Part 4: The Basic Principles of Aerodynamicstularislogo

by Victor Wanchena

You gather speed rapidly, 40,50, 60 mph. At this speed you feel the force of the wind pushing your body as you, to quote Neil Young, “Collide with the very air you breathe.” In the area of aerodynamics, motorcycles are for the most part a triumph of brute force over finesse. Street and race bikes alike just punch a hole through the air and leave masses of turbulence in their wakes, eating up power and slowing the machines down. This brings us to Rob, his bike the Tul-Aris, and air. For those of you just joining our program, this is the fourth in a series of articles about the Tul-Aris, a home built Gran Prix race bike powered by a 700 cc two-cylinder 2-stroke motor, created by the astrophysicist and engineer Dr. Rob Tuluie and his small team of enthusiastic supporters. One of the Tul-Aris team members, Mike Weston, also joins us in the discussion. We hope this can shed a little light on some of the principles and design elements used to make a motorcycle do one thing, go fast.

Drag is quite simply the retarding force that air exerts on any body moving through it. Anyone who has ever ridden a motorcycle or even stuck their hand out the window of a moving car has a basic idea of what drag is all about. Drag is typically represented as a drag coefficient. The drag coefficient or Cd is determined by dividing the force exerted on a body by the square of the speed times frontal area times the density of air, or Cd=F / ((speed2) x frontal area x d/2), sorry about the big math. The reason that we even bother to calculate Cd is that it is relatively independent of vehicle speed and a good indicator of how “aerodynamic” a vehicle
is *see note. From this formula you get a number like 0.6 which incidentally is the Cd for an average faired sport bike. For comparison, the Cd of a Ford Probe is roughly 0.34.

If you want to go fast on a bike then the name of this game is low drag. When cruising around town at anything close to legal speeds drag is not really a concern, but on a race track where most of the time the Tul-Aris will be above 100 mph, and top speeds routinely top 170+ mph, minimal drag is a must since the horsepower required to push a bike along rises at a quickly increasing rate. The force requirement basically goes up as a square of the speed you’re traveling, which means that the horsepower requirement goes up as the cube of speed. In other words if your bike uses bhp at 30 mph to push you through the air then it would take bhp at 60 mph and 64 hp at 120 mph. Now strap in for some really big math. A typical motorcycle (with the rider upright, no fairing) can roughly obtain a top speed (in mph) of 30 times the cube root of its power (in hp), or Vmax = 30 x (power)1/3. For a faired sport bike, with the rider tucked behind the fairing, the factor increases from 30 to about 35. The other forces such as frictional losses in the bike’s drive train or the rolling resistance of the tires consume little of a bike’s total power output at 100+ mph speeds.

When your hand was out the car window in the breeze with the palm forward much more force was felt on your hand than if you turned it sideways. This is the first way to reduce drag, by reducing what is called the frontal area. The frontal area is simply the surface that faces into the wind. The more frontal area your bike has the more drag your bike has. The streamlined bikes used for top speed runs are a good example of low frontal area and low drag coefficient or Cd. They are built as narrow and thin as possible to keep the frontal area to an absolute minimum. They are long and gently curved, similar to a teardrop, to reduce Cd. Low frontal area is a basic design goal for Rob, keeping the Tul-Aris small and narrow. For the home engineer the simplest way to determine the frontal area for a given machine is to take a photo of it head on from a measured distance. Then the total forward facing area is measured from the photo.

Components like the front wheel, mirrors, and fairings or windshields all add frontal area to a bike. And then obviously there is how the rider sits on the bike. When you sit in an upright position on a bike your body creates much more frontal area than when you sit in a crouch and therefore more drag as you move through the air. When a road racer sits up when braking into a corner their body creates extra drag and thus slows them down, but when back on a straight-away they once again crouch to minimize their frontal area for maximum speed.

As the Tul-Aris takes shape Rob must shape all these parts for a minimum of drag and frontal area. Not only is the front of the bike important, but how the air rejoins at the rear is critical to low drag. A conventional cooling system for a bike produces considerable drag despite the relatively low volume of air that flows through it. This is because the intake and exhaust routes for the cooling air are often very inefficient and bring the air to an almost standstill compared to the speed of the surrounding air. This so-called cooling drag can be a substantial amount of the bike’s total drag.

For motorcycles, the other major contribution to drag is the turbulence created by the bike. As air flows over a bike it swirls around the different parts of the bike trying to fill the partial vacuum created behind the bike. A good example of turbulent drag is the buffeting felt by a rider behind a windshield or large faring. The buffeting is caused by air rushing to fill the partial vacuum that has been created on the backside of the faring. This turbulent drag is not only the largest source of drag most often it is also the easiest to cure.

Turbulence creates a sort of pulling force or suction on the bike. So a design goal for Rob and other engineers is to minimize the disturbance of the air as it flows over a bike. Blending shapes together and eliminating sharp corners and intersections between different areas lessens the turbulence. Giving air a smooth route to follow can be seen in the much more organic shapes of modern sport bikes. One of the most critical areas for a bike is the tail section. To keep the airflow smooth the tail should fill the void that is created behind the rider and bring the air flowing all around the bike gently back together. This can be seen on Rob’s first home built racer the Tul-Da, which featured a cone shaped tail section and a rear seat that was raised slightly in relation to the gas tank and handle bars to keep a smooth transition from the riders back to the tail section.

For Rob as well as the average guy looking to make his race bike faster there really isn’t any access to a full-scale wind tunnel like the major manufactures use. A careful eye towards the basic principles of aerodynamics as mentioned above can lead to significant improvements to a bike’s high-speed performance. More in depth study of a bike’s problem areas can be made with a small-scale model in a water tank (by matching scale and speed to achieve similar Reynolds numbers, see below), or coast-down testing with careful subsequent data analysis. The work continues in Rob’s secret lab and we will keep you posted in the coming months as the Tul-Aris takes shape. Remember what the good doctor says, “Ride fast and take chances.”

M.M.M.

*For those keeping score, the Cd is not completely independent of speed, but is a function of the Reynolds Re number (which is basically a measure of how turbulent the flow is). Typically, Cd decreases with increasing Re until Re ~ 1,000,000, at which point Cd increases due to the onset of flow separation and turbulence.

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