Starting torque of a motorcycle and rider

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.

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?

russ_watters
Mentor
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'.

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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.

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?

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?

i'm also interested in knowing the friction force on the rear wheel.

It involves a rope and a fish scale.

What do you mean by 'friction force'?

It involves a rope and a fish scale.

What do you mean by 'friction force'?
the friction which will oppose the motion of wheel

μMg

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".

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 dont know the fish and rope method

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.)?

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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By your calculations it will take a 1852.2 N force to get the bike moving. Does this seem reasonable to you?

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?

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?

jack action
Gold Member
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.

Spinnor