Some­thing spe­cial

The UK couldn’t quite leave the Tri­umph unit en­gine be­hind when ISDT duty cropped up – meant a few su­perb chas­sis were de­vised though.

Classic Dirtbike - - Contents - Words &Pics: Tim Brit­ton

As the chimes of Big Ben her­alded the end of the Swing­ing Six­ties and the dawn of what would be par­tially re­ferred to as the decade style for­got, there were changes afoot – or pos­si­bly ‘awheel’ in the in­ter­na­tional en­duro scene. Hith­erto in or­der to qual­ify for the tro­phy con­test, teams had to be mounted on ma­chin­ery man­u­fac­tured in their home coun­try. The vase teams, plus club, mil­i­tary and in­di­vid­u­als, could ride what­ever mo­tor­cy­cle they liked. For 1970 the restric­tion was lifted for the tro­phy teams, largely at the re­quest of the Amer­i­can teams it has to be said. You see, off-road sport was quite the big thing in the USA and li­able to be a big money spin­ner.

So, de­spite the fact there was no ac­tual need to ride Bri­tish-built mo­tor­cy­cles for na­tional hon­ours, the home teams were mounted on what be­came pos­si­bly the most suit­able and ar­guably the best-look­ing bikes they’d ever been is­sued with.

In The Mo­tor­cy­cle of June 1970 there was a fea­ture on the build of these su­perb Tri­umphs by Eric Cheney and his team and it seemed the world was against the build. Al­most at the last hour a sup­ply of wheel rims had been sourced and donated on the prom­ise the sup­plier – Vic Nunn Wheel­build­ing – would have them re­placed once sup­plies of the cor­rect high ten­sile rims were avail­able again from Dun­lop.

With rims now on hand, the bikes could be fin­ished for the ISDT try-out at the Welsh Two­day Trial. And what bikes they were, nickel plated frames car­ried oil in the tub­ing and housed the well-proven unit Tri­umph en­gines. For the Welsh, all en­gines would be 490cc but af­ter­wards, to com­ply with ISDT ca­pac­ity regs, two units would be bored out to 504cc. In­side the gear­box a slightly tweaked clus­ter would give a lower bot­tom gear in case of nadgery go­ing so the bike could be plonked, clutch fully home, like a tri­als bike but the Day­tona spec of the en­gine would re­act quickly once the throt­tle was wound on and the higher gears gave a de­cent turn of speed – so de­cent in fact one team mem­ber qui­etly told me his own sim­i­lar team bike had been ‘of­fi­cially’ timed at 100mph on the way to an im­promptu prac­tice ses­sion… But, we di­gress, feed­ing fuel into the en­gine was a 28mm Con­cen­tric car­bu­ret­tor pro­tected from the el­e­ments by two pa­per air fil­ters. These were eas­ily change­able by lift­ing the qd seat.

For 1970 ig­ni­tion and light­ing had gone 12v with a pair of coils seated in a spe­cial holder de­signed to be eas­ily ac­ces­si­ble in the event of an in-event-in­ci­dent. A Zenor Diode heat sink re­quired by the ex­tra power cre­ated from 12v electrics was at­tached to the front mud­guard sup­port while the ca­pac­i­tor used in place of a bat­tery was mounted in the air­box on a spring. The ma­chines fea­tured lots of mod­i­fi­ca­tions to the frame which, be­ing as it was orig­i­nally an MX one, wouldn’t have or need. For in­stance, few MX bikes use a cen­tre­stand, none car­ried an air bot­tle for tyre in­fla­tion and rock guards too would be un­usual on a MX frame.

Sus­pen­sion was taken care of at the front with Cheney’s own style of tele­scopic fork and Koni oil filled dampers at the rear. Now the rims were avail­able the front one was laced into Cheney’s own Elek­tron cast hub while the rear wheel used the very pop­u­lar BSA qd hub. More oil is car­ried in the swing­ing arm tube and a small tap al­lows a drip feed onto the chain. Fin­ish­ing off the bike was a two gal­lon Gold Star style petrol tank and al­loy side pan­els which the fea­ture claimed were an­odised red.

The ac­tual story of this bike seems a bit of a mys­tery and even the owner Bryn Richards is un­sure ex­actly what it is. He’d been told at the time of pur­chase it was a prac­tice or spare team bike for the UK squad but can’t find any proof other than cir­cum­stan­tial. He does know that when he bought the bike it was an­odised blue, though colours are easy to change, and the ma­chine had

spent a lot of time in the Isle of Man. The ISDT it had been built for was in Spain at El Es­co­rial – and fea­tured in the iconic film ‘On Any Sun­day’ – but the bikes were also used in the Isle of Man in 1971 and some on to 1972 in Czechoslovakia. So af­ter the de­struc­tive fire which re­duced his bike to a melted ruin and re­quired the sourc­ing of many parts, the fin­ished ar­ti­cle is as close as could be to what it was orig­i­nally.

Nat­u­rally there are some dis­crep­an­cies to the spec but it looks pe­riod and we hope this will suf­fice to keep the faith. That the frame was sal­vage­able at all was a tes­ta­ment to the orig­i­nal build and Si­mon Cheney had warned Bryn it might be too far gone. A re­place­ment en­gine unit, to Day­tona T100 spec, was sourced and re­built though with­out the spe­cial gears. A petrol tank and side pan­els wouldn’t be a prob­lem as they are pretty much stock fare for Cheney ma­chines but nei­ther Bryn nor Si­mon could quite re­call what the forks were off orig­i­nally but they look re­mark­ably like the cor­rect units. At the rear end of the ma­chine in­stead of Koni dampers a pair of Rock­shocks sit in place. As for hubs and rims, the front one is a mis­cel­la­neous light al­loy one in­stead of an Elek­tron cast­ing and the rear has an oil in frame con­i­cal hub with some big holes for light­en­ing bored in. In­stead of high ten­sile steel rims though both wheels wear al­loy ones. 

Though not com­pletely orig­i­nal, the re­build has kept the spirit of the era alive. Note the tor­tu­ous route for the ex­haust sys­tem and the small si­lencer.

By all the rules there should be bet­ter en­gines than Tri­umph’s 500 for ISDT use but it has an en­vi­able record of suc­cess. Rear brake light is well pro­tected from the wet and the bracket is bolted to the pri­mary case. The forks are re­place­ments and seem to be cor­rect but no one is sure where they orig­i­nated, or more cor­rectly which bike they orig­i­nated on – bought on the in­ter­net. Hub is pos­si­bly a Grimeca Note the ISDT mods such as hav­ing a ca­ble al­ready in place in case the orig­i­nal is dam­aged.

Orig­i­nally the bike wore Koni rear dampers but now Rock­shocks sit in place. As bought and rea­son­ably orig­i­nal... More ca­ble rout­ing, think about it, hard­est job when chang­ing a ca­ble is to feed it through the frame and up to the bars… Af­ter the fire, show­ing how­much was de­stroyed. Re­build un­der way at Cheney’s work­shop.

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