Interference Drag

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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/)

Interference drag occurs wherever the fluid flows around two or more adjacent objects interfere with each other, resulting in regions of high-speed turbulent flow that carry away power and thereby create drag.

The classic example discussed in most aero texts is the area in which the wing joins the fuselage. Flows over the wing and along the fuselage are crowded together where they join, forcing the flows to accelerate.

The example most relevant to motorcycling is that of the extended landing gear (“undercarriage” if you are English) of an airplane. The first example that caught my eye was that Boeing’s pioneering jet bomber, the B-47, more than doubled its drag when it extended its gear prior to landing. In another case, a flight crew who found themselves unable to retract the gear of a familiar B737 commercial twin became more and more troubled as they realized that the lowered gear was more than doubling their fuel consumption at 250 knots and 20,000 feet.

Those numbers are both interesting and suggestive, but there’s more. Those situations were for the condition of fully extended landing gear. But the peak drag caused by the gear occurs during retraction, just before it begins to enter its gear bays in wing or fuselage. The reason for this is the interference drag—regions of chaotic accelerated flow—between two bodies, a) the aircraft and b) the wheels with their appurtenant struts, as they come ever closer to it.

This may explain descriptions I have read of “hot rock” wartime pilots who on takeoff liked to begin retraction the moment the tires unloaded. A fair number of them then felt their aircraft sink back toward the runway just enough to bend (and thereby ruin) their prop blades. As the gear retracts, its drag increases—perhaps just enough to reverse the climb long enough to whack the props.

Every motorcycle, save for the very few fully streamlined roadrace bikes of the postwar era up to 1958, and Bonneville record seekers, pushes ahead of itself something that looks a lot like aircraft landing gear and struts—the messy front wheel, the fork legs that join it to the bike’s frame, and the brake caliper(s) and disc(s), with their cables, hoses, and possibly, wiring as well.

To the oncoming air, the front end of even the slickest superbike is messy. (Jeff Allen/)

The process, after takeoff, of reducing drag by retracting the gear and gradually pulling up the flap system and leading-edge slats is called “cleaning up the aircraft” and is essential for rapid climb.

Practical motorcycles, because their speed is limited by things like curves, police, and other obstacles, are not greatly troubled by the strong interference drag of their exposed front wheels. It is only when a) the racing application requires lowest possible drag so that high speed may be obtained, or b) if society dictates maximum permitted fuel consumptions for all classes of vehicles, as a means of reducing the carbon dioxide release caused by energy production. Electric vehicles must be included because roughly 60 percent of US electricity is still produced by combustion—principally of natural gas and coal.  Finally, a third reason to reduce aero drag is to increase the range of electric vehicles.

Various schemes exist by which the aero drag of motorcycles can be reduced. Most dramatic are the demonstrations made by Craig Vetter of what can be done in a few minutes using cardboard and duct tape to build a fairly smooth enclosure for the front wheel.

A scheme that leaves the 1958 FIM rules in place (they originally specified the exposed front wheel for racing, but production bikes have since then have tended to have the same look) is that chosen by the late Peter Williams for the Norton he brought to Daytona in 1972, a year later by Kawasaki for the fairing of its 750 H2-R roadracer, and lately for Aprilia’s MotoGP bike; to make the region immediately behind the front wheel as a flat plate, perpendicular to the direction of motion.

Will we see new shapes at the front of motorcycles in the future to combat interference drag? (Jeff Allen/)

When I once went to talk to aero people at MIT they explained this to me as follows. Before more compact aircraft radars were developed, aerodynamically “dirty” arrays of dipoles had to be mounted on the noses of aircraft. One way to reduce their drag was to place immediately behind them a flat surface, which would push a “haystack” of stagnant air ahead of it. The dipole array, being located within that haystack, encountered slower-moving air and so its drag was reduced. Presumably this is what Peter discovered at the MIRA tunnel in Britain, and Kawasaki developed at its tunnel in Gifu, Japan.

For the motorcycle application, substitute the front wheel and associated parts for the 1944 radar dipole arrays.

As described on this site not long ago, Pierre Terblanche’s “Motorcycle of the Future” also leaves the front wheel exposed in 1958 FIM fashion, but moves outward the sides of the fairing behind it, hoping to attach the turbulent mess streaming off the “IDP” (interference drag producer) to those sides, thereby restoring some order to the flow and perhaps even reducing its drag somewhat.

What I am hoping is that national governments will continue to largely ignore motorcycles as being too few to justify the cost of setting and imposing fuel consumption standards on them.

I admit that it would be hard to put across the usual “gritty-in-the-city” visual message of a classic Harley Sportster if it were 100 percent invisible inside one of Mr. Vetter’s effective streamlined shapes.

Could we get used to such change? Think of how much aircraft shapes have changed—from, say, the Vickers “Gunbus” of World War I to the Spitfire or Mustang of WWII, and then only 10 years later, the further upset of shapes designed for supersonic operation.

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The title says it all .