December 30, 2014

FLO Cycling - Studying Tires Part 2, Elevation Data

Posted by FLO Cycling at Tuesday, December 30, 2014 Labels: , , ,

I’ve learned a lot since I posted “Studying Tires Part 1, Logging Data.” I mentioned that I believed there was either a shift in the barometric pressure, or that the absence of temperature and humidity were the reason for the shift in my driveway's elevation. Today I have the results.  

A 12-Hour Backyard Plot
The graph below shows the barometric pressure over a 12-hour period. I placed the logger in my backyard out of the sun to try and minimize and temperature variation. 



To make things easier to see, I calculated the elevation for each barometric pressure reading. The graph below shows the elevation of my backyard over the 12-hour period. 



I was quite surprised that there was a shift of almost 80 feet over the day. It made the 22-foot shift from the other day look pretty good. The last time I checked the seismic activity in Las Vegas, it  was pretty low, so this got me thinking =].  

Barometric Pressure Records
I figured there had to be  a history of barometric pressures. A few minutes on Google and I found a website that records the barometric pressure at major airports for a one week period and displays a graph. When I compared my 12-hour log with McCarran International Airport - the airport in Las Vegas, where I live - my graph matched theirs. Here is a screen shot of the last seven days at McCarran International Airport. 



This graph shows a shift in elevation of over 700 feet. That's a massive difference.  

The Meteorologist
I needed a bit of help. A good friend from university is a special type of energy trader. He's one of the smartest guys I know. Luckily for me, he hired a meteorologist to work on his team. I reached out to my friend and the meteorologist and I ended up exchanging a few emails.  

During our discussion, he mentioned that it’s easy to see a shift in elevation of 60 feet. A source he referenced mentioned that this shift is commonly up to 150 feet. This is still a lot better then the 700 feet I saw in Las Vegas, but still not great. Even if you correct with temperature and humidity, you are still likely to see a 15-foot variance.  

What Now
The meteorologist recommend a combination of things I could try in an attempt to get more accurate elevation data. He brought up the idea of using GPS, barometric pressure, temperature, humidity, and hi-resolution topographic maps. I like the idea but ultimately it may be a bit overkill for what I am doing. Regardless, I will have to find a way to get an elevation that is accurate enough for my research.  

Next Up
Next week I should have an update on mounting the logger and sensors to my bike. It’s time to get handy in the garage.  

Take care,

Jon
Read More

December 23, 2014

FLO Cycling - Studying Tires Part 1, Logging Data

Posted by FLO Cycling at Tuesday, December 23, 2014 Labels: , , ,

For a while now, I've wanted to study the aerodynamic performance of tires on the road. We've done a bit of work in a wind tunnel, which is great, but it's not the real world. In order to get the job done, we needed a way to log data on the road. Quite some time ago I started developing a logger on my own by programming a microcontroller and building sensors.  While I got pretty far, there were a number of things that weren't where they needed to be.  The biggest concern was accuracy. The picture below is of the logger I built.

Microcontroller that I developed to read yaw angles, rider speed, and wind speed.

Once I realized accuracy would be a problem, I put things on the back burner for a while.  Last year during Interbike I sat down with Ryan Cooper from Best Bike Split and we talked about his work with the Trek Factory Racing Team. He mentioned that math models were now able to predict accurate yaw angles of the riders, rider speed, elevation, and a number of other things. We also discussed the logger I had been developing and Ryan mentioned he would be interested to check his results against the results I was able to measure on the road. That conversation got the ball rolling again and I went on a search for a logging system that would get me the accuracy I needed.  

Weather Stations are Great Loggers
After doing quite a bit of research, I found that some of the best data loggers were weather stations. They have the ability to sit in remote locations (battery powered), can collect large amounts of data over time (onboard memory storage), and collect multiple data points (versatility). This all seemed like a good fit, but I  just needed to figure out how to mount it on the front of a bike!  

Onset's Hobo Micro Station
Luckily I found a system that worked for me. I found a company called Onset that had a micro weather station that took four inputs and would mount well on my bike. I ordered the micro station and five sensors.  
1.  Temperature sensor (Altitude-related measurements)
2.  Humidity sensor (Altitude-related measurements)
3.  Barometric pressure sensor (Altitude-related measurements)
4.  Wind direction sensor (Yaw angle measurements)
5.  Wind speed sensor (Relative velocity measurements)

A few days ago, the micro station, temperature senso,r and barometric pressure sensor arrived. The wind speed sensor, humidity sensor, and wind direction sensor (on special order from Germany) are still on the way, but I was able to test the parts I did have.

With an initial set up, I was able to test the micro station's logging capability, the software package, and the sensors, I mounted the micro station in the back of my truck and went for a drive up to Lee Canyon outside of Las Vegas.  

Micro station mounted in my truck and a dog who was ready for a walk.

The micro station was recording temperature and barometric pressure every second for just under four hours. The flat line in barometric pressure displays a time when my truck was parked while I was walking my dog. Remember, as elevation increases, barometric pressure decreases. The black line in the graph below shows the barometric pressure. The blue line is the temperature. There are a few jumps which relate to the sensor being in and out of the sunlight.


Calculating Elevation
The reason I got the barometric pressure sensor was to get accurate elevation data. If you use the barometric formula - which there are a number of variations of - you can solve for altitude. The barometric formula I used was the one below. I believe I will have to modify this to incorporate temperature and humidity but that will be another day.

I needed to solve for height since I had the current barometric pressure so I rearranged the equation to the one below. I dusted of the math skills on this one. =]

With the new formula, I was able to calculate the elevation for each barometric pressure I recorded. The results are graphed below. The hardest part was putting the above equation in a Excel! Here is what I found.  


Results
To be honest I am pretty impressed with the data logger. For the most part, the elevations are spot-on, but the data did leave me with a few questions. For starters, I noticed that the elevation of my driveway - my starting and stopping point - were different. I am no meteorologist, but my guess is that there was a shift in the barometric pressure over the four hours I was gone, or that not including temperature and humidity values in the equation was the problem. Today I am logging data for 12 hours in my driveway to see if my theory on barometric pressure shift is correct.

That's it for today. I will report back soon with the results of the 12-hour log in my driveway.  Hopefully by then I'll have a few more parts to test out.

Take care,

Jon
Read More

December 18, 2014

FLO Cycling - Tire Pressure and Temperature

Posted by FLO Cycling at Thursday, December 18, 2014 Labels: ,

Have you ever wondered what happens to your tire pressure when you ride through different temperatures? How much does it change? Should you pump your tires up inside or outside? These are all things I recently started thinking about. There are a few rules of thumb but I wanted to know why. If you've had some of these same questions, or I've sparked your interest, I hope you enjoy the article below. 

How a Tire's Air Temperature Changes
It's important to know that there are two ways for a tire's air temperature to change.  

1.  Ambient Air Temperature Change: If you pump your tires up inside your house where the air temperature is 72 degrees, and then move the wheels outside where the air temperature is 30 degrees, the temperature of the air inside the tires will eventually drop to 30 degrees as long as there are no mechanical interferences.



2.  Mechanical Change: The three most common ways that a tire's air temperature will change mechanically is from the sun's radiation, from friction when the tire deforms during riding, and from brake heat. When the sun's rays hit a tire, it naturally increases the temperature of the tire and therefore the air inside the tire. When you ride a bike, the tire deforms near the contact patch. The deformation causes friction and increases the temperature of the tire and therefore the air inside the tire. Finally, when you brake, the friction of the pads and the rim creates heat that transfers to the air in the tire.



The Quick Rule of Thumb
If you read about tire pressure changes due to temperature changes, you will find that people say for every 10 degrees Fahrenheit, tire pressure will change approximately 2%.  That means if you start at 70 degrees fahrenheit and increase the temperature to 80 degrees Fahrenheit, you will increase the tire pressure 2%. Likewise, if you start at 70 degrees Fahrenheit and lower the temperature 10 degrees to 60 degrees Fahrenheit, you will lower the psi by 2%.

Proving that Tire Pressure Changes Approximately 2%
To prove this we need to discuss the Ideal Gas Law and do a bit of math.  

The Ideal Gas Law states the following.  


The units for the variables above are as follows.



Note: To make things simple, I will use degrees Fahrenheit and and psi in the discussion. The math below will show all of the conversions if you want to see how we move between the different units.

Setting a Base Line
I am going to assume that we start with 100 psi at 70 degrees Fahrenheit. Knowing this, I can calculate the tire volume we will use for each future calculation. Since we know the amount of gas will not change, we can assume that n = 1 and remove it from the Ideal Gas Law equation.  



Now that we have the initial air volume, we can calculate the change in tire pressure for every 10 degrees Fahrenheit. I will show one example below for an increase in ten degrees to 80 degrees Fahrenheit and then provide tables with multiple values. Note: Remember that these changes are for the first cause of tire temperature change, a change in the ambient air temperature.  



The difference is 1.888 psi or approximately 2% which is what the rule of thumb says. The table below shows a number of different psi values when starting at 100 psi and 70 degrees Fahrenheit.  



To show that the 2% rule works at a different psi I also started with 50 psi at 70 degrees Fahrenheit. You can see the table below.



Tire Pressure Recommendations and "Cold Tire Pressure"
When you read a recommended tire pressure from a manufacturer, what does this mean?  The tire pressure recommendations are "cold inflation pressures". Cold tire pressure means that the tire has not been ridden and has been sitting for a while. A car before the sun breaks in the morning that has been sitting overnight is a good example of cold tire pressure. If you plan to ride on a winter's day but store your bike inside your house, you may experience a drop in tire pressure when you go outside. It is true that the mechanical friction will raise the tire pressure and eliminate some of the drop. Ultimately, the temperature effect is not nearly as dramatic as many might think.

So What Does all of This Mean?
Ultimately, I don't think that you really have to worry about tire pressure and temperature changes.  This is especially true if the ambient temperature where you pump your tires up is close to the ambient temperature where you are riding.  

If however, you are going to experience an extreme drop in temperature you may want to leave your bike and pump outside for a while before you pump your tires up or add a few psi for the upcoming change. The opposite is true if you plan to raise the temperature to an extreme after you start your ride. I've ridden at 110 degrees Fahrenheit after leaving a house that was 70 degrees Fahrenheit. In reality, I could have dropped the psi a bit to account for the increase but to be honest I thought more about the heat of my body than the temperature of the air in my tires.  

Today a few new toys showed up.  We will be logging temperature and a few other things in an upcoming tire study.  It should be pretty cool.  



Take care,

Jon


Read More

December 10, 2014

FLO Cycling - Tire Pressure and Elevation

Posted by FLO Cycling at Wednesday, December 10, 2014 Labels: ,

I was recently at 11,000 ft elevation.  \While I was sucking wind running up a set of stairs, I started thinking about what happens to tire pressure when elevation increases or decreases. If you've read the title and these first few sentences, I bet you can guess what this article is about.  

Pressure Background
Before we get started, here is a quick background on the different types of pressures.  

Atmospheric Pressure - The Earth's atmosphere is made up of five primary layers. These layers and their distances from sea level are as follows:
  • Exosphere: 700 km (440 miles)
  • Thermosphere: 80 to 700 km (50 to 440 miles)
  • Mesosphere: 50 to 80 km (31 to 50 miles)
  • Stratosphere: 12 to 50 km (7 to 31 miles)
  • Troposphere: 0 to 12 km (0 to 7 miles)

All of these layers contain gases and gases have mass. Assume that you were to create a tube that had a cross sectional area of one square inch. If this tube extended all the way to the top of the exosphere, gravity would act on all of the gas in the tube and pull it towards the surface of the earth. The pressure that would be exerted on the surface of the earth from this column of air is called atmospheric pressure.  

If we were above sea level, say on a mountain top, then the column of air would be shorter and therefore would exert less pressure on the surface. If we were below sea level, say in Death Valley, then the column of air would be longer and the pressure would be greater.  Since tire pressure is measured in psi I will use state atmospheric pressure in psi. Here is a list of atmospheric pressures at different elevations.  


Gage Pressure - When you use a tire pump you are reading the pressure from a gage.  This gage value is the pressure increase (order decrease, technically a vacuum) over the atmospheric pressure. If you have a deflated tube at sea level, the pressure inside the tube and outside the tube is 14.7 psi. If you were to hook up your pump you would read a value of 0 psi. The minute you start pumping, you increase the pressure on the inside of the tube relative to the pressure outside the tube. This difference in pressure is what you are reading on the pump's gage.  

Gage from the Silca Superpista Ultimate
How Tire Pressure Changes as Elevation Increases or Decreases
The easiest way for me to visualize what happens when we increase or decrease elevation is to assume we start at one elevation with 0 psi in gage pressure. Let's say that we have a tube at 0 psi at sea level. The atmospheric pressure outside and inside the tube is 14.7 psi.  Now let's close the tube and head to a higher elevation. As we increase the elevation, the atmospheric pressure outside of the tube decreases but the pressure inside the tube stays at 14.7 psi. If the pressure outside of the tube is lower then the gage pressure would increase. Remember, gage pressure is the increase in pressure over atmospheric pressure. 

We can assume that the tube is sealed. Even though the gage pressure has increased we haven't added any air to the tube.  

The same is true if we start at a higher elevation and move to a lower one. If the tube was inflated to a gage pressure of 90 psi at 10,000 feet and we moved down to sea level, the gage pressure would now read 85.4 psi.  


How Important is This?
To be honest, the differences in psi over a 15,000 ft range is quite small. When you take temperature into consideration, (we will talk about this in an upcoming article) the difference is even smaller. Ultimately, if you ever find yourself thinking about the psi of your tires as you change elevations on your ride, know that it's nothing to worry about.  

I hope you enjoyed this article. If you have any questions, please let me know.  

Take care,

Jon
Read More

December 1, 2014

FLO Cycling - How Does a Tire Support Its Load?

Posted by FLO Cycling at Monday, December 01, 2014 Labels: ,

Last week I wrote a blog article that discussed why you reduce the air pressure in a tire when you widen the wheel or increase the tire size. While writing the article, I started thinking about how a tire supports its load. My first guess was the air pressure, but that's incorrect. It plays a role in the equation, but there is another part of the story.

If you haven't read last week's article, "Why Do You Use Less Tire Pressure for a Bigger Tire or Wider Wheel?" I suggest you do. It will likely provide some background that will help with this article.

Tire Pressure, What Does it Do?
When you pump up a tire, you increase the air pressure. My first thought was that by increasing the air pressure, the tire would support its load. The load exerted from the road to the tire would be opposed by the tire pressure pushing down on the road. However this is not the case.

The purpose of the air pressure in the tire is to hold the tire casing under tension. As we increase the tire pressure, the threads of the casing are placed under more tension.

 This is difficult topic to explain and visualize, so I've put together a video to help with the understanding. I even use a high-speed camera to film a sidewall explosion in slow motion.  Fun stuff.  


How A Tire Supports Its Load, The Basic Explanation
As stated above, the purpose of tire pressure is to increase casing tension so the tire can support itself. If you have a flat tire, there is no casing tension and the sidewall cannot support the load placed on the wheel. The picture below shows a cross-section view of a flat tire. The sidewalls of the tire are folded and the rim makes contact with the ground. Flat means there is no air, the casing tension is too low, and the sidewalls cannot support the load.  

As we increase the air pressure in the tire we also increase the casing tension of the tire. At some point the rim will leave the ground and the sidewall will have enough casing tension to support the load on the wheel.  

How A Tire Supports Its Load, The More Complicated Explanation
A tire that is in an unloaded state can be seen in the picture below.  


You can see that the force arrows created by the internal pressure act perpendicular to the internal tire surface. In this example, all of the threads in the tire casing experience the same tension.  

When a tire is loaded, there is a flat section that makes contact with the road, which is known as the contact patch. This flat section changes the circular shape of the tire. The section of casing that is the contact patch experiences a reduced or eliminate casing tension. This means the threads next to the contact patch (the sidewalls) have to increase their casing tension to make up for the reduction in casing tension experienced by the contact patch. When a tire is loaded you will see a bulge on the sides of the contact patch.  This bulge is created by the increased casing tension in this section. The video above gives a visual explanation of this.

Where the unloaded tire has a uniform circular shape, the loaded tire does not.  As the tire leaves the ground and makes its way towards the rim, the angle of the tire is very close to zero. The picture below shows what I mean.  


Since the angle of the threads next to the contact patch are close to zero, most of the force (load) is transferred horizontally and eventually makes its up to the side wall. The increased tire pressure allows the casing and side wall of the tire to support the load placed on the wheel. Without the proper tire pressure and resulting casing tension, the tire could not support its load.

That sums up today's article. I hope it made sense. Please let me know if you have any questions or comments.  

Take care,

Jon
Read More

November 28, 2014

FLO Cycling - Why Do You Use Less Tire Pressure for a Bigger Tire or Wider Wheel?

Posted by FLO Cycling at Friday, November 28, 2014 Labels: ,
During the last several years, bicycle rim width at the brake track has been increasing.  Manufacturers who are producing the new wave of high-tech wheels are ignoring the old 19mm width standard, and commonly producing rims that are 23-25mm in width. This is done to improve both performance and aerodynamics.  

 This increased rim width has manufacturers asking their customers to inflate their tires to a lower psi, and the obvious question is why. Why does a wider rim and/or tire require a lower psi? We get this question so frequently that we decided to write a blog article to explain the science behind it.


Tire Background
Before starting, let's look at the basic parts of a clincher tire.  

Casing: The fabric that is used to shape the tire and support the air pressure in the tire.
Tread: The rubber coating put on the casing for grip on the road and to make it airtight.


Bead: This is the metal hoop that hooks into the clincher hook on the wheel.

Parts of a Tire
The casing is the important part for today's discussion. The casing is made from a thread that is stitched together in a diagonal orientation to form a bias ply tire. This bias ply is what holds the tires structure when inflated with air. Without it, the tire would stretch like a ballon.   

Bias Ply Weave
Casing Tension and Hoop Stress
In order to understand why we use a lower pressure for a bigger tire or wider wheel, it's important to understand hoop stress. To understand hoop stress, it helps to have a few visual examples. The picture below shows a large propane tank. It has a cylindrical body and cupped ends.

If we were to cut a section of the tank (Section A), we would get a circular shape like the one below.



There are three dimensions for the section. There is the outer diameter, the inner diameter, and the mean (average) diameter. For our calculations we are concerned with the mean diameter.

When the propane tank is full of propane, the pressure of the gas exerts a force on the wall of the tank as shown below.   


This force from the internal pressure creates a stress on the wall of the propane tank.  Wikipedia describes stress as follows:
"Stress is the force per unit area on a body that tends to cause it to change shape."
In the case of a thin-walled vessel, (like the propane tank) the stress that the wall of the tank experiences is called hoop stress.  

So how does this relate to a clincher tire? If we were to section a clincher tire when it is mounted on the rim, you will see a shape that is very similar to the section from the propane tank. Yes, the rim and tire is not perfectly round but the principle is the same. As we inflate the inner tube, (increase the internal pressure by increasing the air pressure) the casing of the tire is stressed. When stressed, the fibers of the bias ply weave are placed under tension. This tension is referred to as casing tension.   


 Calculating Casing Tension
To determine the casing tension, we need to define the equation for hoop stress.  



In the above equations we define the following as:

Wall thickness = the thickness of the tire casing
Mean diameter = the tire size
Pressure = the pressure the wheel is inflated to

We eliminate thickness since we can assume all of the tires in the calculations below have the same wall thickness.  

Setting the Base
As a starting point, we will use a 20mm tire at 120psi to calculate the casing tension.  



From the calculation above, you can see that the casing tension on the tire is 8,273.709Pa.  When calculating the appropriate psi for a new tire diameter, we will want to keep the casing tension at a constant 8,273.709Pa. To get the psi for each new tire size, we will want to rearrange the hoop stress equation used above to solve for pressure.  


Now that we have solved for pressure, let's use a 23mm tire as an example and calculate the tire pressure needed to create the same casing tension we saw in the 20mm tire.  


 As we can see, the tire pressure that creates the same casing tension for the larger 23mm tire is less then 120psi. Since bigger tires do not have thicker tire casings, we want to make sure we are not creating too much casing tension. This is why we keep the casing tension the same when we calculate the new tire pressure. It is also why you will see tire manufacturers lower their max tire pressure recommendations for wider tires.  Here is a list of tire pressures that relate to 120psi for a 20mm tire all the way up to a 30mm tire.  Remember, the tire casing tension is the same in each example.  



Lower Tire Pressures for Wider Wheels
As we widen the rim of a wheel, we by default widen the tire. It is not by a huge amount, but it does get wider.   The picture below shows what I mean.

The tire on the wider rim (left) is wider at the mid point then the tire on the narrow rim (right)

Since the tire gets wider with a wider rim, we increase the dimeter in our pressure equation above. This results in a lower tire pressure for a wider rim with the same size tire. The proper way to calculate it is to determine the percentage that it increases. Each wheel will be different and it will depend on what wheel you are comparing it to.

I hope this blog article was easy to follow and that you enjoyed it. I wanted to thank our good friend Tom Anhalt over at http://bikeblather.blogspot.com/ for helping me sort out my thoughts and teaching me a thing or two about casing tension. Please let me know if you have any questions or comments below.  

Take care,

Jon

Read More

November 10, 2014

FLO Cycling - What I've Learned from My Time in a Wind Tunnel - Part 2

Posted by FLO Cycling at Monday, November 10, 2014 Labels: ,


This article is a follow up to last week's article FLO Cycling - What I've Learned from My Time in a Wind Tunnel - Part 1.  I suggest reading the previous article before you read this one, especially if you want a bit of background on wind tunnels. Here we go with the rest of the lessons I've learned.



Lesson Three: Should We Test Products Alone?
This lesson really depends on what you are testing. When testing wheels, you are faced with the question of testing the wheel by itself, with a bike, or with a bike and rider. There are literally endless possibilities and each is unique. Someone with shaved skinny legs will affect the air flow around the back wheel in a different way than someone with thick hairy legs. Is there a water bottle on the bike? If so, what shape does it have? Is it a TT frame or a road frame and who makes it? The list goes on and on.



Different people have different opinions but personally I like to single out wheels by themselves. I feel I can get a better understanding of what is going on with the wheel.  That’s not to say that testing additional features is bad. To be honest, it makes a lot of sense, but I feel because of the number of options, specific testing is best done with individuals and their gear. I don’t think it’s fair to add a frame and rider and then assume that it works for everyone. I think it gives us an idea, but I don’t think it’s the answer.  

You do have to take things into consideration when you test items individually. A wheel is part of a system. So without the system the results will be different. Remember the discussion about what the air sees. The wheel on the back of the bike sees the air differently than the wheel on the front. Additionally, when the air gets to the back wheel it has already passed the front wheel, the bike and the rider.  Air that was laminar (traveling in a smooth flow before it interacts with anything, or clean air) can now be turbulent (air that is not traveling in smooth flow pattern, or dirty air). Clean air and dirty air modify the drag of an object in a large way.

This matters because reducing drag is about saving time. If the drag is reduced more when a wheel is tested by itself than when a wheel is tested on a bike, the calculated time savings are exaggerated. I don’t think this is deceitful if it’s clearly spelled out. We openly admit to calculating time savings by testing wheels by themselves but will also tell you that you have to test the whole system (you included) to get an accurate answer on time savings.  

Again, for me it comes down to clearly stating what you are doing. If you make it clear, the reader understands what is going on.  

Lesson Four:  Attachment And How You Sweep
First we have to discuss attachment. Watch the video below for an explanation.



Now that you understand attachment, let’s talk about how it works in practice. The longer the air stays attached to a wheel as you sweep it through yaw angles, the more you will reduce the drag. Attachment is a good thing. If I start a wheel at zero degrees yaw, I can assume that the air is attached on both sides of the rim until it hits the spokes.  As the yaw angle increases, we eventually get to the point where the flow detaches. Let’s assume that the flow detached at 15.3 degrees. It would make sense that if we moved the wheel back to 15.2 degrees the air would reattach. However, it doesn’t. It’s a fact that once the air detaches, it take longer to reattach when going back in the opposite direction.  

So why does this matter? Well, how do I report my results? Do I start the test at zero degrees and sweep it through different yaw angles until the air detaches? Or, do I start the test with the flow detached and sweep it until the air attaches? The first option would give me a better result than the second. The best answer is to sweep in both directions and take an average. In the real world, we experience a number of different situations and the average seems like a fair approach.  

I learned this after talking to some people much smarter than I am. Swaying the results can make a big difference so make sure you ask the question if you don’t see it spelled out. Our most recent results do not include an average because we didn’t run full sweeps. I wish I had read this article before I made my last trip =]. The next time we will take an average.  

Lesson Five: Small Things Can Make A Really Big Difference
The last time we visited the A2 Wind Tunnel we tested a number of different tires. While testing we noticed that the same tire put on in the reverse direction can make a big difference in the drag. Why? I’m not quite sure. I can see a couple of grams but this was way more than couple. Up to 20 grams for our tests and the guy running the test had seen far worse.  

Tire pressure is another factor that makes a difference. We have not extensively studied tire pressures but we plan to in the future. I’ve heard through the grapevine that 90psi is the magic number. In the wind tunnel I am sure it’s pretty accurate but what happens on the road with different rider weights may be a different story. We are working on a new testing protocol which, I will talk about later.  

If I had to take a guess at why small things like tire direction and psi make such a big difference I would say it's the tires shape. Increasing or decreasing tire pressure changes the shape of the tire and each tire leaves the mold looking a little different than the next.  This may explain the difference in drag from one side of a tire to the next.  

Make sure when you are in the wind tunnel to look for the little things. It may save you from scratching your head. Then again it may have you scratching your head even more.

Lesson Six:  What’s The Equation To Solve For A Fast Wheel?
Science can be broken down into two categories. The first allows you to develop a mathematical formula so that you are able to predict results and see changes before you perform the experiment. The other category does not allow you develop a formula. The only way to get more info on a topic is to perform more experiments. A part of aerodynamics falls into this category. There are equations in aerodynamics, but there isn’t one that allows you to solve for a shape with the least amount of drag in various conditions. Therefore you have to test each shape you design to see how it performs. 

Lesson Seven: Black Magic
Since there is no equation that solves for the fastest wheel, and small things make a big difference, a lot of what we do in a wind tunnel is guess and check work. That’s not to say that general rules don’t apply. People with experience can predict behaviors with a high degree of accuracy. There are, however, things that seem like black magic.  

The Continental GP 4000 S II tire is a good example. It’s a very fast tire when tested but understanding exactly why is very difficult. Modeling the surface of a tire in CFD is very difficult so testing plays a big role in tire design. Sources have told me that even the engineers at Continental aren’t exactly sure why the tire is so fast. I don’t want to say they got lucky because a lot goes into designing a product. However, I do believe you can have a very fortunate educated guess.  

Spokes in wheels are another example. There have been a number of studies on spokes but they are very hard to completely understand. The air leaving the spokes is much different than the air entering the spokes and understanding how that works is nearly impossible. Speed changes, yaw angle changes, dirty air, it all adds up.  

There are people working on these difficult areas like Matthew N. Godo at Intelligent Light.  He’s doing amazing work. We interviewed him a while back if you are interested in reading about him and his work.  

That’s A Wrap
Wind tunnels are amazing places and we will continue to use them when we design products. I do, however feel that real world testing will change as technology evolves. We are currently developing a new testing protocol that we believe will make testing in the real world very accurate and more accessible to the general public.  Since wind tunnel time is so expensive ($600-$900/hour), the general public does not readily have access to them. In order to understand more about aerodynamics, I think it is important to collect as much data as possible and our new testing protocol plans to do that. We will have more on this shortly.  
Thanks for reading my thoughts. If you have any questions about them, please let me know.

Take care,


Jon
Read More

November 3, 2014

FLO Cycling - What I've Learned from My Time in a Wind Tunnel - Part 1

Posted by FLO Cycling at Monday, November 03, 2014 Labels: ,

The first time Chris and I visited the A2 Wind Tunnel we were under the assumption that testing a wheel was a simple as placing it in the wind tunnel and reading the results. What we didn't know was that there a large number of things that can alter the results. We also didn't really know how to perform a wind tunnel test since there is no standard way to test a wheel. Each company defines their testing protocol. After getting over this initial learning curve, we did our best to define how we test wheels so that it's clear to whoever is reading the results. I wanted to write this article to discuss the things I've learned through experience. This will be part one of a two-part series. If you ever find yourself in a wind tunnel or you are reading wind tunnel results, the lessons below may help you bypass some of the learning curve I went through.  

A Little Background. How A Wind Tunnel Works
A wind tunnel has a testing section which is not connected to the rest of the wind tunnel. The brown section in the picture below is the testing section. 
The brown irregular shaped section of the floor is the testing section.

 The reason it is not connected is so that it moves freely as the air passes over the test object. Good wind tunnels use test sections that have six degrees of freedom. If you are unfamiliar with what six degrees of freedom means, I've created a short video to help you understand.



In order to measure the force created by the moving air (drag force), the test section houses a number of force sensors. When the test object is mounted to the testing section, it enters the air stream. As the air interacts with the test object, it creates forces that are transferred to the free moving testing section and read by a computer. The computer records the measurements gathered by the force sensors and then computes the drag. It’s also important to note that anything on the test section will change the results. For example, if you were standing in on the test section with your wheel you would change the drag.  Even a mouse would have an effect.

Lesson One: The Truth About Tare
When testing objects in a wind tunnel you need to hold them in place. A bicycle wheel, in our case, needs to spin, so something that looks like an upside-down fork is used. That upside-down fork will also create drag. This additional drag is called tare.     
Definition of Tare - "an allowance made for the weight of the packaging in order to determine the net weight of goods.”

Since weight and drag are both forces they can be replaced. The package weight is equivalent to the drag produced by the mounting fixture.  

 The picture below should help with a visual.  

The wheel is mounted to two metal uprights using bolts and nuts.
Well this raises an obvious question. When we test our object do we remove the drag created by the mounting fixture? The quick answer is yes, but you have to think about it for a minute. Let me explain why. When you test cycling products in a wind tunnel, you run them through a sweep. This means that you turn the object in the airstream to simulate air hitting the object from a different angle. When the object is straight into the airstream we are at angle zero. If we rotate five degrees, we say the wind is hitting the object at a yaw angle of five degrees. The pictures below will help with a visual.  



At zero degrees the air sees all of the mounting fixture. Note: When I say the air “sees” something I mean that it is in the path of the air. For a visual example, think of someone squirting you with garden hose. When the water hits you it "sees" you.  f you were to stand behind a wall blocking the water from hitting you, the water would not "see" you. When testing a wheel like a disc, one of the sides of the mounting fixture moves behind the wheel as you increase the yaw angle. If you were to turn the wheel 90 degrees into the air stream, one half of the fixture would be directly behind the wheel and out of sight of the air. Removing the tare here isn’t entirely a fair representation because when you run the mounting fixture by itself the disc is not in the way. This leaves you with the option to calculate the drag created by the mount by testing it alone in the tunnel. These drag values can then be subtracted from the drag value of the object you are testing.  

So we are left with a bit of a dilemma. If we remove the tare we can potentially make the wheel look better than it is. If we don’t remove tare then we potentially make the wheel look worse than it is. The best way, in my opinion, is to clearly explain what you have done and make sure you keep it standard for each test. Don’t treat the baseline wheel any different then the wheels you're testing.If we compare apples to apples and it’s explained, the reader has a clear picture of what's going on.  I encourage those testing to spell out their testing protocol. As a company, we chose to remove tare. That being said, I don’t think one is necessarily better then another unless you plan to change the mounting fixture.  

When you are reading future wind tunnel data or heading to the tunnel on your own, make sure you keep this in mind.

Lesson Two:  When And When Not To Compare Data
Wind tunnels are great for comparative studies. Testing one wheel versus another with the same testing protocol gives us a good indication of which creates less drag. The same would apply to one person riding a bike in two different positions. This is why wind tunnel fitting makes a lot of sense. The rider is continually tweaked and the results are compared.  Eventually you find a fast, comfortable and efficient position that is optimal.  

Wind tunnels are not good when you compare results from two different studies or you look at the results from someone else and assume they will work for you. It’s a known fact that the best aero helmet on one person may be terrible on someone else. Don’t get caught up in the marketing. If you keep in mind that a wind tunnel should be used for comparisons you will be able to weed through a bunch of the marketing. For more on wind tunnels used for marketing, feel free to read my blog article.

 That's it for today. You can check out Part II here.  you can sign up for all of our blog content by signing up in the box at the top right of this page. Please feel free to comment and ask questions below.

 Take care,

 Jon
Read More
Subscribe to: Posts ( Atom )