The cottage industry of motorcycle manufacturers that is so prominent in Great Britain has an abundance of talented engineers, who all have their own dreams and ideas of what they believe will work in the hugely competitive world of off-road motorcycling. We witnessed this in trials with John Shirt Snr and the Majesty
Yamaha project, and Graham and Nick Beamish (Beamish Suzuki) with their own ideas for a production trials motorcycle. After Sammy Miller blew the dinosaur four-stroke single cylinder trials machines of old into extinction with his two-stroke Bultaco in 1965 it appeared that four-stroke trials motorcycles were dead and
buried forever. In 1978, however, a certain Alan Clews had other ideas ticking away in his head.
Alan Clews was very fond of the four-stroke engines, using production stock including engines and components he purchased when the mighty BSA Empire collapsed. He had tasted success in the world of Motocross, or Scrambling as it was called then, with his own machinery powered by the four-stroke motors. Quite by accident he had taken a British trials title in 1973 with Jack Matthews in the Sidecar class on one of his early ‘Clewstroka’ machines.
A RETURN OF THE FOUR-STROKE
Back in ‘The Day’ both Trials, a winter sport and Motocross, a summer sport, were very seasonal, which also affected sales of machines. CCM machines had tasted success and were a very well respected brand name in the Motocross world. The machines, using four-stroke motors, were produced by a dedicated work force but as with any seasonal product what do you do with that workforce during the close season? An avid off-road fan, Clews had had sat back and watched the Honda trials project take shape in the capable hands of Sammy Miller in the early seventies which had produced the TL 125cc and 250cc. The trials world was set alight when Rob Shepherd won the 1977 British title under Miller’s guidance. When the bombshell, that Honda would not be continuing with Sammy Miller, was dropped it looked like the four-stoke return would once again be a dream. As it happened Honda did continue with Shepherd, who would go on to take the first ever WTC win in the FIM world championship series, but disposed of the services of Miller. Still very passionate about the fourstroke trials engine, Miller approached Clews with a view to producing a new motor designed just for trials. After looking at development and tooling costs of a new motor Clews declined the opportunity to peruse the venture with Miller.
As the winter months of 1977 rolled into 1978 Clews was given the opportunity to look at one of the Honda trials machines and he went away with many thoughts running through his head. He spoke about a CCM trials project with his good friend Jim Sandiford, who was the successful UK importer of the Spanish brand. He purchased a full front end of a Montesa Cota from Sandiford and the CCM four-stroke trials project was born. The idea of a trials project would also answer the question of what to do with the dedicated work force during the remainder of the winter months.
Clews knew that the marketing of a new fourstroke trials ‘Iron’ would be the easy part as he had the motocross network of dealerships already in place, but the manufacture of a machine would not be so easy. With the end of the Miller Honda trials team in 1977 Clews went straight to ex-team member Nick Jefferies and offerd him a contract to help with the development programme of the proposed new machine. Jefferies had learnt so much from the Honda project that Clews knew he would be a valuable asset to the development of the new machine.
Both Clews and Jefferies tested many of the new models on the trials market before they sat down and discussed which direction they would go in but they both agreed on the most important factor, that it would have a four-stroke motor.
After discussing many options they went for a hybrid BSA B44/B50 engine using a bore and stroke of 79mm x 70mm, which produced an engine capacity of 343cc, the same as the old BSA B40. The specialised Alpha Engineering Company would produce the ‘Big End’ of the motor using a B40 connecting rod pressed in with a steel crankpin with a caged roller bearing for a smooth operation. The BSA B44 casting would be used for the cylinder head and barrel. The engine lubrication would be taken care of with the oil carried in the tubular steel frame. Smooth operation is essential in a trials motor and the prototype flywheel weights would start at 5½lbs on either side of the crank. Magnesium Italian Marzocchi forks would take care
of the front suspension and at the rear they contacted Girling, who they had worked closely with in Motocross.
After the burning of much midnight oil the first four-stroke prototype trials machine was presented for testing to Nick Jefferies on Saturday 4th February 1978. In secret over the weekend Jefferies went to his local practice area and gave the machine a long workout. As expected with any prototype machine unseen problems soon came to the surface.
The motor was over-heating and the low-speed power was like an electric light switch, either on or off. To help eradicate this problem the flywheel weight was increased to 7.8lbs on each side and to help the motor run cooler the oil capacity was increased. The rear shock absorber positions were also changed. After more testing the handling was improved as the steering head angle was pulled in by two degrees and the swinging arm fulcrum point was lifted to improve ground clearance. When Jefferies had applied the power at the first test it pulled the suspension down, which in turn reduced the all-important ground clearance. Even though the exhaust system worked well it was changed to reduce the output noise. To help with starting, and the power output of the motor in general, a 24mm Amal carburettor was fitted and Clews applied his four-stroke knowledge to fine tune its performance in certain areas.
CCM – Clews Competition Machines. TRIAL MAGAZINE
Whichever way you look at the hybrid four-stroke BSA B44/B50 air-cooled single cylinder motor, it’s ‘Big’.
It’s 100% concentration for Nick Jefferies as he stays ‘Feet Up’ on the second prototype at the SSDT.
The fuel tank and seat were easily removable for maintenance. 70