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1. Introduction to Aerodynamics

Aerodynamic properties of a motorcycle or any other type of vehicle are studied in great depth in order to better understand the characteristics of that vehicle at different speeds. The traditional example of aerodynamic analysis involves design of airplane wings, where we generally expect that the attached airplane leaves the ground after a certain speed. However, the study of aerodynamics encompasses a much broader field, including design of cars and motorcycles, neither of which we expect to take off the ground; in fact, we would hope for our designed motor vehicle to guarantee a good grip to the street even at higher speeds. Conversely, we also do not wish for our motor vehicle or any specific portion to overly resist the elements, so as to negatively affect performance. These tend to be the main issues when considering aerodynamics in respect to cars and motorcycles.

While we will not go into great depth in our introductory discussion and analysis of aerodynamics, we will cover two critical characteristics:

  1. The Bernoulli Principle
  2. Drag

1.1 The Bernoulli Principle

Definition: The law that pressure in a fluid decreases with the rate of flow.

The Bernoulli Principle can be explained by many examples - the most relevant example for us pertains to the shape of an airplane wing. In this simplified example, the bottom of a wing is flat, while the top is curved. As the air flows through the volume above the wing, its velocity increases, whereas the air flowing through the volume below the wing does not experience any change in velocity. The reason for the increase in wind velocity above the wing is the gradually decreasing size of the corridor through which air can flow, which is in turn caused by the increase in thickness of the wing. This principle is shown in Figure 1.1, which depicts a Venturi tube.

According to the Bernoulli Principle, the higher the velocity, the lower the pressure. For the example of the wing, this translates into high pressure on the bottom and low pressure on top of the wing, which causes a net lift (= makes airplanes fly). (See Figure 1.2). One explanation for the reciprocal relationship between pressure and velocity is that more energy is used as particles accelerate, which leaves less energy to exert pressure.

Figure 1.1: Flow in a Venturi tube. The same amount of fluid exits the tube as enters the tube. In the narrowing portion, the velocity of the flow thus increases.
Figure 1.2: Analysis of a wing in I-DEAS. Blue and green regions denote areas of low velocity (high pressure); orange and red regions denote areas of high velocity (low pressure). Since there is more pressure below the wing, the net force lifts the wing.
Click image to enlarge, or click here to open
An airflow similar to that of a wing can be observed on a motorcycle, or a car for that matter. However, the lift effect tends to be opposed by a good amount of drag and mass, so that motorcycles and cars do not lift off the ground. For race cars, which tend to not only be designed very aerodynamically, but also drive at speeds comparable to that of small airplanes, the lift effect is actually a problem. For this reason, many race cars are equipped with wings that essentially have the same shape as airfoils on airplanes, but are inverted to cause a downward lift force as opposed to an upward lift force. While even a race car would not lift off the ground without this attached wing, the road traction would be impeded tremendously to the point of causing slipping tires, etc.

Figure 1.3: Analysis of a motorcycle cross-section in I-DEAS. Blue and green regions denote areas of low velocity (high pressure); orange and red regions denote areas of high velocity (low pressure). While there is generally less pressure above the motorcycle, motorcycles do not tend to lift off the ground.
Click image to enlarge, or click here to open

1.2 Drag

Simply and rather obviously stated, drag slows down a moving vehicle. For motorcycles, this is due to many factors, ranging from the frontal area of the bike + rider, the shape, whether cross-sectioned from top or side, to minute details like clothing, positioning of boots and arms, etc.

Figure 1.4 outlines a few shapes and their associated aerodynamic drag values. While we will not go into detail about what this value represents, we note that a higher value is due to more and a lower value due to less drag (resistance). From the figure it appears that a teardrop shape experiences the least amount of drag. This is due to 2 factors, both of which should be taken into consideration when designing the shape of a motorcycle:
  1. The frontal area is curved, which allows the wind to transition smoothly from its straight path to around the object.
  2. The rear converges at a point.

Figure 1.4: Drag Coefficients
It is not difficult to implement a design that takes care of the first point. Cars, trucks, and motorcycles are already designed and built to counter the effect of drag in the frontal areas by making them transition between a sharp edge and the full body. However, it is uncommon for the same vehicles to have a similar tear-drop design in the rear. Trucks storage trailers for example are as boxy in the front as they are in the rear. Understandably, they are designed for optimum storage capacity, which is why such trailers do not assume a teardrop shape. Passenger cars, station wagons excluded, tend to approximate the teardrop well enough to counter most of the extreme drag. Due to their size and availability of space, airplanes and most ships tend to be designed in the teardrop style to begin with.

The drag in the rear of a vehicle is the result of the presence of a wake. It is caused by displacing the vehicle in a volume of air at higher speeds, where the air is "pushed out of the way" in the front of the vehicle, and a lack of air in the rear of the vehicle causes air to be "pulled in" from the sides and top and bottom. The larger the area in the rear, e.g. for a truck trailer, the more air needs to be pulled in, which results in an increase in drag. Should the rear of the vehicle be shaped to allow the stream lines to adjust gradually, the smaller the wake. This principle is outlined in Figure 1.5.

Figure 1.5: Wake behind 2 differently shaped objects