Motorcycle Center of Gravity Motorhead Myths

[Hugh Janus]

Kevin Cameron (Robert Martin/)

As a young adult, free at last to embrace the motorcycle, I began to drink in its culture through the British magazines on offer at a nearby news kiosk. Magazines and newspapers in every language were crowded together on four walls and some jostling was required to reach the titles I wanted. Soon I learned the litany of British motorcycle design: that a low center of gravity was the key to handling.

It didn’t occur to me to ask myself, “What is handling?” because I had no experience—only interest. Only gradually would I learn that handling was something like rapid, stable, and predictable maneuverability. I learned also that motorcyclists were divided by strong and contradictory opinions. Riders of large, heavy machines tended to look down upon riders of British twins and singles as upstarts, newbies ignorant of what’s really important. The buy-British crowd loved the lightness, quicker maneuverability, and stronger acceleration of the bikes they rode, and regarded the heavyweights as relics of a 50-year tradition. Just beginning to appear in 1960 were small Japanese machines, hardly more than overgrown mopeds at first.

For riders of heavy bikes, having a low center of gravity makes the machine feel lighter and requires less muscle to reliably support at stoplights or during parking maneuvers. The lower most of the weight is located, the less it moves off-center for a given angle of tilt, and the less it feels as if it might get away from you if it tips too much. You feel this apparent lightness by standing over the bike and rocking it from side to side; the lower the CG, the more manageable the bike’s weight feels. Outstanding in this respect is Honda’s Gold Wing, which feels much lighter than its 800 pounds because its six cylinders and its heads are at the same height as its longitudinal crankshaft.

Honda’s Gold Wing feels much lighter than its 845-pound weight because of the low center of gravity. (Jeff Allen /)

On the other hand, no one is going to pretend that the long wheelbase and weight of touring machines give them “rapid, stable, and predictable maneuverability.” Stable and predictable they are, but quick-steering they are not. That’s fine for their purpose, which is to cruise the highways in comfort.

Another specialized activity in which low CG works is drag racing. Because front wheel lift sets the upper limit to acceleration, drag racers are built as low as possible, with their major masses far forward. This is a specialization not practical on street or highway because the low engine touches the pavement at even a modest angle of lean and the extreme weight on the front wheel makes low-speed steering heavy. Dragsters have traction not available on the street because their tires are preheated before each run.

Let’s set aside the special cases of touring and dragbikes and consider the British twins that invaded the US after World War II. What many riders liked about them was their more athletic nature. They accelerated strongly and responded quickly to rider inputs. They made their riders feel more like they were on a quick-turning quarter horse than at the throttle of a thundering locomotive (clearly, both have their appeal). Elements of the difference were lower weight, shorter wheelbases around 54 inches (as opposed to over 60 for many heavyweights), and steering geometry biased toward quick response (around 27 degrees of rake and 4 inches of trail) than toward all-day cruising stability (30-32 rake, 4.5 inches trail). Another element was generally lighter wheels, whose lower gyro resistance to steering reduced control forces at the bars.

Did British bikes have low CGs? In fact they did not, as their engines had to be mounted high enough not to ground during sporty turning. The British-made machines most praised for their handling—the classic single-cylinder roadrace bikes like Norton’s Manx and the Matchless G50 had tall vertically mounted engines—every one with its complex and heavy OHC head up high, right against the bottom of the gas tank.

Off I went to the races as a know-nothing, to find that unquestioned beliefs and second-hand magazine wisdom weren’t much help.

Things happened that those ideas couldn’t explain. Around 1971 everyone was nodding and smiling in agreement that the path to stability and control was longer swingarms. But the next year, there was our heroic Master of Wobbles, Yvon Duhamel, at an Ontario Motor Speedway test day, turning the gas to accelerate off corners with the prescribed long swingarm and spinning—not hooking up and going forward. Sliding out. Going slow.

But when they switched to a shorter swingarm, turning the gas threw more weight on the back tire, it hooked up, and his lap times dropped. Could it be that there is a difference between handling and performance? The long swingarm certainly killed some wobbles and made bikes of that time easier to handle. But they went slow.

The late Hurley Wilvert, a great rider/engineer in the best American tradition, rode a comparison test between his own Kawasaki H2R (whose low frame he had designed in the British low-CG style) and a bike with a much higher-set engine, built by team tech Randy Hall. Wilvert told me the stopwatch and his own perceptions didn’t lie. As much as he wanted to believe his own bike was on the cutting edge, he was faster on Hall’s tall bike. On his own low bike, when he fed throttle while leaned over to exit a corner, the tire spun rather than hooked up. But on the tall bike, during acceleration the extra height transferred more weight onto the rear tire, making it hook up and go forward. Sure, the front kept lifting up and letting the bike head for the outside, but it was definitely better to go faster on a bit of a handful than to go slow on a tour bus.

In 1984, Honda brought its first NSR500 to Daytona, built in approved low-CG style with the fuel tank slung under the engine and the exhaust pipes (which are mostly empty space) routed over the top with the rider protected from their heat by an insulated dummy “tank.” Honda was funding a French group that was applying Formula 1 concepts to bikes. Very exciting.

The new NSR’s problems didn’t appear until later in the Grand Prix season, when rider Freddie Spencer noted that it was sluggish in direction changing on twisty tracks. A test was laid on, consisting of a slalom course made with traffic cones. The rider would ride the slalom at successively higher speeds until he began to knock down some cones. Baseline was the best slalom time Spencer could achieve on last season’s three-cylinder NS3, a fast-maneuvering bike. Both bikes were tested with full fuel. When Spencer entered the cones on the new bike at the NS3′s highest entry speed, the bike knocked down the cones; it could not be manhandled through the direction change as quickly as the triple.

Next, they removed all but a quart or two of fuel from the new bike’s underslung tank, and added roughly 35 pounds of lead weights above the engine, simulating a full top-mounted tank. This time, the new bike went through the cones just as quickly as had the triple.

Why? Engineers had imagined that as a bike rolls in changing direction it is pivoting on a line through its two tire footprints. If that were true, putting the fuel on the bottom would make it easier to pivot because it would be swinging through a shorter arc than if it were up on top of the engine, farther from the ground.

But that’s not what happens. At one track I could stand and watch bikes coming straight toward me, then turning right. In light rain I could see the trace of their travel as a line on the track. As the riders countersteered to their left to make their bikes flick to the right, the tire footprints moved to the left as the tops of the bikes tilted to the right: They were rolling, not around a line connecting the tire footprints, but around the center of gravity of bike, fuel, and rider, which is about 22 inches off the pavement. They were pivoting around their roll axes.

Seen in this way, the slalom test result makes sense. Putting the fuel under the engine moved it farther from the roll axis, creating more resistance to the motion. Putting the fuel back above the engine reduced its distance from the roll axis by roughly half. No wonder fuel tanks in MotoGP have been changing their shape, growing a “foot” that extends back under the rider’s seat, where the fuel’s mass is right on the roll axis.

At Laguna one year American Honda set out one of its RC45 V-4 750 Superbikes as a display. I went to it and measured its crankshaft height, wheelbase, and distance from front axle to crankshaft. Then I went to see Rob Muzzy and asked if he’d mind my doing the same with one of his team bikes. Result? Another upsetting, funny-shaped brick to somehow be fitted into the temple of knowledge. The Kawasaki, with its wide four-cylinder inline engine, carried its crankshaft some 2.5 inches lower than did the V-4, which was only two cylinders wide. How could this be? Doug Chandler’s riding style was high corner speed, so the Kawasaki was built for stability: long and low. Because Chandler’s apex speed was high, he braked and accelerated less. Miguel Duhamel, getting the “terrible 45” turned in point-and-shoot style, needed instant weight transfer to generate prompt rear traction to accelerate early and hard.

Honda’s RC45 carried its crankshaft 2.5 inches higher than the Kawasaki ZX-7RR that it competed against, suiting Miguel Duhamel’s point-and-shoot riding style. (Cycle World Archives/)

For me the final nail was the discovery that sometimes braking distances are shorter if a bike is made taller. As a bike accelerates from one corner to the next, its tires cool significantly, losing some grip. If the rider just pulls the lever at his/her braking point, the cooled front tire may lack grip and slide or lock. But make that bike just a bit taller, increasing the brake-induced transfer of weight to the front, and the extra load makes it grip.

Most road and street riders wisely do not ride at a level that brings all of the above into play, but it does partly explain why powerful sportbikes are built as tall as they are, with higher riding positions than in the past. The rest of the explanation, of course, is that today’s much grippier tires allow lean angles that were impossible 20 or 30 years ago. That requires engines and footpegs to be placed higher to allow cornering clearance.

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[Catteeclan]

And asimo's built by Honda.