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Motorcycle Power-to-Weight Ratio and Acceleration

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Mass, which we measure as weight on a scale, determines how quickly a machine and the load it carries can accelerate, as seen from the familiar relationship: Acceleration equals thrust divided by weight. When we were mad youths, we added up the weight of machine, fuel, rider and gear. We divided that by (claimed or suspected) engine horsepower to get the power-to-weight ratio. As an example, if a given bike weighs 375 pounds, the rider and gear are 180 pounds, and fuel on board is 15 pounds, the total is 570 pounds. If the engine’s peak power is 100 hp, the power-to-weight ratio is 570 ÷ 100 = 5.7 pounds per horsepower. This number tells us that removing 5.7 pounds of weight is equivalent in performance terms to adding one extra horsepower. This is why, in “The Grand Prix of Gibraltar,” Peter Ustinov’s audio spoof of Formula One racing, the manager of the US team (whose entry is actually a prototype dream car called “Cabana Club”) says, “If there’s time before the race, we plan to remove the armrests and fit a lighter dash clock.”

Back in the 1950s, Moto Guzzi engineer Giulio Cesare Carcano applied power-to-weight logic remorselessly to that company’s single-cylinder 350 GP roadracer, getting its weight down to 216 pounds by 1957. For five straight years, 1953-57, that light, simple machine was able to defeat the higher-revving and more powerful but heavier and less maneuverable four-cylinder 350s of Gilera and MV.

Making the Bike Do What We Want

A special case of power-to-weight ratio is the one relating our own muscular power to vehicle weight. Any of us can yank a 200-pound MX bike around smartly, but it can be a different story threading through slow traffic on a 900-pound tourer or a top-heavy 550-pound ADV rig. The heavier a bike is, the more effort is required to maneuver it, especially at low speed. Honda, at its last redesign of its Gold Wing tour bike, reduced its weight significantly. Experienced riders on such heavyweights display admirable skill, but great weight can be a deterrent to first-timers.

What are Mass Properties?

Weight is a single number. What does the scale say when we roll our bike onto it? Mass properties describe how that weight is distributed. If we concentrate mass close to the center of a bike, rather than locating it farther from the center, we can reduce the effort required to lean it over into a turn and set it turning. If we are carrying a 24-pound ladder and want to change direction, it takes effort and planning, but if we redistribute that 24 pounds into a cannon ball 5-1/2 inches in diameter, we can turn on the proverbial dime.

The same relationship also applies to the effort involved in rolling a motorcycle over for a turn. The closer the major masses are to the center, the easier and quicker the roll-over will be. A big lesson hit the industry in 1984, when Honda built its first NSR500 four-cylinder GP bike in Elf fashion, meaning the heavy fuel tank (full fuel of 6.2 gallons weighed just under 40 pounds) was carried under the engine and the much lighter exhaust pipes routed over the top, under an insulating cover (Elf is a French gasoline company that funded years of motorcycle chassis research). When the vastly talented rider Freddie Spencer complained that the new bike was slow in roll-over, a slalom course was laid out using traffic cones. Freddie ran the three-cylinder bike from the previous year through the course to set a baseline entry speed. When the 1984 bike was ridden into the slalom at the same speed, it knocked down the cones. Verdict: something was slowing its ability to change direction.

Related: Motorcycle Agility and Rotating Masses

As a check, the underslung bike was drained of all its fuel save for a quart, and ballast weight equal to a full tank was placed among the pipes above the engine. In this condition, the new bike whizzed through the slalom as fast as the ‘83 triple had.

What did this teach the engineers? It showed that bikes roll around their center of mass, which is located nearly 2 feet above the pavement. Carrying the fuel under the engine rather than above it moved it farther away from the center of mass, meaning that for a given level of rider effort, the bike rolled over less quickly. The center of mass is the point at which an object, if supported from it, would be in balance around all axes.

The industry rushed to equip itself with mass properties rigs that could measure a bike’s polar moments, its resistance to rotation about the three axes of roll, pitch, and yaw. When I visited Erik Buell’s factory, there were two such machines, one for whole motorcycles and a smaller one for wheels. Mass centralization was a priority for Erik because he knew that made his bikes more responsive to rider control inputs.

Mass props machines consist of a turntable on low-friction bearings that is free to oscillate against springs, rather like the escapement wheel of a mechanical watch or clock. The bike or assembly of interest is placed on the turntable with the axis of interest vertical and centered on the turntable axis. The turntable is set to oscillating and the oscillation frequency around that axis is noted. Comparison with that of a test mass of known polar moment, plus some arithmetic, reveals the polar moment of the bike around that axis. Today the same information can be obtained by processing the data from on-board IMUs as a vehicle is maneuvered.

Making Weight More Manageable for the Rider

Rapid maneuverability in roll can be important to accident avoidance, but there can be a different priority in the case of especially heavy motorcycles: the rider’s ability to manage the weight at a standstill or during parking. If the rider accidentally allows the bike to tip to one side, the higher that bike’s center of mass is located, the greater the “overturning torque” felt by the rider. We don’t enjoy the anxiety of feeling that a heavy bike is about to get away from us. Therefore it is usual to locate the major masses of such heavy machines as low as is consistent with being able to lean over far enough for ordinary maneuvering. And so we find quite limited maximum angle of lean, just over 30 degrees, in such bikes. That’s far from the 45–55 degree capability of more sporting machines, and the 60-plus degree capability of outright roadrace bikes on their “tires of tomorrow.”

Related: Fundamentals Of Unsprung Weight

Big and heavy motorcycles have gained nimbleness since the later 1970s by applying experience from motorcycle sport. In olden times it was considered safest to give tour bikes conservative (sluggish) steering geometry to guarantee stability. But as chassis, and fork assemblies in particular, have become stiffer, it has been possible to quicken steering geometry without loss of stability, and to reduce steering delay by use of larger, stiffer fork tubes and axles, plus more rigid clamping in fork crowns.

Mass Properties and Weight Transfer

Acceleration: For sporting and racing motorcycles being ridden close to their limits, weight transfer during acceleration or braking is important. As a rider throttles up to exit a turn, the speed of the exit depends on rear tire thrust, and that depends on the applied load. If there is less weight transfer to the rear tire because the bike’s wheelbase is longish or its center of mass is on the low side, the maximum thrust available from the rear tire will be lower than if the weight transfer from acceleration puts nearly all the weight on the rear tire. What the rider perceives is either 1) too little load on the rear, letting the tire spin, or 2) with enough weight transfer to make the tire hook up, the bike accelerates.

The upper limit of acceleration on a motorcycle is set by point when that front wheel is lifted off the pavement by the engine’s torque on the rear wheel.

The upper limit of acceleration on a motorcycle is set by point when that front wheel is lifted off the pavement by the engine’s torque on the rear wheel. (Jeff Allen/)

Braking: The reverse situation affects braking. If the bike is too low or too long, less weight will transfer to the front and the front wheel may lock rather than grip during hard braking. But if the bike’s weight is in the right place, weight transfer can put 90-plus percent of it on the front tire, giving it the load it needs to generate maximum stopping force.

These concerns can seem remote from everyday riding, the concern of racers as they try to set up their chassis to achieve best lap time. But because production bikes may be called upon to turn, stop, or accelerate hard in order to avoid traffic threats, these concerns must remain an important basis for design.

A Special Kind of Weight: The Wheels

Today’s abundance of horsepower tends to conceal the importance of weight, but in times past, when less power was the norm, the special importance of wheels had to be considered. Why? Because during acceleration, the engine must not only accelerate the weight of the wheels in a straight line, it must at the same time accelerate the weight of tire, tube, and rim whirling around the axle centerlines. Therefore, weight in tires and rims counts double.

In the 1980s I watched countless vintage racers lever on the biggest tires that would fit, sure that more rubber guaranteed more grip and higher corner speed. Yet wider tires also added weight where its effect is doubled. The same was true of rim diameter; the British regarded the 1950s-60s Italian move to 18-inch rims, down from the 19, 20, and 21-inchers of tradition, as radical and odd. Yet each inch of reduction meant that a 3.14-inch length of tire and rim were no longer present, and no longer had to be accelerated twice.

In search of better performance wheels became lighter and smaller in diameter.

In search of better performance wheels became lighter and smaller in diameter. (Jeff Allen/)

The pursuit of light weight must be considered against other goals, such as structural stiffness. More than one of the Japanese majors has built ultralight versions of their racebikes in hope of realizing higher performance. But test riders dashed those hopes by reporting high-speed instability resulting from chassis flabbiness. Fortunately, designs stiffer than traditional twin-loop or cradle tube frames came into being, providing the necessary stiffness at a lower weight penalty. I was stunned when I put one of the once-revolutionary Norton “Featherbed’’ steel tube frames on a scale and saw the dial whirl around to 42 pounds.

All of the above is not to denigrate the classic designs of the past, which some of us continue to own and enjoy. As technology advances, new levels of performance become possible.

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