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Leics. LE7 3RE
UK

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  The Scott 3 speed clutch.

First an anecdote.

In the mid 1980’s I found myself in Durban, South Africa over a weekend.  I had done my business calls on Friday, but had another call the coming Monday, so had to stay in Durban over the weekend, rather than fly up to Joburg as usual.  I considered what I could do over the weekend and consulted my VMCC membership list.  I phoned a member and asked if there were any activities on over the weekend.  We compared notes on our interests and learned that he owned a Scott.  He told me that he had problems with the clutch as it both slipped when in drive and dragged when disengaged.  He was very nervous about stripping it, as the previous owner had given him strict warnings that to set up a clutch was a “Black Art” and he should touch it at his peril.  I duly spent part of my weekend rebuilding his clutch.

Ok lets start at the beginning.  If you have similar problems with your clutch, I suggest you proceed thus:-

1)                  Remove rear chain

2)                  Remover outrigger with sprocket

3)                  Loosen special nut that secures clutch centre to gearbox sleeve gear, if you have long box spanner.  You can use the primary chain connection to stop the clutch revolving whilst you do this by restraining the flywheel. 

4)                  Remove primary chain

5)                  Remove any rear mudguard fixings that attach to undertray.

6)                  Loosen undertray rear securing bolts.

7)                  Loosen gearbox lower fixing nuts.

8)                  Remove undertray front securing bolts.

9)                  Swing down undertray

10)               Remove gearbox from tray

11)               Remove clutch spring nuts and springs

12)               Remove all plates and  clutch basket. It is better to keep all items in order.

13)               Remove clutch backplate retaining nut and backplate to expose clutch release mechanism.

14)               Remove clutch release mechanism.

15)               Clean thoroughly all items in paraffin using a scraper and stiff brush to remove hardened deposits.

16)               Examine condition of clutch release worm gear.  The lever is made in brass and can wear badly. It works against a steel inner, but sometimes this is made of brass also.  Where both items are brass, the wear rate is generally greater.  If it is in reasonable condition, then lubricate and refit.

17)               Lubrication of clutch release and clutch plates is effected by using “Dry Moly Paste”. Get an old 20mm paint brush with short stiff bristles (about 25mm long) and having put some dry moly on a piece of card, work it into the brush bristles. Now brush on a light surface coating on to the working surfaces of the clutch release components before refitting.

18)               Check out the condition of the thrust race components. Using point contact balls on flat faces as a thrust arrangement is not a normal procedure as the high point loads can produce premature wear.  If you have significant wear here, it is possible to replace the original thrust race with an alternative modern thrust race.

19)               Next item is the clutch rear plate and clutch hub assembly.  The plain clutch plates, including the back plate are inclined to become burnished and so become less effective to grip the friction surfaces of the friction plates.  It is good practice to break the glaze by using some medium / fine emery cloth in a circular motion until the surface has an even matt finish. Now wash out all grit carefully.           A good alternative is to have all plain plate surfaces lightly sand blasted.  This saves hand work BUT the rear mounting plate / hub assembly has a bearing surface in its construction upon which the clutch basket roller bearing runs.  If you decide to sand blast this plate, be sure to mask off this bearing surface with masking tape and inform the operator to be sure not to blast this bearing surface.  Paint a thin deposit of dry moly on to the splined hub including the sides of the splines

20)               Clutch basket.  Check condition of primary drive sprocket and inner diameter of rear wall that comprises the outer bearing track of the roller bearing that allows the clutch basket to revolve freely on the back plate when the clutch is withdrawn and the plates unclamped.  Be sure to check the condition of the rollers and the cage, often referred to as the “necklace”  Check that the “Ferodo” coin type friction inserts are serviceable and are still at least 1.5mm longer than the thickness of the clutch basket back wall, so that no metal to metal contact will take place. Paint dry moly on to the bearing surface and on to the sides of the drive slots where the tongues of the friction plates engage.

21)               Drive tongues.  The drive through a clutch is effected by clamping together two sets of plates, one being driven via internal dogs / tongues and the other set via external dogs / tongues.  The plates were manufactured by being stamped out by press tools.  The ideal characteristic of metal for this process, if damage and rapid wear to expensive tools is to be avoided, is one that is soft and plastic in nature.  This type of metal is not ideal for use in clutch applications, as the drive tongues can easily become deformed by the operating pressures.  Unfortunately, the realities of manufacturing costs have dictated that this type of metal is used.  There are modern alternative metals and processes that can be used, but anyone needing to enhance overall clutch capacity should contact us, as we are dealing with standard clutches here. 

22)              Plain plates.  Check that they are flat within 0.5mm.  If several plates are not flat, they will together act as disc springs and the clutch may never free completely and so will “drag”  Now give very careful attention to the inner dogs.  You may remember from school, the law of physics of “Force times distance”, well all we need to appreciate, is that the inner dogs on the plain plates that engage in the clutch centre hub, are at a smaller distance from the clutch centreline of rotation, than those of the friction plates, which engage in the clutch drum outer rim.  You will notice that the deformation of the dogs / tongues on the plain plates is greater than on the friction plates for this reason.  When the drive faces of the dogs are deformed, they will be spread, as though they have been hammered, as indeed they have.  They drive faces will have spread both sideways and on diameter.  In some cases, they are so deformed that the increase in diameter results in the deformed section becoming tight in the bottom of the slots in the clutch inner hub.

                                                                                  

           You should carefully file off this deformed metal so that each plain plate will slide without sticking in the inner clutch hub splines. Now look carefully at the faces that drive on the sides of the spline slots in the clutch hub.  You will notice that as force has been applied, the drive faces of the plate have been indented to the profile of the slots of the hub.  You will see that a part of the original face remains as a step forward from the deformed section.  This can grip the Outside diameter of the hub in use and prevent free sliding of the plate.  File this step section level with the deformed front face so that the plate will be free to rattle a bit on the hub.  The dogs will also have deformed sideways so as to appear to be noticeably wider than the original metal section. DO NOT reduce this deformed side thickness by filing, as now it is wider, it will deform more slowly in the future.  It will not impede the free action of the clutch. Brush dog edges only with dry moly, do not get on operating faces, the plates are now OK to assemble

23)               Friction plates.  The traditional Scott pattern consists of a plate with dogs on its outer edge which are driven by the clutch drum and a number of holes into which “Ferodo” friction coins are fitted.  The dogs will not suffer deformation to the same extent as the plain plates, but depending on engine power and usage, they will be deformed and they should be treated in the same way as the procedure for plain plates.

24)               Alterative plates.  I use clutch plates bought from Ian Pearce, which have a continuous ring of friction material bonded on.  Both the plain and friction plates are slightly thicker than the Scott standard item and I have found them unaffected by oil contamination and give excellent service.  Ian also will have such a ring of modern friction material bonded onto a customers clutch basket / friction plate so as to upgrade the entire clutch friction arrangement.

25)               Pressure plate.  The Scott pressure plate is adequate for all normal conditions, but if the engine is uprated for output, or a sidecar or heavy pillion passenger is to be carried, then some clutch slippage might be experienced.  Tightening the special bolts that preload the springs can have some effect, but the pressure plate tends to deform like a dish under spring pressure.  In this case most of the driving load is taken at a smaller radius nearer the hub.  Remember the force times distance bit?  A simple solution is to screw am extra plain plate to the inner face of the pressure plate to double its thickness and make it more resistant to bending.

26)               Springs pressure.  The clutch serves to transmit movement and power, but it can serve a second very beneficial purpose.  It can act as an effort limiter.  If you wind up the clutch springs tightly, are you applying more force than is needed to transmit the engine power?  If you are, then the force taken to operate the clutch lever will be excessive and the wear on the clutch withdrawal worm assembly will be greatly accelerated.

27)               You should now be able to assemble your clutch and rebuild into your bike by reversing the dismantling procedure.

28)               Adjustment.  Tighten springs so the tops of the threaded studs are about 4mm below the top of the nuts.  Pull in clutch and with bike on stand, rotate the rear wheel.  If the clutch pressure plate does not rotate true, then adjust the individual springs until truth is achieved.  Take the bike out and try it.  The clutch should drive up a good hill without slipping, but if you let the clutch in quickly, it should no be a violent engagement, as the clutch will slip a little on engagement. If you make a mistake with a gearchange, then the ability of the clutch to slip slightly under unusual shock loads, will protect the gearbox internals from damage.

29)               Broken clutch cable.  Years ago you saw the sign of a man who did long journeys on his bike, was that he had duplicate cables taped to the working cables in case of failure.  I remember seeing a gearbox where an owner had suffered a broken clutch cable.  Not having a spare, he put it in gear, pushed and jumped on.  He completed his journey by stamping through the gears.  The damage to his box was quite dreadful.  If you find yourself in similar circumstances, slack off the clutch springs, so you can continue at a modest speed, but when you stamp the gear through, the clutch will slip and save damage.

30)                Options.  The worm type of clutch release mechanism is prone to wear and is heavy in operation. A smoother and lighter alternative has been known for many years and is available. A modification to the gearbox to replace the bronze output bush with a needle roller is advantageous and an oilseal is easily fitted to the gearbox if desired. 

Service intervals.  If you want your clutch to operate properly, you should service it every 5000

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email roger@mossengineering.co.uk or richard@mossengineering.co.uk