A little light work
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.