During the summer of 2000 Perry Leask and I began speculating on the feasibility of building a lightweight supermono racer utilising a motocross engine.
We argued that a machine producing 70bhp and weighing 100kg (0.7bhp per kilo) would compete favourably with the traditional Yamaha and BMW racers that produced 80bhp but weighed in at 130kg (0.61bhp per kilo).
Even Ducati’s Supermono, which at around £16,000 is the famous Italian marque’s state of the art single cylinder racer which produces 75bhp, weighs in at 135kg (0.55bhp per kilo). Aside from the obvious power to weight ratio there would be far greater advantages in handling, corner speed, braking, acceleration, aerodynamics and general efficiency. A smaller more efficient engine, breathing through a single carburettor would produce less unwanted heat, would require a smaller fuel load and suffer less paracitical losses from smaller water and oil pumps. It would require a smaller radiator and therefore less coolant. This would give further improvements to power to weight ratio and handling benefits when considering real race masses. It would also be slimmer to aid aerodynamics and lighter to require less substantial mountings.
Being more compact it could be positioned in the chassis to give more freedom to consider weight distribution, centre of gravity, chain alignment, ground clearance, the positioning of ancillaries and the passing of air 'through the bike'. A lighter machine would run smaller wheels and tyres, require only a single front disc and be far more conservative with tyre wear. Less rotating mass both in the engine and wheel assemblies would result in a quicker steering bike with less inertia.
The Late Nights Begin...
A Husqvarna engine was the obvious choice as we already had supermotard engines producing 67bhp in the confines of a dirt bike chassis and with exhaust designs compliant with stringent noise regulations. We had also built a deep knowledge of potential reliability problems from our off road experience, having recorded British Championship wins with Husqvarna in both Motocross and Enduro. We saw an opportunity to speed and control our movement into road racing, at the same time strengthen our position as Britain’s number one Husky tuning company. In September, we purchased a used Spondon TZ250 style chassis and the late nights began! We finalised engine position and delivered the chassis to Nu Trak components in Bolton to manufacture and heat treat an engine cradle and carry out minor frame modifications. We scrutinised the rear suspension geometry and made modifications to improve the linkage ratio progression curve. Perry re worked a GSXR600 Ohlins rear shock, which we fitted along with an Ohlins steering damper. The upside down WP forks also received Perry’s attention being extensively re valved and re sprung, the stanchions titanium nitride coated and the Spondon adjustable triple clamps anodised. Turning to the engine, data acquisition from previous supermotard testing showed that the very basic lubrication system resulted in alarmingly high oil temperatures when run in a road race environment. We decided to drop the oil out of the engine into a heat exchanger to allow it to cool and deairate before being filtered and fed back to the oil pump. Given that pressure from the downstroke of the piston is used to exhaust oil from the crank area back to the gearbox (i.e. scavenge), great consideration of the position of oil ways and breathers was necessary. We chose a longer con rod, which gave a safer rod/stroke ratio and angulation that would assist reliability at sustained high revs and achieve an increased dwell around TDC*.
*A short con rod which often benefits a dirt bike can accelerate the piston away from TDC too fast at high rpm, chasing away from the combustion charge too quickly. The relativity large cylinder bore that requires more time to complete combustion worsens this situation. Con rod length and rod/stroke ratio can define how an engine “feels” on the track more than performs on the dyno and selection is less than straightforward in the Husky’s case with the unusual offset bore. Previous motors built with longer con rods had proved less than successful, their deficiency attributed to a lesser acceleration given to the inlet charge, a problem countered by camshaft selection, valve timing and altered inlet length.
