Difference between rolling resistance and tractive force

[R Power]

Hi friends
Let me talk in context of cars wheels...
See!, tractive force is what ground offers us depending upon the weight of wheels or particularly car. It is the maximum force that ground offers us opposite to the force we apply to ground in the form of torque(t=rxf)... if we aplly more torque(or force) than that, we will create a wheel slip. So this tractive force is then the friction having it's max value= u x Rn(normal reaction)...This is what atleast i imagined and found on googling too...

But then what is rolling resistance? ... when i googled or saw in my textbooks i found simply it's resistance to rolling it is what due to which a wheel stops and it is also in opposite to direction of wheel spin or we can say in same direction of car moves....actually we can't say it rolling resistance because it is what helps a wheel roll...it's better we say it rotating resistance that's why when we release the gas pedal the torque decreases and decreases and finally becomes 0 due to rotating resistance . This also has direction opposite to wheel spin or same as that of car direction i.e same as that of tractive force...
Actually rotating resistance(or rolling res if u want to say) is what makes a wheel roll as long as we apply torque and same stops the wheel when we stop applying torque!
Then what is difference between these two terms.........?
Plz help me...

 

[arildno]

1. The tractive force (friction) is, indeed, as you imagine it to be. Think of it as a "glue" that makes the wheel stick a bit to the ground.

2. ROLLING FRICTION, however, is due to that the two surfaces in contact (the wheel&te ground) DEFORM a bit, and do not wholly regain their shape.
Thus, there is a loss of elastic energy involved.

Usually, when hard surfaces are involved, the rolling friction is order of magnitudes less than the traction, but with a rubber wheel on the car, the rolling friction is not wholly negligible.

 

[R Power]

Can u tell me the difference in their directions if i am wrong.....

 

[xxChrisxx]

Rolling resistance is a tricky thing, because it not like friction. It isn't one 'force' acting to prevent the rolling of the wheel it's a mixture of many things all acting to retard rolling.

Rolling resistance does not make a wheel roll. It wouldn't be a resistance then would it :P? (This is now getting into a minefield of explinations that may or may not confuse you, I shall attempt it anyway.)

What acutally causes a wheel to roll is static friction (coefficient). If you push a wheel with low static friction it will not roll, it will slide. eg. a car tyre on ice. The higher the static friction the more likely the object is to roll as opposed to slide.

Friction prevents sliding. Rolling resistance prevents rolling.
Rolling resistance consists of adhesion and deformation.

 

[rcgldr]

2. ROLLING FRICTION, however, is due to that the two surfaces in contact (the wheel&te ground) DEFORM a bit, and do not wholly regain their shape.

They regain their shape, but there is hysteresis involved. The forces involved in the "return" path are less than the forces in the "deform" path, so mechanical energy was converted into heat.

 

[Bob S]

The rolling resistance coefficient RRC for automobile tires is about 0.01. So the force required to push the car is F=RRC x mg, where m is the mass of the car. The tractive force (static friction coefficient) of a car is I believe ~0.8 x mg drawbar pull (for all-wheel drive).
Bob S

 

[R Power]

The rolling resistance coefficient RRC for automobile tires is about 0.01. So the force required to push the car is F=RRC x mg, where m is the mass of the car. The tractive force (static friction coefficient) of a car is I believe ~0.8 x mg drawbar pull (for all-wheel drive).
Bob S

so u think the surface of road doesn't contribute in friction coefficient...
Secondly, how can u say tractive force is static friction because in order to move a body we always need to give a force greater than static friction, but in car if we exceed torque(force) than available traction we will create a wheel slip

 

[nucleus]

Here are the formulas for a hydraulic drive. For a mechanic drive change to gear ratios.

http://www.parker.com/literature/025 Engineering Data FINAL.pdf

 

[arildno]

They regain their shape, but there is hysteresis involved. The forces involved in the "return" path are less than the forces in the "deform" path, so mechanical energy was converted into heat.

And hysteresis involves permanent deformation on the microscopic level.

 

[R Power]

guys please tell me difference bw rolling resistance and tractive force & not how it occurs...!
also tell me the difference in direction of the 2

 

[xxChrisxx]

We have told you.

Tractive force is what pushes the car forward, occurs opposite to the direction of motion.

Rolling resistance prevents the wheel from trying to roll, this is a moment that occurs opposite to the wheel rotation.

 

[R Power]

We have told you.

Tractive force is what pushes the car forward, occurs opposite to the direction of motion.

Rolling resistance prevents the wheel from trying to roll, this is a moment that occurs opposite to the wheel rotation.

u are wrong...u say tractive force pushes the vehicle forward then it must act in direction opposite to wheel rotation which will make the wheel roll(not rotate)...then how can it act opp to motion...it should be in dir of motion

 

[xxChrisxx]

Tractive force is in the direction opposite to VEHICLE MOTION. (not wheel rotation)

The tyre pushes backwards, which as a reaction the car moves forwards. A tyre rotating clockwise, makes a force pointing to the left, but the car will move to the right.

 

[R Power]

ok u say tractive force is force with which the tyre pushes the ground and the result of which is a reaction in opp dir which creates tyre roll and car moves forward......
but the reaction is rolling friction...then it should be equal and opp to tractive force......is it so?

 

[xxChrisxx]

Er, I don't think you are getting it.
The reaction to traction is not friction...Can I suggest you go and read up on Newtons law on force pairs.

The reaction to the traction of wheel pushing on ground is the ground pushing on the wheel.
Friction is a completely separate force pair. Thats why you can have less friction than traction or te other way round.

 

[Bob S]

so u think the surface of road doesn't contribute in friction coefficient...
Secondly, how can u say tractive force is static friction because in order to move a body we always need to give a force greater than static friction, but in car if we exceed torque(force) than available traction we will create a wheel slip

Hi R Power-
The tractive force is basically just the drawbar force a vehicle (i.e., tractor) can apply to a drawbar. I used the approx static friction of a tire on dry concrete (~0.8) to estimate 0.8 mg so that a 1000-Kg vehicle with all-wheel drive would have a drawbar pull of 7848 Newtons. A 2WD vehicle would have ~1/2 of this. Beyond this the tires would start slipping.
Bob S

 

[R Power]

tractive force= u mg
then how to calculate rolling friction?
And is static firction the tractive force or max traction avaialable?

 

[Bob S]

tractive force= u mg
then how to calculate rolling friction?
And is static firction the tractive force or max traction avaialable?

The tractive force is the force on a drawbar =~0.8 mg, limited by the static friction of the tires on dry concrete. The force to push a vehicle to overcome tire rolling resistance (hysteresis of flexion) is about 0.01 mg, where mg is the weight of the vehicle.
Bob S

 

[R Power]

bob
can u tell me direction of tractive force and rolling friction in comparison to wheel rotation?

 

[R Power]

but in the book i read "Fundamentals of vehicle dynamics by Thomas D Gellispie"
directions are just opposite to what u showed above...

 

[sganesh88]

xxChrisxx
You are wrong. Tractive force acts in the forward direction. Its an external force from the ground to the car. Could be either kinetic friction-during slipping or static friction during pure rolling. Rolling resistance acts in the backward direction.

 

[sganesh88]

Rolling resistance occurs due to a shift of the resultant normal force in the forward direction. There is an offset between the vertical from the COM of the wheel to the ground and the effective point of application of normal force. So it applies a resistive moment to the wheel's rotation-the kind we do by pressing the brakes. Now static friction or kinetic friction acts opposite to the vehicle motion decelerating it so as to make v=r*w again. Similar to what happens when the wheel's rotation is decelerated by pressing the brakes.
So if the wheel is moving through a frictionless surface, rolling resistance cannot decelerate its total motion. It can only decelerate the wheel's rotation.

 

[sganesh88]

Generally we talk about forces which act on the vehicle. And if what you have drawn above is the free body diagram of the wheel, you need to represent the forces acting onthe wheel. Not what the wheel exerts on the ground. right?

 

[sganesh88]

I was doing this because he thought that friction was the reaction force to traction. When in fact it's a completely separate force pair.

He's right. The reaction is traction whose nature is friction.
http://en.wikipedia.org/wiki/Traction_(engineering)

Except I didnt really know how to show the RR, which is why I stated that it's best just to think of it as a moment action to stop the rolling movement.

He understands that RR is a moment. But what he didn't get is the force which resulted in the moment. He thought it was friction which decelerates the wheel. If that's the case, it will act along the direction of motion and hence cannot decelerate the vehicle as a whole. Its actually the effective normal force that contributes to the rolling resistance moment..

 

[sganesh88]

I thought it would help him realize that although the tyre is pushing backwards, the car moves forwards due to a reaction.

Reaction is friction. If the surface is frictionless, there is no action or reaction. The wheel and surface will mind their own businesses without interfering.

 

[xxChrisxx]

Reaction is friction. If the surface is frictionless, there is no action or reaction. The wheel and surface will mind their own businesses without interfering.

You know... that is a bloody good point. It seems I've managed to confuse myself more than anything, i'll clean up the crap posts and come back when I've acutally thought this through.

It's embarassing really as I know that, but said something blatantly silly regardless. Apologies to you R Power, as I've confused the situation needlessly when you understood in the first place.

 

[R Power]

S ganesh now i know i was thinking right. xxChrisxx totally confused me as he got himself confused...
So i thought right of tractive force and it's direction.
And i also understand the torque due to displacement of normal reaction due to deformation of tyre at front opposses rolling but then...i still have a confusion:

I think friction or tractive force reaction which acts in the forward direction as this: This friction helps in rolling of the vehicle. This friction doesn't help in rotating tyre because it doesn't give any torque in direction of rotation( opp to in case of a wheel rolling down a hill where friction acts backwards)... so as long as we provide torque to tyre from engine this friction helps in rolling and as we stop the engine ( no torque) the same friction helps in stopping......!
Remember and note carefully i said: This friction doesn't help in rotating but rolling totally different than the case:- in which we give force to a sphere, it slides a bit and then friction creates a clockwise torque and makes it rotate and due to forward linear motion component rotation becomes rolling.
But in case of a car we don't apply any force to wheel but a torque so no friction gives a clockwise torque for rotation but the friction makes the wheel roll ( i think u can imagine how).
So this was what i thought and imagined.....
Am i wrong ?

 

[sganesh88]

i still have a confusion:

The next paragraph doesn't say what your confusion is.

 

[xxChrisxx]

The reason I get comfused tis the way you state your questions R Power. It's very difficult to read, and then comprehend what you are acutally trying to say. Which meakes it moe difficult to gauge what you actually know.

I'm going to have to deal with this slowly by braking down your post into manegable peices as I don't want to make a mistake again.

You say:

But in case of a car we don't apply any force to wheel but a torque so no friction gives a clockwise torque for rotation but the friction makes the wheel roll ( i think u can imagine how).

Do you that forces and moments are connected? When you apply a torque to a wheel about it's centre of rotation, you are acutally applying a force at the contact patch.

Also why do you seem to be making a distinction between rolling and rotation.

 

[R Power]

tell me whatever i wrote in next para, is it all correct?