With the same tire type (same inner tire and pattern) and the same tire pressure, the larger the wheel, the smaller the rolling resistance.
The rolling resistance of pneumatic tire can be regarded as a rigid wheel (without rolling resistance) climbing a constant angle slope. The slope angle is determined by the ratio of the pressure deformation of the tire to the diameter of the wheel. The larger the wheel diameter, the smaller the slope angle.
On the other hand, when encountering obstacles, the larger the wheel diameter, the smaller the horizontal impact brought by the obstacles, that is, the smaller the speed loss. In the extreme case, a 10 inch wheel and a 29 inch wheel hit a 6-inch kerb at the same time. For a 10 inch wheel, it likes hit a wall,tricycle will stop at once, while a 29 inch wheel can still pass at a certain speed.
Therefore, whether it is plane rolling resistance or obstacle crossing resistance, it is the tricycle with large wheel diameter that has less resistance.
Larger wheel diameter can also increase grip, directional stability and high-speed stability. Therefore, it is very popular that mountainous bike with 29er and it has shown its advantages in various competitions.
So what are the disadvantages of large diameter wheel? First of all, it is heavier so it consumes more energy when accelerating and decelerating. Secondly, it needs more steering force, which is also a contradiction with directional stability.
By the way, let’s talk about the wide and narrow tires also. Most riders think that the narrower the wheel, the smaller the resistance. In fact, this is not entirely correct. If the same type of tire only changes the width, the wide tire actually has lower rolling resistance. This is because when the air pressure is the same, the grounding area of the tire is the same. The grounding area of the narrow tire is a long and narrow area, which makes the whole tire more “out of round”, so the rolling resistance is greater. So why is the higher the speed, the narrower the tire? This is because narrow tires can use higher tire pressure, which reduces the grounding area, and lower windward area leads to less wind resistance.
Finally, the type of tire is very important to the rolling resistance! Therefore, tube tires (road tricycles) and vacuum tires (mountain bikes) are widely used in medium and high-end tricycle. The latest Shimano XTR mountain bike with tube tires. From my experience, the rolling resistance of XC vacuum tire of mountain bike is almost no higher than that of bare tire. Don’t forget that there is twice the tire pressure between them.
