WORKSHOP: CLUTCHES

We get to grips with springs, plates, baskets and thrust washers. Yup, Scoop is checking his clutch

Steve Cooper on how to sort them

The use of the clutch dramatically advanced the earliest motorcycles. Prior to its introduction, riders had to kill the engine whenever coming to a complete halt and re-start the motor again to get going. The clutch negated this tiresome palaver at a stroke and made those early machines substantially easier to use and infinitely more popular to would-be riders. From that point onwards the clutch was refined and updated to a point where most riders from the latter half of the 20th century rarely ever gave this key device a second thought. Well, until it went wrong, that is! If you can grasp that the humble clutch works as make-or-break switch between the engine and the rear wheel then you begin to get an idea of the stresses and strains it has to cope with. Now factor in that it acts as arbitrator between the rear wheel and that torque amplifier, better known as a gearbox. Beginning to see just how a clutch works? Not only does it have to carry these two jobs, but it also has to function as a variable ratio drive as you move away from a standing start, slipping the clutch until engine speed and road speed achieve something like parity. Oh, and you like to pull the occasional wheelie as well? Your well-being is now being taken care of by a handful of springs and an aluminium basket holding some metal and fibre plates. Impressed? Well, if you’re not, you should be! When we stripped Project Yamaha CS3C’S engine, it was painfully obvious the bike had been abused… badly. Yet, as the saying goes, it’s an ill wind that blows no one any good. Therefore, my misfortune and mechanical disasters will hopefully illustrate some of the obvious and not so obvious issues to look out for as and when you take a clutch apart.

If a clutch is in tip-top condition then the chances are it’ll perform just as the manufacturers intended. The beauty of Japanese clutches is that they were all designed from first principles, i.e., if a bike makes so many ft/lbs of torque at the crank, when the gear ratios are taken into account the clutch as an entity must be able to cope with the resultant level of increased torque. The total area of friction and plain plates is specifically calculated to do just this. In the Japanese bike world many bikes received their own purpose designed clutches, which means a Yamaha RD125 will run a smaller clutch than, say, an RD200. Prior to the arrival of Japanese machines, our own industry often saddled their bikes with the one-size-fits-all approach when it came to clutches. This, and the frankly perverse use of adjustable pressure plates, often gave the end user a clutch, which could slip and bind at the same time! Taking the time to inspect clutch components and replace as necessary means you’re very unlikely to ever suffer the same diametrically opposed faults.

Thanks to Wemoto for the clutch plates

The clutch-boss, or centre, is where the steel clutch plates slot on to. There’s some evidence of wear or chatter to the alloy dogs, but it’s more visual than physical so we should be OK to re-use this component

8/ This is the kick-starter pinion, which lives on the back of the clutch basket. The teeth of the dogs (upper area) show witness marks, but no appreciable wear. The teeth of the drive gears are in a similar condition, so we’re OK here

7/ In addition to the clutch push-rod moved by the actuator there’s also another one, which pushes on the inner face of the pressure plate, and it needs to be in good condition. The same applies to any plate washers that bear down on the thrust bearing along with the splined washer and nut that hold the primary drive cog on the end of the crank

6/ Often overlooked, but vital to smooth, vibration-free operation ,this phosphor/ bronze bearing at the heart of the clutch basket needs to be in tip-top condition. Just another reason why fresh oil is vital to both gearbox and clutch

9/ The steel clutch plates need to be flat and free from rough edges. A minor tidy up is permissible, but if they are obviously warped, corroded or blued, they need to be replaced. Check for flatness on a piece of glass sheet using feeler gauges; your manual will have the specifications

4/ Now this really isn’t healthy! The fingers of the friction plates have seriously eaten into the sides of the clutch basket. It’s an instant case of sticking plates, random clutch lift and jerky transmission

3/ Here’s one that someone else broke earlier! Apparently insignificant fractures can quickly become serious failures

2/ Easily damaged or broken if mishandled, the pressure plate is effectively the lid of the clutch system. Check for fractures or damage to the alloy towers that home the clutch spring screws

5/ Hard to believe, but this second clutch basket is even worse. How did anyone control a bike with a clutch like this?

OEM clutch parts are often either fiercely expensive or no longer available. Fortunately, the likes of Wemoto offer high-quality plates and springs

10/ No question about these friction plates, then! There’s normally a wear (thickness) limit specified. Before installation, soak new clutch plates on gearbox oil overnight 11/ These cushion rings damp down clutch noise. They also assist with plate separation, due to their C-shaped cross section. If they’re a standard fitment don’t build a clutch without them 12/ Ensure the spring screws have good threads and are all the same length/diameter. Clutch springs have a minimum set length, so replace all of them if one or two are below par. Five new springs are significantly cheaper than ten new clutch plates!