Aero Drag on Motorcycles

[Hugh Janus]

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/)

Imagine air impinging perpendicular to a flat plate at velocity “V.” It doesn’t matter whether it’s the air moving or the plate, but rather the relative velocity of the two. As the oncoming air stops against the flat surface, the kinetic energy it has is converted into pressure energy. That pressure is called “Q”—the dynamic pressure. It is, as you’d expect, proportional to the density of the air (engineers call that “rho” because using Greek letters makes their calculations look cool). It’s also proportional to the square of the velocity, V. So Q is equal to 1/2 rho times V squared.

I save time by looking it up on a table I keep handy. If a MotoGP bike in the recent Sepang preseason test moves at 335 kph (that’s about 208 mph) that’s 305 feet per second. Here on my table it says Q will be equal to 0.052 times the local still air pressure: roughly 5 percent. Sea level pressure is nominally 14.7 pounds per square inch, so Q acts on each square inch as an extra 14.7 x 0.052 = 0.764 lb./sq. in. Multiplying times the 144 square inches in a square foot, we get 110 pounds of drag per square foot of flat plate area.

If we estimate the frontal area of our MotoGP bike as 4.5 square feet, we find that if Q acted on an equal square footage of flat plate it would produce a drag force of 4.5 x 110 = 495 pounds.

We know that 1 horsepower is the ability to lift 550 pounds 1 foot in each second. To get the power consumed by overcoming that 495 pounds of drag, we first find the foot-pounds per second that will be required. We multiply the 495-pound drag force times the 305 ft./sec. bike speed to get 150,975 ft.-lb./sec., then divide by 550 to get the horsepower required, which rounds out to 274.

Trying to Make Sense of the Various Losses

That’s obviously too much, but we can reduce drag by altering our flat-plate frontal area into a rounded shape that guides the flow around itself more gradually and produces a smaller energy-rich turbulent wake. If we regard the drag in flat-plate form as 100 percent, we may reduce drag to something like 60 percent of that under the basic 1958 FIM rules that require full exposure of the front wheel and essentially forbids streamlining behind the rider. The percentage of flat-plate drag achieved by a given shape is called its “coefficient of drag,” or Cd.

In our above example, if we manage to reduce Cd to 0.6 our aero horsepower requirement at 208 mph drops to 0.6 x 274, or 164 hp. That’s actually reasonable, as there has to be power left over to overcome tire rolling resistance, and to deal with the friction losses between engine crankshaft and rear wheel. I reckon each transmission of power—by primary gears, transmission, and drive chain—eats 2 percent. A total of 6 percent of 277 hp is 16.6 hp—which becomes heat delivered to the engine oil and to the drive chain, equivalent to 16.6 kitchen toasters.

Aprilia’s efforts to smooth the surfaces at the front of its MotoGP racers includes smoothing the flow over fork legs and brakes. (MotoGP/)

One estimate of rolling resistance says 0.02 times the weight. If bike, rider, and fuel total 570 pounds, then 0.02 x 570 = 11.4 pounds. To get horsepower required to overcome that we multiply times velocity (305 ft./sec. from the example above) to get 3,477 ft.-lb./sec., and dividing by 550 we get 6.3 hp consumed by this crude estimate of rolling resistance. Got a spring scale, a bike, a rider, and a level paved road?

Now we subtract for the losses, which are 164 hp for aero, 16.6 hp for driveline friction, and 6.3 hp for rolling resistance, giving us 187 hp of loss. If I estimate the peak power of a MotoGP engine as 277, that leaves 90 net hp still accelerating the bike the instant before the rider brakes for the next turn. No circuit on the GP calendar allows these bikes to reach their actual maximum speeds.

I cheerfully admit that the above is playing with imaginary (estimated) numbers. Some will, for example, remind us that it’s usual to estimate 15 percent loss between engine and rear wheel, but that would send the heat from 41.5 kitchen toasters (that’s 31,125 watts) into the engine oil and drive chain. Is that reasonable?

Why All That Drag?

A well-streamlined car can be given a Cd of 0.30 and the great airships of the 1930s dropped to 0.05. Why do MotoGP bikes have such an embarrassing Cd? The bulk of the bike is limited by rule to being only half-streamlined (no streamlining behind the rider), and that shape has to plow through “dirty” air already considerably disturbed by passing over the irregular shapes of front wheel, fork, and brakes.

Aprilia currently leads the way in reducing that disturbance by adding flow-smoothing fairings to fork legs and brakes. The blue-blazer boys of the FIM in years past were very strict in enforcing its 1958 rules, so this is something of a departure.

What of those stegosaurus vanes that are showing up on more than just the Ducatis in preseason testing? (MotoGP/)

More than one team is now ducting air into the normally low-pressure region behind the front tire.

The “stegosaurus vanes” sticking up and slightly outward on some seatbacks are slotted airfoils whose purpose may be to waste competitors’ wind tunnel time or to guide some higher-energy air into the bike’s wake to prevent or reduce vortex shedding.

Where did I get the figure of 277 hp for a MotoGP engine? A good BMEP (Brake Mean Effective Pressure is stroke-averaged net combustion pressure) at peak power is 200 psi, and if these 61ci engines are peaking at 18,000 rpm, this rough estimate gives us 277 hp. But everyone has lately begun saying these engines make 300 hp, so that either requires they actually peak at 19,500 or that their peak-power BMEP is 216 psi. Nobody likes a party pooper so I’ll duck the charge by saying they make 300 hp at the PR-director’s pen point. Pen-point power has a nice ring to it.

Source

[Sir Fallsalot]

Disappointing from the title i was expecting to see one of these being pulled behind a motorcycle i wanted to see if the mint one lasted longer than the chocolate one

[XTreme]