# Direction of friction of each wheel and total moment when a car turns

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• JM7
In summary: The direction of static friction of each wheel is aligned with the direction of the wheel during acceleration (driving wheels) and braking (driving and driven wheels) on a straight line. 2) The static friction of any wheel becomes a combination (or vectorial addition) of any tangential or longitudinal friction force (mentioned above) and a lateral friction force after the car starts to turn.Static friction provides centripetal force when a car turns. The direction of static friction is aligned with the direction of the wheel during acceleration and braking. The static friction of any wheel becomes a combination of any tangential and longitudinal friction force and a lateral friction
JM7
Static friction is known to provide centripetal force when a car turns.
Assuming uniform circular motion, my questions are
1. Is the static friction of each wheel points toward the center of turning circle or it's the combined forces of all four wheels that has to point toward the center of turning circle from the center of mass of the car?
2. Would the total moment of frictional force of all four wheels with respect to the center of mass be zero (no net torque so no angular acceleration)?
3. Is there any condition for the relative values between each frictional force e.g. the frictional forces of the two rear wheels have to be the same or the frictional forces of the inner (front/rear) wheel are always higher than those of the outer (front/rear) wheel?

Welcome!

1. The static friction of each wheel points toward the center of the turning circle.

2. Ideally, the total moment of frictional force of all four wheels with respect to the center of mass is zero. In reality, some cars are over-turning, and some are under-turning, according to the location of the center of mass, and how the tires deform under lateral loads.

3. The frictional forces of the inner (front/rear) wheel are always lower than those of the outer (front/rear) wheels, since there is a rolling torque or moment (the center of mass is higher than the contact patches).

https://en.wikipedia.org/wiki/Vehicle_dynamics

https://en.wikipedia.org/wiki/Automobile_handling

https://en.wikipedia.org/wiki/Body_roll

https://en.wikipedia.org/wiki/Weight_transfer

https://en.wikipedia.org/wiki/Understeer_and_oversteer

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1. Why is the direction of static friction of each wheel not tangential to the direction of the wheel? Consider a front-wheel drive car which is moving along a straight path, the direction of the static friction of the rear wheels are opposite the motion of the wheels (and the car). Would the static friction of the rear wheels change direction quite suddenly by 90 degrees when the car starts to turn?
2. Shouldn't the static friction of all four wheels be added up to be the centripetal force mv^2/R pointing towards the center from the center of the mass?

JM7 said:
1. Why is the direction of static friction of each wheel not tangential to the direction of the wheel? Consider a front-wheel drive car which is moving along a straight path, the direction of the static friction of the rear wheels are opposite the motion of the wheels (and the car). Would the static friction of the rear wheels change direction quite suddenly by 90 degrees when the car starts to turn?
2. Shouldn't the static friction of all four wheels be added up to be the centripetal force mv^2/R pointing towards the center from the center of the mass?
1. The direction of static friction of each wheel is aligned with the direction of the wheel during acceleration (driving wheels) and braking (driving and driven wheels) on a straight line.
For a non-braking driven wheel, there is the rolling friction, which makes it freely roll over the pavement.
The static friction of any wheel becomes a combination (or vectorial addition) of any tangential or longitudinal friction force (mentioned above) and a lateral friction force after the car starts to turn.

2. The lateral static friction of all four wheels should be added up to equal the centripetal force mv^2/R pointing towards the center from the center of the mass.
https://en.wikipedia.org/wiki/Circle_of_forces

http://www.formula1-dictionary.net/traction_circle.html

https://en.wikipedia.org/wiki/Cornering_force

You are welcome

JM7 said:
1. Is the static friction of each wheel points toward the center of turning circle or it's the combined forces of all four wheels that has to point toward the center of turning circle from the center of mass of the car?
2. Would the total moment of frictional force of all four wheels with respect to the center of mass be zero (no net torque so no angular acceleration)?
3. Is there any condition for the relative values between each frictional force e.g. the frictional forces of the two rear wheels have to be the same or the frictional forces of the inner (front/rear) wheel are always higher than those of the outer (front/rear) wheel?
Welcome to PF.

@Lnewqban is obiously simplifying his replies for your question. As you can see from his avatar, his force vector involves a very signifcant tantential acceleration as well as the radial centripital acceleration...

Lnewqban
1. But if the static friction of each wheel points toward the center (from your earlier answer), doesn't that mean that the tangential component of the static friction (of each wheel) becomes zero?
2. So are the following conditions required at the same time
(a) the static friction of each wheel points toward the center
(b) the vectorial sum of the static friction of all four wheels points toward the center
Thank you.

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JM7 said:
1. But if the static friction of each wheel points toward the center (from your earlier answer), doesn't that mean that the tangential component of the static friction (of each wheel) becomes zero?
2. So are the following conditions required at the same time
(a) the static friction of each wheel points toward the center
(b) the vectorial sum of the static friction of all four wheels points toward the center
Thank you.
Conditions 1 and 2 above happen only when there is no acceleration or braking during the turn.
Otherwise, we have two components of the actual friction force acting on each accelerating or braking tire, one points toward the center of the turn and the other points tangentially respect to the described circular trajectory.

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## 1. What is the direction of friction for each wheel when a car turns?

The direction of friction for each wheel when a car turns depends on the direction of the car's motion and the orientation of the wheels. In general, the front wheels experience a sideways friction force towards the center of the turn, while the rear wheels experience a sideways friction force away from the center of the turn.

## 2. How does the direction of friction affect the car's turning ability?

The direction of friction plays a crucial role in the car's turning ability. The friction force between the wheels and the ground provides the necessary centripetal force to keep the car on its curved path. If the direction of friction is not aligned with the direction of motion, the car may skid or lose control.

## 3. What factors influence the direction of friction when a car turns?

The direction of friction is influenced by various factors, including the speed of the car, the weight distribution of the car, the type and condition of the tires, and the road surface. These factors can affect the amount and direction of the friction force, thus impacting the car's turning ability.

## 4. How does the direction of friction impact the total moment when a car turns?

The direction of friction also affects the total moment, or torque, that is generated when a car turns. The friction forces on the wheels create a moment arm, which contributes to the total moment. If the direction of friction is not aligned with the direction of motion, it can create a larger or smaller moment, impacting the car's stability and handling.

## 5. Can the direction of friction be controlled or adjusted in a car?

As a scientist, I cannot recommend any specific actions for adjusting the direction of friction in a car. However, factors such as tire pressure, weight distribution, and driving techniques can influence the direction of friction and can be adjusted to improve the car's turning ability and overall performance.

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