This is a multi-disc clutch that intermittently disengages the driving force by changing the gap between the clutch plate and friction plate by operating the clutch lever. The clutch plate engages with the clutch boss and the friction plate engages with the clutch housing, but if the clutch boss or the clutch housing is worn, it may lead to malfunction during disconnection. If there is any discomfort at the lever just before the clutch engagement, check the condition of the clutch boss and housing as well as the plates.
Clutch boss and clutch housing are worn down by continuous striking by the plates.
The function of the clutch is to separate the clutch plate from the friction plate when the clutch lever is depressed, and to bring the two plates into close contact when the clutch lever is released to disconnect power. The reason for combining several clutch plates and friction plates is to increase the contact area and make it difficult to slip. If the half-clutch condition, in which both plates rotate while rubbing against each other, continues for a long time, the friction material on the friction plate surface will wear out.
The convex of the outer circumference of the friction plate engages the dog of the clutch basket, while the inner convex of the clutch plate engages the spline with the clutch boss. When the clutch is disengaged, the convexes of both plates slide away from the surfaces of the dog and spline, and when the clutch is engaged, the convexes engage and strike the dog and spline, causing stepped wear. When the engine is started, the clutch boss and clutch basket rotate together, but during neutral, the main axle to which the clutch boss connects is idle, so no driving force is transmitted.
By removing the nut from the main axle, the clutch boss and clutch housing can be removed. Rotation of the crankshaft is transmitted in the order of clutch housing, friction plate, clutch plate, clutch boss, and main axle.
Although there is a difference between wet and dry clutches, the majority of motorcycles use what is called a "multi-disc" clutch. The counterpart to the multi-disc type is the single-disc type, which is used in motorcycles such as the Moto Guzzi and former BMW, and in cars with manual transmissions.
The feature of the multi-plate system is that friction plates with friction material on the surface and metal clutch plates are alternately superimposed, and both plates rotate with the clutch housing and clutch boss, respectively. Both plates rotate with the clutch housing and clutch boss, respectively, and the power is interrupted by opening or closing the gap between the 2 plates by operating the clutch lever.
The clutch plate and friction plate adhere to each other under the pressure of the clutch spring, but to prevent slipping under the strong torque generated by the engine, the clutch plate and friction plate transmit power to the clutch boss and clutch housing, respectively, by means of convexity and concavity. The clutch plate has bumps on the inner diameter, and the friction plate has bumps on the outer diameter.
When the clutch is engaged in first gear while the motorcycle is stopped, there is a considerable difference in rotational speed between the clutch housing, which rotates with the crankshaft, and the clutch boss, which is connected to the transmission and has not yet rotated. By gradually engaging the clutch, the rotational force is transferred from the friction plate to the clutch plate, and then to the main shaft and driveshaft to start turning the rear tire.
At this time, the convex and concave parts of the 2 types of plates are pressed strongly against the clutch boss and clutch housing. The impact will be even greater if the engine speed is increased and the clutch is engaged vigorously in a clutch engagement like a race start. Even when the clutch is engaged, rough opening and closing of the throttle will cause the plates to strike the clutch boss and clutch housing.
These operations cause stepped wear on the plate contact surface of the clutch boss and clutch housing, resulting in a variety of effects.
- Point 1: The clutch plate and friction plate of the multi-disc clutch are crimped by the clutch spring while mechanically engaging with the clutch boss and clutch housing to transmit power.
- Point 2: Stepped wear of the clutch boss and clutch housing due to being struck by the clutch plate and friction plate during clutch disconnection
Stepped wear can also cause noise and judder during clutch engagement.
The dogs of the clutch housing are subjected to the force of being chopped by the outer convex part of the friction plate every time the clutch is disengaged, resulting in stepped wear in the direction of rotation. Although wear can be reduced by handling the clutch carefully, for example by half-clutching and not opening and closing the throttle unnecessarily while the clutch is connected, it is inevitable that stepped wear will progress with increasing mileage due to the structure.
The stepped wear of the clutch basket dog is easy to correct with a file, but the unevenness of the spline of the clutch boss is difficult to correct because it is difficult to get the tool in. If you try too hard to correct the bumps, the bumps will become tighter and have an adverse effect on the disengagement of the clutch.
Some motorcycle manufacturers refer to stepped wear of the clutch boss or clutch housing as "pitching". It is not clear whether this is due to the regular longitudinal indentation of the area struck by the plate or due to the symptom of the body rocking back and forth during clutch engagement, but stepped wear causes irregular and unnecessary movement when the clutch plate and friction plate are disconnected.
In the friction plate and clutch housing, the convexity of the outer circumference of the plate strikes the concavity of the housing, resulting in stepped wear on the struck surface. When the clutch lever is squeezed to widen the gap between the clutch plate and the friction plate, the plate may get caught on the stepped part of the clutch housing, resulting in poor clutch disengagement. The same applies to clutch disengagement, where the convexity of the outer circumference of the plate may catch on the worn part of the housing, interfering with a smooth clutch engagement.
If the stepped wear progresses further, even if the crimping force of the pressure plate is released by squeezing the clutch lever, the gap between the clutch plate and friction plate will not open sufficiently, and the clutch will always be in a half-clutch state. This can also lead to problems such as difficulty in getting out of neutral when stopping, and a large shock when shifting from neutral to first gear.
Careful clutch operation is effective in controlling stepped wear of the clutch boss and clutch housing. If the half clutch is used for too long, the contact surface of the clutch plate and friction plate will overheat and cause problems, but moderate use will reduce damage to the clutch boss and clutch housing.
To make good use of the half-clutch, it is also effective to reduce friction loss in the clutch lever and clutch cable. If the control of the half-clutch area is difficult to understand due to poor lubrication of the cable, the clutch engagement will inevitably be messy. The same applies to greasing the clutch lever pivot bolt. If the initial stage of the lever stroke is smooth, the control of the half-clutch will be easy, and if the impact at the clutch engagement can be reduced, the stepped wear of the clutch boss and clutch housing can be suppressed.
On the other hand, the difference between wet and dry types also affects step wear. The importance of clutch handling and clutch cable lubrication is the same for both, but the wet type, which is immersed in engine oil or transmission oil, certainly has a lighter impact when the clutch is disengaged. Dry clutches have a sharper touch than wet clutches, but the force of striking the housing is also transmitted directly. In addition, there is no oil film to mitigate the impact on the contact surface, so it is better to be gentle when operating the clutch.
- Point 1: Stepped wear of the clutch housing can lead to problems such as difficulty in disengaging the clutch and shifting gears.
- Point 2: To prevent stepped wear, it is important to operate the clutch carefully and also to reduce the friction loss of the clutch cable and clutch lever.
For motorcycles stored for long periods of time, pay attention to the tension between the clutch plate and friction plate.
Even if the mileage is low, rust may occur on the clutch plates with age. If this is the case, there will be no discomfort in disengaging the clutch, but it will cause contamination of the engine oil and transmission oil, so if you notice it during maintenance of the clutch, replace it as a set.
If the clutch plate and friction plate are left in close contact for a long time, the rust will cause the two types of plates to become one. When this happens, the clutch cannot be disengaged because the plates do not come apart even when the clutch lever is held, and when the gear is engaged, the rotational force of the crankshaft is suddenly input to the transmission, causing the motorcycle to stall. If the rust is at this level, the sticking cannot be removed without using a scraper or flathead screwdriver.
If the clutch boss or clutch housing has stepped wear, it can be improved by smoothing out the unevenness with a file or sandpaper if it is minor. However, it is important to note that grinding to match the concave part caused by the plate striking will increase the clearance in the rotational direction, which may result in further accelerated wear.
Apart from the clutch boss and clutch housing, another factor that can affect clutch operation is rust on the iron clutch plates. This is common in motorcycles that have been stored without riding for a long period of time. The clutch plate rusts due to moisture in the crankcase and bites into the friction material of the friction plate, causing the clutch to remain connected even when the clutch lever is pressed.
If the clutch plate and friction plate become one, the motorcycle body will try to start riding vigorously as soon as the gear is shifted into first gear from a stop, and in many cases, the motorcycle will stall on the spot. In this case, too, the engine power is transmitted to the transmission with no half-clutch at all, and a large force is applied to the clutch boss and clutch housing.
If the clutch sticking is minor, the sticking may be relieved by the impact of vigorous engine rotation input, but if the plates stick tightly to each other, the motorcycle will only stall even if you put it into first gear over and over again, so you have to disassemble the clutch and force it to come off. If the clutch plates are noticeably rusted, or if the friction material of the friction plate is delaminated, do not force the clutch to be reused, but replace it with a new part.
- Point 1: If the clutch plate and friction plate are sticking together in a long-term storage vehicle, replace both plates with new plates.
