A lit­tle light work

Classic Dirtbike - - Contents -

How did Suzuki pro­duce such a light mo­tocrosser? By ded­i­ca­tion, sound prin­ci­ples and cre­ativ­ity. We look at an RN370 from 1972.

How light is too light and what is needed to create such a light bike? The first ques­tion is a dif­fi­cult thing to quan­tify, as gen­eral think­ing in the clas­sic pe­riod had it that bikes were be­com­ing too light, yet weigh­ing in at nearly 300lb – 136kg give or take a gram. Once two-strokes hit the scene, weights dropped dra­mat­i­cally though. Most rid­ers will re­alise that the key to suc­cess in off-road sport is a good rider on a light bike and us club­men al­ways knew the works bikes were go­ing to be way lighter than ones we could buy.

Take Jeff Smith’s cham­pi­onship BSAS for in­stance, all made with vary­ing de­grees of ti­ta­nium, mag­ne­sium and other such ex­ot­ica un­ob­tain­able to mere mor­tals. Then, when the Ja­panese came on the scene and money was start­ing to be poured into off-road sport, or at least the MX side of it, good­ness knows what sort of met­als would be used when the fac­tory men­tal­ity was ‘a bike had to last two 40-minute plus two lap mo­tos’, so the fac­tory could be seen to be win­ning.

For years it was as­sumed the only way to build such light ma­chines was to em­ploy ex­ot­ica but, as in so many fields, the truth is a lit­tle dif­fer­ent from the folk­lore. Yes it is pos­si­ble to use space-age and aero­nau­ti­cal met­als to make light ma­chines, but it is also pos­si­ble to use or­di­nary ma­te­ri­als and good prac­tice to do the job. This was the way Suzuki chose to go.

Yes, you could ar­gue ti­ta­nium fas­ten­ers and mag­ne­sium cast­ings are ex­otic but they were avail­able even in the Thir­ties and ac­cepted by all. Seam­less mild steel tub­ing cer­tainly wasn’t ex­otic and this is the ma­te­rial Suzuki chose for the frame of their 1972 RN. Us­ing thin gauge steel, the frame was lighter and lower than pre­vi­ous in­car­na­tions and it housed an en­gine both nar­rower and shorter with cas­ings cast from mag­ne­sium help­ing to achieve an all up weight of 180lb.

This feath­er­weight pro­jec­tile could be the

rea­son the FIM in­tro­duced a min­i­mum weight for GP bikes. Hints that it may be too light for safety may well have masked the in­abil­ity of other fac­to­ries to meet the chal­lenge and for 1973 Suzuki had to dras­ti­cally add weight to bikes al­ready built for that sea­son.

Be­fore that though, their star man Roger de Coster picked up an­other world cham­pi­onship win on their light­weight ma­chine. Look­ing over the bike it is clear some­one has re­ally thought about how to care­fully re­move weight from al­most every as­pect of it and no part was deemed too unim­por­tant to re­ceive at­ten­tion. But that isn’t the only con­sid­er­a­tion for a GP ma­chine, it has to be easy to work on too in case there’s a spill on the track and things need to be re­placed quickly. It doesn’t re­quire much ef­fort to change things but it does mean the fac­tory lis­tened to those in the pad­dock.

Now then, this isn’t the first 370 RN we’ve fea­tured as we had one of the three 1970 fac­tory ma­chines in is­sue 41 of CDB – what d’you mean you missed it… get on to our back is­sues depart­ment, it’s a great tale – and the sim­i­lar­ity is ob­vi­ous be­tween the two bikes yet there are sub­tle dif­fer­ences which mark ma­jor changes.

Most ob­vi­ous of these is the re­vised tank mount which did away with the aero­las­tic bungee cord to hold it in place. When the owner of this bike got it as a mostly com­plete ma­chine there was ev­i­dence it had seen a hard life and even the en­gine had been run loose in the frame, which as well as elon­gat­ing the mount­ing holes in the cases also wore the mat­ing faces so there was no way the mo­tor would sit prop­erly in the frame. The frame it­self had been al­tered with the shock mount­ing points moved as sus­pen­sion ideas changed dur­ing this volatile rac­ing pe­riod. As part of the re­build, the owner Clive Bussey re­made the mount­ing points and re­paired the cases as per pe­riod images.

That said, the in­side of the en­gine wasn’t in bad con­di­tion at all, but Clive did re­place

all the bear­ings and a cou­ple of gears in the five-speed box. “The rest of it, as in the crank, the big end and the pis­ton and bore, were in good con­di­tion,” he says. Less good was the fuel tank which had sev­eral splits in the pa­per thin alloy, luck­ily re­pair­ing such things isn’t a prob­lem to a lad who made a sim­i­lar tank from scratch for an­other bike.

Speak­ing of pa­per thin, as well as the frame tub­ing be­ing thin gauge, the han­dle bars are also ex­tremely thin too. Luck­ily they’re bent up from a length of chrome molyb­de­num tube, so re­silient to the stresses and strains of a GP sea­son oth­er­wise they’d not have sur­vived. It would be nice to think there’s a cup­board in the Suzuki race shop with a few spares tucked away just in case

Also of ex­tremely thing gauge is the ex­haust sys­tem and it is a work of art loop­ing down un­der the en­gine, slip­ping sen­su­ously up be­hind the frame tubes and end­ing in a small di­am­e­ter pipe about mid-shock level. Fab­ri­cated in a cor­ner of the race shop maybe? No, like all Suzuki fac­tory MX team ex­hausts of the pe­riod it was made by an elderly cou­ple in their house. Luck­ily for Clive this pipe was sal­vage­able and pretty much un­dam­aged con­sid­er­ing where it is in re­la­tion to stones and what­ever be­ing thrown up from the track.

With the two team bikes to com­pare, it was easy enough to spot some dif­fer­ences be­tween the 1972 bike and its ear­lier rel­a­tive. The 1970 ver­sion had valanced rims on the wheels but by 1972 these were un­valanced which of­fers an im­me­di­ate weight sav­ing and the added bonus that it is much harder for mud to build up on the wheels which would al­ter the bal­ance and add to tir­ing the rider out.

On the sub­ject of the wheels, which have mag­ne­sium hubs and brake plates, and bear­ings only just big enough for the job, all the spac­ers are alu­minium but the dis­tance pieces are ma­chined from engi­neer­ing plas­tic. Suzuki were ac­tu­ally one of the pi­o­neers of plas­tic com­po­nents on their team bikes and from com­po­nents as ba­sic as mud­guards the Ja­panese fac­tory added not only wheel spac­ers but fuel cap, num­ber­plates and on this bike, a large vol­ume air box.

The min­i­mal­is­tic theme con­tin­ues through­out the RN as weight and mass was re­moved from as many com­po­nents as pos­si­ble, fork slid­ers thinned and waisted, spin­dles hol­lowed, alu­minium case screws, ti­ta­nium en­gine bolts and so it goes on. Even the sus­pen­sion bolts got at­ten­tion and in­stead of a hexagon head as a nor­mal bolt would have, there was sim­ply a shoul­der to stop the bolt pulling through the frame and a cir­clip on the other end to stop it go­ing back the other way. Those bolts which had to have a hexagon head and a nut were hol­lowed out to ease frac­tions of a gram off them and not a thread longer than they needed to be. This is only what the scene ex­pected of course

as we all knew works bikes were hand- built beasts bristling with such re­fine­ments and likely to be snarling beasts when un­der power and only able to be rid­den by su­per-he­roes.

And the re­al­ity Clive? “It’s an easy to ride ma­chine,” says Mr Bussey, “lots of power yes but it is man­age­able, when you think about it, a works bike has to be man­age­able as there’s lit­tle point in the rider hav­ing to fight the bike as well as the track, while try­ing to keep ahead of the op­po­si­tion.” Yes we know the tal­ent of a works team mem­ber is such that prob­lems can be over­come but gen­er­ally that’s be­cause cir­cum­stance has dic­tated this – a spill per­haps – rather than a dif­fi­cult to ride bike be­ing de­signed at the out­set. Just such a sce­nario would be­fall Suzuki at the start of the 1973 sea­son when de Coster was set­ting out to de­fend his cham­pi­onship.

All the work done by Suzuki re­sulted in a bike weigh­ing 180lb and thanks to an un­der­stand­ing of things such as air-box vol­ume and its ef­fect on en­gine per­for­mance – the 1972 box was much less re­stric­tive than pre­vi­ous ones – the fac­tory had great hopes for suc­cess in the com­ing sea­son. At least un­til the FIM placed a min­i­mum weight limit on GP bikes. Suzuki had al­ready built and despatched their team bikes to Europe for the sea­son when news of the new rule broke.

With no time to re­design or build new bikes, some­how around 30lb – or just over 13kg – had to be added to the bikes. Ti­ta­nium bolts were re­placed with steel, alloy too be­came steel and even the frame tubes and en­gine cav­i­ties were filled with lead. The re­sult met the re­quire­ments but ac­cord­ing to an in­ter­view with de Coster “...the 1973 bike wasn’t as good and lead to Suzuki ex­per­i­ment­ing with longer sus­pen­sion…” Leg­end has it the weight limit was in­tro­duced af­ter pres­sure from Euro­pean fac­to­ries but Suzuki and de Coster shone in ad­ver­sity and he took his third world cham­pi­onship. )

Sub­tle changes mark out that this is the 1972 open class RN, as op­posed to the 1970 ver­sion.

In the 1972 GP se­ries, this is the view most other rid­ers would have of de Coster and his fac­tory Suzuki, though I doubt they’d be ad­mir­ing the slim lines of the bike. With mag­ne­sium hubs and non-valanced rims on ei­ther end, not just the rear, un­sprung weight is as low as pos­si­ble.

A lit­tle bit of grooved plas­tic keeps the rider’s toe on the brake. Though the alloy tank re­sem­bles the ear­lier ver­sion, it has a bet­ter method of hold­ing it in place than a bungee cord. If metal, even alloy, is deemed too heavy try plas­tic for the spac­ers. Springs keep the rear of the pipe from flex­ing too much yet al­low a de­gree of move­ment in the sys­tem to stop it frac­tur­ing. Front forks are slimmed down as far as they can be and still stand up to the job, though in GPS the team found some plas­tic tube helped pro­tect them from dam­age. Beau­ti­fully crafted rear brake pedal uses the clutch cover as a stop. De­vel­op­ments in­clude a lower frame to keep what en­gine weight there is as low as pos­si­ble.

Mag­ne­sium cast­ings are used for all of the en­gine in 1972, ear­lier mod­els had alu­minium crankcases and mag­ne­sium side cases. Note ti­ta­nium bolts and alu­minium case screws. Yel­low has been a Suzuki colour for some time, tank looks good.

We al­ways knew fac­tory bikes were spe­cial but only close in­spec­tion shows how spe­cial. A rub­ber strap, clipped to the in­side of the tank and loop­ing round a bracket re­places the bungee cord of ear­lier mod­els, much neater.

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