Curves and balance of forces
Forces acting in curves: P weight force, Fcf centrifugal force, Fa friction force, R support reaction constrain, α roll or fold angle
Safely traveling a curve at high speed is not all that obvious. We have to deal with a force that is not normally present and that appears precisely and only in curves... centrifugal force.
In a curve, therefore, we must not only pay attention to what we see, such as the roughness of the ground but also to what we can only perceive, such as the action of this force.
If you stand, legs together, and someone pushes you sideways strong enough, you're definitely going to fall. The only chance to keep standing is to put yourself in the "Tower of Pisa" position in the direction of the push: your weight, downward, will balance the lateral thrust and you will remain in balance, which is precisely what happens in a curve when the bike-rider system takes on a certain roll angle.
The position to be taken by the body is described below:
In a curve, therefore, we must not only pay attention to what we see, such as the roughness of the ground but also to what we can only perceive, such as the action of this force.
If you stand, legs together, and someone pushes you sideways strong enough, you're definitely going to fall. The only chance to keep standing is to put yourself in the "Tower of Pisa" position in the direction of the push: your weight, downward, will balance the lateral thrust and you will remain in balance, which is precisely what happens in a curve when the bike-rider system takes on a certain roll angle.
The position to be taken by the body is described below:
- bike folding and with a certain steering angle,
- head and look towards the exit of the curve,
- the rotation of the head must be accompanied by the rotation of the shoulders, with the arm inside the curve more stretched than the outer one,
- the rotation of the shoulders must be accompanied by the rotation of the torso and pelvis, with the external knee facing the bike frame,/li>
- weight offloaded on the foot outside the curve and on the inner arm,
- maintain the centrality of the weight and the torso bent on the handlebar,
- inner arm more stretched, outer arm bent with raised elbow.
Leaving aside all the reasons to assume this posture, it is interesting to photograph the rider in a curve and highlight what forces act on him and their effect.
There are four: weight force and centrifugal force, which are considered to be applied in the center of gravity of the system; the constraining support force and the friction force applied to the wheels.
By applying the equilibrium conditions, some useful results can be obtained.
Let's start by saying that since forces in the same direction but with opposite orientation must have the same intensity, the frictional force must be equal to the centrifugal force. If, on the other hand, the centrifugal force were greater, the equilibrium condition would not be respected, the wheels would not hold and we would slide sideways to the ground: the centrifugal force prevailed over the friction force. From the equilibrium condition and making some calculations you get the first interesting result: the ground friction required to hold on to the curve is exclusively defined by the travel speed and radius of the curve. The rider's weight or posture has no influence.
Considering the action of the moments, calculated with respect to the contact point of the wheels on the ground, the centrifugal force induces an outwards rotation, while the one of the weight tends to rotate the system inwards. Their balance establishes the second important result: the inclination of the bike-rider system with respect to the ground (roll angle) also depends exclusively on the speed and the radius of the curve.
It is therefore clear how important it is to follow the right trajectory and have adequate speed; they mathematically define the friction required by the ground and how much you have to lean the system inward.
For any information, to suggest a topic or to simply have your say, find the contacts you can write to.
There are four: weight force and centrifugal force, which are considered to be applied in the center of gravity of the system; the constraining support force and the friction force applied to the wheels.
By applying the equilibrium conditions, some useful results can be obtained.
Let's start by saying that since forces in the same direction but with opposite orientation must have the same intensity, the frictional force must be equal to the centrifugal force. If, on the other hand, the centrifugal force were greater, the equilibrium condition would not be respected, the wheels would not hold and we would slide sideways to the ground: the centrifugal force prevailed over the friction force. From the equilibrium condition and making some calculations you get the first interesting result: the ground friction required to hold on to the curve is exclusively defined by the travel speed and radius of the curve. The rider's weight or posture has no influence.
Considering the action of the moments, calculated with respect to the contact point of the wheels on the ground, the centrifugal force induces an outwards rotation, while the one of the weight tends to rotate the system inwards. Their balance establishes the second important result: the inclination of the bike-rider system with respect to the ground (roll angle) also depends exclusively on the speed and the radius of the curve.
It is therefore clear how important it is to follow the right trajectory and have adequate speed; they mathematically define the friction required by the ground and how much you have to lean the system inward.
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For any information, to suggest a topic or to simply have your say, find the contacts you can write to.