Simplify. And Add Lightness

[Hugh Janus]

The benefits of light weight and simplicity in racing motorcycles. (Robert Martin/)

Walking down from the shop last night I was humming The Beach Boys’ 1963 hit “Little Deuce Coupe” when I became curious about the relative weights of that famous coupe and the Thunderbird mentioned in the song. A few keystrokes later and I’d found that the T-Bird outweighed the coupe by around 40 percent. No wonder that “When somethin’ comes up to me, he don’t even try…”

As anyone who’s ever played around with rockets and other things that accelerate knows, the rate of acceleration is the thrust divided by the mass of the vehicle. Reduce the mass by 40 percent while keeping the thrust the same, and the result is that the deuce coupe accelerates 40 percent faster with equal power.

As soon as Detroit realized that Americans in the mid-to-late 1960s would buy thousands of factory hot rods, people mostly stopped building or even thinking about hand-built street coupes weighing 2,200 pounds. Machine shop, fab, and welding skills required? Forget that! Installment plan, here we come! Detroit was soon cranking out 7-liter monsters that could move mountains, which is just what those giant pressed-steel “lumber wagons” were.

Yet weight remains as important as ever. If your bike with you on it weighs 600 pounds, and its engine makes 150 hp, that’s a power-to-weight ratio of 600/150 = 4 pounds per horsepower. That in turn tells us that the 4-pound pair of boots on your feet is costing you one whole horsepower. One whole horsepower is required just to accelerate those boots, while the other 149 get on with the big picture.

In the early 1980s the weights of big-engine bikes had bloated their way toward 600 pounds, so Suzuki’s very light, original GSX-R750 was a dead cert to leave them behind, which it did. Weight crept up again, so Yamaha’s release of the first R1 literbike, hailed as “feeling like a 250,” taught the same hard lesson over again: Acceleration is the thrust divided by the weight.

My doc (and any bike I might ride) wants me to weigh 167 pounds instead of my actual 200—a difference of 33 pounds. If every 4 pounds of weight reduction is equivalent to a horsepower, that’s like a gain of 8.25 hp. That makes me remember other terrible numbers. The four smokestacks on Yamaha’s two-stroke RZ500 four weighed 42 pounds! Later, when titanium pipes hit Supersport, I was handed a complete four-cylinder system, muffler included: 6 pounds! That’s a difference of 36 pounds.

Weight growth affects air forces too. Here, let’s build a large number of small and inexpensive but extremely agile lightweight fighters. Then the process begins. Why limit ourselves to that tiny radar? Put on this larger array. Except that increases fuselage diameter, drag, and weight. It also sucks more electricity, so now we need more generator pads on the engine(s). Growing weight shortens range, so we have to fill every empty space with fuel, plus valves, lines, and pumps. Gosh, now we need to beef up the landing gear, and our airplane is starting to handle like what it’s becoming—an air tanker. No problem! An enlarged wing and bigger control surfaces will fix that right up. Hm, takeoff distance is growing because acceleration is dropping, so now we need a serious thrust increase from the engine…

It doesn’t end until someone with the authority to change things pulls up a fresh screen and says, “Do it over. From the beginning. Get it right.”

I was also impressed by the big differences in approach to racing motorcycle design in the 1950s. The English wanted more than anything else to win the Isle of Man TT, so they built engines and chassis rugged enough to finish the seven 37.5-mile laps. Speeds were generally high, so wheelbases tended to creep up, and engines were biased toward peak power (nothing below 5,500 rpm, all done at 7,000). Meanwhile at Guzzi, engineer Giulio Carcano, charged with winning the 350 GP championship over a variety of European tracks, some quite twisty, designed for acceleration. His engines featured wide torque ranges and his bikes weighed as little as 216 pounds. Guzzis took five successive 350 titles, 1953–1957 inclusive.

Around 1980 Honda’s Shin’ichi Miyakoshi, impressed that the lap time difference between 500s and 250s in European GPs was small, thought about the idea of a 100 hp 250: “What if we built a bike with the weight and agility of a 250 and the power of a weak 500?” They built it and Freddie Spencer rode it. In 1982, at the very fast Belgian Spa circuit, his 108 hp Honda topped the Yamaha and Suzuki opposition, all making 25 percent more power. Freddie’s lap times showed that their extra power was just waste heat. A year later he was 500cc world champion.

Weight is just one of a great many motorcycle variables, but if we don’t watch our motorcycle’s diet, weight creeps up and performance is lost. Remember: Acceleration equals thrust divided by the weight.

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