Difference between rolling resistance and tractive force

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Discussion Overview

The discussion revolves around the concepts of tractive force and rolling resistance in the context of car wheels. Participants explore the definitions, differences, and directional aspects of these forces, touching on theoretical and practical implications in automotive dynamics.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants describe tractive force as the maximum force the ground can provide, dependent on the weight of the vehicle and the friction coefficient.
  • Others argue that rolling resistance is not a single force but a combination of factors that impede the rolling of a wheel, including deformation and hysteresis.
  • There is a suggestion that rolling resistance helps a wheel roll, leading to a debate about terminology, with some preferring "rotating resistance."
  • Participants express differing views on the direction of tractive force and rolling resistance, with some stating that tractive force acts opposite to vehicle motion while rolling resistance acts opposite to wheel rotation.
  • Some clarify that static friction is involved in tractive force, while others question how this relates to the forces required to initiate motion without slipping.
  • There are references to specific coefficients for rolling resistance and static friction, with calculations provided for context.

Areas of Agreement / Disagreement

Participants do not reach consensus on the definitions and directional relationships between tractive force and rolling resistance. Multiple competing views remain, particularly regarding the nature of these forces and their interactions.

Contextual Notes

Some participants highlight the complexity of rolling resistance, noting that it involves multiple factors and may not be easily defined. There are also references to specific texts that may present differing interpretations of the forces involved.

Who May Find This Useful

This discussion may be of interest to those studying automotive dynamics, physics of motion, or engineering principles related to vehicle performance.

  • #31
xxchrisxx
Now here u got wrong!
By applying force i meant apllying force through COM as if to push the wheel whereas applying torque is a different thing. When we aplly torque to wheel force is applied a distance r (radius) apart form COM.
Consider 2 cases:
1. in first case u push a wheel in a line of COM.
2. in second case u give the wheel a torque on it's top point.
Now configure the direction of friction in both cases and you'll know what i wanted to say and how in first case friction gives rotation and the linear push u provided makes that rotation a roll and in 2nd case friction provides no rotation but still gives a roll.
First of all think about this what i have wrote.
Then i will tell you how in case2 a roll is generated without rotational torque by friction.
 
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  • #32
sganesh88 said:
The next paragraph doesn't say what your confusion is. :confused:
My confusion is that maybe u and many books say that wheel stops due to displacement of normal reaction as result of tyre deformation at the front where it is pushing the ground and this is rolling resistance but i say that the friction which is in forward direction as a result of tractive effort (which helped in rolling) also helps to stop the wheel when we stop applying torque. So i want to ask am i correct?
 
  • #33
R Power said:
xxchrisxx
Now here u got wrong!
By applying force i meant apllying force through COM as if to push the wheel whereas applying torque is a different thing. When we aplly torque to wheel force is applied a distance r (radius) apart form COM.
Consider 2 cases:
1. in first case u push a wheel in a line of COM.
2. in second case u give the wheel a torque on it's top point.
Now configure the direction of friction in both cases and you'll know what i wanted to say and how in first case friction gives rotation and the linear push u provided makes that rotation a roll and in 2nd case friction provides no rotation but still gives a roll.
First of all think about this what i have wrote.
Then i will tell you how in case2 a roll is generated without rotational torque by friction.

That's clarified that, I'm just working through the points one by one to clarify what you know and what your acutal confusion is.

What do you mean by rotation and roll?

Are you making a distinction from it just spinning where it is (spinning and not moving like it were off the ground) and roll as in rotating but moving forward?
 
Last edited:
  • #34
yeah u can say that spinning as if we were off the ground.
In first case friction provides spinning and your linear push and spin provided by friction produces a roll and in case 2 friction provides no spin but still a roll is generated.

It was just simple what i asked in the beginning but u made it this thread too long due to misconceptions(e.g action reaction and friction forces are different force pairs) and u also gave wrong directions of tractive force and RR. Please don't mind.

Let me make clear what i want to ask as simply:
1.Does friction due to tractive force helps in stopping the wheel apart from RR?
2.Why direction of RR is in backward direction? Since it generates an anti spin torque due to displacement of normal reaction as a result of tyre deformation so direction of RR should be forward same as that of tractive force.
3.How to calculate it. I mean friction due to tractive force= u N
then what is RR=?
 
  • #35
The answer you are looking for was more simple than I thought. I'm sorry about confusing things, I should have clarified what you meant before answering incorrectly.

I'll leave sganesh88 to answer your questions, as he has done a much better job than me in this thread and will give you better answers.
 
  • #36
R Power said:
It was just simple what i asked in the beginning but u made it this thread too long due to misconceptions(e.g action reaction and friction forces are different force pairs) and u also gave wrong directions of tractive force and RR. Please don't mind.
Cool. People can misunderstand your question. Happens all the time. And what if the thread gets too long? The end result is we need to understand the concept clearly.

1.Does friction due to tractive force helps in stopping the wheel apart from RR?
Tractive force's nature is frictional. Generally we refer Tractive force as the one that makes the vehicle accelerate. (though the same friction helps the vehicle to decelerate too. Its all semantics) Even if the engine stops giving power to the wheels, its the combined effect of internal friction, air resistance, gradient resistance and rolling resistance at the wheels that will bring the vehicle to rest. Not the tractive force.
2.Why direction of RR is in backward direction? Since it generates an anti spin torque due to displacement of normal reaction as a result of tyre deformation so direction of RR should be forward same as that of tractive force.
Torque decelerating the tire's rotation needn't always be due to a force in the forward direction. It can also be in the vertical direction. Its the normal force that contributes to the resistive moment. Thats why i said in one of my posts above that in a frictionless surface, rolling resistance moment cannot reduce the velocity of the vehicle *much* as the horizontal component of the normal force is negligibly less. Rolling resistance involves loss of energy due to hysteresis but deceleration of the vehicle due to friction. Just like kinetic friction at the brake pads produces an energy loss at the wheels and static friction brings about deceleration of the vehicle during normal braking.

3.How to calculate it. I mean friction due to tractive force= u N
then what is RR=?
refer:
http://en.wikipedia.org/wiki/Rolling_resistance#Physical_formula
 
  • #37
1. clear o.k.
2. Unclear
Imagine wheel rotates in clockwise direction
U say that RR is due to normal component basically due displacement of normal component. So, normal component is displaced a bit forward(in the direction of motion), so as to provide an anti spin torque and also because tyre deforms at the front where it first pushes the ground. So this torque will be in upward direction (as displaced normal force) which will be opposite to wheel rotation, then how can it's direction be backward which is clockwise (i.e direction of wheel rotation).
3. Clear

BTW u r from India i guess! Me too!
 
  • #38
So this torque will be in upward direction (as displaced normal force) which will be opposite to wheel rotation, then how can it's direction be backward which is clockwise (i.e direction of wheel rotation).
Answer this question. When we apply brakes, what is the flow of processes that end up in decelerating the vehicle?
Ya i am from India. :)
 
  • #39
When we apply brakes friction is produced between brake shoe surface and tyre which gives an anti spin torque to decelerate.
All the kinetic energy of wheel gets converted into heat energy.
 

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