### May 8, 2011

# FLO Cycling - Wind Tunnel Results and Cycling Wheel Aerodynamics Tutorial

**Introduction**

When we set out on our journey to design aerodynamic cycling wheels, we asked ourselves, “what makes a wheel aerodynamic?” We soon learned there wasn’t a real answer. After hours of research, computational fluid dynamics testing, and a trip to the A2 wind tunnel, we now know what makes a cycling wheel aerodynamic. Below are several paragraphs discussing how we designed our FLO Cycling wheels and of course our

Be sure to check out our latest wind tunnel results on the all new FLO wheels. You may also find Part 1 and Part 2 our tire study of interest. In our tire study, we studied the combined resistance of aerodynamics and rolling resistance on nearly 20 tires.

**w****ind tunnel results**. For those who are interested in understanding the science that goes into designing aero wheels, we have added an additional section to the end of this blog post titled "**Aero Wheel Tutorial.**" As always, questions and comments are more than welcome!Be sure to check out our latest wind tunnel results on the all new FLO wheels. You may also find Part 1 and Part 2 our tire study of interest. In our tire study, we studied the combined resistance of aerodynamics and rolling resistance on nearly 20 tires.

**FLO Cycling Design**

The picture below shows the progression of aerodynamic wheel shapes. The V-Notch design was one of the earliest aerodynamic wheel shapes on the market. This wheel displayed a reduction in aerodynamic drag but is known for being unstable in crosswinds. Many manufacturers still use this technology today.

Designers next released what was known as a toroidal shape. The toroidal fairing flared out and got wider than the brake track before coming to a point. This design showed an even greater reduction in aerodynamic drag and improved crosswind stability

**FLO Cycling Symmetrical Design**

In our opinion, the largest downside to both the V-Notch and early toroidal design is that they come to a point. The leading edge on the front half of any wheel is the tire. A tire is a bulbous circular object. If we want to create even side force on the front half and the back half of the wheel, we assumed that the leading edge on the back half of the wheel would have to also be a bulbous circular object. By removing the “point,” we have been able to create a balanced aerodynamic system. On a FLO Cycling wheel, the leading edge on the front half of the wheel (the tire) is nearly identical to the leading edge on the back half of the wheel (our wide toroidal shape). This design reduces the aerodynamic drag even more than the early toroidal shapes and greatly improves crosswind stability. FLO Cycling wheels are nearly symmetrical from front to back. The result of this symmetry is superb aerodynamics and superior crosswind stability.

**Net Low Drag Technology**

When we sat down to design our wheels we observed the aero data published by other companies and noticed a trend. Their wheels became very aerodynamic at a specific yaw angle. Unfortunately, a few degrees in either direction of that yaw angle caused the drag to rise quickly. This produces good results in a controlled environment like a wind tunnel, but doesn't necessarily help in the real world. We'd all love to ride our next race at the optimal wind angle, but mother nature simply won't let that happen.

Our goal was to produce wheels that became aerodynamic and stayed aerodynamic for as long as possible. We knew that roughly 80% of a cyclist's time is spent riding in yaw angles between 10 and 20 degrees. During our CFD analysis we tweaked our wheel shapes to be as aerodynamic as possible throughout that 10 degree sweet spot, and not at a specific point. The wind tunnel results were remarkable. As an example, our FLO DISC wheel experiences negative drag from 12 to 24 degrees of yaw.

A lot of companies claim to have the fastest wheels in the world. At specific angles, using specific tires, and in a wind tunnel, they certainly do. They also say that at that specific angle the wheel will save you X amount of seconds at your next race. Unfortunately, the wind will not be blowing only at 12.6 degrees of yaw at your next race.

At FLO Cycling we wanted to define a useful term. Since we know we spend roughly 80% of our time racing between 10 and 20 degrees of yaw, we feel it is best to calculate a weighted average of drag reduction. A cyclist would then have an excellent idea of how many seconds a FLO Wheel will save them regardless of the wind angle. We call this value the

**Net Drag Reduction Value (NDRV)**.
Below are the

**NDRV**of our FLO wheels and the optimal reduction value assuming mother nature is only blowing at the perfect angle all day. These values are the amount of grams saved when compared to a standard box section rim (Mavic Open Pro - 32 Spokes).**FLO CLIMBER**

**NDRV:**78.3 grams - 31.3 seconds over a 40km time trial OR 2 minutes and 21 seconds over an Ironman.

**Optimal Reduction:**88.3 grams - 35.3 seconds over a 40km time trial OR 2 minutes and 39 seconds over an Ironman.

**FLO 60**

**NDRV:**175.3 grams - 70.1 seconds over a 40km time trial OR 5 minutes and 16 seconds over an Ironman.

**Optimal Reduction:**210.1 grams - 84.0 seconds over a 40km time trial OR 6 minutes and 18 seconds over an Ironman.

**FLO 90**

**NDRV:**172.1 grams - 68.8 seconds over a 40km time trial OR 5 minutes and 10 seconds over an Ironman.

**Optimal Reduction:**202.1 grams - 80.9 seconds over a 40km time trial OR 6 minutes and 04 seconds over an Ironman.

**FLO DISC**

**NDRV:**253.7 grams - 101.5 seconds over a 40km time trial OR 7 minutes and 37 seconds over an Ironman.

**Optimal Reduction:**319.9 grams - 128.0 seconds over a 40km time trial OR 9 minutes and 36 seconds over an Ironman

Without further ado, here are the FLO Cycling Wind Tunnel Results...

**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 headwind 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 its 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. Anytime there is a difference in pressure on opposite sides of an object, the high pressure side pushes the object towards the low pressure side. Think of 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, two 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 the 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-lb 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,

Jon and Chris Thornham

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## 45 comments :

Hi Guys,

Thanks for sharing, results look sweet and seem to back up the review posted earlier! As a aero wheel newbie, I'm a little confused why your FLO 90 is less aero than your FLO 60 in the "sweet spot" that you talked about during the post. Aren't deeper rimmed wheels supposed to give you more aero benefit (which is why they tend to cost more)? It looks like it performs slightly better as you get out over 20 degrees of yaw, but there doesn't seem to be much benefit to the deeper wheel besides that. Your thoughts would be greatly appreciated!

Jonathan,

We were surprised at first when we saw the 60 vs. 90 results. Here are my thoughts.

When you look at the 60 vs 90 your first instinct tells you that our 90mm wheel is not that aero. That is because a 60mm wheel is typically more aero than a 90mm wheel. If however, you look at the results of the 90 on it's own, you will see that it is a very aero wheel.

I think the amazing thing is what has happened with the 60mm wheel. Using our new fairing shape, we have been able to significantly reduce the drag of a 60mm wheel. So it's not that our 90mm wheel is not aero, we've just hit a perfect storm with the 60mm wheel.

If you look at the new firecrest aero data, you will see that the 404 and 808 also have very similar profiles. Both the FC 404 and 808 bottom out at around 40 grams of drag. This is exactly where the FLO 60 and 90 bottom out.

I think your decision to use either wheel would be about the same. In the long run, the NDRV for both wheels is only 3 grams apart. This is almost negligible.

If you have any additional questions, do not hesitate to ask.

All the best,

Chris

Hey Guys,

First off-Congrats on the fruits of your labors. Second-Thanks so much for breaking down the 'aero' data so a total newb can understand it. The wheels look great, perform as advertised and they're priced where even I can afford one. Best of Luck to you all.

VAJeff,

Very kind words. Thank you! We really wanted to explain what we did so that everyone could understand it. There seems to be a lot of mystery surrounding products at times. We have been open and honest with our customers since day one and wanted to not hide anything when posting our Wind Tunnel Results. I am glad that you like the results.

All the best,

Chris

Very impressed guys. Great report, great results. Can't wait for the pre-order to open up.

I have to say, I really appreciate the time you took to write up the Aero Wheel tutorial. Great information about what looks to be a great wheelset.

How are the anodized breaking surfaces holding up? Are they still black? What are your thoughts on keeping them black versus the standard silver?

Would you happen to know how much clearance (in millimeters) is between the drive-side spokes and the cassette on the Flo60?

Sorry about the barrage of questions - I'm eager to get a pair of these on my bike!

RayV,

Thank you very much. We appreciate the comments. We can't wait for the pre-order to start either!

Take care,

Chris

outdoorWI,

Thank you for your questions. See my answers below in

boldI have to say, I really appreciate the time you took to write up the Aero Wheel tutorial. Great information about what looks to be a great wheelset.

Our pleasure. We are glad you enjoyed it.How are the anodized breaking surfaces holding up? Are they still black? What are your thoughts on keeping them black versus the standard silver?

We haven't seen any wear to date on the black braking surfaces. It looks like they will hold up fairly well. 99% of our customers have said they would prefer black brake tracks so we are going to stick with them.Would you happen to know how much clearance (in millimeters) is between the drive-side spokes and the cassette on the Flo60?

I don't have an exact measurement but we have had no tolerance problems to date. Even our disc wheel is tolerance free between the cassette and "spokes" or fairing.Sorry about the barrage of questions - I'm eager to get a pair of these on my bike!

No problems at all. Feel free to ask all of the questions you want.

Thanks for the support,

Chris

Hi Chris,

The numbers look awesome, thanks for posting the data. A quick comment, then a couple of questions. Your earlier comments were a bit off--the new Zipp 404 is actually quite a bit higher in drag, bottoming out around 85 grams versus 45 grams, so in the case of the Zipps, while the drag profiles are the same, the 808 is actually a significantly faster wheel than the 404. In the case of your wheels, it appears that you'll be selling a lot more 60's than 90's. With the lower drag force, lighter weight, and similar (if not better) aerodynamics, it's hard to imagine going for the 90. Well, except that it looks more badass.

A couple of quick questions:

-what tires were used for testing

-at which tunnel did you test, and is the tare removed from the results?

Thanks,

John

Sorry, one more quick question:

what tire was used for the wheels, and do you know the actual width of the tire mounted on these rims?

Thanks,

John

Results look great! Just a couple of questions 1) What front wheel did you pair the disc with for the test?, and 2) given the results, in what situation would you recommend the 90 over the 60?

Hi Chris,

The numbers look awesome, thanks for posting the data. A quick comment, then a couple of questions. Your earlier comments were a bit off--the new Zipp 404 is actually quite a bit higher in drag, bottoming out around 85 grams versus 45 grams, so in the case of the Zipps, while the drag profiles are the same, the 808 is actually a significantly faster wheel than the 404. In the case of your wheels, it appears that you'll be selling a lot more 60's than 90's. With the lower drag force, lighter weight, and similar (if not better) aerodynamics, it's hard to imagine going for the 90. Well, except that it looks more badass.

The aero data that I have seen seems to have the FC 404 bottoming out around 40 grams. Take a look at the pictures I posted here under the name "Canadian".http://bit.ly/j5dTTLA couple of quick questions:

-what tires were used for testing

We tested with Michelin Pro 3 Races Tires-at which tunnel did you test, and is the tare removed from the results?

We used the A2 wind tunnel. We did remove the tare. I think you will find that most manufacturers do this. That is because we are testing the wheel and not the tare.Thanks,

John

We appreciate your compliments and interest. Take care,

Chris

John,

We used Michelin Pro 3 Race Tires. It is 23mm wide. Take a look at this picture from tririg.com. It shows how the tire mates perfectly with the rim.

http://i1210.photobucket.com/albums/cc409/flocycling/FLO_Smooth_Transistion.jpg

Clinton,

Thank you for your compliments. All wheels are tested by themselves. This is the norm for all wheel companies.

The FLO 90 wheel is still a great choice. It's a very aero wheel and really only a couple grams of drag different than the FLO 60. The FLO 90 makes a great rear wheel choice and I would imagine a better wheel to add a wheel cover too. You can add a wheel cover to a FLO 60 as well however... Our FLO DISC uses a lenticular shape. The beginning of our FLO DISC uses the FLO 90 profile. I would imagine using a FLO 90 with a wheel cover would give you nearly the same aero advantage as our FLO DISC.

I hope that helps,

Chris

I have no clue if you guys can answer this question, but would the 60 or 90 be a better rear wheel? These tests demonstrate the capabilities of these wheels when they are the leading edge of a bike, it seems like. Is there any research out there that discusses how aerodynamics of a back wheel change compared to that of a front?

Thanks!

Greta results guys! Once again, I am excited and cannot wait to see the wheels in action. Have you had a chance to try them in the rear of a Specialized transition yet? I have not heard confirmation on whether or not the disc or FLO90 will fit it's goofy shaped chain stays. That's all I'm waiting on to make my decision to wait for your wheels!

Andrew Alseth,

We have not done any specific tests as a rear wheel. There are simply too many variables to take into consideration. For example, the wheel will behave differently on every bike. The wheel may be more aero on a Felt than it is on a Cervelo. The crank you use could also be a factor.

I think that both the 60 and the 90 would be great choices as a rear wheel. Both wheels have nearly the same aero advantages when you look at the calculated NDRV. I will say this... Our FLO DISC uses a lenticular shape. This means it is bulged. The bulge follows our FLO 90 profile. So if you were to buy a wheel cover for our FLO 90, then I would assume you could get nearly the same aero advantage as our FLO DISC wheel.

I hope that helps. Let me know if you have any additional questions.

All the best,

Chris

Nick,

Thanks again for the compliments and support. We still have to test our wheels in a list of frames. We will be sure to post our "do our wheels fit your bike report" before the pre-order.

Thank you for your patience,

Chris

This looks great, I am looking forward to the pre-order. I assume the "climbers" are the version without a fairing? Do you guys have a price estimate on those yet? I would be interested in getting a set as everyday trainers since I will obviously have to set up my brakes to accomodate the new width. Will they be a regular part of the line, or just available during the pre-order?

Jason,

Our FLO CLIMBERS will be available at all times and not just during the pre-order. They are as you mention essentially our race wheels without the carbon fiber fairing. Pricing is estimated at $199 for front wheels, and $249 for rear wheels.

Thanks for the interest,

Chris

Hey guys,

Great post, thanks for the tutorial. Being a mechanical engineer (and bike dork) I eat this stuff up. The one area where I wasn't able to follow your explanation was where you gave the side force vector a y-component that is in the direction of travel of the wheel. Could you offer a little more explanation as to how that comes about?

Thanks!

-Rob

RobLindqu,

I'm glad you liked our tutorial. Let me try and answer your question. Take a look at Figure G. When the yaw angle is 20 degrees the "absolute" or "total" side force acts perpendicular to the yaw angle. So imagine the side force you are seeing in that picture is the absolute side force. Now do a head to tail addition of X and Y vectors. You'll see that the Y component actually pushes forward. Figure I shows this in better detail. You will see that "Side Force" is perpendicular to "Drag".

Trust me... It's not a very intuitive concept. It took us a while to figure this out.

Does that make sense? Let me know if you have any more questions.

Chris

Rob,

We published this comment the other day and for some reason it did not make it up. I apologize for the delay. Good question. When we have a yaw angle greater then 0 degrees our side force also acts at an angle on the wheel. Remember the side force and drag force always act in the same direction but the wheel rotates. When this happens our side force vector has an x and y component. We chose to call the direction of motion the y-component since we did the same with our drag vector. This makes the addition and subtraction more intuitive. If this doesn't help let me know.

Take care,

Jon

Will you have a Campy compatible hub?

michael,

Yes Campy Hubs will be available.

Thanks for writing,

Chris

Hi guys,

I'm surpriced you guys (engineers, right?) didn't do the complete math in this process...

The thing is, with one exception that I know of (BTR), no-one measures the true drag of a wheel. You know, the one that takes into account WHERE the axial drag vector hits the wheel in relation to the hub. The resulting torque of this offset must be added to the axial drag in order to get the resistance the rider does experience on the road. TotalDrag=Drag*(1+offset/tire_radius)

In the end, the 90mm rim might be some/a lot better than the 60mm, please do the math...

If this is news to you and you feel that this could improve your marketing, please send me a set of 90mm's, :-)

I'll help you with the guerilla marketing...

Wbr,

Nicko (@Slowtwitch)

Guys, from memory I think I made a typo, sorry.

Total_drag=axial_drag * (1+offset/tire_radius)

( all horizontal components)

Nicko,

I'm not sure I understand your equation. You mentioned that no one in the industry is using this calculation. We have chosen to stick with the industry standard for calculating drag. This way our numbers are comparable to the other companies using the accepted calculation and people can easily see how our wheels compare.

I hope this makes sense,

Chris

That's what I'm saying, no-one does it correctly...

The (horizontal component of the) drag vector typically hits the wheel at some distance ABOVE the hub, right?

Now do the torque equation of the wheel around the contact patch: Thrust*tire_radius - drag*(tire_radius+offset) = 0. Right?

So Thrust = Drag x (1 + offset/tire_radius)

My point is that the drag number is NOT what the rider must overcome, it's the required thrust that is important. This has been lost on Zipp, HED, Tour Magazine, Matt Godo/Fieldview and others that claim drag numbers from testing or calculations. Mind-boggling...

Nicko,

Yes, we are both mechanical engineers. I understand your thinking but ultimately it is flawed. You are stating the the horizontal component of drag (simply the drag force) hits the top of the hub above center and creates a moment opposing the motion of the rider. That is true but the wind hits the entire wheel in the horizontal plane. It's not just the hub. Even if it were just the hub any moment created by the air hitting the top of the hub is counter acted by the wind hitting the bottom of the hub. One moment opposes motion and the other aides it. This equates to a balanced effect for the rider.

Torque is also not a variable in the drag equation. Drag can create torque but it is not part of it. The folks at the A2 wind tunnel have an incredible facility. It was created to measure drag to as true of a value as possible. Obviously there are factors such as tare to consider but I can assure you that our drag numbers are calculating drag correctly and that there is no missing component as you stated. I hope that makes sense. Please let me know if you have any other questions.

Take care,

Jon

Jon, thanks for taking your time with this.

It seems you misunderstood my model. I am not talking about any drag on the physical hub, I am talking about the drag vector (from all forms of fluid drag components) for the whole rotating wheel. This drag vector does typically not intersect with the rotational axis of the hub, right? Or are you saying that the resultant force from the wind hitting all parts of the wheel does balance out to act through the hub?

You state that "Torque is also not a variable in the drag equation".

How is this torque counteracted? Remember that the rotation of the wheel is forced (kinematic constraint). So the torque must be balanced out by some other torque, right? I'm saying that torque is created from a horizontal force at the contact patch, superimposed on the other forces there. The reaction force to this horizontal force comes from the hub, requiring thrust.

Nicko,

I've read what you wrote several times trying to understand your thinking. You start by talking about fluid drag and end talking about torque. Understand these are two separate models. Yes there are a number of forces on the wheel. Since it is moving something is out of balance. In this case we have created an angular velocity. However this doesn't really matter for our drag model. Yes the wheel is spinning to account for motion but the combination of forces such as gravity etc aren't relative to the drag. Try to separate the two models.

I really am trying to understand where your going with this but I don't know if I'm doing a good job. As for the torque on the wheel from air think of a water wheel. If the water only hits the bottom paddles the wheel rotates. If we laid the wheel on its side and the water hit both sides the wheel would not move. The forces are balanced. If we spun the wheel with an outside power source the water still moves and hits the wheel the same. The wheel either moves towards the water or away from it. The forces essentially balance each other.

I hope this helps. Also understand we calculate the body axis drag of the wheel. The gyroscope in the wind tunnel measure forces in x, y, and z planes. We gather the forces from by using equations to calculate side force and drag force.

Take care,

Jon

I think I see now were I was not clear enough: the torque I'm talking about is from the side view of the wheel, in the rotational direction. You might have thought of the STEERING torque.

Another way to look at what I'm saying is to monitor the electric power required to spin the wheel in the wind tunnel stand. Some of that is from the rolling resistance, sure, but not all. This required extra energy is what I'm talking about but in a different form.

If you search "watts to spin" you'll find a lot of discussions.

Nicko,

Side force has no effect on rotation. MHow do "watts to spin" effect the grams of drag? If you can, derive the equation that shows how the two are related.

Take care,

Jon

Jon,

Look at the wheel from the side and model everything in 2D. All forces and torque's in that view are relative to the rotation of the wheel as it rolls down the road.

The drag vector does NOT affect the wheel through the hub. Maybe the folks at the A2 wind tunnel give you that information but it's incomplete... there must be more "drag contribution" from the parts of the wheel that are above the hub since they meet the air at a higher speed than the bottom parts.

"Watts to spin":

grams = P x 100 / v,

grams = extra grams of drag on top of "axial drag"

P = mechanical power required to rotate the wheel

v = air speed [m/s] = rotational speed [m/s]

Nicko,

The top of the wheel meets the air at a higher speed but the lower half meets it at a lower speed. The horizontal center of the wheel at an instant is the only part of the wheel that see the speed of the bike.

Take care,

Jon

Chris G,

I'm sorry... By mistake I deleted you last comment and I can't bring it back. I'll post it below and answer your question...

"What air speed did you use in your testing?"

We used 30 mph in the wind tunnel. This is the industry standard value.

Sorry again about accidentally deleting your comment.

Let me know if you have any additional questions.

Take care,

Chris

Hi,

I am having trouble deciding on a disc or 90 rear wheel (I'm set with a 60 for front). Money aside, are there good reasons why not to go with disc ? If so, would it make sense to go for the 60 and get a separate wheel cover that could be installed and removed as need ? What is not an option for me is purchasing a disc and a 60 for rear, in addition to the front.

I plan on using the wheelset on race day, and possibly the last training session before the race only. These will be installed on a cervelo p2.

Pablo Smiraglia,

There are a few reasons why you would not want to OR cannot use a disc wheel. I'm going to make the assumption that you are racing triathlons.

1. There are some races where DISC wheels are not allowed. This is usually because of high winds. Kona is one of these races and I believe the 70.3 in Hawaii is another. However, most other races will allow you to use a disc wheel.

2. If the bike course consists of mostly climbing, you may be better off going with a lighter wheel. That said, there are very few triathlon courses that have enough climbing to justify not using a disc.

Typically, a disc wheel will always be your fastest wheel from point A to point B. Most people rarely train on a disc but since you are planning on only racing on your wheels I would think the FLO DISC would definitely be your best option.

As you mentioned, you also have the option of adding a wheel cover to a FLO 60 or 90. If you ever did decide to train using your race wheels, the FLO 60 and 90 are much better training options than a FLO DISC. Then on race day you could put your wheel cover on.

I hope that helps. Let me know if you have any additional questions.

Take care,

Chris

Guys,

The aerodynamic data (NDRV) above and on page 5 of the "Aerodynamics" section of the website are inconsistent.

Which are more recent? More appropriate to be relied upon when deciding between purchase of the 60s or 90s?

Thanks!

Dan

Dan Swenson,

Thanks for pointing this out. The data in this blog article was from our first trip to the A2 wind tunnel. We produced better results during our second trip. The data on our website is the most current and accurate data. I will work on updating this article so it is current.

Thanks again for pointing this out.

Take care,

Chris

Since ordering a pair if flow 45 carbon clincher wheels I have been trying to understand the physics of the design relative to the wind tunnel results. I appreciate the attempt you both made to explain some of the key concepts some of which I did not understand (I'm an electrical engineer with a cursory understanding of Newtonian mechanics). I found the following two web sites regarding the physics of a sail boat "tacking" into a head wind to be highly illustrative:

http://newt.phys.unsw.edu.au/~jw/sailing.html

http://physicsbuzz.physicscentral.com/2015/05/the-physics-of-sailing-how-does.html

However, I am still amazed at how sensitive the negative drag effect is to the tire that is mounted on the rim (large aero test on Flo carbon 60 with several tires) even within a given tire size. Fortunately, the yaw angles at which the loss of negative drag occurs is beyond 10 degrees for most 23 mm tires which as you measured do not occur with a lot of frequency in normal riding. Given my great appreciation for tubeless road tires, I will probably use Schwalbe's pro one 23 mm tires which have one of the lowest rolling resistances of any road tire out there right now. The combination should be terrific for road racing.

Unknown,

Thank you for sharing those links. They are definitely helpful.

I hope you are enjoying your new wheels!

Have a great day,

Chris

Hi Chris, great post! I was hoping to find out some details about the facility where you performed drag measurements. Are they obtained using a force transducer? Also, was the wheel rotation driven by a motor or just allowed to rotate freely, and were the arms which hold the wheel in place designed with an aerodynamic profile to limit interference with drag measurements? Thanks, I'm designing a wheel test stand for use in measuring drag on racing wheels in a wind tunnel right now so any additional insight into this setup would be great.

Robert,

Thanks for writing. Let me address your questions.

The A2 wind tunnel has a large series of load cell underneath the wind tunnel that measure all of the forces. Simply blowing on the wheel will pick up a measurement.

The rotation of the wheel was driven by a motor.

The arms are not designed to limit drag. We remove tare from our results. This means you run the stand by itself in the wind tunnel, then subtract that drag from your final results.

I hope that helps!

Chris

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