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.
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.
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.
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.
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.
Tying It Together
At this stage, running the engine will cause the clutch drum and its drive plates to turn, which will then try to drag the driven plates and hub along with it. Unfortunately, there's not enough friction between the plates to accomplish anything except create heat and make noise. What we need is something that allows us to lock and unlock the plates together as required. That something is the pressure plate, which works in conjunction with the clutch springs to accomplish the task.
As a rule, the pressure plate will be located at the top of the clutch pack (though some designs place it at the bottom) and in most instances, the plate will also have a mechanical connection to the clutch hub.
Most often, several small coil springs are positioned evenly around the pressure plate's circumference. In this arrangement, the springs are held in place by bolts that pass through them and the pressure plate into the clutch hub. Alternatively, one large diaphragm spring fitted atop the pressure plate
may be used. These have grown in popularity, mainly because they provide lots of pressure at the clutch without requiring a Godzilla-like grip at the lever.
The springs do the heavy lifting. When the clutch is engaged, springs apply tension to the pressure plate, which in turn compresses the drive and driven plates together, causing the basket and hub to turn as one. Power flows from the crankshaft to the clutch basket, through the drive plates to the driven plates, and then from the driven plates to the clutch hub and into the transmission.
Pulling the clutch lever causes the clutch release mechanism to work against the pressure plate compressing the springs. As the springs compress, the tension placed by the pressure plate against the clutch pack is relieved. Without spring tension to hold them together, the plates are free to slip past one other, which disconnects clutch basket from the hub, and the engine from the transmission. Understand that it doesn't take much to relieve spring tension; as soon as the clutch release pushes against the pressure plate and those springs start to compress we only need a few thousandths of an inch between the plates before they disengage.
Car vs. Bike?
After all that, you may wonder why the majority of motorcycles use such a Rube Goldberg-style contraption to transfer torque, especially if you're familiar with the common and fairly simple automotive-style clutch.
A clutch's ability to transmit torque is directly related to its size and the amount of spring tension on the pressure plate. As torque flow through a clutch increases, we have to increase the surface area of the clutch or the amount of spring pressure applied to the clutch pack.
We could use one big automotive-style clutch plate, but given modern motorcycles' prodigious torque output the thing would probably end up being the size of a dinner plate. Or we could use very stiff springs to put a lot pressure on a smaller plate, but you'd need forearms like Popeye to pull the thing in. However, if we build a clutch with a large number of small plates, we achieve the surface area required, and it won't take a whole lot of spring pressure to clamp the thing together. So that's what manufacturers do.
As we mentioned, clutches can be run dry, and in certain applications (primarily racing) where an external clutch can be easily serviced without draining oil, that has some advantages. However, dry clutches tend to be grabby and noisy at idle, which makes them more of a sport bike affection than a real advantage on the street.
Wet clutches have a sweeter, more progressive engagement, and are quieter than their dry counterparts. They also simplify the primary drive architecture because there's no need to prevent the oil from reaching the clutch. This is why they're the clutch of choice for the majority of motorcycles, including most sport bikes.
When It Goes South
Clutches are subject to only two malfunctions: they can slip or they can drag. And yes, they sometimes do both at the same time.
The more common problem is slip. Slip occurs when the clutch can no longer handle all the torque it's being asked to deliver. Typically, the first sign of a slipping clutch is a tendency for the engine to flare, or rev up slightly when the clutch is released in the higher gears. As the situation deteriorates, you may notice engine rpm increases disproportionately to road speed when you twist the throttle, especially in the higher gears, where the clutch enjoys the least mechanical advantage and the strain on it is greatest.
Clutch slip can be caused by a variety of things including normal wear and tear and abuse, but one that always jumps out is improper adjustment. All clutches need some amount of slack in their release mechanism. Somewhere around 3mm (1/8th of an inch) generally works well; much less than that and there's a chance that thermal expansion will cause the release mechanism to apply pressure to the clutch pack when the clutch is at operating temperature, causing premature wear.
Once a clutch starts to slip its days are numbered. You may be able to nurse the bike along for a few miles, but you better start making plans to replace it (see the March 2009 issue of Cruiser for the full skinny). As a rule, you'll be replacing the drive plates, the clutch springs and quite possibly the driven plates as well, although they can certainly be reused if they're up to snuff.
Clutch drag happens when the plates fail to completely release. A dragging clutch will make it difficult to change gears, and if it's bad enough, may allow the bike to creep forward when in gear. Drag occurs when there's too much free play in the clutch release mechanism or when a mechanical problem prevents the clutch plates from fully separating. Outside of the obvious things, like an improperly adjusted clutch or broken release mechanism, drag is often caused by wear to the clutch hub and basket. If the fingers or splines develop grooves or notches, the plates may hang up or twist in them and fail to fully release when the clutch is disengaged. Overheating the clutch can also swell the plates and cause drag, though everything should work normally once things cool off.
I should mention that, by nature, wet clutches suffer from something called viscous drag, particularly when they're cold. Viscous drag occurs when the viscosity of the cold oil creates enough drag between the plates to transfer some torque, even though you're holding the clutch in. If the drag is severe, punching the bike into gear can result in a loud clunk and maybe a small leap forward, or even a stall. Normally, this is only a problem when the bike is started for the first time on a cold day, or too heavy an oil is used, but the solution here is simple. Just let the bike warm up for a minute, preferably with the clutch disengaged, during the first start of the day, before putting it in gear.
As you no doubt now realize, clutches aren't particularly complicated devices. They don't demand much attention and most will put up with enormous amounts of abuse before calling it quits. Had they not been developed, it's unlikely that the motorcycle-or any other form of internal combustion vehicle, for that matter- would have progressed very far. -MZ