Mass Centralization

Weight loss isn’t just a post-holiday resolution.

Marc Marquez Honda RC213V

I have been thinking of Honda’s use of the term “mass centralization,” and it occurs to me that this is an entirely natural process that has happened by itself. Honda PR people just like the sound of it and adopted it.

Motorcycles are naturally mass centralized because of the vertical “sandwich” of rider, fuel, and engine. The only masses that are not centralized are the wheels, but the developmental events of the past 40 years have greatly lightened wheels, tires, and brakes. No one company has led this process; really, racing has accomplished it.

Years ago, I weighed front wheels that were as heavy as 50 pounds. Rear tires weighed up to 13 pounds, with an inner tube adding another three. Race tires today weigh seven to 10 pounds, and tubes are no longer used, taking nearly 10 pounds of weight away from positions very distant from the bike’s center of mass.

The disc-brake revolution first gave us 12-inch discs, which were 7mm thick, had wide pad tracks, and weighed several pounds. At the 1972 Tokyo Motor Show, Honda displayed a race bike with much thinner 5.5mm discs, and since then, pad tracks have narrowed (to bring the heating rates of inside and outside diameters closer together, minimizing disc distortion), resulting in much lighter discs on production bikes. Race bikes have adopted carbon-carbon discs, whose density is only 20 percent that of iron. Another big chunk of mass, formerly located far from the bike's center of mass, now gone.

Before 1973, the usual production wheel consisted of a hefty hub, 36 or 40 steel wire spokes, and a steel rim. For racing, the steel rim became aluminum, a metal whose density is only 36 percent that of steel. English racer Peter Williams had experimented with cast "artillery wheels" on his Matchless G-50 in the late '60s, and in 1973, Elliott Morris produced his seven-spoke cast magnesium racing wheels. Classic-era-rider Phil Reed hand-carried a set of Morris mags to MV Agusta. Soon there were also Shelby-Dowd mags, Campagnolo mags, Dymags—lots of choices. Rims grew in width from the 2.13 inches of an old-tech WM-3 to rear widths as large as 6.5 inches, but wheels became ever lighter. Today, forging magnesium wheels, rather than casting them, gives greater fatigue strength even in a much lighter product, shaving further weight. Such wheels, in hands accustomed to the 50-pound car wheels of the '70s, feel like paper.

Honda RC166 studio image

The front brake calipers on YamahaTZ750s were cast iron to make them rigid enough not to let the handlebar lever come in too far. But they weighed a whacking 4 1/2 pounds apiece! In 1974, riders on those bikes at Daytona found their mass, located ahead of the fork legs, made the front end feel heavy and clumsy. Right after second practice, those heavy calipers were moved behind the fork legs, putting them much closer to the steering axis. And there they stayed throughout the 750's racing lifetime. When Yamaha supplied much lighter aluminum calipers on the 1980 TZ750, riders discarded them as too springy (aluminum has only one-third the stiffness of iron). But in all the years since, caliper design has evolved until today's Brembo MotoGP caliper weighs just one pound but supplies the stiffness necessary for a firm lever. That change took seven pounds away from a forward position far ahead of the machine's center of mass. That was a big contribution to mass centralization.

The rider—a very substantial mass—has steadily moved forward from a starting position behind a rather long fuel tank to a present position more above rather than behind the tank. The fuel itself, at first concentrated in a wider, taller fuel tank, began to move downward to place some mass under the rider's seat. All of these changes have naturally increased the modern motorcycle's mass centralization.

What other parts do motorcycles have that are normally far from their center? No one mounts a battery in the seat back or up on the front fairing stay; batteries have always been central. The chassis already surrounds the center of mass. That leaves the fork and swingarm, both of which have been made as light as possible, extending away from the center. The change from conventional to “upside-down” forks has placed the larger-diameter tube closer to the machine’s center, with only the smaller sliders extending to the front axle. Today’s swingarms have to be hefty to provide twist resistance, but they are probably no heavier than the old tube-and-lug constructed steel arms. Axles have gained stiffness from geometry, being made very large, hollow, and thin-walled (stiffness of tubes increases at something like the fourth power of diameter; make the axle 10 percent bigger in diameter, with the same wall thickness, and you get a 46 percent increase in stiffness).

This is why I have concluded that mass centralization has been a naturally occurring process in the development of racing motorcycles, leading to similar changes in production bikes. The result is that the motorcycle becomes more like a 24-pound cannonball and less like a 12-foot ladder of the same weight. Which one can you rotate faster to a new direction? It’s no contest.