December 30, 2014
Posted by FLO Cycling at Tuesday, December 30, 2014 Labels: Aerodynamics , en , FLO Research & Development , Tire Study
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.
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.
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 UpNext week I should have an update on mounting the logger and sensors to my bike. It’s time to get handy in the garage.
December 23, 2014
Posted by FLO Cycling at Tuesday, December 23, 2014 Labels: Aerodynamics , en , FLO Research & Development , Tire Study
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.
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.
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.
December 18, 2014
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.
December 10, 2014
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.
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.
December 3, 2014
December 1, 2014
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.