Reconciling Centralized Mass and Efficient Structure

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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/)

In response to my recent review of BMW’s new R 1300 GS, reader “Hipsabad” quoted two apparently contradictory statements, saying, “I’m not sure how to reconcile these…”

The first was my observation that BMW had made a “concerted effort to move components inward” and the other was my admonition to “Remember the rule of efficient structure. Move the material outward, away from the centerline.”

Yes, the first statement says BMW moved components inward, while the second, speaking now of efficient structure, said “Move the material outward.”

The reason to move components inward, clustering them as close as possible to the center of mass, is to make the vehicle respond promptly to control inputs. You can change direction a lot more quickly carrying a 24-pound cannonball than if you are toting a 12-foot ladder of the same weight. The components in question are nonstructural, such as the fuel and battery.

Moving mass closer to the center of mass allows the motorcycle to respond quickly. (BMW/)

In the second statement, calling for structure to be moved away from the centerline, the goal is not mass centralization for quick maneuverability. It is to achieve a structure that is both adequately stiff and light in weight.

An everyday structure of this kind is a paper box, whose strength is in material far from its center. That allows little material to provide impressive stiffness (if the top of the box is taped shut!).

Think of the frame of BMW’s R 1300 GS as a box with stiff sides. (BMW/)

The first motorcycles had single-plane frames inspired by the bicycle. In the early 1920s along came Howard R. Davies, who noticed that when he raced the motorcycles of his time, their rear wheels wagged from side to side, causing unnerving self-steering and even instability. He saw that as a first step; he could stop the tail-wagging by continuing the two lower rear axle stays forward, past the seatpost, then under the engine, to join the downtube from the steering head. This step away from a single-plane chassis stopped much of the tail-wagging.

After World War II, the McCandless brothers built a chassis for Norton as twin loops—in effect making a kind of box of it, much more strongly bracing the rear wheel to the steering head. Yes, this did move some metal away from the center of mass, but the effect of doing so increased chassis rigidity and controllability enough to make it worth doing.

On this new chassis—later named “the Featherbed” by appreciative rider Harold Daniell—the late Geoff Duke defeated the four-cylinder Gilera 500s in 1951, despite the fact that his single-cylinder Norton was making 25 percent less power.

To recap, my statements described two different goals: the first, to enhance rapid maneuverability by grouping heavy components close to the vehicle’s center of mass; the second, to move structure away from the center to increase its stiffness in relation to its weight.

That is seeking two different goals by two different means. I hope that clears it up.

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