Minimoto Garage - A Beginner's Guide to the Clutch

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A Beginner's Guide to the Clutch
Big thanks to Jaz, from MiniMotoClub Forums for this guide
The following information is assuming you are working on a Polini engine fitted with either standard or Bi-Zeta 2 shoe clutch, although the theory is the same for any centrifugally engaging clutch. The pictures show a varied range of clutch types.

On the left hand side of the engine (as you are sat on the bike) behind where the chain travels around the small sprocket is the clutch. The clutch is ‘dry’ meaning that it does not run in oil like most road going bikes and, because the engine has no gearbox, there is no gear oil to drain anyway.
The clutch casing is attached to the engine crankcase by four bolts. It maybe necessary to first remove other exterior items such as a chain guide or guard and, if fitted, any lower bodywork. Once these items are out of the way the four bolts can be removed and the cover will come away from the engine case. It is not usually tight but may require some gentle persuasion using a soft mallet or similar tool.

Once free from the engine the clutch casing can be moved towards the rear of the bike allowing enough chain slack to remove the chain from the front sprocket. On some water-cooled models you may also have to remove the small rubber belt that drives the water pump from a pulley next to the sprocket.
Having completed the above the clutch casing should be free from the bike.
Note that the casing contains the clutch drum on its inside. The drum is connected to the sprocket (and water pump pulley on some models) and both should rotate freely on the bearing that is pressed into the casing. The inside face of the drum should be smooth with no ridges. Ridges can be formed by excessive drum wear characterized by a wide, flat ‘trough’ where the clutch shoes have worn into the drum.

Drums are available in three sizes of diameter, 80, 79 and 78mm. As the clutch friction surface wears and becomes thinner a smaller drum can be accommodated. The shoes can be adjusted to offset the wear but this is not as good as using a smaller drum. Alternatively, replacement shoes can be fitted.
The sprocket is simply screwed into the drum on a conventional right hand (clockwise) thread. To remove it the drum has to be suitably gripped and, using a special sprocket spanner, the sprocket undone. Often easier said than done! This is not necessary to carry out clutch adjustments.

Looking at the engine you will see the clutch assembly that is mounted on the crankshaft. The whole clutch assembly can be removed.
Firstly, the nut on the end of the crankshaft has to be unscrewed. A special holding tool is available that prevents the crank from turning when slackening the nut, the nut takes a 13mm spanner and is a conventional anti-clockwise thread to undo (see picture). Once the nut has been removed the clutch assembly is ready to be released from the crank. A special puller is available that mounts to the backing plate through two threaded holes either side of the retaining nut. Both tools are available from minimoto specialists.
The clutch assembly comprises of a backing plate, two clutch shoes, two springs and the associated retaining parts. At one end each clutch shoe pivots on a pin that is part of the backing plate. At the other end each shoe is retained by a threaded pin that passes through a spring that holds the shoe in place. Sounds complicated when written down but makes sense when you look at it.
To strip down follow these steps;
Remove the plastic spring guides. These are two blocks, usually made from black plastic mounted at the side of each spring. These can wear to form ridges that the springs’ coils snag on impeding clutch engagement. When this happens they need to be replaced.
Release the spring adjuster nuts, collared washers and springs. The collared washers are sometimes referred to as ‘top-hats’ because of their shape. It’s worth noting the original position of the nuts by measuring with vernier calipers or similar so that on re-assembly you can set the adjustment as it was.
Release the shoes by removing the circlip and, if required, the spring retaining pin. Some shoes (as the one pictured) can accommodate weights that take the form of threaded grub screws. It is also worth noting regarding the pivoted end of the clutch shoe that both the pin and the hole in the shoes can wear so that the shoe will move with so much play that the clutch will engage from the wrong end.
When the engine is running and the crankshaft is turning the backing plate the shoes are affected by centrifugal force. Depending on the type of springs, their adjustment, the weight of the shoes and the rate at which the engine is turning (revving), the centrifugal force will eventually be greater than the force of the springs that are holding the shoes in the opposite direction. When this happens the shoes begin to move outwards and make contact with the clutch drum. The engine begins to drive the drum that, in turn, drives the front sprocket that drives the rear wheel sprocket that drives the wheel!

The clutch performs the important role of transferring the engine power to the rear tyre. Its operation, therefore, is fairly critical and that is why you see so many posts at MiniMotoClub regarding it.

SPRINGS – There are several different spring weights available. With regard to the springs, the term weight refers to the spring’s strength. A light or soft spring is one that is easy to compress. A heavy or hard spring is one that is difficult to compress. The spring weights available for a minimoto clutch are relative to one another and there is a range of several springs from the softest to the hardest. Polini grade theirs using a numerical index while Bi-Zeta use a colour code. I have not got the definitive answer to how many types there are but I have at least six separate weights to choose from in my toolbox. Visually, it can be difficult to tell between them but (normally) the harder the spring the thicker the wire is. The spring length, material and frequency of turns per given distance plays a part in this as well but I will avoid unnecessarily burdening you with a complicated physics lesson on spring pre-load. In a nutshell, a heavy spring will resist the centrifugal force more than a light spring. A heavy spring will result in the engine needing higher revs than a softer spring before the clutch engages. What are the best revs I hear you saying? It depends on your engine type, the way it produces its power, rider weight, and rider preferences and, to some extent track conditions. With so many factors it is impossible to give an exact rev figure to use.
SPRING ADJUSTMENT - Once fitted, you are able to adjust the springs so that the clutch engages at higher or lower revs. The nut that retains the clutch shoe through the spring can be turned to either compress or extend the spring length. This assembly has been designed for this purpose; the nut is not to be tightened all the way down. The nut should be of a locking type so that it grips the threaded bar and cannot vibrate loose. At least one thread of the bar should always protrude from the top of the nut to ensure it is locked onto the bar. The lock nut requires the use of a 7mm spanner to turn it. The more the spring is compressed the higher the engine will rev before engaging the clutch. The adjustment available is quite wide and, at the extremes, it is possible to adjust springs to within the range of a heavier or lighter spring. As a fairly reliable guide, one-sixth of a turn, corresponding to one side of the nut, increases (or decreases) the clutch bite by 150 revs per minute (rpm).

CLUTCH SHOE WEIGHTS – Bi-Zeta clutch shoes allow for the fitting of additional weights. Polini shoes are easily modified to take weights. Each shoe has a threaded hole in it, into which can be placed a small weight that takes the form of a threaded grub screw. It might not seem much but the effect of these weights can make a big difference to how your clutch operates. Each shoe must carry the same amount of weight to ensure balanced engagement of the friction material. Weighted shoes engage earlier than un-weighted shoes and you will have to change or adjust your springs to compensate. The purpose of weights is to reduce the amount of clutch slip between the initial bite and full engagement. The heavier the weight, the less slip you will have. Too much clutch slip and your engine works too hard to achieve drive and will not make the most use of the engine’s power. Too little clutch slip and your engine bogs as it tries to drive. The differences in weights are tiny and represented by using different sized screws.
Like spring weight and adjustment, it’s a balancing act based on your engine type, the way it produces power, rider weight, rider preferences and, to some extent track conditions. Many riders believe that weights are an unnecessary complication and that all the adjustment needed can be achieved with spring adjustment.

Once you have taken in the principles of Spring Weight, Spring Adjustment and Clutch Shoe Weights you are ready to experiment with setting it all up. A properly set up clutch is the best tuning you can do to a minimoto.

Setting up your clutch requires making lots of adjustments, one at a time. At the very least, you will have to remove and replace the clutch case and drum assembly every time. It is possible to replace the springs with the clutch assembly in place but some prefer to remove it.

All you need now is a rev counter. It is not essential but extremely useful when making the adjustments to know the rpm at which the clutch is engaging by keeping one eye on the rev counter and the other on the rear wheel. Only buy a rev counter from a minimoto specialist. My advice is to buy one that is proven to work. They start at about £50 [$85].
This is the basics! Once mastered, you can move on to different friction material.

Three types of backplates. The first two are Bi-Zeta and the third one is Polini. The blue one is Bi-Zeta’s lightweight plate.
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