Does Streamlined Mean Pointed?

While it’s natural that we’d come to think of pointy-looking supersonic aircraft and rockets as highly streamlined (they go real fast, right?), the shapes of such craft solve quite different problems from those facing motorcycles in their modest speed range up to around 200 mph.

It’s impressive to see the films of the 1970s US ABM called “Sprint” exploding upward out of its silo, turning abruptly toward the incoming ICBM while accelerating at 100G to reach a quarter-mile terminal speed of 1,900 mph in about 9/10 of a second. And what did Sprint look like? It was a pointed cone, flying sharp end first.

Or have a look at the 1964 SR-71 “camera truck” (aka recon aircraft), with its needle nose and pointed engine intake shock cones. In its day, cruise was Mach 3.17.

It’s no surprise that industrial artists, seeking inspiration for the endless succession of yearly new car, truck, and motorcycle models, dip into the excitement of the supersonic world.

Then some of us see photos from the preseason MotoGP tests in Malaysia, where most of the prototype fairings are plain black. My goodness! Their shapes are as smoothly rounded as the nose of a Boeing 747, and there is a minimum of the scoops, points, and jagged edges so often featured on new-model sportbikes. Why is this? Don’t they know this is the 21st century?

The difference arises from the contrast in conditions between low subsonic and supersonic flows. Just as we can see much of the energy of a boat’s engine being carried away in the wave action of its wake, so in supersonic flight through air, energy is carried away by the conical shock wave generated by the foremost parts of the vehicle’s shape. A long pointed nose helps to reduce this energy loss. Where that shock cone intersects the ground near us, we hear the bang or rumble of a sonic boom. But the short, rounded nose of a subsonic vehicle has less surface area, and so lower skin friction at subsonic speeds.

At the low subsonic speeds of motorcycles, drag is produced by the combination of skin friction and energy lost to a large area of turbulent wake. That was what I saw one morning in 1993 in a misty first 500 practice at Eastern Creek International Raceway, near Sydney. The mist let me see the whirling mass of energy-filled eddies that trailed behind each bike. That whirling mass would have been much smaller, and the drag lower, had each bike had the streamlined shape of a fish, which is thickest near the front, then tapers slowly enough to keep the flow attached, almost all the way to its tail, from which trails a small turbulent wake. The tapered tail performs the function of closing the vehicle’s wake.

The outstanding example of a human-made vehicle with very low drag is the rigid airships, from which so much was expected up to the early 1930s. Where a highly streamlined car has a drag coefficient of just over 0.30, and a racebike more like 0.45 to 0.60, the great zeppelins had ultra-low coefficients of around 0.05. Drag coefficient relates the drag of the subject vehicle to a flat plate, normal to the flow, having the same frontal area as the vehicle. When you see the expression “CdA” that is the drag coefficient, Cd, multiplied times the frontal area, A. Rigid airships had low drag because their long, tapering tails kept the air flowing around them attached until, at the point of flow separation, the remaining turbulent, energy-filled wake was of very small diameter.

How does all this work? The fundamental principle of streamlining is that as a vehicle passes through the fluid (air, in our case) minimum energy is lost if each air molecule displaced by the vehicle’s motion is put back where it originally was, with its original energy, after it has passed. What I saw at Eastern Creek was anything but that. The large turbulent wakes of those 500 GP bikes carried away much of the engine’s power in random motion.

When Harley-Davidson went to the Caltech wind tunnel in the 1967-68 off-season to seek a drag reduction (with which Calvin Rayborn would win the Daytona 200 in 1968 and ’69), the work produced a fairing that has since boosted the top speed of nearly every kind of motorcycle on which it has been used, including little 250s. It has a smoothly rounded nose and is widest up front, tapering inward as much as possible toward the rear. Because of the 1958 FIM rules that ended full streamlining, it was no longer legal to have the long tapering tail that reduces drag so much for aircraft, birds, and fish. This has given motorcycles drag coefficients similar to those of a bread truck. The scoops and sharp edges beloved by stylists only make this worse disturbing flow and causing premature flow separation.

Builders who rely on intuition continue to bring cars and bikes to Bonneville that caused a young aerodynamics engineer to say 20 years ago, “Many of the shapes I’ve seen here would probably go faster backward.” What he meant is that minimum subsonic drag is associated with shapes that are rounded, with maximum cross section close to the front, leaving the rest of their length to provide a slow taper down to the minimum possible wake. The taper is slow because that’s necessary to keep the flow attached to the vehicle’s skin, keeping the flow smooth and minimizing turbulence.

Intuition derived from looking at missiles and supersonic aircraft tends instead to choose shapes that are pointed at the front and grow steadily wider toward the rear—the opposite of what actually works. When a friend was leaving for Europe in the late 1960s with his Yamaha TD1-B, I saw that he’d mounted his fairing in “snowplow” style—narrow at the front and growing wider toward the rear. You won’t find that on starting grids today! It makes a bike’s shape act even wider than it actually is.

Look at the streamlining on bikes being built for 300 mph at the speed meets now being held on the runways of closed-down USAF bases: rounded at the front, tapering away gradually to leave behind the smallest possible wake. Like a zeppelin. Like a fish. Like a bird.