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The ma­chine used to win a world cham­pi­onship al­ways ex­cites in­ter­est… must be some­thing spe­cial af­ter all, mustn’t it?

Classic Dirtbike - - Contents - Words: Tim Britton Pics: Mor­tons Ar­chive, Nick Ni­cholls col­lec­tion

… a world cham­pi­onship-win­ning CZ. We look at a pro­file of the ac­tual Ceezee Joel Robert won his world cham­pi­onship on in 1964.

The Six­ties hadn’t quite got to the Sum­mer of Love or even started swing­ing when 20-year-old Bel­gian Joel Robert made his first of many in­deli­ble marks in the his­tory of mo­tocross. In Au­gust 1964 Mo­tor Cy­cling got hold of a fea­ture about his cham­pi­onship­win­ning ma­chine, de­ter­mined to un­earth a few se­crets these fac­tory teams keep to them­selves.

The press were to be dis­ap­pointed, as they found out the ma­chine was ex­pertly main­tained and if not built re­gard­less of cost – this was the East­ern bloc af­ter all. The team bikes were in tip-top con­di­tion and pretty stan­dard fare as well as be­ing ba­si­cally what a racer could buy but with the afore­men­tioned race-shop at­ten­tion to de­tail, though that could also be achieved in a pri­vate en­vi­ron­ment.

It seems CZ had re­lied on sim­plic­ity rather than revo­lu­tion­ary con­cepts to pro­duce their win­ning ma­chines. The bike in this fea­ture is Robert’s ma­chine and shows the ro­bust­ness of the en­gine which had been a CZ thing for some time and would aid re­stor­ers in the then fu­ture as these en­gines are vir­tu­ally in­de­struc­tible.

Re­ly­ing on years of good engi­neer­ing prac­tice, cou­pled with the ex­pe­ri­ences gained in com­pe­ti­tion, CZ pro­duced a mo­tor­cy­cle which was pow­er­ful, flex­i­ble and strong enough to out­last the rigours of a GP sea­son. Mo­tor Cy­cling learned the en­gine was a sim­ple two-stroke in unit con­struc­tion with a four-speed gear­box and the re­sul­tant unit has a peak power out­put of 26bhp at 5900rpm.

Surely there must be some trick­ery in­side the en­gine mas­querad­ing as stan­dard? No, sorry, and I quote: “…the en­gine em­ploys a clas­sic loop-scav­enge sys­tem…” which ba­si­cally means the fresh com­bus­tion charge is drawn into the crank­case as the pis­ton rises and is pushed through trans­fer ports opened and closed by the pis­ton as it trav­els up and down the bore creating pres­sure at ei­ther end of the scale. These trans­fer ports are formed in both the bar­rel and the crankcases.

In a move to pro­vide en­gine rigid­ity and pis­ton sup­port, the bar­rel has an ex­tended skirt or spigot ex­tend­ing deep into the crank­case mouth. Aha, the bar­rel and cases must be mag­ne­sium or other un­ob­tainium mustn’t they? No, it is cast iron for the bar­rel and alu­minium alloy for the cases and head, so hardly fancy. The head does have a twin plug ar­range­ment and a squish band to pro­mote a thor­ough mix of the com­bus­tion charge so the plugs can ig­nite it and pro­duce the right amount of power from the mix­ture squeezed at 10.5:1 com­pres­sion ra­tio.

The back­bone of an en­gine is the crank­shaft and CZ had made an ex­tremely rigid unit with small di­am­e­ter fly­wheels for snap ac­cel­er­a­tion as well as mak­ing the crank­case pump­ing much more ef­fi­cient. To keep the crank turn­ing eas­ily, CZ used a dou­ble row bear­ing on the ig­ni­tion side with two ball bear­ings on the drive side.

They also moved the clutch to the crank­shaft which means it op­er­ates un­der much less torque than if it was on the gear­box main­shaft. A down­side is this ar­range­ment gives the gear­box it­self a harder time be­cause there’s less chance of the clutch slip­ping. CZ re­alised this, so did a lot of re­design work in the clus­ter. Be­cause there had been is­sues with ear­lier mod­els this re­design of the clus­ter in­cluded such things as us­ing nee­dle roller bear­ings rather than bronze bushes. Why? Be­cause nee­dle rollers don’t need a lubri­ca­tion hole in the shaft, so in­stantly a weak point is elim­i­nated. This ro­bust 250 unit didn’t have a chain as pri­mary drive, us­ing in­stead spur gears deemed too noisy for a road bike but ac­cept­able on a race ma­chine.

Some­thing Mo­tor Cy­cling found in­ter­est­ing was the gear selec­tion method, or rather it and it’s ac­cess. Ac­cept­ing in the heat of the mo­ment a scram­bler is more con­cerned

with find­ing the next gear than the niceties of a smooth change, what was wanted was a ro­bust and fool­proof chang­ing sys­tem. Even bet­ter, as far as a me­chanic was con­cerned, the sys­tem could be in­spected and re­paired with­out split­ting the en­gine. What was used was a large cam-track plate with a small con­nect­ing rod which en­sured the gear selec­tion was pos­i­tive and with no chance of go­ing past the stop into a false neu­tral, no mat­ter how big the boots of the rider were. In­spec­tion was through an ac­cess plate on the top of the en­gine.

This drive to sim­plic­ity wasn’t only re­stricted to the en­gine unit but the cy­cle parts too. A steel frame of tubu­lar loop de­sign with a strong but light swing­ing arm used a front fork con­structed from light alloy slid­ers and steel stan­chions with plenty of travel. At the rear were oil damped sealed units.

Help­ing the bike weigh in at a use­ful 218lb was the use of Elek­tron alloy for the wheel hubs – ad­di­tion­ally low­er­ing the un­sprung weight too – and a fuel tank and seat pan laid up from glass fi­bre. It seems the fac­tory had ex­per­i­mented with alloy sheet for the seat pan but found it dif­fi­cult to man­u­fac­ture the cor­rect shape eas­ily so used the glass fi­bre and were sur­prised to see a weight sav­ing too.

In or­der to stop muck and dirt play­ing havoc with the en­gine in­ter­nals as their rid­ers blasted around the scram­bles cour­ses of the world, CZ fit­ted two air fil­ters in­side the in­duc­tion cham­ber, thus en­sur­ing max­i­mum pro­tec­tion.

Along with the re­port, Mo­tor Cy­cling had been sup­plied with some line draw­ings, clearly show­ing the cham­pi­onship en­gine and how it was laid out. The writer ex­plained the main photo had the cylin­der laid out at the front so as to show its re­la­tion­ship with the rest of the en­gine and begged the read­ers’ for­give­ness if it seemed mis­lead­ing.

But with­out the rider…

…a bike goes nowhere and in Joel Robert was some­one who was tak­ing the CZ to the top. Not that it had been a fore­gone con­clu­sion, as in an in­ter­view with the young Bel­gian cham­pion Mo­tor Cy­cling learned prior to gain­ing a works CZ af­ter a cheeky re­quest to the fac­tory, Robert had been com­pletely off-form and thanks to ma­chine fail­ures had done more walk­ing than rid­ing. But, once on the Cze­choslo­vakian ma­chine he was on form again, and at 20 be­came the youngest ever world cham­pion.

Roberts’ rise to su­per­star­dom was par­tic­u­larly rapid and from the 14-year-old who rode on pri­vate land be­fore com­ing of age to com­pete, it only took six years to be world cham­pion.

There were those who doubted the Bel­gian’s tem­per­a­ment and felt he was more likely to fall foul of the au­thor­i­ties look­ing af­ter scram­bling in Bel­gium. In­deed, his fa­ther had done just that and re­ceived a life­time ban for an in­ci­dent with of­fi­cials. Luck­ily, young Joel was drafted into the Army for his na­tional ser­vice and the en­forced dis­ci­pline of the mil­i­tary tem­pered his zest and al­lowed him to ma­ture into a top racer.

Robert’s route to star­dom in­volved a

se­ries of small ca­pac­ity field bikes, then at 16 a 250cc Zun­dapp which was re­placed with a 250 Greeves for 1960. It was on this ma­chine his tal­ent be­gan to show when he man­aged fourth in the Bel­gian light­weight cham­pi­onship. Great things were promised and Joel went to the 250cc Bri­tish MX GP at Shrub­land Park in 1961. It wasn’t a great de­but as a fly­ing stone left him con­cussed and he barely man­aged to get back to the pad­dock, de­tuned and out of the run­ning. The next year how­ever, he did the full sea­son of GPS and man­aged ex­cel­lent plac­ings when the Greeves could stand the pace. When the Vil­liers Star­maker en­gine was in­tro­duced this seemed the an­swer to his prayers, yet still wasn’t up to the job and the costs of keep­ing it go­ing were un­fea­si­ble.

It was at this point Robert cheek­ily sug­gested to the CZ fac­tory they loan him one of their works ma­chines. The fac­tory did, it ar­rived in Au­gust 1963; by Au­gust 1964 Joel was world cham­pion and no one was more sur­prised than him. Re­ceiv­ing his draft pa­pers just be­fore Christ­mas in 1963, he wasn’t even sure he would get leave to at­tend GPS. He did, went to Spain, was medi­ocre, man­aged to make the Bel­gian home round and… well… dom­i­nated the event to get his first win. The next three rounds saw his name at the top and even bet­ter it was with vastly dif­fer­ent cir­cuits from mud to bone dry, a great boost for his con­fi­dence. He lapsed a lit­tle af­ter that but was back on form for three con­sec­u­tive wins, in­clud­ing the Bri­tish GP where our pho­tos of him are from; the fi­nal saw the ti­tle go to Robert. He did a ‘just to be cer­tain’ win in Rus­sia but was un­chal­lenge­able at that point.

Of those wins in that sea­son, it is pos­si­ble the Czech GP showed his met­tle, in the first leg he went from sixth to first; in the sec­ond leg he and Torsten Hall­man shot away from the field and Robert was on the Swede’s rear wheel for 50 min­utes un­til a balk by a back­marker made him stall his en­gine two laps from the end. Hall­man van­ished, Robert fired up and not only caught the Swede, but passed him to win the sec­ond leg.

Joel Robert was on his way... )

Youngest ever world cham­pion... at that time at least. Joel Robert and his CZ. Don’t you just love ex­ploded en­gine draw­ings? These show the CZ as Joel raced it.

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