Transmitting Power: Your Clutch System

You'd go nowhere without a clutch

A typical multi-plate clutch assembly
Illustration by Jim Hatch

Clutch: n. 3: a coupling used to connect and disconnect a driving and a driven part of a mechanism." See, there's really nothing sexy to say about a clutch; even Webster's Collegiate Dictionary addresses the process as briefly as possible.

Manufacturers bandy about a new motorcycle’s rear wheel horsepower all the time, but we’re never told how it’s transferred to the wheel from the powerplant. So we’ll attempt a brief explanation here.

To transmit modulations of the engine’s power to the final drive, you must have a clutch. The path flows like this: from the crankshaft, power typically goes through the primary drive to the clutch. From the clutch, power is then transferred to the gearbox input shaft, where it goes to the final drive at the selected speed. In more basic terms, a clutch connects two separate rotating shafts so that they can be locked together to spin at the same speed, or be uncoupled to spin at different speeds. The clutch, when operated with a modicum of finesse, permits a spinning engine to gently engage a non-spinning transmission by introducing and controlling slippage between them. Moreover, all manual clutches operate in pretty much the same manner, be they wet, dry, single or multi-plate.

Clutch system
Illustration by Jim Hatch

The Real Deal
This liaison of power is usually located between the primary drive gear and the gearbox in the motorcycle's crankcase. (see illo 2). The assembly is comprised of spring-loaded plates (called friction drive plates and steel driven plates) inside a clutch basket (housing, see illo 1). The cable running from the clutch lever on the handlebar is connected to an inner release gear. Here's how it works: When you pull in the clutch lever at the bar, the cable or hydraulic mechanism activates a pushrod to press clutch springs against a pressure (spring) plate, which moves it away from the friction drive and steel driven clutch plates. This relieves the spring pressure holding those plates together, allowing them to float apart and spin freely. The running engine is in this way unhooked from the gearbox.

When you release the clutch lever, return springs move the pushrod back against its seat. The clutch springs force the pressure plate toward the clutch hub, causing the drive plates and driven plates to lock together tightly, engaging the clutch with the gearbox. The engine’s primary gear turns the clutch basket, whose slots engage tabs on the friction drive plates. At this point the clutch basket, drive plates, driven plates, clutch hub and gearbox input shaft all spin together, transferring engine power to the gearbox and thus the rear wheel.

Different Strokes
There are several different clutch variations, but most motorcycles run multi-disc wet or dry clutches. These are also known as countershaft clutches because the clutches are mounted on the transmission countershaft. In most cases the clutch is chain- or gear-driven from the crankshaft, which offers the advantage of a low operating speed for smooth high-speed shifting. The biggest complaint with this configuration is its relatively large mass and frictional area, which requires stiffer springs to activate fully. Often, that can mean a lot of lever pressure from the rider.

  • A multi-plate clutch provides a compact yet high-friction coupling between the engine and the gearbox. Multiple plates increase strength and friction in the engaged clutch. Depending on engine size and the weight of the bike, four to eight sets of plates may be housed in the clutch basket.
  • A few bikes use an automotive-style, single-plate clutch known as a flywheel clutch. BMW and Moto Guzzi are often associated with this type—it's mounted on the crank and spins at engine speed. While the flywheel clutch can make high rpm shifting rough, its single plate makes the lever easier to activate.
  • The phrase "wet clutch" means the clutch runs in an oil bath. Otherwise the only difference between a wet and dry clutch is the type of friction material on the plates. Running a wet clutch covered in oil has several advantages:
  1. Since the primary drive needs lubrication anyway, it's less costly than running two separate oil systems.
  2. Debris from clutch wear can be drained with the oil.
  3. Oil helps keep the clutch cool.
  • Since a dry clutch is designed to run without an oil bath, it needs to be a separate, sealed unit to prevent oil entry. Oil can cause clutch slippage and ruin the friction discs.

Adjust Your Slip
Because clutches wear and cables stretch, the clutch must be adjusted periodically to assure proper operation. Incorrect adjustment can cause problems like clutch failure, clunky shifting, lack of power and difficulty in finding neutral. Methods of adjustment vary from machine to machine, but there are usually set threaded adjusters on the cable (hydraulic systems are often self-adjusting). Simple "free play" adjustments can be made at the adjuster—sometimes at the handlebar, sometimes down the cable. Check your owner's manual for locations.

  • You will know the clutch is properly adjusted if there is a small amount of free play at the lever. By free play we mean the distance the lever moves before resistance is felt. If there is no detectable free play, you can't be sure the clutch is fully engaging.
  • Insufficient free play can cause clutch slippage by keeping the plates from closing fully (which can lead to clutch burning and failure).
  • Too much free play may not allow the clutch plates to completely separate, causing clutch drag. If there's an excessive amount of free play, check the bike while stopped, with the bike in gear on a level surface. Hold the clutch lever fully against the grip and without the brake, rev the engine to see if the bike creeps forward. If it does, there's too much free play and the clutch can't fully disengage.
  • The friction material on the friction plate can wear out, and when it's gone, the clutch will start to slip. (But it's only when plates are slipping that wearing occurs.)
  • Once a properly adjusted clutch starts to slip, it should be replaced promptly since the remaining friction material may soon be worn off, stranding you.
  • Some wet clutches are sensitive to the oil you use. Using modern automobile oils with the friction modifiers used to improve mileage may cause your clutch to slip prematurely.

Read your owner’s manual thoroughly to familiarize yourself with the intricacies of your clutch system. It may not be the sexiest aspect of your cruiser, but being aware of its components and characteristics can keep that beautiful horsepower flowing to the rear wheel where it belongs.