Would you pay £30 to save 1kg?

Nealh

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The 1.7kg G370 Bafang front hub is available at topbikekit last I looked they had over 200 in stock, It is rated 42Nm so comparable to a 80Nm mid drive.
 

Woosh

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Woosh are you going to sell a kit with the G370 Bafang front hub ?
yes, but with the situation of Corona virus, I can't say when.
 

Nealh

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Like the Aikema's the G370 also utilises a change to LHS cable exit.
 

mike killay

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The 1.7kg G370 Bafang front hub is available at topbikekit last I looked they had over 200 in stock, It is rated 42Nm so comparable to a 80Nm mid drive.
I have difficulty following this.
Why should a crank drive in bottom gear only produce half the Nm of a hub drive.
(Don't get me wrong, I have a hub drive and am slowly getting to prefer it to my crank drive bike.)
 

MikelBikel

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Jun 6, 2017
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"Overvolting", I thought you took a dim view of such practices Tony? ;)
"Rotor spins twice as fast", does this mean twice the noise & wear?:(

High rpm motors in 20in wheels are great for folders, don't mind extra odd kilo if it means bike will carry two, yes please!:D

Bafang leak details of dream motors, like the "3-speed motor for small wheel folders", but no show so far.
'Bird in the hand is worth two in the bush'.

If I could have a wheel each for 16in Brompton, a 20in Dahon, a 26in Marin and a 29in Raleigh, swapping the same 48-52v battery between them, I'd be v.happy. Cheers :cool:
 

Woosh

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"Overvolting", I thought you took a dim view of such practices Tony? ;)
yes. If you want a motor to be best at going up x% gradient at y kph, then just get one that is optimally built for that purpose. When you move one of the parameters, like applying a higher or lower voltage, you lose that optimal yield. That means the motor will shed more heat, become noisier and wear out much more quickly.

"Rotor spins twice as fast", does this mean twice the noise & wear?:(
that's correct in the same motor, but not quite if spinning twice the speed lets you reduce the rotor size by square root of 2.
The noise of the motor is proportional to the amount of power shed through air resistance and friction. For the same power, if you spin the motor twice as fast, you can effectively make the rotor 40% smaller and keep the same optimal yield (86% for popular motors) so the speed of the air between the rotor and stator remains little difference.
 
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BazP

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I have difficulty following this.
Why should a crank drive in bottom gear only produce half the Nm of a hub drive.
(Don't get me wrong, I have a hub drive and am slowly getting to prefer it to my crank drive bike.)
Like you I don’t quite understand the comparison but I’m sure someone will explain.
If the power ratings are at the final drive shaft then they are just working at opposite ends of the chain.
 

Woosh

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I have difficulty following this.
Why should a crank drive in bottom gear only produce half the Nm of a hub drive.
(Don't get me wrong, I have a hub drive and am slowly getting to prefer it to my crank drive bike.)
what is fairly constant is the maximum power delivered by the motor.
The torque is equal to power (in Watts) divided by the angular speed (in radians). The faster the wheel turns, the lower the torque.
If your bike has a crank motor, the power is entirely transmitted to the rear wheel by the chain.
For example, if you have 44T at the chainring and you are on 11T at the rear cog, the ratio front to rear is 44T/11T = 4. Your torque is going to be 4 times smaller at the rear wheel than at the cranks. If your motor delivers 100NM at the chainring (44T) and you are on the bottom gear (11T), then the torque at the wheel is 100NM * 11T/44T = 25NM.
Therefore, if you ride at 44/11 front to rear, you won't get as much help from the crank motor as from say an XF08C geared hub motor which gives up to 45NM max.
If you ride on the top gear (28T) with the same setup, the torque at the backwheel will be 100NM * 28T/44T=63NM, you will be able to climb steeper gradient with the crank motor than with an XF08C rear hub motor.
 

mike killay

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Feb 17, 2011
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what is fairly constant is the maximum power delivered by the motor.
The torque is equal to power (in Watts) divided by the angular speed (in radians). The faster the wheel turns, the lower the torque.
If your bike has a crank motor, the power is entirely transmitted to the rear wheel by the chain.
For example, if you have 44T at the chainring and you are on 11T at the rear cog, the ratio front to rear is 44T/11T = 4. Your torque is going to be 4 times smaller at the rear wheel than at the cranks. If your motor delivers 100NM at the chainring (44T) and you are on the bottom gear (11T), then the torque at the wheel is 100NM * 11T/44T = 25NM.
Therefore, if you ride at 44/11 front to rear, you won't get as much help from the crank motor as from say an XF08C geared hub motor which gives up to 45NM max.
If you ride on the top gear (28T) with the same setup, the torque at the backwheel will be 100NM * 28T/44T=63NM, you will be able to climb steeper gradient with the crank motor than with an XF08C rear hub motor.
Strange, I would have thought that the torque would be greater in a lower gear not less.
I am thinking in terms of a car, why change down gear on a hill?
 

Woosh

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Strange, I would have thought that the torque would be greater in a lower gear not less.
I am thinking in terms of a car, why change down gear on a hill?
top gear/bottom gear: sorry if I got the wires crossed.
with CD motor, torque is highest at minimum speed, when you ride on your largest rear cog.
with geared hub motor, torque is also at highest at minimum speed but independent from gear.
 

Nealh

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Aug 7, 2014
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I have difficulty following this.
Why should a crank drive in bottom gear only produce half the Nm of a hub drive.
(Don't get me wrong, I have a hub drive and am slowly getting to prefer it to my crank drive bike.)
I'M no engineer though believe some of the difference is down to final drive, hub is direct and mid drive is indirect as it has to go via the drive train.
 

BazP

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Oct 8, 2017
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what is fairly constant is the maximum power delivered by the motor.
The torque is equal to power (in Watts) divided by the angular speed (in radians). The faster the wheel turns, the lower the torque.
If your bike has a crank motor, the power is entirely transmitted to the rear wheel by the chain.
For example, if you have 44T at the chainring and you are on 11T at the rear cog, the ratio front to rear is 44T/11T = 4. Your torque is going to be 4 times smaller at the rear wheel than at the cranks. If your motor delivers 100NM at the chainring (44T) and you are on the bottom gear (11T), then the torque at the wheel is 100NM * 11T/44T = 25NM.
Therefore, if you ride at 44/11 front to rear, you won't get as much help from the crank motor as from say an XF08C geared hub motor which gives up to 45NM max.
If you ride on the top gear (28T) with the same setup, the torque at the backwheel will be 100NM * 28T/44T=63NM, you will be able to climb steeper gradient with the crank motor than with an XF08C rear hub motor.
Sorry but I can’t get my head around equating the above to my 15/11 - 48 Gearing on my Bosch motor.
But in the real world: I am in Spain at the moment and have just hired two Decathlon Rockriders for three days. These have 250W x 42NM hub motors and we have just done three 30 mile off road routes. My impression is that there is nowhere near as much help as with the Bosch gen 3 motor and I found the power delivery somewhat jerky, a boost within the first pedal revolution then a very noticeable step down. I couldn’t get used to this over the three days.
This is the first time I have ridden a hub motor but it has definitely put me off ever owning one. The bike I had was only a week old by the way.
The above are only my personal findings and in no way do I want to start another crank vs hub argument.
 

Woosh

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what controller or controllers do you have on the hub bikes?
 

Nealh

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With hubs it really is dependant on the rpm and also the current. A low rpm hub will climb so much better then a faster one, thus would provide superior torch and nm through the gears.
 

BazP

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what controller or controllers do you have on the hub bikes?
There was one controller, sorry don’t know the make. It only had three buttons, on/off, plus and minus. It had three modes 1,2 and 3. Number 3 seemed about equal to between Tour and Sport on the Bosch but a lot more jerky. Any slackening off of pedalling on the flat resulted in a sort of indecision by the motor as to whether it should be on or off.
We are hiring the same bikes again for two days next week ( they are the only models available) so could get more information but I assume that these are peculiar to Decathlon parts.
 

Woosh

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there are two things that make a ride bad: insufficient power range and poor controller.
The controller's Amps rating is the key to good performance: you need 36V 17A controllers to match Bosch Active Line and 36V 20A controllers to match the Bosch CX Line. On top of this, the pedal sensor needs to have at least 12 magnets, anything less, the bike will not be responsive, the controller also should be programmed to respond within 60 degrees of crank rotation (one sixth of a turn). The firmware needs also to operate in 'current control' mode. A lot of cheap £15 controllers don't meet those criteria.
 
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what is fairly constant is the maximum power delivered by the motor.
The torque is equal to power (in Watts) divided by the angular speed (in radians). The faster the wheel turns, the lower the torque.
If your bike has a crank motor, the power is entirely transmitted to the rear wheel by the chain.
For example, if you have 44T at the chainring and you are on 11T at the rear cog, the ratio front to rear is 44T/11T = 4. Your torque is going to be 4 times smaller at the rear wheel than at the cranks. If your motor delivers 100NM at the chainring (44T) and you are on the bottom gear (11T), then the torque at the wheel is 100NM * 11T/44T = 25NM.
Therefore, if you ride at 44/11 front to rear, you won't get as much help from the crank motor as from say an XF08C geared hub motor which gives up to 45NM max.
If you ride on the top gear (28T) with the same setup, the torque at the backwheel will be 100NM * 28T/44T=63NM, you will be able to climb steeper gradient with the crank motor than with an XF08C rear hub motor.
That's very interesting Woosh, I don't wish to start yet another crank vs hub motor debate but for my clarification would you agree with these (huge) simplifications using your examples:

A typical 250W crank drive motor will provide between 25NM and 63NM at the back wheel depending on the gear selected.
A typical 250W geared hub motor will provide around 45NM at the driven wheel, no user selectable gears are available.

The rotors of the motors can provide similar power albeit at possibly different RPMs.

Since the hub motor has fixed gearing this has to be a compromise between maximum assisted speed vs maximum torque available at low speeds for steep gradients.
 
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Nealh

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There was one controller, sorry don’t know the make. It only had three buttons, on/off, plus and minus. It had three modes 1,2 and 3. Number 3 seemed about equal to between Tour and Sport on the Bosch but a lot more jerky. Any slackening off of pedalling on the flat resulted in a sort of indecision by the motor as to whether it should be on or off.
We are hiring the same bikes again for two days next week ( they are the only models available) so could get more information but I assume that these are peculiar to Decathlon parts.
That's not a controller it is handle bar display and can be LED or LCD.
A controller is a pcb/etc in a small ali box or integrated into the motor or battery that regulates the current asked for.
 

MikelBikel

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Jun 6, 2017
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yes. If you want a motor to be best at going up x% gradient at y kph, then just get one that is optimally built for that purpose. When you move one of the parameters, like applying a higher or lower voltage, you lose that optimal yield. That means the motor will shed more heat, become noisier and wear out much more quickly.
..
The noise of the motor is proportional to the amount of power shed through air resistance and friction. For the same power, if you spin the motor twice as fast, you can effectively make the rotor 40% smaller and keep the same optimal yield (86% for popular motors) so the speed of the air between the rotor and stator remains little difference.
Thanks for clearing them up.
I thought hub gears might benefit from the flexibility of 2 speeds to cope with hills and still be as efficient on level ground, but yes, with limited power, every gradient needs the right gearing. CVT hub motor anyone? Oops, more losses ! ;-)
And I thought the motor noise was from the gear whine, and the windings vibrating under strain of electromagnetically induced torque. I'd forgot about the air swirling inside, oops.
Back in my box now. :cool:
 
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