Question on forces/friction acting on wheel

[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

 

[Doc Al]

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.

In order for the road to serve as a 'pivot', the tire must push against the road. It's static friction which allows such a push. The road, in turn, pushes the tire (and car) forward.

 

[MrNerd]

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

Your welcome for the explanation. I feel proud of myself if I can get someone to understand something. I also feel proud of them, too.

I suppose it could be considered as a pivot for leverage. If I were to think of it in a similar way myself, I would probably go with a frog jumping off a log. It's not exactly the same thing, but it illustrates the point for the most part.

Try thinking of a spinning wheel again. Now imagine a treadmill with little to no resistance, so it can simulate the effect of traveling better. If you were to place the spinning wheel on the treadmill, the friction would cause the wheel to grab hold(not really, but you get the idea) and pull the surface in the direction of rotation. If you think of friction as the nanoscopic pits and mountains in the surface of a material(I'm pretty sure this is the correct definition/visual), it becomes a lot more similar to the frog on a log, or in this sense, maybe more like a gear meshing with another gear. I assume you know how a gear transfers power, but for the most part, it's like pushing on a part that sticks out to produce an imbalance of force(thus torque), with the roundness enabling it to continue as long as it continues to spin. On a toy gokart I once had(by toy I don't mean vroom vroom race with gas engine play, it was more like a 4-5inch long model thingie, maybe for playing with), the little steering was fashioned in a similar manner. The steering wheel connected to a rod with a little gear on the end. At the gear, there was a rectangular piece with grooves matching the gear teeth. This rectangular piece was connected to the little tie rods(they connect the tire to the steering mechanism on a car), which enable the wheels to steer. By turning the steering wheel, gear at the end of the rod would be turned. The gear teeth would push the rectangular piece to the side, and the tires would move. A car tire on the ground can be envisioned in the same way, with the friction visualization stated above. The nanoscopic pits and mountains act like the gear teeth. A mountain on the tire's surface can fit into a pit on the ground's surface(it's not exact, but imagine as if this is a spot where this happens) A mountain over a mountain doesn't really do much since the scale is so tiny, but in order to move, the mountains do have to go over the other mountains). The pit is surrounded by a higher elevated surface, so it receives opposition when moving horizontally(like a gear). When the tire has torque applied to it from the engine, it the mountain pushes on the mountains(or simply not pitted part, it will still have the same effect). The tire is still having the torque applied, so the force is kind of moved upward a bit to get over the mountain(like a little ramp). If the mountain can't get out of the pit, resulting from a hook or something, the mountain will just break off, or the hook will break off, thus wear and tear from friction(I assume this is what happens with hooks, at least). If the molecule is bonded weakly enough with the attached surface, then it can break off too. This is why you have to replace brake pads, as they rely on friction to decelerate the car, and it wears the pads down. In the process of going up, they still exert the horizontal force like a gear tooth. With a ramp, if the object is forced up, it still has to exert the horizontal force. If the force wasn't there, there wouldn't be any point in having to keep the ramp in place, as it wouldn't go anywhere. Place a small ramp on a slippery surface(or at least able to slide without too much difficulty), and roll a ball at it. It should move back as a result of the balls horizontal motion. This principle is what enables the friction to take place. So, in effect, it could be argued that the tire drives the ground as well as the car. This comes from Newton's Third Law of Motion. However, the mass of the Earth is HUGE compared to a single automobile(even a fully loaded truck freight truck). So no effect is noticed on the Earth's motion.

In short, a tire can be envisioned as a gear. The gear is meshed with the ground. The ground and tire have the little mountains and pits(causing friction) that enable the transfer of power. If you need another analogy, place a spinning gear on a surface that is grooved to fit the gear's teeth. Notice that the teeth will push against the surface's teeth(remember, it's an analogy of friction). Because of Newton's Third Law of Motion, a force against the surface means the surface exerts an equal force on the gear/tire. As a result, it is the same as an imbalance of forces on a round object, creating rotational motion. A car has axles that can turn in place, so the car doesn't have to roll around itself(That might be a fun amusement park ride, though). The car simply rides along with the wheels, which drag the car with them. Voila! Self-powered(by the object itself or a person on it, at least) transportation!

If you would like to learn about how a car functions itself(like the machinery and such), you should probably obtain a book on the matter, or take a class. Transmissions alone can fill an entire book, so a forum won't be able to completely cover it.