Anyone who has ever ridden a soft, high-damping downhill mountain bike tyre can confirm that rolling resistance can be a formidable opponent as soon as the gradient drops. Every pedal stroke fizzles out as if you were riding over dough.
But what is rolling resistance anyway?
When a rider sits on a mountain bike, the tyres are pressed in at the point of contact with the ground (ground contact area) by the force of the rider's weight. Energy is lost every time a tyre rolls. The loss is caused by the permanent deformation of the tyre as it passes through this contact patch. In the process, mechanical energy is converted into thermal energy. How high the rolling resistance of a tyre actually is depends on several factors. Designing an MTB tyre that rolls easily is not an impossible task for most manufacturers such as Schwalbe, Continental, Maxxis or Michelin. But low rolling resistance is unfortunately not the only requirement for a tyre. The fact that braking, cornering and steering forces have to be transmitted creates a conflict of objectives. Reducing rolling friction by ten per cent would also increase the braking distance by the same amount under certain conditions. A bad trade-off. The Tyre structure decides whether it rolls easily and smoothly or sticks to the floor.
Decisive for the rolling resistance of MTB tyres
1. rubber compound
The rubber compound of MTB tyres accounts for the lion's share of rolling resistance. A Schwalbe Hans Dampf with a Pace-Star rubber compound, for example, rolls at 29.7 watts. The same tyre with a grippier Trail-Star compound, on the other hand, uses 41.8 watts. A whopping 29 per cent difference. A Schwalbe Rocket Ron Evolution Addix Speed has a rolling resistance of 17.2 watts, while the same tyre in the Addix Performance version has a rolling resistance of 19.6 watts.
2. carcass
The structure of the tyre carcass can influence the rolling resistance by around ten percent. A fine carcass with 127 ends per inch (EPI) or threads per inch (TPI) is more flexible than a coarse 30-EPI carcass. Casings with low EPI values, i.e. fewer threads per inch, tend to be used for low-cost MTB tyres, as they are easier to manufacture because less energy is required to form them.
3. size of the wheel and air pressure
The bigger the wheel, the easier it rolls, as the BIKE tests on the roller dynamometer prove. This means that 29er tyres are generally better than 26 inch tyres. With smaller MTB tyres, tyre deformation has a greater effect. The tyre therefore becomes less round when rolling. Only on the roller or on asphalt does a high air pressure in the tyre provide less resistance. The opposite is true off-road. A tyre rolls much better with less air pressure!
4. tyre width
Wider MTB tyres roll better than narrow ones. Although the ground contact area is the same for the same tyre pressure and weight load, it has a different shape. With wide tyres (left), the surface area is shorter and therefore also the braking lever arm. In addition, the narrow tyre deflects deeper, which means that more material has to be deformed.
5. profile
The tyre tread has the least influence on rolling resistance. Particularly on smooth surfaces, chunky tyres with a lot of space between the lugs roll somewhat worse than tyres with a high positive component. This means that the higher rolling resistance of chunky tyres is more noticeable when pedalling on asphalt than off-road.
Below you will find a PDF with a table of different MTB tyre models and their rolling resistance.
If you want to delve deeper into the subject of rolling resistance in MTB tyres, you should read the thesis by BIKE test manager Peter Nilges. An extract entitled "The truth about rolling resistance" is available here.
