We are republishing some of our best blog content. After nearly three years of writing and roughly 150 articles, surely some of our newer fans have missed some of our best stuff. This week we are going way back to our Aero Wheel Tutorial. If you’ve ever wondered why an aero wheel is faster, read on!
Aero Wheel Tutorial
Let’s start by defining some terms…
A yaw angle is the angle at which the wind interacts with the wheel. Take a look at the pictures below. In Figure A, the wind (blue arrow) is hitting the wheel at 0 degrees. This is known as 0 degrees of yaw, and what you experience when the wind is blowing straight at you. In Figure B, the wind is now interacting with the wheel at a 20-degree angle. This is known as 20 degrees of yaw and the cyclist would a feel a combination of head wind and side wind.
Let’s start with a visual. Imagine a canoe moving through a calm lake. The front of the canoe is the first part of the boat to cut through the water. It is therefore defined as the “leading edge.” A wheel in the wind is no different. Remember, air is a fluid just like water.
A wheel can have two leading edges. The tire at the front of the wheel, and the carbon fiber fairing at the back of the wheel. When a wheel is at 0 degrees of yaw, the front of the wheel is the only leading edge. This is because the back of the wheel is “hiding” behind the front of the wheel (see Figure C). When a yaw angle of greater than 0 degrees is introduced, we now have two leading edges. Figure D shows the wind at 20 degrees of yaw. The back of the wheel can no longer “hide” behind the front of the wheel and sees it’s own air. It is therefore defined as a leading edge.
Drag is defined as the force on an object that resists its motion through a fluid. Let’s use another water example. If you stand in waist deep water and try to run forward, the force you feel holding you back is drag. Air also has drag, just not as much as water.
Lift, or “side force” is one of the most important forces to consider when designing aerodynamic cycling wheels. To help you better understand the three main components of lift, let’s shift our focus to the skies and talk about airplanes.
The wings of an airplane allow it to fly, but how? To answer this, let’s look at the forces acting on an airplane (Figure E). Thrust is the force generated by the engine of the airplane to move it forward. Drag is the force exerted by the air that resists the forward motion of the airplane. Let’s ignore these two forces for now.
Gravity is the earth’s attractive force that wants to keep the airplane on the ground. Lift is the force we need to create in order to get the plane off the ground. To take flight, we need the lift force to be greater than the gravitational force. Lift is generated by the wing. A wing has three major components that contribute to the lift force it produces. Those three components are:
1. The shape of the wing.
2. The wing’s angle of attack.
3. The velocity or speed of the wing.
The shape controls the way the air (fluid) moves around the wing. By controlling the airflow, we can create areas of high pressure below the wing, and areas of low pressure above the wing. Any time there is a difference in pressure on opposite sides of an object, the high pressure side pushes the object towards the low pressure side, like a balloon. The more air (pressure) you blow inside of the balloon, the bigger the balloon gets. This is because the high pressure is pushing the inside of the balloon out. In order to take flight, we have to create a high enough pressure under the wing to lift the plane off of the ground.
The angle of attack is the angle that the wing moves through the air. This is the same as the yaw angle of a wheel. As you increase the angle of attack, you increase the lift force until you reach the critical angle of attack. The critical angle of attack is the angle that produces the maximum lift. Think of sticking your hand out of the window of a moving car. By turning your hand up or down (changing the angle of attack), you can make your hand rise or fall. If you turn your hand too far in either direction, it no longer moves up or down but instead straight back.
Finally we have the velocity or speed at which the wing travels through the air. The simple answer here is, the faster you go, the more lift you create.
When designing effective aerodynamic race wheels there are in our opinion to very important points to consider. The first point is the reduction of drag. In order to be fast, the wheel must reduce aerodynamic drag as much as possible. The second point is the ride quality and stability of the wheel. Anyone who has ridden deep wheels in a strong side wind knows they can be a challenge to control. Therefore, it is important to design a wheel that has good crosswind stability.
Side Force (Lift) and Drag
In the world of cycling, lift is called side force. Figure F shows a wheel at 0 degrees of yaw. In this case the wheel only experiences drag. Since the wind flows evenly around both sides of the wheel, side force is equal to 0.
Figure G shows a wheel at 20 degrees of yaw. Thinking back to our airplane example, we have increased the angle of attack. This produces a higher side force on the side of the wheel facing the wind and produces lift.
Let’s now consider the side forces that a standard training wheel experiences. Because a standard training wheel has very little rim depth, it generates very small side force. For sake of argument, the drag is more or less equal to the side force. An aero wheel however, has a much deeper rim profile and an increased surface area. The increased surface area generates a higher drag force. An efficient fairing shape will increase the side force. The key is to design a fairing shape that produces a higher percentage of side force relative to drag.
Why do we want side force? Let’s start with vector forces. When a force pushes on a surface at an angle, a portion of that force pushes the object in the X direction and a portion of that force pushes the object in the Y direction. Take a look at Figure H.
Let’s look at the vector components of side force and drag acting on a wheel. Figure I shows that the Y component of side force actually opposes the Y component of drag. In theory, if we can generate a side force high enough relative to drag, the Y component of side force will be greater than the Y component of drag. When this happens, the wheel will actually be pushing you forward. This is known as negative drag.
Here are two numeric examples.
Standard Box Rim Wheel
Total Drag Force = 100g
Drag Force Y Component = 93.97g
Total Side Force = 100g
Side Force Y-Component = 34.20g
Resultant Drag Force = (Drag Force Y-Component) – (Side Force Y-Component)
Resultant Drag Force = 93.97g – 34.20g
Resultant Drag Force = 59.77g
Total Drag Force = 150g
Drag Force Y Component = 140.95g