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Peter Watson sheds lightthe workings of thenotorious Velocette cl

MVSTERIEOF THE MECHANIS'YOU can always tell an experiencedVelo rider bythe way he puts the bikeinto neutral just before he comes to ahalt: explained Geoff Dodkin. Thehighly respected Velocette dealer wasstanding behind the counter of hisLondon shop which was strewn withthe component parts of a Velocettesingle's clutch, dismantled for myedification. 'You should never hold theclutch in on a Velo: he concluded.

Although l'd learned a lot that wasnew to me about the infamous andunorthodox desiqn in our two-hourteach-in, this much was very familiar.It echoed those last-minute

instructions, occasionally spiked withthreats for emphasis, that I've alwaysreceived upon making off aboardborrowed Velo singles.

The Velocette clutch - and especia Ilyits thrust bearing, whose life isshortened by prolonged engagement- once loomed large in motorcyclingdemonology. Even today it's probablymore misunderstood than themajority of married men. Given theunconventional manner in which itoperates and the demands it makesupon an owner, that's hardlysurprising.

As a basic design, itwas introduced

in 1922 when Veloce Ltd built nothimore powerful than lightweighttwo-stroke singles displacing less250c,c. Forty-eight years later, asmotorcycle production at Hall Grground to a halt in 1970, a clearlyrecognisable descendant was se .the transmission requirements of41bhp 500cc Thruxtons with a topspeed of around 110mph.

The Velocette layout floutsconvention in numerous ways, mnotably by mounting the gearboxsprocket outboard ofthe clutch. Sofamiliar relationship between primand final-drive chain lines is reverse

Clutch is disengaged when pressure from handlebar lever is applied to thrust gate at point shown byarrow. Pressureis transferred to frontplate by three pins acting against small coil springs. Spring holder is threaded into front plate

18 Classic Bike

coil springs

front

plate~J.lrz:----~~t~&springholder ,1.Öitlrff11n11

thrustrace

-: I 111111111111. IdH't1lI/thrustpins

back ~_----r~nplate- , .

Cross section of operating mechanism

This naturally calls for a vet» si imclutch.lt also rules out a conventionalactuating rnechanism such asthe longpushrod which passes through thegearbox mainshaft to lift a deeppressure pleite in its centre and allowthe clutch plates behind it to separate.

Since you have already begun to askwhy, I had better tell you. The initialimpetus was provided by the heftyoffset to the .Ieft of the two-stroke'scrankcase.ln orderte fit a clutch to anexisting engine design it was simplycheap and convenient to adopt thisodd-Iooking layout with its cork-facedchainwheel.rsinqle-plate clutch,

Had it later been dropped, ratherthan developed, it would doubtlesshave been dismissed as a passingeccentricity ivplcat of the 1920s whenall manner of strange designs enjoyedabrief life. Ihstead, its expedientconception is forgotten by many wholaud virtues unearthed later to justifyits continued existence.

Doubtless. there were rnornents inthe development of the ohc K seriesand the later pushrod singles :....especially the introduction of thespring-frame MSS 500 in 1953-whenthe clutch begged to be binned. Ashortage of cash was probably themost powerful argument againstchange. :

In their ultimate form, the clutchcomponents and operation are simplydescribed. Inside the gearbox shell isa cable-operated bell crank. Whippingin the clutch lever causes the crank tomove a short pushrod which protrudesthrough the gear case.

August 1986

This rod iri turn bears on the free endof a hinged "thrust cup' ~ Geoff Dodkinprefers tocall it a 'gate' =whose.shapeis reminiscent of a frying pan with ahole in the jnlddle. Into this housingslots the three-piece thrust bearingconsisting of a bevelled-face steelwasher, a high-tensile brass ball-raceand a plain steel thrust washer.

As the gate moves on its hinge, thisother washer bears on three short'thrust pins'. These pins-reminiscentof big-end rolters=-Iift the clutchsprinqcarrier, and the front clutch plate intowhich it is threaded, acting aqainst 16(20 in the esse of the Thruxton) smallcoil springs. Because the gatehingesout from the.qearbox shell on one side,the clutch plates on either side of thechainwheel are notfreed instantly, butpeel off after one revolution.

That spherical bevel on the face ofthe inner thrust washer is intended toprovide cornpensatlnq allqnrnent,Given the shortage of space, ,everythinghas to be adjusted withlnfine tolerances, These cornrnence withthe original, assembly of thetransmission. There is a choice of twopushrod lengths, and the hinge bfthegate - secured bya wire clip -visshimmed out so that it is parallel withthe face of the gearbox shell atprecisely half lift.

The bell crank and pushrodarrangement replaced an original facecam and quick-thread rnechanisrn in1927. Pre-war friction plates werecork-faced, but in post-war years the22 inserts were filled with Ferodomaterial. Naturallv the number of

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'"-<:Jl

Qo:Jlo~oz

t .Adjustment. The1J special tool is'l i passed through a

i hole in thesprocket to

engage slot inspring holder.

Spring edjustmentis then affected by

turning the backwheel: anti-

clockwlse forrnore grip,

clockwise formore free

movement

plates increased, first to four and then,with the launch of the spring-frameMSS, to six.!

The whole clütch is splined to thesleeve gear of the gearbox andsecured by a large nut that requires aspecial peg spahner for rem oval andreplacement. The Ferodo-linedchainwheel is supported on a largediameter, but naturallv rather narrow,ballrace.

1'11 return to rriodifications in' amoment where :~hey impinge onpresent-day restoratlon, but first aword aboutthe perennial task otclutchadjustment. Because adjusting 'aVelocette clutch requires an entirelydifferent technique to that employedon conventional designs, you are weiladvised tofollowslavishlythe drililaiddown in any handbook. Unfortunately,the very last thing most owners arewilling to do is read their handbook.That's why Veloce used to 'hide' theclutch cable adjuster on road modelsunder the fuel tank.

Cornpensatinq for slip or drag is afairly weird procedure for theuninitiated. What you do is to put themachine on its centre stand aridremove the gearbox sprocket cover onthe outside of ttie primary chaincase.The sprocket itself sports three holesin its face.

Producing Velocette tool KA 62/2 -or a convenientsix-inch nail - youpoke this through one of the holes.Wh at you're hoping to locate is one ofthe slots in the castellated springcarrier I mentioned earlier. With thegearbox in neutral, you achieve this by

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poking away while rotating the rearwheel.

For slip, you wanttowind the carrierin an anti-clockwise direction. You dothis by wheeling the bike forward withthe tool/nail in position. Then youcheckto see ifthe clutch is still slippingon the kickstart.lfyou got it rightfirsttime you can now slacken offthe cableadjuster- modern cables have it in theold racer's position just above thegearbox - and then replace theadjusting tool. Now walk the bikeforward until you getthe right amountof free movement at the handlebarlever.

Adragging clutch-dearme, I knewou'd bring that up-does not merely

involve reversing the direction inhich you move the spring carrier in

the front plate, but a completelydifferent sequence where you beginby slackening off the cable andremoving it from the handlebar lever.

Whatyou're doing, as you screwthespring carrier in an anti-clockwisedirection, is to compress those coilsprings, of course. You must do thatonly enough to prevent slip, because. you overdo ityou'lI clamp the thrustpins up againstthe thrust race. Undera constant heavy loading its life is, asThomas Hobbes might have put it,nasty, brutish and short.

If by now you are shaking your headsadly at the folly of man, but have notquite decided to cross a Velocettesingle off your list of desirablepurchases, take heart. There is worseto come.

According to Geoff Dodkin, the onlyenemyfaced bythe Velocette clutch isman. This creaturewill not only refuseto read a handbook, but also delightsin tinkering with the clutch mechanismuntroubled by the damage hisignorance may cause. Worse still, thebreed persists in mixing early- andlate-type clutch components.

As we've seen, the introduction ofthe more powerful spring-frame MSSto complement the 350 MAC in 1953necessitated a beefed-up clutch. Butmany folk are unaware of just howmany parts were changed. You needa sharp eye and a ruler to spot thedifference in many cases, becauseitems aren't marked in any way.

Later-type clutches feature plateswith thinner inserts, longer springsand longerthrust pins. Then of coursethere's the difference in the length ofsleeve gears, sleeve gear nuts, sleevegear distance pieces ... it goes on andon. With a mixture of incompatibleparts, you'lI never get a clutch that

20

Velocette primary drive deviates from convention in having the gearboxsprocket outboard of the clutch. Holes for clutch adjusting tool can be seen inthe sprocket of this 500cc MSS model

works as it should.'Ifyou buy a bikethat's an unknown

quantity you must assume that theclutch parts are wronq,' says Dodkin.'Ifyou assumethatthey're going to beright, you'lI get caught out.'

Weturnedto an examination ofthatclutch sent in for overhaul. It rapidlyproved the expert's point. 'One of theproblems with this clutch is that he'sgotthewrong thrust pins in it.' Dodkindeclared. 'He's also got pattern insertplates lined with a material that swellsand expands so thattheywon't lettheclutch disengage.' Simple. When youknow, of course.

Iwondered how much th is customerwas going to have to pay for arefurbished unit. Weil, friction platescost f.7.36 each and he'd need three,while at least one plain plate wasstarting to crack up: they're f2.53apiece. New inserts in the chainwheelcost less than f6.50, but a completethrust race is f16.67, which shouldteach you to adjustthe clutch properly.

Not that this was a genuinebasket-case horror story. Justconsider what you have to do ifsomeone's lost the pushrod, or thoseshims from under the hinge of thegate.lfthose are missing, you can betthat the two circular shims under thesleeve gear nut have gonetoo, butyoushould always replace those anyway,says Dodkin.

The condition ofthe chainwheel ballrace - which must be a tight fit in thecentre of the wheel - is obviouslycritical in terrns of both clutch actionand primary chain life. The same goesforthe state ofthe thrust race and thesleeve gear bush itself. Geoff Dodkinhas experimented with rollers in thethrust race instead of ball bearings, butdiscovered no advantage. Theresimply aren't any miracle cu res. With

virtually no room for distortion, plainclutch plates must be dead flat, andeven slightly bent springs have to bediscarded.

The mythsthat surround this designare, maintains Dodkin, purely theresult of ignorance. Myth suggestsotherwise, for instance, butthe sleevegear nut must be done up dead tight.Dodkin will even seil you a usefultubular peg spanner for the task, plusan inexpensive adaptor to get the nutstarted in critical situations.

Thus far, you may feel that I havelaboured the clear disadvantages ofthe Velo clutch. There is the up side,too. Everything is commendablycornpact, with short, stiff shafts. 'Whatyou have to remember is that thisclutch was designed when there wasvery little traffic about,' Geoff Dodkinreminded me. 'But it's stood the testof time.'

According to Dodkin, the antithesisofthe precision instrumentthat is theVelo clutch might be found inside thechaincase of a 16H Norton. I like thecontrast, for there is a unit that can beripped apart, thrown in the gutter andthen slapped back together againcovered in grit and road dirt. And it willwork.

Thoroughbreds, this comparisonimplies, must be treated with respect.And you have to respectthe Velocetterecord, forthis is the clutch - properlyprepared and meticulously adjusted-thatwent into quite a few racewinnersand record breakers. When weil set up,it's light and smooth.

But it is an acquired taste, and oftenendured for the sake of those otherinestimable Velocette qualities.Maybe, as Geoff Dodkin says half-jokingly, Velocettes should by rightsbelong to only one class of rider:professional engineers.

C/assic Bike