Downhill right speed
The trajectory of the center of gravity along a bumpy path. At low speed (continuous line) the bike-rider system sinks between roughness. At high speed (dotted line) the system sinks less so the shocks are attenuated and you have the buoyancy effect.
What is the right downhill speed on a mountain bike?
As a rigorous scientific narrative requires, the first step must be to define the terms that are used to avoid any kind of misunderstanding. In this case, we must define what is meant by "right", because you can attribute different meanings to it. Therefore, if we intend to go downhill safely, but looking for that minimum of adrenaline and a lot of fun, without having to win a competition at all costs, we can define "right speed" as the one that minimizes the travel time of a path, still riding safely.
Answering the initial question with a numeric value is impossible because it is highly subjective. The right speed depends on the degree of training, skill, age, unscrupulousness, technique, and more. Without considering the morphological variability of the trails, which undoubtedly plays an important role, the terrain conditions also affect the rider's performance.
Having said that, we can identify what we need to keep in mind, going downhill, to see if we can accelerate a little or if we need to slow down to reach our personal speed. To do this we need to know how speed acts and affects riding.
As a rigorous scientific narrative requires, the first step must be to define the terms that are used to avoid any kind of misunderstanding. In this case, we must define what is meant by "right", because you can attribute different meanings to it. Therefore, if we intend to go downhill safely, but looking for that minimum of adrenaline and a lot of fun, without having to win a competition at all costs, we can define "right speed" as the one that minimizes the travel time of a path, still riding safely.
Answering the initial question with a numeric value is impossible because it is highly subjective. The right speed depends on the degree of training, skill, age, unscrupulousness, technique, and more. Without considering the morphological variability of the trails, which undoubtedly plays an important role, the terrain conditions also affect the rider's performance.
Having said that, we can identify what we need to keep in mind, going downhill, to see if we can accelerate a little or if we need to slow down to reach our personal speed. To do this we need to know how speed acts and affects riding.
Speed defines the following four quantities:
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.
For any information, to suggest a topic or to simply have your say, find the contacts you can write to.
- the kinetic energy of the bike-rider system, which allows you to overcome the shocks against the roughness of the ground: each impact consumes a little of this energy, so the more we have and the more shocks we can overcome. As a result, the higher the speed, the less likely it is to make the wheel tilt against an obstacle, which can lead to tipping.
- the range of parabolic trajectories. Imagine the classic movie scene, in which the car smashes through the guard-rail and ends up in a precipice. As soon as it no longer touches the ground, the car falls into the void describing a parable in the air. Let's go back to our mountain bike and imagine falling into the void after overcoming an asperity of the terrain. The bike-rider system travels, like the car of the film, along a parabolic fall trajectory that will be elongated proportionally to the initial speed. Therefore, at high speed, you will lose altitude more slowly and this is an advantage. In fact, in a real ride along a rough path, after an asperity we do not fall into the void but we probably have to face another one immediately after, and if we have not lost altitude the impact will be greatly mitigated. That's why it's said that at high speed, sinking less in the roughness of the ground, it's like floating on it.
- the inherent stability of the bicycle. The bicycle, for a set of dynamic processes of which a description is given in the book "The science of Mountain Bike riding: the physics behind MTB skills", is intrinsically stable, that is, it manages to keep itself in balance without the action of the rider. It is possible to see this by pushing a bike at a certain speed: if you let it go it will keep traveling alone resisting any disturbances of its motion. This characteristic is more visible as the speed increases.
- the centrifugal force in curves. As illustrated on the page dedicated to the balance of forces in curves, the thrust of the outward centrifugal force is balanced both by the weight force of the bike-rider system, through the inward roll movement, and by the friction force exerted by the ground on the wheels. Therefore, at high speed, to counteract the thrust of the centrifugal force, it is necessary to bend more in corners, making the journey more difficult, and you must be sure that the ground guarantees the grip of the wheels.
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.
For any information, to suggest a topic or to simply have your say, find the contacts you can write to.