The clutches used in the majority of manual transmission vehicles, from mopeds to big bikes, are wet-type multi-plate clutches that operate while immersed in oil in the engine. Even if oil is adhered to the steel plate, the steel plate may rust depending on storage conditions, and even if rust does not occur, the steel plate and friction disc may stick together. When the shift pedal is put in first gear, it will gunk up and stall, remove the clutch cover and inspect the inside.
Clutch with two types of plates interrupting power
The pressure plate is the outermost clutch component. When the clutch spring pushes the pressure plate, the friction plate and steel plate are crimped together to transmit the engine power to the transmission and drive the rear wheels. Originally, the pressure plate should be removed by hand by removing the bolt and spring, but during a long period of neglect, the steel plate was stuck on and had to be removed with a tool.
The friction disk and steel plate under the pressure plate were also integrated, so we inserted a tool into the claw on the outer circumference of the friction disk and carefully peeled them off one by one. If genuine parts can be obtained without fail, it is acceptable to replace them, but in the case of a half-century old model for which parts supply is a concern, even if they cannot be reused, we want to work with the utmost care.
The clutch transmits and disconnects the rotation of the crankshaft to and from the transmission. The majority of motorcycle clutches are wet multi-plate, with some dry multi-plate and dry single-disc clutches. Dry clutches are to wet clutches what single-disc clutches are to multi-disc clutches.
In a wet clutch, the clutch is immersed in a wet condition, specifically engine oil or transmission oil, and the oil is expected to cool the clutch and cushion the impact when the clutch is interrupted. In contrast, a dry clutch interrupts in air, and since there is no oil film, the clutch has the advantage of a clear feeling when interrupting.
To transmit engine power, a multi-disc clutch consists of two types of discs called friction discs and steel plates that are stacked alternately. The larger the displacement and horsepower of the engine, the more discs and plates are used to increase the contact area.
For example, the 50cc era Honda Monkey and Gorilla manual clutches have only one friction disc and one steel plate each, but the 2018 Yamaha YZF-R1 uses 10 friction discs and 9 steel plates. The clutch spring keeps the two types of plates in close contact, transmitting the engine's power without slipping.
When the clutch lever is squeezed and the pressure applied to the two plates is released, the power is also disconnected. However, if some drag remains, the power may not be completely disengaged, causing symptoms such as difficulty in shifting and difficulty in getting neutral when stopping. Some engines use a component called a cushion ring between the two plates.
The Yamaha HS1, a 2-stroke 90cc model from the 1960s, shown here, is another machine that uses cushion rings. Four rubber cushion rings are built inside the four friction discs. These rings are crushed by the steel plates from both sides when the clutch is connected, and when the clutch lever is depressed and the clutch spring retracts, the elasticity of the rubber pushes the steel plates away from the friction discs. Many Yamaha vehicles use cushion rings, and the RZ250 of later years also seems to have contained a ring.
- Point 1 - The wet-type multi-disc clutch used by the majority of motorcycles interrupts the friction disc and clutch plate in engine oil or transmission oil.
The main cause of clutch sticking is rust and friction material deterioration due to moisture in the engine.
There are various materials used for friction materials for friction discs, starting with cork materials in the old days, and including rubber-based, paper-based, and chemical fiber materials. In the case of a wet clutch, the clutch will be crimped in oil with a steel plate, so it is important that the friction force not to slip even if there is an oil film, as well as not to burn out or break even if the clutch is halfway rubbed in a half-clutch. If the friction material peels off from the base of the friction disc due to clutch tension, the disc cannot be used.
Peeling off the sticking clutch one by one, the friction material peeling off of the friction disk was limited. The black ring further inside the inner diameter of the friction disc is the cushion ring. It is slightly thicker than the friction disc and contacts the claw inside the steel plate. When the clutch lever is squeezed and the disc and plate are stripped, it pushes the claw of the plate apart to reduce clutch drag. Some clutches do not use these parts at all.
The claw of the outer circumference of the friction disk hangs on the notch on the outer side of the clutch outer (clutch basket), which is driven by the crankshaft. In contrast, the convex part of the steel plate hangs on the concave part of the outer circumference of the clutch boss inside the clutch basket. By alternately overlapping the disc and plate and crimping them together with the clutch spring, the rotation of the clutch outer is transmitted to the clutch boss, turning the main axle (transmission) to which the boss is connected and driving the rear wheels.
If clutch tension is mild, it may be possible to reuse the clutch. If rust or friction material remains on the steel plate surface, clutch operation will become unstable, so sand the surface with sandpaper until the surface is free of irregularities.
The cushion ring helps the clutch disengage when the clutch lever is squeezed, but when the lever is released, the two types of plates stick together perfectly. And after years or decades of this condition, the friction disk and the steel plate may become completely stuck together, and the clutch will not disengage when the clutch lever is squeezed.
A typical cause is rust on the steel plates. Wet-type multiplate clutches are immersed in engine oil or transmission oil, but not all of the disc parts are completely submerged in oil. The steel plate, which is the steel plate, may rust due to changes in humidity inside the crankcase, as well as inside the clutch cover, where it is not submerged in oil but exposed to the air. In the case of out-of-print or old cars, the friction material of the friction plate may be altered and stick to the steel plate.
When the friction disc and steel plate stick together, the clutch does not actually disengage when the clutch lever is depressed. If you squeeze the lever, the clutch spring compresses and the pressure plate, which presses the two plates together, is released, but the engine and transmission remain connected because the plates are integrated by the tension, and there is no half clutch or anything, so if you put the car in first gear If you put it in first gear, it will suddenly stall.
You will be surprised at the behavior of the clutch lever when you suddenly release it from your grip, but the biggest difference from when you miss the half clutch is that the car will try to start when you put it in gear even with the clutch lever in your grip.
If the clutch tension is mild, a sudden shock when the gears are connected can cause the disc and plate to separate and the clutch to disengage, but if the clutch is badly rusted or sticking, the problem will not be resolved unless the clutch is disassembled.
Conversely, once the sticking friction disc and steel plate are stripped, the clutch function is restored. However, depending on the degree of plate rust, friction material deterioration, and damage, parts replacement may be necessary. In the sticking example shown here, the rust on the steel plate was mild and the friction material of the friction disc was not damaged, so the rust on the plate could be removed and the plate could be reused.
To prevent the disc and plate from sticking together, a known technique is to tie them apart with tie wraps or other means while holding the clutch lever when long-term storage is anticipated. However, since the clutch spring remains compressed, some people are concerned about spring sagging. The best way to deal with this is to ride regularly to prevent the plate from sticking, but if you find that as soon as you grab the clutch lever and put it in first gear, it jerks!! and stalling, suspect the possibility of clutch sticking.
- Point 1 - Prolonged immobility can cause the clutch to fail to disengage due to rust on the steel plates and deterioration of the friction material on the surface of the friction disk.
