Minimoto Garage - Clutch Setup and Adjustment

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Clutch Setup and Adjustment
Big thanks to Pacific Pocketbikes, along with Vince Rolleri for this guide.
Since all currently manufactured pocketbikes use a two-shoe, friction-type stall clutch, we will focus on this specific version.

Most bikes provide for fairly straightforward clutch access. On a standard Polini motor, the clutch is on the left side of the bike, and is accessed by removing the four M6 socket-head cap screws (SHCS) which retain the clutch housing. Note that the removal of fairings, footpegs, and other bits may be required to access the housing.

Inside the clutch housing you will find the clutch ‘bell’ (aka ‘drum’), which is the driven part of the system and is connected directly to the pinion gear. The bell is retained by the pinion itself, and should first be inspected to ensure the inside surface (where the friction material makes contact) is smooth and even, and that the movement smooth. Excessive wear (grooving, channeling, or any roughness) will necessitate replacement of the bell for peak clutch performance. If the movement has rough spots, it may be time to replace the bearing that supports the bell.

With the clutch housing removed from the engine, the clutch assembly will now be visible. Residing directly on the crank, the assembly is comprised of a main ‘backing plate’ which holds two clutch ‘shoes’. Each shoe pivots on a pin that is fixed to the backing plate and is preloaded against radial movement by a spring. The spring pushes (in compression) on a bolt that is fixed to the ‘free-end’ of the clutch shoe, thus retaining it from rotating out and contacting the clutch drum when it is not spinning.

When the engine starts to spin, the centripetal force causes the spring to compress and thus the shoe rotates out to contact the clutch bell and transmit the engine’s power to the pinion. The small, black, plastic bits next to the springs are there to keep the springs themselves from bowing out radially under centripetal force and need to be replaced when they become ‘imprinted’ by the springs to assure smooth spring action. So far, so good?

Now, with a general understanding now of how the system works, there are four basic adjustments that can be made which, together, will dictate the manner by which the power is transmitted from engine to rear wheel: clutch spring size, clutch spring preload, clutch shoe material, and clutch shoe weight. I put these in the order that I would rate them for importance.

The most basic clutch adjustment is the clutch spring size (actually, the thickness of the wire used to fabricate the spring). The primary effect of spring size is to dictate the spring’s working range with respect to the force it can apply in retaining the clutch shoes. All else being equal, a thicker spring will require more force to compress the same distance compared to thinner spring. There is also a secondary effect that I refer to as ‘immediacy’. A smaller (lighter / thinner) spring will transition from just barely engaged to fully engaged over a smaller rpm range than a larger (heavier / thicker) spring. This is based on the spring constant which is larger for a thicker spring.

For setup, it is requisite you have a spring large enough to provide adequate spring preload given the range over which you can adjust (description to follow). Otherwise, it is simply rider preference on the immediacy that will dictate the spring you choose. The slower the engagement, the easier the bike will be to ride at the expense of some outright acceleration.

Closely related to the spring size is the spring preload. This is the amount of force the spring is ‘pushing’ onto the clutch spring bolt that retains the clutch shoe. The preload sets the rpm at which the clutch engages. For example, to have the clutch engage at a HIGHER engine speed, the spring preload is INCREASED by tightening the adjusting nuts (the nuts that hold the clutch spring bolt in place). Figure an increase of ~150-200rpm per flat of the adjusting nut (one full turn = 6 flats).

So, if the bike feels like its ‘bogging’ coming out of low speed corners and the clutch engages well before the engine’s powerband, it will be necessary to tighten the adjusting bolts, thus shifting the engagement point higher and closer to the onset of the powerband. Conversely, if the clutch doesn't engage until the engine is screaming, or if the clutch feels like it comes in a bit ‘too violently’ then you may need to relax things by loosening the adjusting nuts.

It is important to always have the two shoes adjusted as closely as possible. There are many ways to approximate this, but my preferred method is to completely loosen the bolts. Then, snug the bolts up until each shoe just loses its free play. At this point, you just need to turn the bolts in/out equally until the desired response is attained. Next down my list is clutch shoe material. There are shoes available from several sources, both Polini and BiZeta, as well as the possibility of having your old shoes ‘relined’ (i.e. taking your old shoes and putting new friction material on them – performed by a professional). The material can have serious effects on how the shoe grips, wears, and responds to heating. Again, this is going to be a matter of personal preference. Some shoes (e.g. Kevlar lined shoes) have much longer useful life spans – thus requiring less fettling, so make sure that is factored in when shopping around. A quick word on relines: though the friction material is completely replaced and thus ‘renewed’ the mechanical aspect of the shoe – particularly the bore about which the shoe pivots – can be excessively worn and cause problems. It is very important that the shoe rotates smoothly and with little-to-no play (this can be verified with the spring preload completely removed). If there is excessive play, the shoe will tend to ‘twist’ instead of rotating out and meeting flush with the clutch bell, the result being uneven wear to the shoe and bell.

Finally, the clutch shoe weight will work with spring preload and spring size to dictate the immediacy and engagement point. All else being equal, a heavier shoe will engage earlier and have more immediacy than a lighter shoe. I have found this adjustment to be something mainly done at the onset of clutch setup, as once I have a setup that feels good, I just tweak on the spring preload a bit as the shoe wears and loses some of its weight. Some clutch shoes come in different weights, some support this by allowing additional weight to be added via small set-screws. However, the stock Polini setup does not afford any change (aside from lightening the shoes by removing some material), so shoe weight is not a universally applicable aspect to setup.

There are other adjustments worth mentioning. One is using different size clutch drums. I prefer to work with a clutch that wears as little as possible, but when running the standard Polini shoes, the wear rate is rather high, and this may encourage you to change the size of the drum (make it smaller) to accommodate the ‘shrinking’ of the shoes as the material wears away. Essentially, it allows for a slightly more ‘even’ wear pattern to the shoe – from ‘heel to toe’ so to speak, and reduces the amount of spring preload fettling required.

Another involves the possibility of different weight backing plates. The weight difference changes the rotational inertia of the plate and thus its ability to store energy. The more weight a backing plate has, the more energy is required to change its speed so it takes longer to accelerate the engine. However, the increased inertia tends to have a ‘smoothing’ effect on the way the engine pickup is perceived by the rider. I have also heard that the different weight can affect the maximum engine RPM, however I do not have much first hand experience with this effect.

As a final note, I highly recommend using a tachometer to get some quantitative feedback as to the RPM at which the clutch is engaging. In the long run, it will definitely save some time. Also, I recommend getting input from other riders and using it as a baseline. But, like most setup, “what works for one may not work for another” as the final setup is dependent on too many things (bike, track, rider, etc) and the bottom line is getting the bike to feel right for YOU. That is something only you can good luck and start experimenting!
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