FLO Rolling Resistance and Bike Wheels (Faster by FLO Podcast #58)
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Rolling Resistance and Bike Wheels
Welcome back to “Faster.” Today is part three of our mini-series. And we are talking about rolling resistance. So rolling resistance is one of those things that we sort of discovered a few years into running FLO. And when I first heard of it, it was something that I needed to kind of look up and get more understanding of. A lot of research was currently being done. There’s been some people in this industry that have started a trend sort of in that direction. And we’ve taken a lot of that information. And we’ve expanded upon it in our own way and done some really unique things.
So let’s just start with the basics. Just like we did with the aerodynamics article, I want to start and discuss some terms. So if you think of rolling resistance, what does that even mean? So let’s just kind of break it up. So the first thing we’ll look at is resistance. So resistance means, basically, you’re impeding or you’re slowing, or you’re stopping a force that’s acting on something. So if you think about take both of your hands and you put an elastic band around your hands, and you try to pull those…your hands apart, there’s a resistive force that is…the elastic wants to pull your hands back together. So that elastic force that is pulling your hands back together is resisting your hands from moving apart.
What is Rolling Resistance?
So rolling resistance really what it means is, it’s a resistance of the rolling of a wheel. So as opposed to, like, your hand moving in a straight distance, in a straight direction. Most people think of resistance, like, if you put your hand on top of a piece of sandpaper, and you were to move it, you would feel the resistance. It’d be very rough. So most people think of resistance in a straight line. But there can also be resistance in a rolling motion.
So what is the main component of rolling resistance? Well, it’s something called a…it’s a hysteresis effect. It’s a kind of a very fancy word for that. And the way to think about it is, if you think of the amount of energy that you put into something, the amount of energy that you get out is less. So one thing I like to think of is everyone has done this. You have like a tennis ball or like a bouncy ball when you were a kid, and you hold it at a certain height. And you drop it, and it falls to the ground, and it bounces, and it comes back up. But it doesn’t come back up to the same height as your hand. It’s lower.
The next time it falls, it goes down to the ground, and it goes lower again. And eventually, it stops bouncing. So hysteresis, basically, means that the energy that’s with the ball as it’s falling through the air, from the initial height, has a certain amount of energy. But when the ball hits the ground, and it deforms, there’s a loss of energy. And so when it comes back up, it doesn’t have as much energy to get back up to the original height.
So rolling resistance is the same way. If you think of a tire, anytime you load a bike, your weight and the weight of the bike, there’s a small flat section that sits on the bottom of that tire. So that is a deformation. So as you roll a wheel forward, that deformation is forming… Basically means that you’re deforming the tire at the front of the tire as it rolls. And at the back of that contact patch, it’s called a contact patch, it is expanding again.
So that’s just like a bouncy ball. The original energy that it takes to deform the tire at the front is greater than the amount of energy that you get out when it springs back into place. So the energy that’s used in the bouncy ball example is potential energy. So you have gravity, you have the height, you have the potential energy. So all of that is creating a certain amount of energy.
On a bicycle, the energy that you put in is the watts that you put into the pedals. So, in potential energy, it’s sort of that continual gravity force. And for you, it’s whatever you’re able to put out from an energy perspective. So each time that you’re putting, you know, a certain number of watts into the pedals, you are, basically, loading up that deformation at the beginning. And when it comes out of the back, you’re losing or burning some of your watts in that hysteresis effect that happens in there. So that energy loss.
So that’s why it’s really, really important, as a cyclist, to think about optimize, manage your rolling resistance. Ideally, you would want a rolling resistance of zero. So every watt that you put into it, from a rolling perspective, you’re going to get back out. Now we know that’s not possible. Pneumatic tires solve a lot of problems. But the rolling resistance on them does…is one of the negatives. So in this situation, it’s really important because then more watts that you can reduce, from a rolling resistance perspective, the faster you will go. And the more watts you will use moving you forward, as opposed to overcoming something like rolling resistance.
What is Crr?
So one of the terms that’s used a lot in rolling resistance, you’ll hear it used, is something called Crr, or the coefficient of rolling resistance. So what does that mean? It’s, basically, a number that you use to multiply some forces, and it allows you to calculate the amount of force, and then to which can be converted to watts, the amount of watts that are consumed when you’re using that specific coefficient of rolling resistance.
So it’s a multiplier. It’s a number. It’s dimensionless. It means that it’s not like you can’t measure it in Newtons or anything like that. It’s measured through a number of ways to test certain things. We’ll talk about how we’ve done some of that on the show here. But basically, you want that number to be as low as possible. So if you’re calculating the amount of watts that are consumed by rolling resistance, if Crr was zero, that would mean zero watts were consumed. If it was one, basically, that would mean all of your watts were consumed by rolling resistance. And that’s definitely not something that you want. So you want to get that number as low as possible.
Bicycle Tire Selection and Rolling Resistance
So a few things you can think about when trying to optimize your rolling resistance. Number one is going to be the tire selection. So tire selection is huge. And the reason it’s so important is because tires are made in a number of different ways. There’s different rubbers. There’s different casings that are used. There’s different materials that are used. And you want to sort of optimize your tire selection around a rolling resistance mindset.
Now, I always like to think of tires as a three-legged stool. So what do I mean by that? The first thing you think of when you think of tires, is you want the tire to be durable. So you want it to be good with its rolling that if it hits a rock, or if it hits, you know, a small piece of debris on the road, it’s not going to tear. It’s not going to cut. It’s not going to cause you a lot of problems. But you don’t want it to be so durable that it creates other issues. One of those could be like really, really bad rolling resistance.
So if you have, like, a super durable tire, something that’s not going to have any issues, a really good example of that would be the Continental Gatorskin. It’s kind of like a bulletproof tire, but it’s so stiff and so tough that its rolling resistance is really, really terrible. So it’s got a really, really high rolling resistance. So for example, we did a test back in 2015, which I’ll talk more about and what that study was. But if you look at one of our better tires, and one of the worst-performing cars, which was the Continental Gatorskin, over an Ironman course, if you just change your tires, and tires only, and you only looked at the rolling resistance, the time difference was eight minutes. So you would literally save eight minutes just by changing your tires. So, aerodynamically, the Gatorskin was really, really comparable to something like a GP4000. At the time that we’re using and testing, it was very, very fast. But the rolling resistance was so poor, that it would lose you about eight minutes over an Ironman. So definitely very important to consider.
The other leg of the stool is something that we look at is the rolling resistance, right? So you want to make sure that that number is low. And then the third leg of the stool is the aerodynamic component. So anytime that we look for a tire at FLO, we’re trying to find something, we want to sort of optimize those three things. So there are some tires out there that are extremely low in rolling resistance. But they have kind of decent aerodynamics. In certain yaw angles, not the best. But then they’re not…just not very durable. So they tear easy. They have issues. So if you’re only focusing on that rolling resistance, yeah, maybe the best rolling resistance tire out there, but the other two legs aren’t really that good.
You may have something that’s extremely aerodynamic, very durable, but terrible rolling resistance. That’s, like I said, the Continental Gatorskin is one of those tires that we will look at. Then you may have something that’s just very aerodynamic, not durable, or, you know, any combination that you can think of, you can go down that path. But we try to find one that sort of balances the three. And the best tire that we’ve always found that we use, it used to be the Continental GP4000 SII. Just a great tire. And now it’s been changed. The next version is called the GP5000. There’s the 5000, which is for tubes. And then the 5000 TL, which is for a tubeless tire. We’ve tested both of those. We love both of those tires, and they hit each one of those three legs really, really well. So if you are considering a tire for your bike, definitely consider that tire. And then you really don’t have to think about much more once you get your pressures and everything else sort of set up.
Bicycle Tire Pressure and Rolling Resistance
The tire pressure is another huge component of optimizing your rolling resistance. So if you think of it from a lab setting, the best, like, rolling resistance you can think of would be, like, a hard steel wheel on, like, a very hard steel surface. So there’s, like, essentially zero deformation. There’s very little, in the term, rolling resistance. That’s why if you think of trains, trains use, like, steel wheels on a steel track. They have extremely low rolling resistance. They’re not really worried about the pneumatics. The tracks are generally pretty smooth. So it gets them to move a very long distance, very efficiently.
When you use anything like a pneumatic tire, there’s a lot of benefits. You get things like, it reduces the amount of vibration. You get a better grip for cornering. Trains don’t really corner, so you don’t have to think about that. And they’re just sort of, you know, locked onto the track. We don’t have that in cycling. There are solid tires. If anyone has ever ridden a solid tire, they’re very heavy. They’re very uncomfortable. They don’t get that same dampening component. So, you know, those are some of the huge benefits of tires.
In a lab, if you’re looking at a steel roller setting, or even in the lab, if you have, like, a very, very smooth, polished finish, and you’re looking at a pneumatic tire, the higher your pressure, the lower your rolling resistance. And the reason for that is because that contact patch that we talked about earlier, where the tire deforms at the beginning and then reshapes at the back, is smaller, and smaller, and smaller. So if you think about…it, basically, creates a moment.
So if you think about a…if you have like a two-by-four, and you were to stand that on its end, and you were to bolt that to a sheet of plywood, and that plywood is on the ground. And you were to try to knock over that two-by-four, well, it’s really, really difficult because you have to overcome the moment that’s created by the sheet of plywood on the ground. If you have just a two-by-four straight up in the air and standing on its end, a very simple touch will knock that two-by-four over because the moment is incredibly small.
So the same thing happens in a tire. When the contact patch is larger, it takes more energy to, basically, turn it over, because you have to overcome the moment that’s created by the contact patch. When your contact patch is really, really, really small, like in a lab setting, and you had, like, say a 300 PSI pressure, so your contact patch would be, you know, very, very small, the rolling resistance goes very, very, very low because there’s really no moment to overcome.
What’s different between a lab setting and something on the road is that you do not have a very, very smooth surface. Anytime you’re on pavement, even if it’s new pavement, there are a lot of small bumps that happen. So what happens is your energy, as you’re rolling that tire along the ground, the…as you get higher and higher and higher, and that contact patch gets lower and lower and lower, the pressure in the tire is really high. And so at a certain point, when the pressure gets high enough, that tire hits one of those bumps in the road, as opposed to sort of smooshing and, like, rolling over it and absorbing the bump. What happens is the tire starts to bounce up and down over those bumps.
So each time that the energy is directed in an upward and downward motion, because it’s literally bouncing over the small bumps in the pavement, your energy is now moving up and down as opposed to moving forward. So there’s a specific pressure for each tire, for each rider weight, for each road surface where that point happens. And anytime that that happens, what happens is you get a very large spike in rolling resistance because your energy is now going up and down and it’s being wasted.
That was originally studied by Tom Anhalt. And Josh Poertner was also doing some work, a guy over at SILCA. Tom runs a blog called “Blather ’bout Bikes.” And they sort of coined this term “impedance.” It’s been a big thing in cycling over the last few years. We’ve done a lot of work with it as well. And once you hit that impedance breakpoint, your numbers just get really, really high really quickly. So what we’re trying to do, from a cycling perspective and from a pressure management perspective, is we’re trying to understand and study pressures. So that when you take your weight, your bike weight, the surface that you’re on, your tire size, everything else, and combine it together, that you can pick a pressure that puts you in a very safe optimal range, so you’re not spiking your own resistance.
Using Tire Pressure Charts to Set Your Rolling Resistance
One of the things that we have on our website is for each one of our wheels, with different tire sizes and your rider weight, you can go in and you can look, and you can go through a chart. So let’s say that you’re 160 pounds, your bike weighs 20 pounds, that takes you up to, you know, 80 pounds, 180 pounds. And then, you know, you’re on 28-millimeter tires, and you’re going to be on a, you know, a medium roughness pavement.
We have a chart there that you could add all…you know, you find all the…those correct variables, and it will give you the optimal tire pressure for you to run. How did we get there? Well, what we did was we did a lot of on-road testing. So if you do this in a lab, like I said, it doesn’t really give you those impedance breakpoints. And so we started to get into rolling resistance back in 2015. So six years ago, which seems like a long time ago, now.
Combining Rolling Resistance and Aerodynamics for Cyclists
We had just finished the redesign aerodynamically of our second generation of FLO wheels. And we’re heading to the wind tunnel, and we’re doing this big tire study. And we were looking at it from an aerodynamic perspective. So I think it was 24 tires that we studied in total, and Tom Anhalt got wind that we were doing that and he said, “Hey, I think it’d be really cool if you got all the aero data.” And then I ran all of the tires through my rolling resistance test.
And he said, “If we could combine the data, we may find that, you know, a wider tire, while it may not be more aerodynamic, is faster overall when you combine the rolling resistance.” And I thought, you know, that was a cool idea. To be honest with you, I didn’t think it was going to matter all that much. And I was really wrong. So Tom sent all the data back. And what we quickly realized was that a 25-millimeter Continental GP5000…oh, sorry, it was the 4000 at the time, so it was a Continental GP4000 SII, was slower aerodynamically. But when you added in the additional rolling resistance benefit, it actually made it a faster tire.
So that got us to thinking that, you know, we were really only looking at half of what you needed to make a fast wheel. So we’re only looking at things aerodynamically. And so we realized at that point that what we needed to do was look at, how do you make a wheel that’s aerodynamic, but is also going to optimize for wind resistance? And so we knew that there were some stuff that had been published, that wider tires were faster. And we wanted to know, was there any way to prove that you could do anything to a rim to make a rim faster?
The theory was that if we widened the internal rim width, that that would also help us lower our rolling resistance. And so we started trying to figure that out and prove that. You know, there’s a lot of times you’ll see stuff where some claims are made. And we’ve never sort of been one of those companies to make a claim unless we can prove it. And so we wanted to do it on-road, which was a very difficult process.
Bicycle Tire Height and Width vs. Pressure
So let me walk you through the stages here of what we did. So the first thing we started to do was we wanted to measure tires on different internal rim widths. So we had a summer intern that year, and I think he must have taken, like, 3,000 measurements. We used a very calibrated pressure sensor, and we got width and height measurements for tires, for three different rim widths. And then we were able to, basically, plot that. And what we found was really, really interesting.
So as pressure changes, and as internal rim width increases, the tire gets wider, which makes sense. So you’re widening the internal rim width, and you’re increasing your pressure, both of those lead to an increase in room width. It’s really cool to see that really no matter what the tire is, or what the pressure is, it’s generally a linear increase. So from a…if you use an equation, and you know the…basically the…if you think of a slope, which is y equals mx plus b, if you know what your slope is, for any given tire, you can predict the width on any rim, with an internal rim width, and any pressure. So that was really cool to see.
What we didn’t expect was, I thought that as you increased the tire pressure, that the height would change. And height really doesn’t change. And I also thought as you increase the width, if you’re going to open it up, that you would eventually get to this point where the tire would change height, too, and can either get taller or smaller. And height really is not affected by pressure all that much, or by increasing the rim width. So that was kind of unique to see.
Casing Tension and Rolling Resistance in Bicycle Tires
The next step was to understand what was actually happening to the casing of the tire when pressure increased, when rim width increased, and when tire size changed. So we worked with a group out of Tennessee called the Union University. We’ve worked with those guys in the past on some cool stuff. And what we did was we made a special rim, basically, where we had a hole that came in and allowed us to run strain gauges on the inside surface of a tire.
So the outside surface of a tire is really the sort of the rubber. On the inside, you’re getting very close to the casing. So we scraped away some of the small amounts of rubber, and we mounted these strain gauges to the inside of the surface of the tire. And what we wanted to see was the theory and was that, as you increase the diameter of a tire and if you keep the pressure the same, the casing tension goes up.
So what you can get away with is a lower pressure tire. So if you’ve ever been on, like, a fat bike, some of those pressures you’ll see are, like, two or three PSI, but they feel really, really solid. And that’s because the casing tension, and it’s all based on the math and how it works out, at lower pressures, is the same for a larger diameter tire. So we, basically, plotted that and graphed that, and showed that that was the case. What we also were able to see was that when we increased the rim width, the internal rim width of the tire itself, was that it had the same effect as increasing the width of a tire. So that was pointing us in the right direction what our theory was that if we widen the internal rim width that that would be true.
Measuring Rolling Resistance in Bicycle Tires On Road
The next stage was, how do we actually collect this, improve this on-road? Which came to be a very difficult challenge. When we did all of our aerodynamic data, which we talked about in another episode, I built a computer, and that computer was a…is a, basically, a logging station that collected a bunch of wind data. In this situation, there’s really not a sensor that gives you feedback as to what your current rolling resistance is.
So I started to reach out. There’s a couple of groups. And eventually, we settled on a group out of Calgary, which was in a very, very beta stage of their sensor. So we were sort of ahead of…the questions we had were ahead of the technology at the time. It took us quite a while to even get to the point where we had this beta sensor. And we worked with this group AeroLab out of Calgary. And they were using it originally to study CDA, which is a coefficient of…or the drag coefficient. And so what we were wanting to do is sort of reverse what they’re collecting, because they were able to simultaneously get rolling resistance and aerodynamics. And we wanted to just get the rolling resistance.
So we worked with those guys over the course of about 18 months, and we developed a protocol. And when we had it right, which was, you know, very, very difficult to do, one of the things you need to keep consistent when you’re measuring rolling resistance, because it’s simultaneously getting the drag coefficient and the rolling resistance, is you need to keep your body and the drag portion on the bike identical.
So if you’re going around a corner, you can’t turn your head. You can only look with your eyes. Any movements of anything that takes you out of sort of your original position will throw the test. So it took us a very long time to get that right. But when we did do get that right, we had a group of three riders, what we started to see was some really, really cool things that we could prove on road. The first thing was the theory that wider tires are faster, and you can run lower pressures, yes, 100% true.
The next thing we proved was that our theory, original theory, is that if we increase the rim width, then we lowered the rolling resistance. So we knew at that point that by increasing the internal rim width, that we would lower rolling resistance. And then from that what we did is we sort of optimized a rim around a few things. Number one was, we want the widest internal room width that we could get away from a safety perspective. And then, we then aerodynamically optimized the rim around that internal rim width. So it gave us a really, really wide rim. Our FLO All Sport line is…has a 21-mil internal rim width. The exterior rim width, like, starts at 28 and gets a bit wider depending on the model. And then we optimize around 28-millimeter tires because that was also a very…that was our best tire size for a rolling resistance perspective.
We could go out to 32. But there’s a couple of issues there. Number one, if you got to 32, you get such a wide internal rim width that your tire size, the minimum, really starts…is a 32-millimeter tire. So you want to stay in an area where it’s safe. The other thing is that you start to not fit on certain bikes. So a much wider rim doesn’t fit in brakes, it doesn’t fit through frames and chainstays, seatstays. So that’s something that you definitely have to take a look at.
Rolling Resistance and Cyclist Speed
The other thing that we learned, doing all this rolling resistance testing, was something that we had not heard of before, and I don’t…I think we’re the first ones to find, it was that impedance breakpoint that we talked about, let’s talk a little bit more about that. So what you get is if you are increasing your tire pressure, your rolling resistance decreases continually until you get to a certain pressure where we hit the impedance breakpoint and it spikes.
What we learned was that that impedance breakpoint changes as your speed, your speed that you’re going on the bike increases or decreases. So if you are going faster, the impedance breakpoint happens at a lower pressure. If you’re going slower, it happens at a higher pressure. So what we learned was that your speed as a rider is also very important when you’re considering rolling resistance.
So all of our tables that are on our website include sort of like a maximum speed. Anytime you’re looking at rolling resistance, you are wanting to always be a little bit under on pressure as opposed to over. Because the way that the rolling resistance decreases when you’re increasing pressure before the impedance breakpoint is gradual. It kind of flattens out as it gets close to the bottom. But the way that your rolling resistance increases, after you hit the impedance breakpoint, it, like, shoots straight up. So it’s very important to be slightly under. So all of our pressure calculations include that.
Rolling Resistance is Important for all Cyclists
So what does this mean for you as a cyclist, or as a triathlete, or anybody that’s using a wheel…even if you’re a recreational cyclist? Rolling resistance is important because it allows you to more efficiently get from point A to point B. So if you’re racing, that’s very important. Even if you’re riding recreationally, that’s important. Another thing that really good pressure management does in your tires is if you’re setting a really accurate pressure, you’re eliminating the potential of being overinflated. Overinflation can lead to a loss of grip. So what you get is you start to get a bunch of up and down bouncing, which every time you go up, you’re technically unweighting yourself from the road, which we don’t want to do. And so if you’re going around a corner, you have more potential to skid out because you’re bouncing too much.
Rolling Resistance and Vibration
We’ve also been doing a lot of studies here at FLO to look at vibration as a whole. We’re trying to understand the correlation between rolling resistance, impedance breakpoints, and the amount of vibration that you get in the wheel itself. We’ve got some really cool blog articles that we’ve put up about that. We’ll have some more cool stuff coming out about that. But hopefully, in the future, it will be a lot easier to understand and sort of measure what your impedance breakpoint is, as opposed to following a graph.
If anybody has any questions about rolling resistance, we’re definitely here to answer them. We love talking about this stuff. And like I say, for your next ride or race, make sure that you’re optimizing your rolling resistance with a really good tire by managing your pressure. And like I say, I always say that the fastest athletes are the smartest athletes. So use this knowledge and help make yourself faster. We’ll see you next time.
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