Whatever Happened To Lightweight Motorcycles? | Cycle World

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Whatever Happened To Lightweight Motorcycles?

Despite recent advances in materials and processes, race-sanctioning bodies have made weight irrelevant

jeffrey carver jr riding a harley davidson xr750

Still winning so many decades later: Riding Harley-Davidson's legendary XR750, privateer Jeffrey Carver Jr. took top honors at the American Flat Track Lone Star Half-Mile and finished second at the season-ending Flat Track Finals.

Andrea Wilson

Motorcycles originated as little more than reinforced bicycle frames powered by rudimentary single-speed engines, but evolutionary pressures quickly drove up the weight. This became a real concern by the late 1920s and early ’30s as large, dense arrays of cooling fins became necessary to dissipate the waste heat of ever-rising horsepower. Big-finned air-cooled heads and cylinders were heavy!

When the fabled Harley-Davidson aluminum XR750 was being drawn by Piet Zylstra, H-D’s racing manager wanted a hefty thickness of solid metal above the combustion chambers. This was the engine’s heat sink, serving both to store heat during periods of full throttle and to efficiently conduct that heat to the cooling fins.

Crankshaft weight was another area in which weight was hard to save because, as famed tuner Rob Muzzy once observed, a light crank may make your bike accelerate better but don’t be surprised if it top-ends poorly.

Chassis weight increased as engine vibration intensified, causing everything to crack. Those who remembered the early days of spindly 150-pound motorcycles deplored all this weight growth. In their view, 300 pounds was a ridiculous weight.

As bikes went faster, they needed better brakes, and the lighter the brake, the higher the peak temperature it reaches during braking for a hairpin from top speed. Make brakes too light and they warped and cracked. During the 1930s, drum brakes on bikes racing in the Isle of Man TT events grew to 8 inches in diameter and 1-1/2 inches shoe width.

In the post-WWII era, for reasons of weight it was generally found to be better to build for a given racing class by enlarging a smaller-displacement engine than by sleeving down a bigger one. The classic examples were from MV Agusta (the original company, not to be confused with modern badge engineering). MV and others had tried racing 250cc twins made by putting two of their successful 125cc single top ends on a common crankcase, but the results were powerful yet heavy and slow. Better results came immediately from enlarging the 125 as much as possible, resulting in a 204cc single.

The negative example resulted when MV tried sleeving back its 500cc four to compete in the 350cc class. The dominant design at the time was Giulio Cesare Carcano’s ultra-light horizontal-single Moto Guzzi, which was in effect a powered-up 250. While the rider on the much heavier MV or Gilera four was braking and wrestling the weight of a 500, the Guzzi man was flicking the single’s feathery 217-pound lightness into the corner and quickly accelerating away. In this way, Guzzi won five consecutive 350cc titles.

Nine years later, when Honda considered how to counter the threat of MV’s lightweight 350cc triple, its engineers realized that further hammering on the existing (and substantial) 350cc four wasn’t the answer. So for 1967 Mike Hailwood was given a 297cc version of the 250 six into which so much development had been poured.

A stunning example of what light weight can do was the 1972–’73 clash between the newly arrived (and now dearly beloved) 750cc four-strokes, such as the Triumph/BSA triples and Harley’s new aluminum XR750, and Yamaha’s dinky little 350cc two-stroke twins. It’s fashionable to blame the four-cylinder TZ750 for chasing four-strokes out of the class, but it was the twins, roughly 100 pounds lighter than the big bikes, that first sent them away. Consider just one part, the crankshaft. That component of the 350cc Yamaha weighed 13 pounds, while the crank of the BSA Triple scaled three times that.

dani pedrosa motogp race action

Repsol Honda's Dani Pedrosa won two MotoGP races this past season, both coming in his home country of Spain, and finished fourth overall in the championship.

Courtesy of Honda

Today, when light weight is more easily achieved through recent materials and processes, race-sanctioning bodies have made weight irrelevant by specifying high minimum weights. Dani Pedrosa is not the only one who regards today’s 353-pound MotoGP “sleds” as exhaustingly heavy. But the fashion in rulemaking is to save teams the expense of having to work with exotic materials.

In the early 1990s, 500cc two-stroke GP bikes shrank to as little as 250 pounds dry. The FIM, alarmed by numerous corner-exit high-side rider injuries, felt it essential to appear to be “doing something.” That something was the 287-pound minimum weight it imposed on the class.

Racing has continued along that line ever since, with minimum weights to “level the playing field.” To save weight on his 1950s Guzzis, Carcano eliminated both gel coat and paint from his team’s fairings—too heavy! No point in such things today, when all bikes must weigh the same.