Motorcycle Clutch
n: a device designed to connect the transmission of a motorcycle to the engine, which when properly adjusted will slip at high speed, drag at low speeds and make neutral impossible to find at stops.
The primary drive gear is...
The primary drive gear is on the far right, the mainshaft in the middle, with the clutch hub bushing in place.
Primordial motorcycles connected their engines directly to the rear wheel. Getting under way meant pushing or pedaling until the engine caught, then off you went. Unfortunately, because there was no decoupling device, anytime you stopped, the engine died and had to be restarted. The lack of a clutch also meant a multi-speed transmission couldn't be used, so the bike's velocity was constrained by its drive ratio.
Manufacturers quickly realized that slow, difficult-to-ride motorcycles weren't going to get very far (no pun intended), so they began to install rudimentary clutches (and multi-speed transmissions.) Before long, those basic in-out mechanisms were replaced by spring-loaded friction clutches, identical in principle to the ones we're still using over a hundred years later.
Clutch Convenience
Unlike a steam engine or electric motor, internal combustion engines don't develop much torque until they're running at some speed, and as the load is increased, that speed needs to be maintained or the engine stalls. Because the spring-loaded friction clutch can be gradually engaged or slipped to take up the drivetrain slack, it allows you to balance the engine's rpm (and torque output ) against the vehicle's inertia to get smoothly under way.
The clutch's secondary function is to interrupt the power flow to facilitate gear changing. When torque is applied to a gear set, its natural inclination is to stay locked together. Interrupting the torque makes it much easier to separate them so the transmission can be shifted.
Variants
Motorcycles use several different types of spring-loaded clutches, the most common being the wet, multi-plate clutch, which as the name implies is one that's comprised of multiple clutch plates, and runs partially submerged in oil. Variants include the dry, multi-plate design, which, as you might surmise, doesn't run in oil; and the automotive-style disc clutch, which is generally found only on bikes with crankshafts positioned longitudinally. Since the design is largely confined to certain BMWs and Guzzis, we'll have that discussion another day.
Multi-Plate Minutiae
The multi-plate design starts with the clutch drum, which is sometimes called the clutch basket or clutch outer. The drum rides on a bearing or bushing that fits over the transmission's mainshaft and is coupled via the primary drive directly to the engine's crankshaft. Anytime the engine runs, the clutch drum spins with it; however, since there's no direct connection between the clutch drum and the transmission (at least not yet), you can rev the engine till it bursts and the bike won't move an inch. Additionally, some sort of load damper is often incorporated into the clutch drum. The damper, in the form of springs or rubber bumpers, is designed to minimize shock loadings to the driveline and engine whenever the clutch is engaged.
Before we can transfer torque from the clutch drum to the transmission we'll need to add more parts, starting with the clutch center or hub.
The clutch hub is positioned inside the drum and keyed to the transmission mainshaft most often by splines, although tapered shafts are sometimes used, particularly on older bikes. The hub and mainshaft turn as one, so whenever the bike is in gear, turning the clutch hub will drive the rear wheel. However, the hub and drum share no direct connection, and they'll turn independently of one another-until we link them together.
Clutch plates are that link, and as you might suspect, every multi-plate clutch uses two types; the drive and the driven plates, or as they're more commonly called, friction and steel plates.
The friction plates have tangs along their outer edge that engage with slots cut into the clutch basket. When the engine and basket turn, so do the friction plates. Typically, friction plates are surfaced on both sides with a composite material similar to brake pad lining, although Kevlar and ceramics are also used for high-performance applications.
The driven plates are customarily stamped from steel, and have their centers notched. The notches line up with splines cut into the hub, so anytime the hub rotates the driven plates turn with it, and vice versa.
The plates are installed in an alternating pattern. First comes a drive plate, then a driven one, and so on until the entire assembly, or "clutch pack" is in place. As a point of interest, the terms 'discs' and 'plates' are used interchangeably. Some guys call the friction plates "clutch discs" and refer to the driven plates as "clutch plates," while just as many call them all clutch plates and differentiate between the two by identifying them as fibers (or frictions) and steels.
I'll also mention that many motorcycle clutches incorporate things like "anti-judder" plates or anti-stick springs. Sometimes a particular clutch may have plates that tend to engage intermittently, and this can be exacerbated by a particular engine or driveline design. If judder is present during the design stage, the manufacturer will often incorporate a specially designed plate to eliminate the problem. Likewise, some clutches have a tendency to stick, particularly when they're cold. In those cases, the designer may install anti-stick springs-resembling large O-rings-between the plates to prevent them from binding. Anti-judder and anti-stick springs aren't found in all clutches so don't be too concerned if you dismantle yours and can't find either.