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The Unrideables: The 500cc Two-Stroke Era in GP Racing

GP Racing They are known to this day as the baddest, beastliest, most aggressive motorcycles to ever find their way to a racetrack, known as much for their screaming sound, distinct smell, and wicked performance as they were for launching riders into the air in spectacularly violent crashes.The massive 500cc two-stroke race bikes that once dominated Grand Prix seem almost irresponsibly dangerous now, especially when compared to the ultra-refined four-strokes in use today. But they also made for some of the most exciting and competitive motorcycle racing of all time, giving them a mythic status among both racers and fans that remains to this day.For nearly three decades, those 500cc two-stroke beasts challenged, frightened, and even severely injured the best racers on the planet – a reputation that earned them the nickname “the Unrideables.”Previous to the 1970s, two stroke engines (so named because they have one combustion cycle every two strokes of the piston, rather than every four as in most engines) were very common in Grand Prix racing, but in the smaller 125cc and 250cc classes. Due to their nature, two-strokes accelerate very rapidly and are extremely lightweight relative to the power they create, which makes them excellent performers but also more difficult to control. 

Check out that unique exhaust system! Complex exhausts are a signature part of the 2-stroke 500cc engines that dominated GP racing for nearly 3 decades.

 

At the time, there was not only the perception that 500ccs was a massive amount of displacement for a two-stroke, but that an engine that size could not be made to run reliably. Well it was a huge amount of displacement indeed, but it turns out the reliability issue was wrong – it just took someone to prove that it could be done.

It was British GP racer Barry Sheene who was was first to really master the 500cc two-stroke, due mainly to the creative design of his race bike. His team, led by famed British frame builder Colin Seeley, took the air cooled 2T engine from a powerful Suzuki TR500 and built a new bike around the potent engine. On this new bike, Sheene quickly proved he could ride big two-stroke just as well as he did the 125s on which he rose to fame.

As good as it was, the TR500 engine was only good for around 80HP at the time. But two-stroke development happened fast once the factories figured out that a race worthy 500cc two-stroke was indeed possible. Yamaha hit back next, using their experience with 250cc 2-cylinders to build a 500cc 4-cylinder bike, on which Giacomo Agostini rode to a world championship in 1975.

 

The great Barry Sheene and his now-famous RG500 with a Square Four 500cc 2-stroke engine.

 

But Suzuki was quick to strike back. Encouraged by their success with the TR500, but understanding that they needed a ground-up 500cc two-stroke build, they developed the RG500. The RG500 had a square-four engine developed on the basis of the factory’s 250cc GP bikes, which cranked out over 100 bhp and could reach speeds of up to 175 MPH. The RG500 would become a legend in the skilled hands of Sheene once again, who won World Championships in both 1976 and 1977. Click Here…

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