Rolling resistance is a very important one among a series of forces that need to be overcome when riding a bicycle.
When going uphill, you need to overcome gravity, but as a result, you acquire gravitational potential energy. You will fully utilize this energy when going downhill. In an article on bicycle aerodynamics published by Malizia and Blocken in the Journal of Wind Engineering and Industrial Aerodynamics in 2020, it was pointed out that when cycling at a speed of over 40 kilometers per hour on a flat road, approximately 90% of the resistance you overcome is air resistance. Therefore, road cyclists are fascinated by everything related to air. In addition, the friction between the tires and the road (or rolling resistance) is the main force you need to overcome. Only by overcoming them can you start cycling and ride faster.
The following will tell you what rolling resistance is, how to measure it and the factors that affect it. Although rolling resistance is usually regarded as a concern for road bikes, foreign media will also explain why it is important for mountain bikes and gravel cyclists. Foreign media will also introduce the three best road bike tires discovered in their own tests, which have relatively low rolling resistance.
What is rolling resistance?
According to tire manufacturer Schwalbe, rolling resistance is the energy lost when a tire rolls. This kind of energy loss has multiple sources, among which the most significant one is the energy lost due to tire deformation. There is also friction between the tires and the road surface, which is why it is much easier to ride on smoother asphalt roads than on gravel roads or off-road roads.
The deformation or relative displacement of tires can lead to the generation of heat and sound, as well as the loss of system energy. In the field of bicycles, the width, diameter and tire pressure of tires all affect their deformation. Other factors such as the flexibility of the tire and the rubber ratio used in the tread also play a certain role.
Replacing part of the carbon black in the tire compound with silicon dioxide can reduce the energy loss caused by relative displacement, thereby reducing rolling resistance. This is why high-performance tires usually contain silicon dioxide. It is claimed that graphene has a similar effect. Vittoria has added graphene material to its high-performance tires.
If you participate in a time trial or a race, you might be willing to sacrifice tire life and puncture protection to increase your speed. The treble and sidewall of competition tires are usually thin and may not even contain a puncture protection layer. On the other hand, if you ride on rough roads or simply hate having a flat tire repaired halfway, you might be willing to accept increasing the rolling resistance to obtain an additional puncture resistance layer, which can provide better anti-explosion protection, grip and extend the tire's lifespan.
Why is rolling resistance important?
Rolling resistance is one of the most obvious forces that slow down your riding speed, and it is also one of the easiest forces to solve. Apart from air resistance, roll resistance accounts for a large proportion of the resistance you encounter while cycling, and even exceeds air resistance at lower cycling speeds.
Almost every cyclist hopes to ride faster, farther or with less effort. Reducing rolling resistance is one of the simplest and lowest-cost ways to achieve this goal. As Schwalbe pointed out: "Cyclists have very limited physical strength and they hope to use it as efficiently as possible."
Choosing to replace your tires with low rolling resistance is relatively cheaper, but it can also optimize your existing riding conditions and routes, which are free for you.
How to test rolling resistance?
The tire to be tested is installed on the test wheel and inflated to the predetermined pressure. The tire comes into contact with the test roller and applies pressure to the wheel from above to simulate the driver's weight and cause the tire to deform.
The system determines the rolling resistance of the tire by measuring the excess power or torque required to maintain rotation at the specified speed. Usually, multiple repeated measurements are carried out to ensure accuracy and repeatability. Moreover, there is typically a preheating period before the measurement to maintain the balance between the system and the tires. The rolling resistance of bicycle tires is usually expressed in watts (W), which is a unit well-known to cyclists and enthusiasts. Of course, there are also other units to represent the measured values. Such as the rolling resistance coefficient or Crr, which is the force required to overcome the rolling resistance divided by the wheel load, is usually expressed in N/kg.
Comparison of rolling resistance among clincher tires, tubeless tires and tubular tires.
One factor that affects the rolling resistance of bicycle tires is the friction between the inner tube and the outer tube, which occurs when the tire deforms under load. An evaluation is specifically designed to test the impact of tire rolling resistance and other factors. They tested the differences in rolling resistance when 10 different TPU, latex and butyl inner tubes were used in combination with 25mm Continental GP5000 STR outer tubes, as well as the differences in additional comparisons with tubeless tires.
The conclusion is that it is found that the rolling resistance increases by 0.2-3.8W when using inner tubes compared with using tubeless tire. Latex inner tubes have the least impact on rolling resistance, and the performance of most TPU inner tubes is superior to that of butyl inner tubes. The final conclusion is that the tubeless tire has the highest riding efficiency. If you want to use an inner tube, use the non-vacuum version of the GP5000 clincher tire. This version is faster, lighter and cheaper.
Tubular tires are actually the same as clincher tires, but tubular tires are more expensive and their rolling resistance may be comparable to that of clincher tires using latex inner tubes.
Cotton tires also help to reduce rolling resistance. The type and quantity of glue used, as well as the duration of glue drying, will all affect the results. Therefore, no definite conclusion can be drawn from comparative tests.
Generally speaking, the difference in rolling resistance between tubeless and latex or TPU inner tubes is very small. You can choose according to your own preferences. In addition, the tubeless tire provides extra puncture protection. The presence of self-rehydration fluid enables you to ride home even if the tire is punctured in most cases.
How does the rolling resistance vary with the width of the tire?
Foreign media conducted a comparative test on two versions of Pirelli P Zero Race TLR (26mm and 30mm) with different widths to evaluate whether wider tires are faster. The result is: It depends.
On indoor rollers, when two tires run at the same pressure, the 30mm tire requires less power to maintain the same speed. This is because under the same pressure, wider tires have relatively less deformation due to their wider and shorter contact surfaces. When the actual tire pressure is adjusted to the same level as required, the narrower one is more efficient. Theoretically, if you can perfectly adjust the tire pressure for each width to maintain the same contact area, the rolling resistance should be the same.
However, in actual riding, on better road conditions, wide tires are not slower than narrow tires, and on rough roads that require compliance and grip, wide tires are faster. On such road conditions, they can provide more cushioning compliance and a larger turning surface area. This is why many elite gravel cyclists choose ultra-wide tires to participate in high-difficulty off-road races such as Unbound.
So you should choose the tire size based on the terrain of your ride and your riding goal.
Is rolling resistance important for mountain bikes and gravel bikes?
The short answer is: Important, but not as important as a road car. For many mountain and gravel riders, rolling resistance may not be the top priority to consider. Tires with lower rolling resistance, that is, lighter or thinner, may be more prone to blowouts, which is not worth compromising on for endurance races or challenging tracks.
The area where mountain bikers might be willing to sacrifice some grip in exchange for lower rolling resistance is the XC race. In such races, passing quickly is the advantage. Of course, the route cannot be too difficult, and the same is true for gravel.
So for most mountain and gravel rcycling, grip and puncture resistance are more important than low rolling resistance.
In the field of motor vehicles, the IOS standard has established a unified method for measuring rolling resistance. The standard for bicycle rolling resistance testing is determined by the testers. Of course, experienced testers also provide the most comprehensive and consistent testing methods.
The ultimate conclusion is that there are trade-offs between rolling resistance and other factors such as air resistance, durability and grip. When deciding which type of tire to use and under what circumstances and at what tire pressure, you need to make trade-offs.
