### September 24, 2014

## FLO Cycling - Aero Wheel Tutorial

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…

**Yaw Angle**

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.Figure C |

Figure D |

Drag |

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**

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.

Figure E |

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.

**Wheel Design**

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.Figure I |

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

__Aerodynamic Wheel__
Total Drag Force = 150g

Drag Force Y Component = 140.95g

Total Side Force = 450g

Side Force Y-Component = 153.90g

**Resultant Drag Force**= (Drag Force Y-Component) - (Side Force Y-Component)

**Resultant Drag Force**= 140.95g - 153.95g

**Resultant Drag Force**= -12.95g

**Cross Wind Stability or Yaw Torque**

Imagine a seesaw on the playground. Let’s put a child weighing 50 lbs on one side and a child weighing 70 lbs on the other side. We all know that the 50 lbs child will quickly rise up in the air.

In theory the front wheel of a bicycle is the same. We have the front half of the wheel, the back half of the wheel, and the steering axis. If we push on the front half of the wheel and leave the back half alone, the wheel will turn around the steering axis in the direction of your push. Take a look at

**Figure J**.
If we are going to make a wheel that is stable in cross winds, we want the side force on the front half of the wheel to be equal to the side force on the back half of the wheel. This would be the same as placing a 50 lb child on both sides of the seesaw. This will prevent any turning of the wheel. If the side force on the front of the wheel is greater than the side force on the back of the wheel, any gust of wind will cause the wheel to quickly turn in one direction.

We hope this tutorial has helped you understand the basics of cycling wheel aerodynamics. For more great content, please register for our free monthly newsletter at the top of the column on the right. We send links to all the articles we post during the month. If you have any questions please feel free to ask!

All the best,

Chris

### September 2, 2014

## FLO Cycling - Tire Pressure

We get a lot of questions about the optimal tire pressure to use with FLO wheels. There are a couple of common misconceptions with regards to tire pressure.

- 120psi is the optimal pressure.
- Higher tire pressures equate to lower rolling resistance and make you faster.

Let's talk briefly about those misconceptions.

**120 PSI**

I believe the 120psi misconception has trickled down over the years. I think it was a combination of narrower wheels, the pressure recommendation on the side of a tire, and word of mouth. Over the years the width of rims have changed and tires have changed as well.

**Higher Pressures Make you Faster**

Taken to extremes, this seems to make sense. If you ride down the road with a tire that has really low pressure, it's easy to see that this is much slower. Naturally, if low pressure is slow, then high pressure is fast, right? Actually, it is right, but only up to a certain point. If a tire has too much pressure, it actually, starts to bounce around on the small imperfections in the pavement. All of this bouncing wastes energy and makes you slower.

**So what Should You Do?**

There's a place somewhere in the middle where the correct amount of tire pressure limits the "slowness" created by too little pressure, and limits the bouncing you experience when you have too much pressure. Tom Anhalt, author of the blog Blather 'bout Bikes, has studied tire pressures and rolling resistance at length. When I have a question about rolling resistance and tire pressure he's my go-to guy. We've even shared wind tunnel data with Tom in the past to find the tire that has the optimal balance of rolling resistance and aerodynamics.

When I asked Tom about the optimal tire pressures for FLO wheels, he gave me the following dataset.

**Note:**Never exceed the maximum pressure recommended by your tire manufacturer AND never inflate to a pressure less than the minimum pressure recommended by your tire manufacturer. Riders over 180 lbs or 81kg are advised to inflate their tires to the maximum pressure recommended by their tire manufacturer without exceeding 150psi or 10bar. Riders less than 110 lbs or 50kg are advised to inflate their tires to the minimum pressure recommended by their tire manufacturer. The above data set is a modification of the data found here. The data was modified to adjust for the width of FLO rims.

So how do you use this table? Let give a couple of examples us the standard table.

**Example #1**

**Rider Weight:**150lbs

**Tire Size:**700 x 23c

Knowing you have a 700 x 23c tire, you will start by using the green line. Now find your weight - 150 lbs - on the Y axis and then move over to the right until you hit the green line. Next, draw a line straight down to the X axis. The value on the X axis in this case is about 100 psi. This is your recommended tire pressure.

**Example #2**

**Rider Weight:**170lbs

**Tire Size:**700 x 25c

Knowing you have a 700 x 25c tire, you will start by using the red line. Now find your weight - 170 lbs - on the Y axis and then move over to the right until you hit the red line. Next, draw a line straight down to the X axis. The value on the X axis in this case is about 95 psi. This is your recommended tire pressure.

I hope this article has been helpful. If you have any questions at all, please leave them in the comments section below.

Chris

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