Motorcycle Agility and Rotating Masses

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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/)

A motorcycle’s agility depends upon three main considerations:

1. Its overall weight
2. Its steering geometry, including wheelbase
3. The resistance of its rotating masses to changes of direction.

Weight is basic. The heavier an object is, the greater the effort we must apply to move it in any way.

Motorcycle steering depends upon the lateral movement of the steered front wheel. When we countersteer to roll our bike over for a turn, we are steering its wheels out from under it so that it falls over in the desired direction. We steer the front wheel, and as it tracks to one side, its lateral movement steers the rear wheel via the lever arm of the wheelbase. Thus, the longer the wheelbase, the smaller the steer effect for each inch of the front wheel’s lateral movement.

Dirt-track motorcycles have a very short wheelbase, just 54 inches or so. Their front and rear wheels are essentially as close to the engine as function permits, giving maximum turning response.

Short wheelbases bring the wheels closer to the engine for quick steering response. (Jeff Allen/)

On the other hand, as illustrated by a retired couple out west gliding past Shiprock on their big touring rig, stability is a valued quality. This and the need for room and comfort aboard dictate a long wheelbase, up to 65 inches.

Understanding Rake and Trail

The two numbers that describe motorcycle steering itself are the rake and trail. Rake is the angle by which the steer axis tilts back from the vertical, and trail is the distance by which the center of the front tire’s footprint trails the intersection of the projected steer axis with the pavement. Typical values for rake are:

• Roadracing: 23.5–25.5 degrees
• Sportbike: close to 24 degrees
• Older bikes: 27–28 degrees
• Touring bike: 30 degrees

Older motorcycles often had less stiff chassis, and it was difficult to make stable using the quicker-steering smaller rake angles that are common today. It was, however, quite common in the 1950s–1970s for a team to take matters into its own hands by sawing off the steering head, adjusting it to another rake angle, and welding it back in place. You get answers by trying stuff.

The Gyroscopic Effect and Steering

Now we come to the difficulty of steering the front wheel. Do the classic experiment with a bicycle wheel: Take the front wheel off your bike, hold it by its axle ends, and have an assistant give it a spin. When you try to steer the spinning wheel, it resists. This gyroscopic resistance is proportional to the weight of the rotating object, and each bit of the spinning mass is also proportional to the square of its distance from the rotational axis, and to the square of the speed of rotation.

Notice the strong influence of distance from the rotational axis. In the case of a wheel, the part farthest from the axis is the tire, followed closely by the rim. At a much smaller distance from the axis is the material in the brake discs, commonly 11 to 12.5 inches in diameter.

When a young rider I knew switched from a 250cc machine to a 750, he reported after his first practice, “I’m having to use all my strength to get this thing out of turn 2 and into turn 3,” a right-to-left direction change. The bigger bike weighed 35 percent more than the 250 he was used to, and its front wheel had two brake discs instead of one, as well as a larger-section front tire.

Many examples exist of competition riders whose sincere efforts to steer rather than run off the pavement have bent their handlebars. When the next corner is rushing at you and the bike is slow to roll over, you give it all you’ve got.

This affects all motorcyclists, not just racers. Many have remarked on the drastic difference in ease of steering between a 500-pound sit-up air-cooled literbike of 1975–1982 vintage versus a 600 supersport bike of the later 1990s. The two bikes may have the same horsepower, but the 600 might weigh 100 pounds less. The weight difference helps, but the real biggie in the 600′s quicker steering was the reduction in the weight and diameter of the rotating front wheel. Wheel size dropped from 19 to 17 inches, with a few years of 16-inch wheels in between. Tires went from tube type to tubeless, and the wheels themselves became much lighter. The 7-pound brake discs of the early years (which make great bases for floor lamps!) gave way to the lighter, thinner discs of the ‘90s, which could handle higher operational temperatures thanks to the distortion-free expansion that their floating mounts allowed.

Spinning the Crank Backward

Once you show riders what’s possible, they want more. The carbon fiber or forged mag wheels in today’s racing are lighter still than the cast mags first available in late 1973. Yamaha became aware of what we might call “gyro cancellation” when it adopted jackshaft drive on its early two-stroke roadrace fours. Using a jackshaft from the center of the crank over to a clutch on the right side reversed engine rotation.

Although this seems crazy, a small-diameter (100mm or less) steel crankshaft rotating “backward” at 10,000 rpm can cancel much of the gyro effect of the two wheels (roughly 2 feet in diameter), since the latter are spinning at 1,400 rpm (or about 100 mph). The wheels are six times the diameter of the crank, but are turning only one-seventh as fast. The crank’s weight is about equal to that of one of the bike’s wheels, so to a crude first approximation it looks like reversing the engine’s rotation so it’s opposite that of the wheels can cancel half of the bike’s “gyro resistance” to roll maneuvers. When you’re in a hurry, that’s worth having.

Ducati’s Panigale V4 models have a reverse-rotating crankshaft. The 2022 SP2 model also features carbon fiber wheels; Ducati claims it is the most agile Panigale to date. (Ducati/)

From 1981 onward Yamaha adopted two contrarotating crankshafts in its 500 GP bikes. This had the effect of neutralizing crank gyro effect. Honda switched from three cylinders to four in 1984, but initially its single crankshaft rotated in the same direction as the wheels.

Other Rotating Parts?

What about other rotating parts, such as the clutch and gearbox? The primary gears mean they turn slower, and the dependence of gyro effect on rpm squared greatly reduces the contribution of those parts. Yes, there is some, but it is roughly one-quarter as great per pound as that of the crankshaft.

Honda added an idler in 1987 to reverse crankshaft rotation in the NSR500 GP bike. Rider Mick Doohan later commented that “It helped some, but it didn’t solve all the problems.”

When the MotoGP era began in 2002, Yamaha’s original M1 engine introduced reverse engine rotation. As the years passed, others adopted it as well; today its use is universal in the class. With top speeds of 220 mph, these bikes’ wheels spin at over 3,000 rpm, and their engines spin up at 18,000 revs. That’s a lot of gyro! To steer them riders need a lot of help, in the form of lighter rotating parts and backward-rotating engines.

In the racing of the late 1980s, switching to carbon-carbon discs greatly reduced brake-disc mass (carbon’s density of about 1.5 unsurprisingly resembles that of coal, while steel has a density of 7.8). After much experimentation with 16-inch wheels in GP and 16.5s in World Superbike, tire companies decided they would focus instead on 17-inch wheels, as that would make it easier to apply their racing discoveries to production tires.

The Responsiveness/Stability Trade-off

Some in the academic motorcycle stability community had objected that as wheel diameter shrank instability would rear its ugly head. And indeed it did; the 500 GP bikes I saw in the 1981 season on 16-inch fronts weren’t every rider’s cuppa—they oscillated threateningly after every disturbance.

That’s the way it is with passive systems. The quicker-responding you make the controls, the more they respond to disturbances other than the operator’s commands. Conversely, the more stable you make the vehicle, the more strongly it resists disturbances of every kind, including the operator’s commands.

There are people who are sure that gyro effect is necessary in steering a motorcycle, but the experiment has been done. A test front end was built with two wheels: One was in contact with the road and the other, on the same axis, revolved counter to it, canceling its gyro effect. The experimenters found that this bike could still be ridden normally.

What does any of this have to do with everyday motorcycling? Not much, one could argue, because many riders are happy to pootle along at moderate speeds, using moderate control forces. On the other hand, emergencies do occur, despite our best-laid plans. When they do, we’ll be glad if our motorcycle responds quickly and easily.

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