friction plates – mesh with clutch center to rotate it
- 9 : Steel / plain plates / clutch discs – mesh with the clutch basket
- 10 : Clutch locknut / centre nut
- 11 : Washer
- 12 : Lockwasher
- 13 : Clutch center
- 14 : Thrust washer
- 15 : Clutch basket / outer housing
- 16 : Clutch basket guide
The more power the engine produces the more friction is needed in the clutch plates to transfer the power, this means in the bigger bikes you’ll see bigger clutch discs as well as more of them. Also in the bigger engine motorbikes rather than using a cable from the clutch lever to operate the clutch, a hydraulic piston will be used to force the plates apart due to the increase in friction needed – much the same as your brakes. The arrangement of the plates doesn’t matter as long as each friction plate is between 2 steel plates. Clutches will generally stop working when the plates can no longer grip each other and this will happen for a variety of reasons. A multiplate clutch can either be wet or dry – A wet clutch is immersed in the engine oil to keep it from overheating from the friction produced, the oil causes the plates to slip so more plates are used as opposed to a dry multi plate clutch which as the name suggests is not immersed in oil and will typically have fewer plates also creating more noise with the need for airflow to keep the plates cool. In some cases only a single friction plate is used. The dry clutch design is prominent in Ducatis and contributes to the distinctive sound of the Ducati motorcycle.
You will know when you’ve burnt your clutch out because of the unique foul smell, also you’ll know that you’ve burnt out the clutch by your riding style – if you don’t engage or disengage the clutch properly e.g. you ‘pop’ it not giving it enough time to engage/ disengage the engine properly, although this takes some time to burn out a clutch. Most of the time you’ll get that clutch burning smell because you’ve been doing too many burnouts, wheelies and stoppies. This is the worst way to ruin a clutch and also the most expensive, you’ve basically burnt off all the friction material in the plates and this will have also chipped off leaving chunks of the friction material in your oil, That oil will now also circulate through the engine and cause an engine seizure if you continue to ride it and that means a full engine rebuild (or a new engine as well as clutch). Not only will you have lost the friction material but you’ll have damaged the steel plates and the clutch basket will more than likely be damaged where the teeth on the steel plates will have cut themselves into the slots on the clutch basket. This means that you’ll need a full clutch replacement/ rebuild.
If you get that nasty clutch burning smell – stop immediately! Next drain all the oil out, get a brand new set of friction plates, steel plates and clutch basket and change the oil filter and get a couple of extra new oil filters. Rebuild the clutch and run the engine gently – then replace the oil and oil filter a few times over the course of the next thousand miles to make sure you’re getting as much of the old friction plate material out of the engine as possible. If you take your bike to a garage to be repaired make sure that they do this otherwise in a few hundred miles you’ll then have a seized engine. Other than that, the following problems are the cause of general wear and lack of service on a clutch.
Common causes of problems with motorbike multiplate clutches
When the multi plate clutch slips, or doesn’t engage properly this can mean a few things and as long as you don’t have the nasty burning smell of a burnt out clutch then it’s pretty easy to fix and service the clutch yourself without needing a full clutch replacement, the issues are generally those of wear over time. Here are a few common problems that can occur that will cause the engine to drag or the clutch to slip:
- The clutch fluid levels are too low or too high
- Friction plates are warped or worn
- Steel/ plain plates are warped
- The springs that compress the plates are broken or weak so they can’t provide the correct tension
- Clutch basket/ housing is unevenly worn where the plain plates interlock
- Clutch is not correctly tightened – normally the clutch centre nut
- Clutch bearings are seized or worn – generally this is pretty rare
- The oil being used has deteriorated or is the incorrect standard e.g a heavier oil than recomended can cause the plates to stick together
The most common clutch issue (and the easiest to fix) is simply that the clutch controls and cable are incorrectly set, for example there’s a lack of tension in the cable so that it can’t fully pull the plates apart. Another common cause is from engine modification – if you’ve increased the power then you’ll need to upgrade the clutch, which a lot of people seem to forget since this exerts extra force that the original clutch wasn’t designed to cope with – quick fixes would be additional plates or stronger springs.