Motorcycle Clutches: The Friction Factor

Along with your bike's final drive (which we covered here in February's tech piece), the clutch is a component that doesn't get nearly enough credit. In fact, the lever on the left handlebar gets the nod for smooth starts when it's the assembly in the engine doing all the hard work! And it is a tough job. Without a clutch to couple and decouple the crankshaft and the transmission, you'd have a very hard time starting, stopping, and shifting your bike.

In its simplest form, a clutch is a mechanical device that permits the control of power transfer and motion between two assemblies. In a motorcycle, those two assemblies are the outer clutch basket and the input shaft of the transmission. The clutch basket is turned by the crankshaft, while the input shaft transmits power to the transmission and eventually the rear wheel.

Early clutches (we're talking late 19th and very early 20th century here) were simply a pulley that put tension on a belt running between the engine and the rear wheel. Later, plate-type clutches were invented. These ran dry at first, but nowadays nearly all clutches are of the wet, multi-plate variety described above.

The clutch pack is made up of a stack of alternating steel plates and friction plates. The steel plates have tangs along their inner diameter that engage with the clutch’s inner hub, which is fixed to the end of the input shaft via splines. The friction plates have lugs along their outer edge that slot into the outer clutch basket. Springs (usually of the coil type but occasionally a diaphragm design) press the steel plates and the friction plates together, coupling the outer clutch basket to the inner hub, thereby transferring the crankshaft’s rotation to the input shaft.

Why so many plates? More plates means a greater load capacity for the clutch. To achieve sufficient load capacity on a single-plate clutch (as on older BMWs and current Moto Guzzis), a very large disc must be used, and that takes up a lot of space. Meanwhile, the multi-plate clutch on a Hayabusa uses a stack of smaller-diameter plates and is easier to package. The six springs on a Hayabusa clutch only put about 400 pounds of pressure on the pack. That doesn't sound like much given the tremendous power that clutch has to handle, but that 400 pounds is acting on all 19 clutch plates (10 friction plates and nine steels, plus the hub and pressure-plate faces).

And with friction comes heat, which is why nearly all modern bikes use wet clutches that are bathed in engine oil. The oil cools the clutch pack, keeps the plates clean, and in general provides smoother and quieter operation and longer service life than dry clutches. The oil isn’t needed to lubricate the plates, but it does lower the friction between them, necessitating stiffer springs and/or grippier clutch plates. A dry multi-plate clutch handling the same power can be smaller since oil isn’t reducing the friction between the plates, plus there’s no fluid drag slowing things down. That’s the primary reason Ducati stuck with a dry clutch design for so long, but with ever-increasing engine horsepower (causing issues with smooth engagement and wear) and increasingly stringent noise-emissions requirements, even Ducati has switched to wet clutches.

Clutches haven’t changed a whole lot in the past 50 years, but the pack used to be larger in diameter with fewer plates. Today’s packs tend to be deeper but smaller in diameter. This helps with packaging as well with engine dynamics, as the smaller-diameter pack has less inertia and results in better throttle response. Besides a transition from steel to aluminum for the outer basket, inner hub, and pressure plate, the only other significant advance in clutch technology was the advent of the slipper clutch and its variations. The “Slipper Clutches” sidebar at right explains that technology and its benefits.

The default state of the clutch is engaged, with the clutch springs pressing the plates together. Pulling the clutch lever presses (or pulls, depending on the arrangement) on the pressure plate, compressing the clutch springs and reducing or eliminating pressure on the clutch plates, which allows them to spin freely. However, since the clutch pack and transmission are bathed in oil, there is still some fluid drag between components. That’s why your bike’s rear wheel may turn slowly while your bike is idling in neutral on the centerstand.

The two most common complaints about clutches involve slipping or chatter. Slipping is the result of too little friction, either due to worn or glazed friction plates or improper oil. Some automotive oils contain friction modifiers that will make a clutch slip, while older clutches might slip if modern synthetic oil is used. Modern clutches are happy in any kind of oil, mineral-based or synthetic. Chatter is often caused by too much friction—from a lack of oil between the clutch plates—or by warped steel plates. Sometimes, though, clutch problems aren’t due to any fault of the clutch pack. See the sidebar for the simple stuff to check before pointing your finger at those poor plates.

Twenty years ago slipper clutches were reserved for racebikes, but today they can be found as standard equipment on a variety of streetbikes, including performance machines like the Ducati 1299 Panigale, beginner bikes like the Ninja 300, and even sport-tourers and cruisers.

Slipper clutches incorporate ramps on the inner hub that automatically force the clutch plates apart during hard engine braking, allowing the clutch to slip slightly so as to prevent rear-wheel hop. This is especially helpful on the track when dumping several gears entering a corner (where the slipper, also called a back-torque limiter, also helps abate the effect of engine braking on the rear suspension), but slipper clutches have also proven helpful for street riders as they prevent chassis instability during unintended or sloppy shifts.

A recent evolution is the “slip and grip” clutch. In addition to ramps that push the inner hub out during instances of high engine braking, there are also ramps that force the hub in during acceleration. This puts additional pressure on the clutch plates to help prevent slippage. Since the “grip” ramps squeeze the clutch pack during acceleration, lighter (or fewer) clutch springs can be used, netting a nice easy pull at the lever.

Not all clutch problems are the result of issues with the plates or basket, so if you’re having issues with your clutch there are several things to check before you pop off that engine cover. A frayed, worn, or dry clutch cable that flexes, stretches, or hangs up can cause a variety of disengagement problems. The same goes for a hydraulic clutch with murky or low clutch fluid. Clutch slip, drag, or rough shifting can be caused by too little slack at the lever or the actuating arm on the engine. While you’re checking for proper free play, verify that the lever pivot is properly lubricated and the cable is routed in a way that avoids sharp bends.