The Mystery of Steered Mass | DRAWING THE LINE

We know that we can improve a motorcycle’s performance by reducing its weight. We measure that weight when the bike is stationary by putting it on a scale. At rest and under the effect of Earth’s gravity, we use the words “weight” and “mass” interchangeably.

But a motorcycle is made to move, so it makes more sense to consider inertia when we want to improve performance. Inertia measures an object’s resistance to acceleration—or any change in velocity, in any direction. That resistance is what we really want to reduce when we reduce weight or mass. Since a motorcycle doesn’t just accelerate and decelerate, but also must rotate in roll, pitch, and yaw, it is further useful to be aware of the moment of inertia, which measures resistance to movement around an axis.

We often read about mass centralization and shaving unsprung mass, by reducing the wheels’ rotational inertia, for example. But one mass and inertia we don’t typically read about is steered mass, or steering inertia. Why? Because the telescopic fork is so nearly universal that, in practical terms, we don’t have many options that can demonstrate that when it comes to the fork, we have a steered mass problem.

When we rotate a fork to steer a motorcycle, we are rotating the entire front suspension—all the structure, including the springs and dampers—just to turn the front wheel. Most of that structure is far from the steering axis on either side of the wheel.

To get an idea of what we might do differently, consider my third-generation P3 RADD front suspension. The component farthest from the steering axis is the curved part of what's called the upright, which arcs around the front tire on the right side of the bike. The upright's outermost surface is roughly 4 inches from the steering axis: The outside surface of each of a fork's tubes is about 5 inches from the axis, over their full length.

Why is distance from the steering axis important? Because moment of inertia is the mass times the radius from the pivot axis squared. The steered mass of the RADD P3 suspension is about 30 percent less than a fork, but its steering inertia is reduced a whopping 70 percent because its mass is much closer to the axis.

Reduction in steered mass/inertia gives real performance benefits. On the simplest level, reduced steering mass/inertia can result in lighter steering feel and less steering effort. Steering feedback, or the feel for what the front tire is doing at the contact patch, can also be enhanced by having less mass/inertia to potentially interfere with signals from the tire.

But the key benefit of reducing steered mass/inertia comes in the solution of what can be a fork’s gravest problem—instability characterized by headshake. Visualize that event in slow motion: Some force through the tire or chassis causes the steering to be deflected or rotated. With larger mass/inertia, the fork wants to continue to rotate farther. The rotation may be halted by the rider’s arms, the tire’s damping characteristics, or a steering damper, but other inputs, like fork or chassis flex, may enter the picture. There may also be a resonant response. Rotation may continue all the way to the steering stops, with the fork literally bouncing off the stops.

We’ve found that the P3 suspension, with its significantly reduced steering mass/inertia, shows dramatically improved steering stability in our testing, eliminating headshake even without a steering damper. Not needing a steering damper can further improve steering lightness, feel, and feedback.

We’re not going to hear that steering mass and inertia are problems as long as forks are virtually the only game in town. As long as we’re steering the whole front suspension, we’ll be steering a lot of weight and paying a heavy price. While we’re trying to shave mass and reduce inertia in the rest of the bike, let’s also take a hard look at the steering mechanism.

James Parker designed his first original motorcycle in 1971; his most recent design is the Mission R electric superbike. In between, he worked on multiple other motorcycle projects, including 30 years spent evolving the RADD front suspension system used on the Yamaha GTS1000 and various other prototypes.