1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Difference between rolling resistance and tractive force

  1. Nov 1, 2009 #1
    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 wanna 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...........
  2. jcsd
  3. Nov 1, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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.
  4. Nov 1, 2009 #3
    Can u tell me the difference in their directions if i am wrong..............
  5. Nov 2, 2009 #4
    Rolling resistance is a tricky thing, becuase 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 wouldnt 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.
  6. Nov 2, 2009 #5


    User Avatar
    Homework Helper

    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.
  7. Nov 2, 2009 #6
    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
    Last edited: Nov 2, 2009
  8. Nov 2, 2009 #7
    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
  9. Nov 3, 2009 #8
    Last edited by a moderator: Apr 24, 2017
  10. Nov 3, 2009 #9


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    And hysteresis involves permanent deformation on the microscopic level.
  11. Nov 3, 2009 #10
    guys plz tell me difference bw rolling resistance and tractive force & not how it occurs....!
    also tell me the difference in direction of the 2
  12. Nov 3, 2009 #11
    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.
  13. Nov 3, 2009 #12
    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
  14. Nov 3, 2009 #13
    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.
  15. Nov 3, 2009 #14
    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????
  16. Nov 3, 2009 #15
    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 seperate force pair. Thats why you can have less friction than traction or te other way round.
  17. Nov 3, 2009 #16
    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
  18. Nov 4, 2009 #17
    tractive force= u mg
    then how to calculate rolling friction?
    And is static firction the tractive force or max traction avaialable?
  19. Nov 4, 2009 #18
    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
  20. Nov 5, 2009 #19
    can u tell me direction of tractive force and rolling friction in comparison to wheel rotation?
  21. Nov 5, 2009 #20
    but in the book i read "Fundamentals of vehicle dynamics by Thomas D Gellispie"
    directions are just opposite to what u showed above......
  22. Nov 5, 2009 #21
    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.
  23. Nov 5, 2009 #22
    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.
  24. Nov 5, 2009 #23
    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?
  25. Nov 5, 2009 #24
    He's right. The reaction is traction whose nature is friction.
    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 thats 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..
  26. Nov 5, 2009 #25
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook