Starting torque of a motorcycle and rider

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    Motorcycle Torque
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Discussion Overview

The discussion revolves around calculating the starting torque required for a motorcycle with a specified weight and load, focusing on the concepts of static rolling resistance and friction. Participants explore the definitions and implications of starting torque in the context of a motorcycle powered by a 4kW motor.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to determine the starting torque needed to move a motorcycle weighing 120 kg with an additional load of 150 kg.
  • Another participant questions the definition of 'starting torque', asking whether it refers to initiating movement or overcoming slipping on a surface.
  • Some participants suggest that the concept of 'friction' should be clarified, proposing that 'rolling resistance' is more appropriate in this context.
  • There is mention of using experiential knowledge, such as pushing a car, to estimate the forces involved in starting the motorcycle.
  • One participant expresses confusion about the friction force that the motor must overcome to rotate the rear wheel.
  • Another participant introduces a method involving a rope and a fish scale to measure the force needed to start the motorcycle moving.
  • Concerns are raised about the coefficients of friction and rolling resistance used in calculations, questioning their applicability to the situation.
  • There is a discussion about the relationship between torque, rolling resistance, and acceleration, emphasizing that torque must first overcome rolling resistance before accelerating the vehicle.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definitions and calculations related to starting torque, rolling resistance, and friction. Multiple competing views and methods for determining the necessary torque remain unresolved.

Contextual Notes

Participants express uncertainty regarding the definitions of friction and rolling resistance, as well as the appropriate coefficients to use in calculations. There is also a lack of clarity on the practical application of theoretical concepts in this specific scenario.

Who May Find This Useful

This discussion may be of interest to individuals involved in motorcycle mechanics, physics students exploring concepts of torque and resistance, and engineers working on vehicle dynamics.

mec_divi
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hey everyone I'm interested in finding the starting torque of a bike weighing 120 kg with a load of 150 kg and having the tire specification of 120/70 R17.
starting torque means the amount of torque i need to provide to wheel in order to start moving the bike from rest. power source is a 4kw motor.
 
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First define 'starting torque'. Do you mean to just start it rolling, or to start it spinning (slipping) while on a surface with a 270 kg load applied? Where is the torque to be applied...at the wheel or on the pedals?
 
mec_divi said:
hey everyone I'm interested in finding the starting torque of a bike weighing 120 kg with a load of 150 kg and having the tire specification of 120/70 R17.
starting torque means the amount of torque i need to provide to wheel in order to start moving the bike from rest. power source is a 4kw motor.
This is a pretty difficult thing to calculate from scratch, but you can estimate it based on experience. For example, have you ever pushed a car? Can you guesstimate how much force it took to break the friction? A bike weighs 1/5 as much.

Maybe a better question is why you think "starting torque" matters. What are you trying to use it for?
 
russ:
Be careful with the term 'friction' in this case. I think what you are talking about is 'rolling resistance'.
 
Last edited:
thanks everyone for replying...
my problem is...my bike is standing with a load mentioned now i want my bike to move on the road,so i think static resistance will come into play.i'm confused about how much friction force does the motor have to overcome when it will rotate the rear wheel.
 
russ_watters said:
This is a pretty difficult thing to calculate from scratch, but you can estimate it based on experience. For example, have you ever pushed a car? Can you guesstimate how much force it took to break the friction? A bike weighs 1/5 as much.

Maybe a better question is why you think "starting torque" matters. What are you trying to use it for?
im asking for starting torque because it want to know the reduction ratio i have to provide coz my motor is providing me a peak torque of 20 nm
 
mec:
I think what you are looking for is: The amount of torque needed to overcome the 'static rolling resistance' of your motor bike loaded as described above. There is an easy experimental method to make this determination.
Do you know what it is?
 
AZFIREBALL said:
mec:
I think what you are looking for is: The amount of torque needed to overcome the 'static rolling resistance' of your motor bike loaded as described above. There is an easy experimental method to make this determination.
Do you know what it is?
no please tell
 
mec_divi said:
no please tell
i'm also interested in knowing the friction force on the rear wheel.
 
  • #10
It involves a rope and a fish scale.

What do you mean by 'friction force'?
 
  • #11
AZFIREBALL said:
It involves a rope and a fish scale.

What do you mean by 'friction force'?
the friction which will oppose the motion of wheel
 
  • #12
μMg
 
  • #13
That which opposes the movement of a static wheel is 'rolling resistance'...not friction.
The term 'friction' should only be applied to the condition when one surface is SLIDING against another or is resisting a force applied at the surface between two objects. It is true that it is friction that allows you to apply the necessary torque to the wheel to overcome 'static rolling resistance' but, it is not friction that is causing the need for a force to be applied. That is static rolling resistance.

Do you know how to measure static rolling resistance using a rope and a fish scale?
I do not understand "μMg".
 
  • #14
AZFIREBALL said:
That which opposes the movement of a static wheel is 'rolling resistance'...not friction.
The term 'friction' should only be applied to the condition when one surface is SLIDING against another or is resisting a force applied at the surface between two objects. It is true that it is friction that allows you to apply the necessary torque to the wheel to overcome 'static rolling resistance' but, it is not friction that is causing the need for a force to be applied. That is static rolling resistance.

Do you know how to measure static rolling resistance using a rope and a fish scale?
I do not understand "μMg".
no i don't know the fish and rope method
 
  • #15
Can you visualize how it is possible to find the force needed to start the loaded motor bike moving from a dead stop, using a rope and a fish scale (a device used to measure force.)?
 
  • #16
AZFIREBALL said:
Can you visualize how it is possible to find the force needed to start the loaded motor bike moving from a dead stop, using a rope and a fish scale (a device used to measure force.)?
am i doing it right?
 

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  • #17
By your calculations it will take a 1852.2 N force to get the bike moving. Does this seem reasonable to you?
 
  • #18
AZFIREBALL said:
By your calculations it will take a 1852.2 N force to get the bike moving. Does this seem reasonable to you?
that's what I'm asking what am i doing wrong?
 
  • #19
First- Where did you get the values for the Coefficient of Friction and the Coefficient of Rolling Resistance you used in your calculations and why do you think they apply to this situation?

Second- Are you trying to apply the famous equation F=ma to solve this situation and why do you feel it applies here.

Thirdly- What do you understand that the stated Coefficients mean, in very practical terms.

Fourth- Are you allowing the need for a purely mathematical solution to cloud your perception of the practical approach for solving the problem?

Fifth- Have you drawn an accurate force diagram of the problem at hand?

Sixth: Have you considered all the possible mitigating factors that might effect the solution’s final values?

Seventh- Do you feel you understand the difference between friction and rolling forces?
 
  • #20
What you did in your calculations is not «the amount of torque you need to provide to the wheel in order to start moving the bike from rest», but it is what you are looking for.

The torque you will provide to the wheel will, first, fight the rolling resistance. If the torque input exceeds that, then it will start to accelerate the vehicle. How fast it will accelerate is proportional to the amount of torque in excess. First question on your mind would be, how fast do I want to accelerate?

The answer to this question is usually: As fast as I can.

What will limit your acceleration is the maximum friction force available between the tire and ground; Which is what you calculated. Above that threshold, you will accelerate the wheel (spinning), but not the vehicle.

The only mistake you made in your calculations is that you took the total mass of the bike, but only the rear wheel is powered, so only the portion resting on that wheel should be considered as the normal force.
 
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