Question on forces/friction acting on wheel

[Thevanquished]

i have this burning question which i have thought about for a long time and yet i could not seem to figure it out. here it goes.

when a wheel rotates to move a car or any vehicle, the friction would act on the opposite direction and so the friction (red arrow) would act in the same direction as the motion (black arrow), therefore propelling the car. However, given that this explanation is correct, how would the car travel at constant speed since there's no backward friction and there is always a resultant force (friction acting to the front) acting on the car? If you might say that the air resistance would oppose this force thereby allowing the car to travel at constant speed, i do not think that the air resistance is so huge to match the force, even if so, how then would the car be able to accelerate since the forces are always cancelled?

 

[Doc Al]

Let's ignore air resistance, rolling friction, and similar complications. To accelerate the car (assume no slipping), the ground must exert a forward static friction force on the tires. But when the car is moving at constant velocity, there's no need for a friction force from the ground.

 

[Thevanquished]

hmm... i don't get what u mean exactly... i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

 

[rcgldr]

i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

Friction just keeps the objects from sliding with respect to each other, there's no set force. If the car is going at a constant speed, then the net force is zero (otherwise the car would be changing speed).

If there's a block resting on the ground, friction keeps the block from sliding, but if nothing is pushing the block the friction force is zero. If you push the block with 10 lbs of force and the block doesn't slide, the friction force is 10lbs. If you push with 20 lbs of force and the block doesn't slide the friction force is 20 lbs. At some point if you push the block hard enough it will start sliding, and that is the maximum force that friction can provide (for the given conditions).

 

[russ_watters]

hmm... i don't get what u mean exactly... i mean u can't control the friction acting on the wheel can you? whether moving in constant velocity or not the wheel rotates in the same manner so won't the friction acting on it be the same?

No. The static friction force between the wheel and the ground is determined by the output of the engine. The greater the engine output, the greater the torque on the wheels and the greater the force between the wheels and ground.

 

[Thevanquished]

hmmm ok... but all of you seem to be answering only one part of my question.. for the wheels right... the friction seems to be the one propelling the car forwards and so if like you stated that the greater the torque the greater the friction, won't both forces cancel out and the resultant force acting on the wheel would be 0 and therefore the car would be moving at constant speed. How then does the car accelerate? can someone please answer both of the question (how does it move in constant speed and how does it accelerate) at the same time?

 

[Doc Al]

As I said earlier, if you ignore the complications due to air resistance and other dissipative forces, no tire friction is required for a car moving at constant speed. Of course, to overcome those dissipative forces some friction is required.

To accelerate the car, there must be a friction force on the tires pushing forward. That is produced as a result of the engine creating torque on the wheel. No, the torque due to the engine and the torque due to tire friction do no cancel out--the tire must also accelerate and thus requires a net torque. More importantly, the tires exert an external force on the car, driving it forward.

 

[Thevanquished]

u say no friction is required but no matter what it will still be produced..

 

[Doc Al]

u say no friction is required but no matter what it will still be produced..

Why do you think that?

 

[Thevanquished]

because when 2 surfaces meet there will definitely be friction

 

[Doc Al]

because when 2 surfaces meet there will definitely be friction

Really? I have a book lying on my desk. What's the force of friction on it?

Realize that (in the absence of skidding or deformation) the friction between tire and road is static friction. Static friction will be whatever is required to prevent slipping, from zero up to the maximum value the surfaces can generate for a given normal force.

Let's forget the car for a minute. Imagine just a wheel rolling without slipping along a perfectly horizontal surface at some speed. Assume the usual idealizations (no air resistance or deformation). In this case, there is no tendency to slip, thus the static friction is zero.

 

[Thevanquished]

yes the static friction may be zero but there would still be kinetic friction

 

[Doc Al]

yes the static friction may be zero but there would still be kinetic friction

Nope. Kinetic friction would only exist if the tire slipped (skidded) against the ground. As long as it rolls without slipping, the contact patch of the tire is always at zero speed with respect to the ground. No relative motion of the surfaces means no kinetic friction.

 

[Thevanquished]

ok... if there's no static friction and there's no kinetic friction... how is the care even supposed to move??

 

[Doc Al]

ok... if there's no static friction and there's no kinetic friction... how is the care even supposed to move??

Movement (constant velocity) does not require a force. (Review Newton's first law.)

Acceleration (change in velocity) requires a net force.

 

[Thevanquished]

i know... but from rest how does it even start moving?

 

[Doc Al]

i know... but from rest how does it even start moving?

As stated already in this thread, to accelerate the car requires a friction force on the tires.

 

[Thevanquished]

ok man do u have msn or something? so u can explain better

 

[Dadface]

Your question can be turned around...but from motion how does it even stop moving?The point being made here is that if there is no resultant force the object will carry on in the same way..if it is at rest it will remain at rest,if it is moving at a constant velocity(straight line and steady speed)it will continue moving at a constant velocity.The object will only change its velocity(accelerate) if there is a resultant force.

 

[Chuck St. Lou]

Your original Question;

However, given that this explanation is correct, how would the car travel at constant speed since there's no backward friction and there is always a resultant force.


With out some friction nothing will roll.
Like a brick sitting on concrete, gravity is pushing down on the tire and the normal force is straight up and down and not backwards or forwards.

 

[Thevanquished]

hmm... can anyone explain it in the sense of how forces are acting on it which causes it to accelerate and move at a constant speed instead of the other way round like "because it accelerates that's why it has resultant force and forward friction" and "because it is moving in constant speed that's why it has no friction"

 

[Doc Al]

hmm... can anyone explain it in the sense of how forces are acting on it which causes it to accelerate and move at a constant speed instead of the other way round like "because it accelerates that's why it has resultant force and forward friction" and "because it is moving in constant speed that's why it has no friction"

I'm still unclear as to what you are looking for. Before going any further, do you understand these two statements:
(1) If the tire is accelerating, there must be a net force on it. And the converse: If there's a net force on the tire, it is accelerating.
(2) If the tire is moving at constant velocity, the net force on it is zero. And the converse: If there's no net force on the tire, it's velocity cannot change.

These, of course, are just statements of Newton's laws of motion.

Try this. Imagine a car on a horizontal surface. (Forget air resistance and such complications.) The car is at rest and you want to get it moving. You arrange for the engine to exert a torque on the wheels. If the surface could produce no friction (say you were on wet ice), then the tires would just spin and the car would just sit there.

But friction from the ground acts to prevent the tires from slipping. The tires "try" to slip backward; friction prevents that by pushing forward. That forward push accelerates the car.

Once the car is moving at the speed you want, the tires no longer need to accelerate. They are moving (and spinning) at the exact rate needed to maintain that speed. Thus there is no tendency to slip, and no friction force is generated. (Of course, in real life you know that other forces act to slow the car down, so you have to keep giving it gas.)

 

[Dadface]

Friction plays a major part in all of this.If the car was stationary and there was no friction the driving wheels would simply spin and the car would go nowhere.If the car was moving and hit an ice patch it would skid.The driving force from the engine is transmitted to the driving wheels the tyres of which put a backward force on the road surface this resulting in an equal forward force on the car.If the car reaches a constant velocity the driving force does not disappear it is simply that the other forces ,mainly air resistance,become equal to the driving force making the resultant force zero.If the wheels were considered separately when they reach a constant angular velocity the torque transmitted by the engine is equal in size to the torque caused by friction.

 

[gmax137]

I'm not sure if this matters, but all of the posts that include "assuming no slippage" are apt to reach meaningless conclusions, because the tires *do slip* when the car is accelerating. You see this clearly whenever the driver "burns rubber" but it happens whenever you pull away from a stop (it's just to a lesser extent and therefore less obvious). Otherwise, our tires would last forever.

 

[russ_watters]

"start moving" is acceleration... go back to my post: the engine produces a torque on the tire, creating a static friction force between the tire and the ground.

Perhaps you are just under the misunderstanding that a pair of forces cancells to zero. Not true. Imagine you are pushing an object across a perfectly smooth surface (for example, a plastic puck on an air hockey table). You can say the puck is pushing against your finger or that your finger is pushing against the puck, but really, that's just two ways of looking at the same force. You don't subtract one from the other.

 

[sganesh88]

I guess the main difficulty faced by the original poster has got to do with friction remaining zero when the wheel is in uniform rolling. I would like to put it this way. The road surface "programs" the wheel such that it never tries to slip past it. For such a situation, the solution comes out to be Vcom = R*W(omega). It corresponds to the curve cycloid.
http://en.wikipedia.org/wiki/File:CycloidAnim04.gif
Does any point on the curve try to slip past the surface? Watch the animation again.

 

[Dadface]

I think that a misunderstanding of Newtons third law is a main reason why some are finding this topic difficult.The third law is deceptive, it might look simple at first glance but in fact it is conceptually quite difficult.I will express the law in a way which may be unfamiliar to some.
Forces occur in pairs which...
1. are equal in size
2. opposite in direction.
3. forces of the same type(eg gravitational)
4. act on different bodies.
It seems to be point number 4. that is causing the confusion.If the tyre puts a backward force on the road, the road puts an equal sized forward force on the tyre.Although these two forces are equal in size etc they do not cancel out because they act on different bodies namely the tyre and the road.

 

[Thevanquished]

if they do not cancel out... a car/wheel can never move at a constant speed so yes this is my question. i would like someone to use the same theory which will be able to explain both phenomenon (i.e. moving at constant speed AND accelerating) at the same time and not just one at a time which makes this confusing.

Does any point on the curve try to slip past the surface?

hmm i don't get what u mean by the curve slipping past the surface

 

[Doc Al]

if they do not cancel out... a car/wheel can never move at a constant speed so yes this is my question.

Is your question really about Newton's 3rd law? And why the forces don't cancel out?

If so, then realize that 3rd-law force pairs act on different objects thus never directly cancel. For example: For the car to accelerate, the road must exert a friction force on the tires. Accordingly, the tires will exert an equal and opposite force on the road.

Only when two equal and opposite forces act on the same object will they cancel. That's not the case here.

i would like someone to use the same theory which will be able to explain both phenomenon (i.e. moving at constant speed AND accelerating) at the same time and not just one at a time which makes this confusing.

What do you mean "at the same time"? The car cannot move at constant speed and accelerate at the same time.

I think part of the problem is that you have the idea that the friction force is fixed and unchanging. That's not the case. It varies as needed to prevent slipping.

 

[Thevanquished]

i mean explaining both at the same time... not that it happens at the same time... as in explaining both phenomenon using the same concept/theory which does not contradict each other at the same time

Is your question really about Newton's 3rd law? And why the forces don't cancel out?

If so, then realize that 3rd-law force pairs act on different objects thus never directly cancel. For example: For the car to accelerate, the road must exert a friction force on the tires. Accordingly, the tires will exert an equal and opposite force on the road.

Only when two equal and opposite forces act on the same object will they cancel. That's not the case here.

yes i know that they will only cancel out when they act on the same object... so if in this case they do NOT cancel out, how can a wheel even maintain a constant speed since there is always a resultant/net force acting on the wheel (friction)

 

[Doc Al]

i mean explaining both at the same time... not that it happens at the same time... as in explaining both phenomenon using the same concept/theory which does not contradict each other at the same time

I don't know what you mean. Where have any of the explanations offered contradicted each other?

If you point out what you think is contradictory, then we may have a better idea of how you are thinking.

 

[Doc Al]

yes i know that they will only cancel out when they act on the same object... so if in this case they do NOT cancel out, how can a wheel even maintain a constant speed since there is always a resultant/net force acting on the wheel (friction)

That's where you are wrong. There is not always a net force acting on the wheel. The friction force varies. Sometimes, such as when the car is accelerating, there's a friction force pointing forward. At other times, the friction is zero, such as when the car is moving at constant speed or at rest. At still other times, the friction acts backward, such as when you are braking. Static friction varies as needed to prevent slipping.

 

[Thevanquished]

ok so how are u supposed to control the acceleration then.. since the explanation you are giving seems to be the friction controlling the wheel instead of the "driver"... friction only acts on the wheel when the wheel acts on the floor right? so shouldn't it be the other way round? telling me there's friction when the car is accelerating and there's no friction when the car is moving at a constant speed really dosen't help as it does not explain my question. my question is that how does a car move at constant velocity if there is always friction acting on the wheel. and as people would always say that it is due to air resistance or anything which seems to cancel the forward force of the friction, i would ask them how then does a car accelerate since there is no resultant force due to resistance, people would then answer the question by saying that friction force causes a forward resultant force on the wheel... as you can see this would thus lead to a cycle of question. therefore it would be best if someone could explain both phenomenon using ONE concept/theory as usually answering one would only cause me to question the other.

 

[Dadface]

Hello the vanquished.Think of the forces which are acting on the car only,the road is a different body.Imagine,also, that the car starts moving from rest and that the driving force remains constant.
Initially the driving force will be greater than the resistive forces(mainly air resistance or drag) that oppose the motion and the car will accelerate.As the car picks up speed the drag increases this resulting in a reducing resultant force(driving force minus resistive force) and reducing acceleration.Eventually the resistive forces become equal to the driving force ,the resultant force becomes zero, as does the acceleration and the car is now moving at a constant (terminal) velocity.Friction between the tyres and road is necessary to impart the driving force to the car.
I will describe a different scenario...
A person who weighs 500N steps from an aeroplane to do a free fall jump.A third law pair that applies here is that the Earth is pulling the person with a force of 500N and the person is pulling the Earth with a force of 500N.Although these forces are equal and opposite and of the same type they do not cancel out because they act on different bodies.As the person picks up speed a point is reached when the air resistance becomes equal to their weight this making the resultant force zero and resulting in a terminal(constant) velocity.The person can alter the terminal velocity slightly by changing their body configeration so as to adjust their air resistance.(when they open their parachute they do not fly upwards.This is an illusion caused by the camera person still moving relatively faster having not yet opened his/her parachute)

 

[twowheelsbg]

Let's ignore air resistance, rolling friction, and similar complications. To accelerate the car (assume no slipping), the ground must exert a forward static friction force on the tires. But when the car is moving at constant velocity, there's no need for a friction force from the ground.

Why do the these friction forces are classified as 'ground to tires' ?
This implies that they are pushung back and acting as driving force, which is not correct.
It seems to me that these static friction forces are just counter forces to the driving momentum forces trying to rotate the wheel, emerging into the contact point, no parallel to Neuton third law is rellevant.

 

[Doc Al]

Why do the these friction forces are classified as 'ground to tires' ?
This implies that they are pushung back and acting as driving force, which is not correct.

Not sure what you mean, but friction is surely providing the "driving force" to accelerate the car. Put the car on a frictionless surface and you can step on the gas all you want--the tires will simply spin.

It seems to me that these static friction forces are just counter forces to the driving momentum forces trying to rotate the wheel, emerging into the contact point, no parallel to Neuton third law is rellevant.

Internal forces act to turn the tires. Friction acts to prevent slipping; in so doing, it pushes the car forward.

 

[twowheelsbg]

Not sure what you mean, but friction is surely providing the "driving force" to accelerate the car. Put the car on a frictionless surface and you can step on the gas all you want--the tires will simply spin.

Internal forces act to turn the tires. Friction acts to prevent slipping; in so doing, it pushes the car forward.

I agree with you that friction forces are necessary for acceleration,
but i see no direct link between them - friction forces are not exactly the driving force which pushes forward. if friction was driving force the contact patch would move forward, but it doesn't.

As to the internal forces turning the tyres, my oppinion is that the driving force applied at the wheel axle is pushing forward, which means rotating the tyre , with the contact patch immobialized ( friction forces prevent it from slipping )

 

[Doc Al]

I agree with you that friction forces are necessary for acceleration,
but i see no direct link between them - friction forces are not exactly the driving force which pushes forward. if friction was driving force the contact patch would move forward, but it doesn't.

I don't follow your logic. The contact patch continually changes as the wheel rotates. The entire wheel moves forward.

An external force is required to accelerate the car. Friction from the ground is not a power source, but it is the driving force accelerating the car forward.

As to the internal forces turning the tyres, my oppinion is that the driving force applied at the wheel axle is pushing forward, which means rotating the tyre , with the contact patch immobialized ( friction forces prevent it from slipping )

I suspect a bit of a semantic issue here. Certainly power is applied in creating those internal forces, but they don't directly cause the car to accelerate.

 

[twowheelsbg]

For me, driving force means force, which application produces drive.
If we push the wheel axle, it moves forward, the wheel is rolling - so this means driving force, that drives someting.

Fricition forces as far as i can see only immobialize the contact patch - this is their important role, acting as leverage point, so the axle ( not the contact with the ground ) to be pushed forward by the driving moment ( acting over rotating body over fixed axle )

I may be misinterpretting, but just saying the opposite does not make me leaving my view point. Not at last, i am glad that people with better insight may discuss the question and correct me eventually.

Thanks!

 

[sganesh88]

Fricition forces as far as i can see only immobialize the contact patch - this is their important role, acting as leverage point, so the axle ( not the contact with the ground ) to be pushed forward by the driving moment ( acting over rotating body over fixed axle )

Yes. Realizing that friction is the driving force is something that i haven't yet got accustomed to. But let's respect Newton.
What you call "immobilize" is actually an application of force by the ground on the tire.
A related scenario. Consider a pole fixed to the ground. You curl your hands around it and pull it. The COM of the system comprising of "you and the pole", accelerates as a result of you moving close to the pole owing to it's reaction on you. (read again) So the "actual" reason for the acceleration of the COM of the afore mentioned system seems to be "your" force. But if the Earth hadn't exerted a reaction on the pole it would have accelerated towards you and damn. The COM wouldn't have budged. (that would be the case when the pole is kept-not fixed- on an ice surface), So you should give credit to the ground with humility!

 

[twowheelsbg]

So the "actual" reason for the acceleration of the COM of the afore mentioned system seems to be "your" force.

Dear Sganesh88, thank you for the example, it really cleared out your point.
You are right, that ground deserves the credit, but to quantify correctly, we should interpret correctly. So, the ACTUAL reason for the acceleration of the COM of the afore mentioned system seems to be the ground pull over it, not my force. My force acts as internal for the system and it cancels out with the pole's reaction to my pull, also internal for the system. If not looking for COM, and observe me(the pulling person) as separete system, then we could say that the pole's reaction to me is the actual reason for my acceleration towards the pole. What I am trying to say is that all forces involved are important, but actual is the one at the end of the determined chain and should be used in further equations calculating linear or rotational movement of the object.

As you said showing the humility in front of ground reaction and friction forces is right,
and I agree when speaking of a car with no other supports except the tyre contact patches, there is no other place for push-back or support, except these patches.

 

[sganesh88]

What I am trying to say is that all forces involved are important,

Exactly. The tractive force(propelling friction) is directly proportional to the torque applied at the wheels. You should thank the thumping pistons of the engine for that torque.

 

[Red_CCF]

I don't follow your logic. The contact patch continually changes as the wheel rotates. The entire wheel moves forward.

An external force is required to accelerate the car. Friction from the ground is not a power source, but it is the driving force accelerating the car forward.

Hi

I accept that friction is the force that moves a car forward, but I have trouble visualizing how this occurs given that the contact patch is stationary w.r.t. the ground?

Thanks

 

[Doc Al]

I accept that friction is the force that moves a car forward, but I have trouble visualizing how this occurs given that the contact patch is stationary w.r.t. the ground?

For one thing, the friction involved is static friction. (Assuming the tires do no slip.)

 

[Red_CCF]

For one thing, the friction involved is static friction. (Assuming the tires do no slip.)

Hi

Given that it is static friction, which only exist when two surfaces are at no relative motion, how does it "move" something?

Thanks

 

[MrNerd]

I think I see what you're trying to say.

To propel the car, a force needs to be exerted. This is performed through the engine, transmission, driveshaft, and then wheels, for the most part. The engine produces torque. The torque is transferred to the transmission(through the torque converter on automatic transmissions or a clutch on a manual transmission), which adjusts the ratio to allow for more torque(and thus acceleration) at a lower speed(it needs to rotate more per wheel revolution, reducing maximum speed) or less torque at a higher speed(it spins less per wheel rotation, thus increasing speed but reducing torque). The driveshaft carries this to the wheels. The wheels are where all the discussion is at.

The wheels have a torque exerted on them. The ground/tire contact produces friction, causing a net force. Imagine it as placing a wheel spinning at a constant speed on a tabletop. The tabletop/wheel contact produces the friction which tries to stop the wheel on the bottom. However, there is no force like this(air resistance is technically present, but negligible in this case) on the top of the wheel. Thus, a net rotational force is provided between the top and bottom of the wheel. The momentum of the top of the wheel acts to keep it moving, while the force tries to stop it. As a result, it's something like tripping someone. The top continues to move while the bottom has stopped, until the person falls on their face. The same thing happens with the wheel. However, since it is round, it can be thought of as perpetually tripping on itself. The car is attached to the wheel, so it goes with it it's perpetual tripping action. It's not technically perpetual, since there IS a small amount of friction from the air as it moves, but you get the idea, I think.

If you're asking in another part as to why do cars maintain a constant speed with your foot on the pedal or something, that's fairly easy to answer as well. In reality, there is a frictional force on pretty much everything(even in space, since a true vacuum empty of subatomic particles flying through the air is exceeding rare and temporary, however it is very very very very very small). The engine continues to produce the torque, accelerating the car as stated above. As an object increases, it's friction from the medium it is traveling through increases. This is a result of colliding with the air particles. Because of momentum transfer, a faster car(therefore with more momentum) will transfer more of it's momentum to each air particle, thereby causing what we call resistance(I don't think it's technically friction, but it has the same effect as friction, aside from making air particles change direction). Please note that I never read this anywhere, I thought of this idea myself. But I believe it is what happens. Because the air resistance changed with speed, at some point the engine torque will equal the air resistance. Therefore, there is no net force, as they are equal and opposite. Thus, the car does not slow down.

I hope this is what you wanted, because I don't think I can find a different way to explain it.

 

[MrNerd]

Oh, and there IS always some tire slippage. I learned this in my autos class in high school. I think it's supposed to be like 10% or something, but this seems a bit high now that I think about it. But I'm still pretty sure there is at least a small amount of slippage. Now that I think about it, though, it might just be on turns. When you skid(like stomp the brakes skid so you end up crashing anyway), that is when it's 100% slippage.

And above, for the most part, it works the same way. Even if it's all kinetic friction, it's simply reduced(however by a massive amount). There is still SOME friction propelling the car(or braking). The friction force is F = uN. F is force of friction. u is the coefficient of friction for the surfaces. N is the normal force, in the case of a car it's probably just the weight times gravity. u is difference for kinetic and static, but it's still there. So whether it's slipping or not, there should still be a force.

From the perspective of the wheel, the bottom is not moving, it's the top is just falling, like the example above. Because it's round, the top falling causes the bottom to go up. The wheel is still moving, but the top is falling forward. Like toppling a tilting tower, it's because the friction causes the bottom to not move that the top falls over. In the process, it HAS to go forward in order to get down. But the wheel is round, thus always "falling", so it continues to move forward. So, in effect, the road is taking the wheel with it, but it rolls as a result of being round.

 

[Doc Al]

Given that it is static friction, which only exist when two surfaces are at no relative motion, how does it "move" something?

While the part of the tire that is in contact with the road at any instant isn't moving with respect to the road, the tire as a whole is moving of course. The contact patch is continually changing as the tire rolls.

Another example of static friction creating motion: You are standing then start walking. Your feet propel you along via static friction. You move while the bottoms of your shoes do not. Of course, you soon have to pick your foot up and swing it forward or you'll not get far. (The tire does this by rolling.)

 

[Red_CCF]

From the perspective of the wheel, the bottom is not moving, it's the top is just falling, like the example above. Because it's round, the top falling causes the bottom to go up. The wheel is still moving, but the top is falling forward. Like toppling a tilting tower, it's because the friction causes the bottom to not move that the top falls over. In the process, it HAS to go forward in order to get down. But the wheel is round, thus always "falling", so it continues to move forward. So, in effect, the road is taking the wheel with it, but it rolls as a result of being round.

Thanks very much for the very detailed explanation!

I was wondering, how is this friction force related to the torque by the transmission quantitatively (are they equal or is one smaller than the other)? From this explanation, it seems that the momentum that is causing the "falling forward" effect is due to the torque by the car and not the friction, which seems to be key in serving as a pivot point and not actual motion?

Thanks

 

[Red_CCF]

While the part of the tire that is in contact with the road at any instant isn't moving with respect to the road, the tire as a whole is moving of course. The contact patch is continually changing as the tire rolls.

Hi

When viewing the whole car as one body, friction is the only external force; my main issue is picturing how, as force is a push or pull, friction actually causes the car to move because to me it seems to be just serving as a pivot for the torque provided by the car engine to cause the wheel to turn.

Thanks