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

by BikeBandit 

Today’s 240-plus horsepower MotoGP bikes take an incredible amount of skill for even the best racers in the world to control. But they are nothing compared to the wildly powerful, brutally accelerating, crash-prone 500cc two-strokes used in GP racing for nearly three decades. Check out the history of these beastly race bikes that scare even the best riders on earth! 

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.

But again, Yamaha’s development was fast, and they then developed the OW models, which mated well with the aggressive, tire-sliding riding style of American dirt track prodigy Kenny Roberts. Roberts wheelied and slid his way to an incredible three World Championships in a row, blowing the world away with not only his speed and riding style, but demonstrating the potency of Yamaha’s machines, which by 1980 were twisting out an incredible 130hp.

 

The Yamaha YZR500 became the bike to beat in the skilled hands of great American flat-tracker-turned-GP-racer “King Kenny” Roberts.

Honda, meanwhile, stuck to its guns as a four-stroke company, which they had huge success with in the 125cc and 250cc classes in the 1960s. But as the rules throughout the 1970s kept both four-stroke and two-stroke displacement capped at 500cc, two-strokes had a clear advantage. Even at the height of the two-stroke era, Honda stuck to it’s guns, building the NR500. With unique oval pistons and 8 valves per cylinder, it was unquestionably the most sophisticated engine design in GP, but it remained at a disadvantage due to the huge power differences inherent in its four-stroke design.

Honda finally succumbed to the trend in 1982, finally building their own 500cc two-stroke. But the approach they took was dramatically different from the Yamaha and Suzuki who, locked in a power war, were building machines that were so powerful they became almost unmanageable, winning races almost as often as they launched their riders into spectacular crashes.

Honda’s project was led by a motocross engineer, who opted for light weight and maneuverability at the expense of outright power. The result was the now-legendary NS500, a light and slim race bike with a unique three-cylinder design in an L-configuration. Though less powerful than it’s rivals, the Honda was much more manageable, and Honda’s design philosophy was proven when Freddie Spencer took it to two World Championships.

Honda’s triple-cylinder architecture was unique, and made for a remarkably rideable bike (for being a 500cc two stroke anyway.) But at the end of the day, much of GP racing is still a power war, and it was still no match for the narrow, well-balanced, and massively powerful V4s that dominated the grid. Once Honda went full-bore into both two-strokes and the dominant V4 engine architecture, they became virtually unstoppable on the racetrack. By the late 1990s, Honda’s V4-powered bikes piloted by the brilliant Australian racer Mick Doohan won a remarkable 5 consecutive titles. 

Australian racing champion Mick Doohan mastering Honda’s brilliant NSR500, which he rode to an incredible 5 consecutive world titles.

By the end of the 1990s, a full generation had passed since 500cc two-strokes stormed onto the tracks of Grand Prix – which became apparent when a second-generation GP racer, Kenny Roberts Jr., followed in his dad’s footsteps and won the world championship in 2000. Robert’s Jr. won it on a Suzuki, far advanced from the Suzuki that first proved 500cc two-strokes could even be race-worthy at all. By then, these vicious bikes were weighing well under 300 pounds and putting out 160-180hp – more than double the first generation of engines in the class, which were considered difficult to control even in their time.

But by the 2000s, two strokes had run their course. The motorcycle industry in general was turning away from two-strokes, as government regulations rendered them obsolete on the streets, and as manufacturers began to place more emphasis on four-stroke development that they could apply to their consumer models.

But before two-strokes sang their swan song in GP racing, the 500cc era would give motorcycle racing fans one more special gift – an up-and-coming young racer from Italy named Valentino Rossi. Riding for Honda, and under the tutelage of the legendary Mick Doohan, Rossi mastered the unruly two-stroke beasts, dominating the class for the last two years it remained. 2002 would see a rules change that allowed four-strokes nearly double the displacement at 990ccs, after which the two-strokes were placed at a disadvantage, quickly disappearing from the grid thereafter.

Rossi would go on to become a legend in MotoGP, with a staggering 7 more GP championships in the premier class alone. But Rossi always maintained a fondess for the wild, brutally powerful 500cc two-strokes that posed a constant challenge even to the most skilled racers in the world, a sentiment echoed by many of the racers who were around during that era of racing.

Even still, the 500cc two-strokes – with their otherworldy sound, distinct exhaust smell, and a hyper-aggressive nature unmatched by any of todays ultra-refined bikes – maintain a legendary status, and awe, charm, and thrill spectators and riders alike to this day.