Road holding on wet roots
Friction force Fa on the wheel by a counterslope root, which holds up the bike-rider system
We all know how dangerous wet roots are in mountain biking when you ride a path after it has rained. Experience teaches us that wet rubber on wet root means probable slip. This depends on the "friction force" that in these situations has very low values and results in the loss of grip of the wheels on the ground.
To define friction force, we can say that it is that force that occurs when two surfaces in contact move in such a way as to cause mutual rubbing. This force is present also in the absence of rubbing, that is when the reciprocal movement of the two surfaces is prevented precisely by the friction between them and is what we need in order not to lose grip with the wheels on the ground. Friction, which in this case is called "static", is the force that prevents the surface of the wheel from sliding on the ground, or, in this case, on the root. It depends both on the force which compresses the two surfaces perpendicularly and on a coefficient that depends exclusively on the materials in contact.
In the presence of wet roots and tires, there is a very low coefficient of friction compared to normality and therefore the friction forces will be consequently smaller, increasing the risk of slipping.
To understand which are the dangerous situations, we must start by identifying in what conditions the action of friction is essential for the balance of mountain biking. In general, neglecting all the particular cases that may occur, friction acts in the following three situations:
To define friction force, we can say that it is that force that occurs when two surfaces in contact move in such a way as to cause mutual rubbing. This force is present also in the absence of rubbing, that is when the reciprocal movement of the two surfaces is prevented precisely by the friction between them and is what we need in order not to lose grip with the wheels on the ground. Friction, which in this case is called "static", is the force that prevents the surface of the wheel from sliding on the ground, or, in this case, on the root. It depends both on the force which compresses the two surfaces perpendicularly and on a coefficient that depends exclusively on the materials in contact.
In the presence of wet roots and tires, there is a very low coefficient of friction compared to normality and therefore the friction forces will be consequently smaller, increasing the risk of slipping.
To understand which are the dangerous situations, we must start by identifying in what conditions the action of friction is essential for the balance of mountain biking. In general, neglecting all the particular cases that may occur, friction acts in the following three situations:
- When we ride a path in counterslope, friction acts as shown in the figure and prevents the wheels from sliding down. Doing the math, it can be shown that holding the road depends exclusively on the inclination of the counterslope and the coefficient of friction: the weight of the bike-rider system plays no role. A root that is not horizontal but tilted downwards represents in all respects a counterslope; it is, therefore, clear how dangerous it is in the case of a wet roadbed.
- When cornering, friction is indispensable to balance the centrifugal force, see Curves and balance of forces. Without it, the wheels would slide sideways causing the rider to fall to the ground. In the case of a curve with a bank, the right conditions can be found, in terms of trajectory, speed, and slope of the bank, that allow to cancel out the friction force. By reducing the action of friction, we can generally say that bending using banks reduces the risk of slipping
- In braking or accelerating friction acts in the direction of advancement of the bike-rider system. In the absence of friction, the wheels slide relative to the ground causing the loss of control of the bike and the risk of falling. In this case, the weight of the system is important because it determines the amount of friction. Therefore pumping actions are recommended to get greater adherence.
We can sum it up by saying:
going straight we can fall because of roots in counterslope or because we brake right at the moment of crossing them; the most dangerous roots are those with high slope and facing the direction of travel.
In corners, horizontal roots are also dangerous, but taking advantage of the banks mitigates the risk of falling.
More details can be found by reading "The science of Mountain Bike riding: the physics behind MTB skills" which contains all the topics, addressed to all MTB enthusiasts.
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