Understanding e-bike power/torque
In the UK e-bikes are currently limited to a maximum of 250W assistance if you wish to use them on public roads. However, it seems that the motors do vary in terms of the available torque - from around 40Nm on some of the lighter e-bikes to 80Nm on those that are described as more "powerful".
Now clearly they are not more powerful as they putting out the same 250W max, but what I don't quite understand is what difference that extra torque makes.
I'm used to measuring my power output in watts and thought there was a linear relationship between power and speed, so I don't quite understand where torque comes into it, and what difference this makes in practise.
electric-bike power
asked Feb 14 at 14:29
John MJohn M
543213
add a comment |
In the UK e-bikes are currently limited to a maximum of 250W assistance if you wish to use them on public roads. However, it seems that the motors do vary in terms of the available torque - from around 40Nm on some of the lighter e-bikes to 80Nm on those that are described as more "powerful".
Now clearly they are not more powerful as they putting out the same 250W max, but what I don't quite understand is what difference that extra torque makes.
I'm used to measuring my power output in watts and thought there was a linear relationship between power and speed, so I don't quite understand where torque comes into it, and what difference this makes in practise.
electric-bike power
asked Feb 14 at 14:29
John MJohn M
543213
2
The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31
add a comment |
In the UK e-bikes are currently limited to a maximum of 250W assistance if you wish to use them on public roads. However, it seems that the motors do vary in terms of the available torque - from around 40Nm on some of the lighter e-bikes to 80Nm on those that are described as more "powerful".
Now clearly they are not more powerful as they putting out the same 250W max, but what I don't quite understand is what difference that extra torque makes.
I'm used to measuring my power output in watts and thought there was a linear relationship between power and speed, so I don't quite understand where torque comes into it, and what difference this makes in practise.
electric-bike power
asked Feb 14 at 14:29
John MJohn M
543213
In the UK e-bikes are currently limited to a maximum of 250W assistance if you wish to use them on public roads. However, it seems that the motors do vary in terms of the available torque - from around 40Nm on some of the lighter e-bikes to 80Nm on those that are described as more "powerful".
Now clearly they are not more powerful as they putting out the same 250W max, but what I don't quite understand is what difference that extra torque makes.
I'm used to measuring my power output in watts and thought there was a linear relationship between power and speed, so I don't quite understand where torque comes into it, and what difference this makes in practise.
electric-bike power
electric-bike power
asked Feb 14 at 14:29
John MJohn M
543213
asked Feb 14 at 14:29
John MJohn M
543213
asked Feb 14 at 14:29
John MJohn M
543213
asked Feb 14 at 14:29
John MJohn M
543213
asked Feb 14 at 14:29
John MJohn M
543213
543213
2
The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31
add a comment |
2
The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31
2
2
The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31
add a comment |
2 Answers
2
active
oldest
votes
Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.
Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.
A bit more:
There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.
Re:
[I] thought there was a linear relationship between power and speed.
The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.
If you want to play with the numbers there is an interactive calculator here.
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
add a comment |
Let's assume you ride an electric bike with wheel motor.
- You find the perfect grade on which you climb at 62.5W and 20Nm
torque.
- The grade doubles. You continue your climb at the same speed,
with 125W and 40Nm torque.
- The grade doubles again. You continue your climb with 125W and 40Nm torque at half speed on the "less powerful" bike.
- The grade doubles again. You continue your climb with 62.5W and 40Nm torque at a quarter speed on the "less powerful" bike.
Now, riding on the "more powerful bike" (250W with 80Nm torque), in the third case you will continue climbing at 250W and 80Nm torque at normal speed, and would need another doubling of the grade to go to 125W and 80Nm at half speed.
A motor with higher torque (all else being equal) will give you more power at low motor rpm, even if it is limited at the same power at higher rpm.
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
add a comment |
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2 Answers
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2 Answers
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Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.
Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.
A bit more:
There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.
Re:
[I] thought there was a linear relationship between power and speed.
The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.
If you want to play with the numbers there is an interactive calculator here.
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
add a comment |
Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.
Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.
A bit more:
There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.
Re:
[I] thought there was a linear relationship between power and speed.
The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.
If you want to play with the numbers there is an interactive calculator here.
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
add a comment |
Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.
Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.
A bit more:
There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.
Re:
[I] thought there was a linear relationship between power and speed.
The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.
If you want to play with the numbers there is an interactive calculator here.
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
Very quick answer - the torque numbers that are quoted are maximum torque values which do not correspond to maximum power.
Power = torque × rotational speed, so, at slow speeds the motor unit can provide more torque (and hence more acceleration) while staying within the power limit.
A bit more:
There a decent page here on the characteristics of DC motors. The essential takeaway is that DC motors can provide less torque the faster they spin.
Re:
[I] thought there was a linear relationship between power and speed.
The power at constant velocity is that needed to balance the forces resisting forward motion - rolling resistance and aerodynamic drag for a bicycle or other vehicle. Those resistance forces generally increase with the cube of velocity.
If you want to play with the numbers there is an interactive calculator here.
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
edited Feb 14 at 20:27
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
answered Feb 14 at 15:00
Argenti ApparatusArgenti Apparatus
36k23891
36k23891
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
add a comment |
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
Thanks - I should have paid more attention in physics class :-D
– John M
Feb 14 at 19:19
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
@JohnM YouTube can help you catch up. Power is work over time. youtube.com/watch?v=u-MH4sf5xkY
– Yolo Perdiem
Feb 15 at 4:13
add a comment |
Let's assume you ride an electric bike with wheel motor.
- You find the perfect grade on which you climb at 62.5W and 20Nm
torque.
- The grade doubles. You continue your climb at the same speed,
with 125W and 40Nm torque.
- The grade doubles again. You continue your climb with 125W and 40Nm torque at half speed on the "less powerful" bike.
- The grade doubles again. You continue your climb with 62.5W and 40Nm torque at a quarter speed on the "less powerful" bike.
Now, riding on the "more powerful bike" (250W with 80Nm torque), in the third case you will continue climbing at 250W and 80Nm torque at normal speed, and would need another doubling of the grade to go to 125W and 80Nm at half speed.
A motor with higher torque (all else being equal) will give you more power at low motor rpm, even if it is limited at the same power at higher rpm.
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
add a comment |
Let's assume you ride an electric bike with wheel motor.
- You find the perfect grade on which you climb at 62.5W and 20Nm
torque.
- The grade doubles. You continue your climb at the same speed,
with 125W and 40Nm torque.
- The grade doubles again. You continue your climb with 125W and 40Nm torque at half speed on the "less powerful" bike.
- The grade doubles again. You continue your climb with 62.5W and 40Nm torque at a quarter speed on the "less powerful" bike.
Now, riding on the "more powerful bike" (250W with 80Nm torque), in the third case you will continue climbing at 250W and 80Nm torque at normal speed, and would need another doubling of the grade to go to 125W and 80Nm at half speed.
A motor with higher torque (all else being equal) will give you more power at low motor rpm, even if it is limited at the same power at higher rpm.
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
add a comment |
Let's assume you ride an electric bike with wheel motor.
- You find the perfect grade on which you climb at 62.5W and 20Nm
torque.
- The grade doubles. You continue your climb at the same speed,
with 125W and 40Nm torque.
- The grade doubles again. You continue your climb with 125W and 40Nm torque at half speed on the "less powerful" bike.
- The grade doubles again. You continue your climb with 62.5W and 40Nm torque at a quarter speed on the "less powerful" bike.
Now, riding on the "more powerful bike" (250W with 80Nm torque), in the third case you will continue climbing at 250W and 80Nm torque at normal speed, and would need another doubling of the grade to go to 125W and 80Nm at half speed.
A motor with higher torque (all else being equal) will give you more power at low motor rpm, even if it is limited at the same power at higher rpm.
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
Let's assume you ride an electric bike with wheel motor.
- You find the perfect grade on which you climb at 62.5W and 20Nm
torque.
- The grade doubles. You continue your climb at the same speed,
with 125W and 40Nm torque.
- The grade doubles again. You continue your climb with 125W and 40Nm torque at half speed on the "less powerful" bike.
- The grade doubles again. You continue your climb with 62.5W and 40Nm torque at a quarter speed on the "less powerful" bike.
Now, riding on the "more powerful bike" (250W with 80Nm torque), in the third case you will continue climbing at 250W and 80Nm torque at normal speed, and would need another doubling of the grade to go to 125W and 80Nm at half speed.
A motor with higher torque (all else being equal) will give you more power at low motor rpm, even if it is limited at the same power at higher rpm.
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
answered Feb 15 at 9:07
Calin CeterasCalin Ceteras
1674
1674
add a comment |
add a comment |
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The relationship between power and speed isn't linear at all -- it's approximately cubic. To double speed requires about 8x as much power; to increase speed 25% from, say, 12mph to 15mph requires about a doubling of power. Torque tells you about acceleration, not top speed.
– R. Chung
Feb 14 at 15:37
Yep I shouldn't have said linear - what I meant was that speed would be determined by power output (and aerodynamics of course)
– John M
Feb 14 at 19:31