Broken dreams

Early in the Seventies almost everyone was jumping on the Wankel rotary engine bandwagon. John Nutting rates Suzuki's RE5 as Japan's best effort but poor fuel consumption and performance undermined its initial promise.

Not many survive now but we managed to test two in Mechanics some years ago.

UNLESS you were used to the rare luxury of an expensive BMW flat twin, the idea of a perfectly smooth motorcycle engine was almost unknown to riders in the early Seventies. Even the best of the emerging range of four cylinder machines from Honda and Kawasaki were buzzy, though they were better than the big British twins we'd mostly been accustomed to. So when I first rode a rotary-engined bike in 1973, the silkiness of the power delivery was a revelation. It was Triumph's P41 prototype, a twin-rotor Wankel adapted to a spine-type Bonneville chassis. The BSA-Triumph group was in terminal decline and, keen to get publicity, had permitted David Garside, who was in charge of the project, to lend me the machine for a weekend. Its impact was stunning. Here I was on the M6 whistling along at 100 mph on what looked like a British bike, but decidedly felt nothing like one. The engine, developed from a Fitchel & Sachs under licence from NSU, was so smooth I realised for the first time that there were other types of vibration produced by a motorcycle; the gearbox, drive chain, wheels. And I also realised just how tiring high-frequency vibration could be.

The P41 offered me a glimpse into the future. But whether the Wankel design would be part of that future was still open to question. BSA-Triumph had jumped on the Wankel bandwagon in the late Sixties along with a number of manufacturers. Felix Wankel had been working on the principles of his rotary engine since the 1920s but it wasn't until 1951 when he attracted the attention of Germany's NSU (then one of the largest manufacturers of motorcycles in the world) that he was able to develop a working prototype engine. With design input from NSU's research chief Walter Froede, it first ran in 1957. From the power the tiny 125cc engine made, NSU quickly realised its potential. So did others and a measure of its impact was the speed with which the automotive giants - including Mazda, General Motors, Citroen, Daimler-Benz, Curtiss-Wright, Fitchel & Sachs, Yamaha and Suzuki - were keen to buy licences to develop and commercialise their own designs.

While NSU was first to use the Wankel engine in its cars, followed by Mazda, the motorcycle makers perhaps had more to gain from developing a new type of engine that had the potential to be more compact and smoother than reciprocating engines. By the end of 1974 the bandwagon was fully loaded. Yamaha had shown its watercooled RZ201 at the Tokyo show, DKW in Germany revealed a 294cc, Fitchel & Sachs-engined machine, while in The Netherlands Van Veen revealed its huge OCR2000 with a two-litre Comotor (Citroen-NSU) engine. Suzuki in Japan was, however, the most advanced in its development of a Wankel-engined bike, the RE5 first shown at the Tokyo Show in 1973. The basic merit of the Wankel was that it was simple and compact, having two key moving components - the eccentric crankshaft and an almost triangular rotor. Rotating motion rather than reciprocating made it inherently smooth.
These benefits were immediately obvious from my first ride on the twin-rotor, aircooled BSA-Triumph P41 which subsequently evolved into Norton's Commander police bike, the successful racers and the watercooled F1 sports machine. Turning the Wankel engine into a compliant and unfussy road-going power unit took a wholly different path at Suzuki - one that in the end lost sight of the engine's basic virtues. Using what appeared to an engineering sledge hammer to crack the challenging nut of overcoming some of its inherent design compromises, ensured that it was heavy and underpowered. Suzuki's version of the Wankel was similar to that used in Mazda's cars. It had a watercooled outer casing with a single rotor and a nominal swept volume of 497cc. Cooling was by a huge radiator behind the front wheel.

Lubrication was a cross between four stroke and two stroke, with the rotor's main bearings being fed from a sump under the engine unit by oil that was cooled by a second radiator. The rotor's tips were lubricated by oil fed into the inlet tract and controlled by engine speed and throttle opening. Drive from the rotor was by a double-row chain on the right side to the wet multiplate clutch and a conventional five speed gearbox and chain final drive.

Designers challenge

THE biggest challenge to the designers was metering the fuel and throttle control. By the engine's nature, the combustion chambers change not just volume but overall shape through the cycle and have a much higher surface area relative to their volume. This results in a quenching action that draws heat from the combustion process, reducing thermal efficiency. In addition, the inlet porting into the wall of the casing meant that controlling the fuel at small throttle openings and at idle was difficult. The solution was to use a carburettor with two venturis, a small one with an 18mm diameter connected to the combustion chamber with small ports for better filling, and a larger 32mm one for full throttle opening. Two sets of push-pull cables connected to the twistgrip. Over the grip's first 40 percent of movement, the smaller choke was used. Above that, the CV-controlled throttle came into play. Another characteristic of the rotary engine was its high exhaust temperature. Suzuki overcame this by providing double walls for the exhaust system which opened into two silencers. Ports at the front end of the pipes, under the coolant radiator, allowed air to cool the system. In size and weight, the RE5 was very similar to the three-cylinder GT750 'Kettle', with a 59-inch wheelbase and 560 pounds including a gallon of fuel. Chassis and suspension was almost identical too, except that the 19-inch front and 18-inch rear wheels had light alloy rims. Peak power on the RE5 was 62 bhp at 6500 rpm, slightly less than the 67 bhp at 6500 rpm of the first GT750, later tuned up to 70 bhp.