Motorcycle Fuel Consumption

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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/)

When gasoline prices are up it’s embarrassing to see that various autos capable of carrying five people may deliver fuel economy superior to that of some heavy or high-performance motorcycles. One reason why is that motorcycles are often geared to cruise at fairly high engine rpm, and that generates serious friction loss. Another reason is that motorcycle aerodynamic drag remains high.

Drag and Transmission Design

This drag is why the faster we go, the harder it becomes to accelerate—as aero drag rises, it consumes power that would be available for acceleration at lower speeds. It’s also why gear ratios grow closer together the higher we go in the gearbox. For example, the time-honored Suzuki SV650′s revs drop 28 percent from first to second gear, but they drop a much smaller 11 percent at the upshift from fifth into sixth. This provides a good compromise between the low first ratio necessary to get the bike moving from rest (possibly with a passenger) and the engine’s ability to pull across 28 percent of its rpm range to make the first-to-second upshift.

This situation also leaves us with the engine spinning fairly vigorously at highway cruise. . . . and the faster an engine turns, the more of its power is lost to friction.

Coefficient of Drag

Another cause for many bikes’ not-so-hot fuel economy is that motorcycles have much worse coefficients of aero drag (Cd) than do well-designed autos. One possible reason is that our government does not regulate motorcycle fuel economy (more on this in a bit), while the EPA most certainly does exert steady pressure on automakers to cut fuel consumption.

Moto Guzzi famously used a wind tunnel to develop their motorcycle designs. Years later Harley would too when developing the fairings for some of their racebikes. (Moto Guzzi/)

The coefficient of drag roughly compares the drag of a given vehicle shape with that of a flat plate perpendicular to the airflow and having the same frontal area. The Cd of an unfaired bike with rider sitting up is similar to that of the aforementioned flat plate (Cd = ~1.0), while that of a faired bike with rider tucked in might be much lower, perhaps .45 or so. By comparison, a car’s Cd can range as low as .32 for one with good streamlining.

A vehicle encounters resistance to its motion through a fluid like air because energy from the vehicle is transferred to the fluid and relatively little of it is recovered. In ideal streamlining, every air molecule displaced by the passing vehicle would be restored to its original position, velocity, and direction of motion.

Instead of this “conservative process” (meaning that no energy has been lost to the fluid) what we actually see is a wake trailing behind the vehicle, consisting of swirling, eddying air motion, and containing considerable energy that wasn’t there before the vehicle arrived.

Gone Fishing

There are long-standing social and practical reasons that make it difficult to streamline motorcycles. On the social side, we are accustomed to either completely unfaired bikes  whose many projecting parts transfer extra energy to the wake, or to the partly-streamlined machines that roughly follow the present FIM rules for road racing. Under these rules, the motorcycle’s front wheel must be exposed, its fender coverage is limited, its rider must be completely visible from both sides and from the top, and there can be no streamlining behind some arbitrary station along its length.

Compare that with the shape of a fish, which tapers smoothly inward all the way to its tail. The fish’s motion through the water pushes water radially away from its central axis, and must supply the force necessary for this push. But after the fish’s maximum cross section passes, the inward pressure of that displaced water acts on the tapering tail much as the grip of a wet hand causes an oval bar of soap to squirt out of its grasp. In effect, much of the energy used by the fish to displace water out of its path is recovered as that displaced water squeezes radially inward on its tapering tail. The result is quite low drag.

Streamlining is a lot more than just making a small hole through the air—it's also about making sure the air closes up back smoothly behind you. (Ducati/)

Autos can easily be given fairly fish-like tapering shapes, but motorcycles are too short for this treatment. This lack of a tapering tail and the pressure recovery it can achieve is a primary source of motorcycle drag: the “squeezing wet hand” of the air surrounding the vehicle cannot give the bike a push..

Another tremendous source of drag is exposing the front wheel, fork legs, brake calipers and so on to the airflow. This is akin to the very high drag created when an otherwise fish-like aircraft with retractable landing gear extends its wheels prior to landing. We’ve become used to exposed front wheels on our bikes, so anything else looks odd to us. In racing, one way of reducing front-wheel drag has been to make the fairing behind the front wheel quite flat. This pushes ahead of itself an invisible “mound” of low-velocity air within which the front wheel is protected from high-speed flow.

Another component of aero drag is skin friction, which is just another mechanism by which the motion of a vehicle through a fluid loses energy to that fluid. In skin friction, individual collisions of air molecules with the moving vehicle’s surface accelerates those molecules, dragging them along with it. Seen from the vehicle, this generates a semi-stagnant layer, thin at the nose but growing in thickness toward the rear. This layer of air moves with the vehicle—the boundary layer. Because skin friction operates at an atomic scale, we cannot eliminate it by polishing.

Wake Turbulence

The competitive US trucking industry faces similar problems, a truck’s blocky shape leaving behind a lot of energy in turbulent wakes. All motorcyclists have felt the thumping from the vortices shed from the aft ends of trucks. Guide vanes can direct air in ways that restore symmetrical flow, thus suppressing vortex shedding. The result can be reduced drag and lower fuel consumption. Another method is to collect high-energy air with scoops to form jets that can be directed to the same purpose. It’s possible that the “Stegosaurus tail” seatback wings in MotoGP are intended to accomplish the same goal: reducing the energy carried away by the wake.

Circling back to an earlier point, why do neither the US EPA nor their EU counterparts set mandatory fuel-economy standards for motorcycles? It’s estimated that the approximately eight million US motorcycles annually consume less than one percent of the fuel used by our 260 million light cars and trucks—a literal drop in the bucket. Maybe the cost of regulation is seen as high in comparison to the benefit it could bring. China, with ten times more motorcycles, now requires their fuel consumption to be published.

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