Rolling resistance is a hot topic in cycling. The truth is, rolling resistance matters for all cyclists regardless of whether you ride on road, gravel or even a track. It’s important if you race or simply just enjoy riding on the weekends. Why? It effects things like your speed and grip which matter to everyone. There are a lot of myths surrounding rolling resistance, making a complex concept even harder to understand. This article is meant to explain rolling resistance for you, a cyclist, to understand what it is, how it affects your cycling, and how to manage it. Enjoy!
What Is Rolling Resistance?
Resistance is defined as follows:
the impeding, slowing, or stopping effect exerted by one material thing on another.
To help visualize resistance, picture trying to stretch an elastic band with your hands. The elastic band "resists" your hands from moving apart. Resistance makes it more difficult to move in the direction you want to go. For example, when cycling up hill, gravity resists you from moving up or when pedaling into a strong head wind, the wind resists you from going forward.
When a wheel, like a cycling wheel, rolls on a surface, the force that resists the rolling motion of the wheel is called rolling resistance force. A consumption of watts is required to overcome rolling resistance force.
One of the variables of the rolling resistance force equation is called the Coefficient of Rolling Resistance (Crr). All tires have Crr values. Therefore, the lower the Crr value for the tire, the fewer watts that are consumed when riding your bike. The equation for rolling resistance force is below:
Wrr = Crr*m*g*v
Crr = rolling resistance coefficient
M = mass of rider and bike
G = gravity
v = velocity of bike
What Causes Rolling Resistance?
In cycling, we use pneumatic tires because of their ability to absorb the unevenness of terrain. Their deformation ability, aka the material property of the tire, and the surface the tire is riding on, contributes to hysteresis. Hysteresis is the main cause of rolling resistance in pneumatic tires. What is hysteresis? We’re glad you asked.
Hysteresis in a wheel is explain well by The National Academy of Sciences:
“A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the [rider+bike], it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.”
In other words, the rubber compound in pneumatic tires experiences hysteresis and the energy lost is heat and sound. Therefore, the energy required to deform the tire as it rolls onto the pavement is greater than the energy that is recovered as the tire leaves the pavement which contributes to the rolling resistance. The image below shows where the deformation and recovery occur in a tire as it rolls forward on the road.
Understanding Contact Patches
Contact patches are another piece to the understanding of rolling resistance. A contact patch is the area of the tire that touches a surface. We know that when a cycling tire does not have contact with a surface or it is not under a load, it is perfectly round. The picture below shows a tire on a rim without a load.
When you place the tire on the ground and add a load like, sitting on the bike, the tire deforms where it makes contact with the road surface and creates a flat contact patch. The image below shows a flat contact patch where a tire under load makes contact with the road surface.
How Contact Patches Affect Rolling Resistance
Rolling resistance is affected by the length of the contact patch. To understand this, think about an object with a small base of support. This object is easier to knock over than an object with a large base of support because of the smaller moment. It also takes less energy (watts) to knock over the object on the smaller base. The same is true for tires with shorter contact patches. When you have a shorter contact patch, your rolling resistance is less, and it takes less energy (watts) to roll your wheel forward. There are several factors that affect the shape of contact patch. The image below shows an example of a small and large base of support.
The Shape Of Contact Patches And What Affects Them
A few years back, we wanted to understand the shape of contact patches for different tire and rim widths. To test, we used ink pads to load the bottom surface of tires and then loaded each test tire with a known mass. Then, we studied the contact patches. We determined the following list of items affected the shape:
- Tire Pressure: Assuming the same tire and load, a higher tire pressure results in a smaller contact patch. A lower tire pressure has a larger contact patch in length and width. In theory, a high enough pressure would produce an infinitely small contact patch that is only dependent on the elastic properties of the rubber and not the pneumatics of the casing, but that’s not the case. Read on to learn why. To learn more about casing tension, check out an earlier article on casing tension.
- Tire Width: When comparing a wide tire vs a narrow tire, the wider tire will produce a shorter and wider contact patch. We’re assuming the brand, model and casing tension are the same. The only difference is width of the tire. See image below. For more info on why wider tires need less air pressure see our earlier article.
- Tire Model: We like to think that tires have fingerprints. Different brands and models of the same size tire are different in shape and will produce its own unique contact patch. For more info on tire height and width measurements at different pressures, take a look at our previous article.
- Internal Rim Width: A rim with a wide internal rim width will produce a wider and shorter contact patch compared to rim with a narrow rim width. Again, assuming the casing tension and tires are the same. This is part of the reason why a wider rim is faster.
The images below give an estimate of the contact patch for a Continental GP 5000 25mm vs. a Continental GP 5000 32mm with the same casing tension.
So The Higher The Pressure, The Smaller The Contact Patch The Lower The Rolling Resistance Right? Well, Actually No. Say Hello To Impedance
In theory, the answer is yes. If you are able to create a really small, short contact patch, the moment you have to overcome is negligible because there is so little deformation. For instance, in a lab setting, on very smooth, steel rollers, your Coefficient of rolling resistance (Crr) value continues to decrease as you increase the pressure. Which makes sense given the aforementioned theory.
Another scenario with a smooth surface is indoor track riding. Hour challenge cyclists are known to use very specialized tires and 300 psi, which greatly reduces their rolling resistance.
However, once we leave the lab or the track, surfaces are actually quite bumpy. Even new pavement has a lot of bumps. Our friends at Silca did a really cool study on different pavement conditions and how it affects rolling resistance. We learned that if a tire is over inflated on a rough road surface, the tire ends up bouncing up and down over the bumps. Our goal, as a cyclist, is to transfer the watts put into pedal into forward motion, so, any motion up and down over the bumps is a waste of energy.
If the tire has enough pressure to create bouncing, we hit an area in the Crr vs. Pressure graph known as the impedance breakpoint. Once the impedance breakpoint is reached, additional pressure added to the tire greatly increases the rolling resistance. This is contrary to the lab findings where a continual increase in pressure reduces the rolling resistance. Another thing to point out is that rougher the surface, the sooner you reach the impedance break point.
Impedance not only wastes energy, but the up and down motion from bouncing reduces your grip on the road. If you’ve ever ridden a gravel bike with too much tire pressure and come around a corner, you lose the front end quickly (and hope you’ve got great bike handling skills.) If you lower your pressure and ride the same corner, going the same speed, you can hold it well. The first image below shows a wheel that has low enough air pressure to avoid impedance. The second image shows a tire with high pressure that is experiencing impedance.
The Gear You Use
So, we know that the lower the rolling resistance, the more watts we, as cyclists, save. That means less energy spent. We also know what helps lower the rolling resistance, like, small contact patches, plus the awareness of the impedance break point. What takes all that information into account? Your gear. Tires are crucial because different tires are made from different materials and the casings are woven in different ways. A good tire, with a low Crr value, can save you eight minutes over a 100 mile century, compared to a tire with a poor Crr value. For example, the Continental GP 5000 has superior rolling resistance (a low Crr value) vs. Continental GatorSkin.
Rims also have an affect on rolling resistance. Our 2020 FLO rims have been designed to lower rolling resistance by having very wide internal rim widths. The All Sport/road line has an internal rim width of 21mm and the Gravel/XC Mountain Bike line has a 25mm internal rim width. Wide internal rim widths shorten your contact patch and allow you to run a lower tire pressure, since you are increasing the volume of your tire. Additionally, all of our rims have been designed to optimize aerodynamics for tires. The road line is optimized with tires as wide as 32mm. Our Gravel and XC Mountain Bike rims are the first in the world to be aerodynamically optimized, reducing drag by more than 20% over a traditional Mountain Bike rim. Fast wheels factor in both rolling resistance and aerodynamics. If you aren’t considering both, you are missing half of the equation.
So, what does this mean? Ultimately, rolling resistance is important to a cyclist. When you are managing your rolling resistance, you improve your speed by saving wattage and increasing your grip. In other words, you use less energy to go faster. The best way to do this is with good gear and proper tire pressures. Have fun riding with the understanding of rolling resistance and how it affects you.