Hub or Mid Drive? Any views?

[Deleted member 25121]

We have ridden both up a 15% incline on the North Downs and they are similar, the centre drive has the edge when both bikes are on the 11 tooth sprocket but certainly nothing like 50% better?

I would expect the centre drive to have the edge when used with large sprockets, not small ones. Did you do that comparison when cycling uphill with a large sprocket?

 

[vfr400]

I will dig up the paperwork but my information came from The Light Electrical Vehicle Association Europe. (LEVA EU)

I've just been through some guidelines for approval that give clear guidance on all the requirements and standards. I can't find anything in there that says anything about batteries. I did find that the low voltage directive applies to anything over 50v AC and 75V DC.

Normal Li-ion 48v batteries are 13S, so 54.6v fully charged. The Continental crank-drive system is/was 48V and got approved OK. Also the KTM Panasonic hub-motor was 48v (13S), even though they called it 46v or something like that.

 

[Woosh]

For cycling in hilly areas I'm referring to the torque available to assist the rider in cycling up hill in a low gear with reasonable speed, comfort and cadence.

you see and explain clearly why choosing a motor with good torque.

There is no agreed standard torque measurement for advertisements on rear hub motors as the maximum torque will increase when the speed drops until close to stalling point, it drops exponentially until it stalls, like the proverbial brick.

as I understand, the hub version uses a Shengyi motor. I use some of their models myself, DGW07 on my Camino and DGW22C (C=with cassette fitting) in hub kits. Shengyi will make motors to order, so you won't find the maximum torque on their website.

A lot of users do not know the formula connecting motor yield to RPM, battery voltage and max Amps to power and power to torque, so they don't see clearly the interaction between these elements. For a detailed understanding, you need to get down to formulae.

Starting with power:

Power = 36V (or 48V) * 17A (typical on e-bikes) * motor yield (conversion ratio output power/input power). Motor yield (0%-87%) varies with the wheel's rotational speed or RPM. The motor yield curve is shaped like a wave, best only in the middle, zero at both end.

Power to torque:
power = torque (in NM) * rotational speed (in radians/second)
So for the same power, less RPM = more torque.

Your first job is to define what is your minimal acceptable yield. Too low, most of your input power is turned into heat harming your kit. Let's say 50%, half into useful work, the other half shed as heat.
Look up the test charts that the motor manufacturers give you for the minimum RPM. For 50%, you have typically 100RPM (or 8mph on 26" tyres) for the DGW07 and 80RPM (6mph on same tyres) for the larger DGW22. So, if you have steep hills, you will need the larger DGW22.

For practical evaluation, let's say that we use a 17A controler at 48V at 50% yield.
Power = 48V * 17A * 0.5 = 408W

If you own a DGW07, your torque will be at 8mph/100RPM
T = 408W /(2*pi * 100/60 rad/s) = 39NM

If you own a DGW22:
T = 408W / (2 * pi * 80/60 rad/s) = 49NM.

Shengyi will make custom motors, but the motor dimensions stay the same so the calculations are similar. Google Shengyi motors for other characteristics.

 

[Deleted member 25121]

you see and explain clearly why choosing a motor with good torque.

There is no agreed standard torque measurement for advertisements on rear hub motors as the maximum torque will increase when the speed drops until close to stalling point, it drops exponentially until it stalls, like the proverbial brick.

as I understand, the hub version uses a Shengyi motor. I use some of their models myself, DGW07 on my Camino and DGW22C (C=with cassette fitting) in hub kits. Shengyi will make motors to order, so you won't find the maximum torque on their website.

A lot of users do not know the formula connecting motor yield to RPM, battery voltage and max Amps to power and power to torque, so they don't see clearly the interaction between these elements. For a detailed understanding, you need to get down to formulae.

Starting with power:

Power = 36V (or 48V) * 17A (typical on e-bikes) * motor yield (conversion ratio output power/input power). Motor yield (0%-87%) varies with the wheel's rotational speed or RPM. The motor yield curve is shaped like a wave, best only in the middle, zero at both end.

Power to torque:
power = torque (in NM) * rotational speed (in radians/second)
So for the same power, less RPM = more torque.

Your first job is to define what is your minimal acceptable yield. Too low, most of your input power is turned into heat harming your kit. Let's say 50%, half into useful work, the other half shed as heat.
Look up the test charts that the motor manufacturers give you for the minimum RPM. For 50%, you have typically 100RPM (or 8mph on 26" tyres) for the DGW07 and 80RPM (6mph on same tyres) for the larger DGW22. So, if you have steep hills, you will need the larger DGW22.

For practical evaluation, let's say that we use a 17A controler at 48V at 50% yield.
Power = 48V * 17A * 0.5 = 408W

If you own a DGW07, your torque will be at 8mph/100RPM
T = 408W /(2*pi * 100/60 rad/s) = 39NM

If you own a DGW22:
T = 408W / (2 * pi * 80/60 rad/s) = 49NM.

Shengyi will make custom motors, but the motor dimensions stay the same so the calculations are similar. Google Shengyi motors for other characteristics.

Don't those issues also apply to hub motors?

I think I may have missed your point, I'm interested in the torque available on the wheel using a crank motor.

 

[peter.c]

With a mid drive the less teeth on the front sprocket = less speed and more climbing ability but more strain on the drive components chains etc, the bbs02 with a 36 lekkie and a etm motor is a strong climber

 

[Woosh]

Don't those issues also apply to hub motors?

I think I may have missed your point, I'm interested in the torque available on the wheel using a crank motor.

I have already answered that question in post #5, 72NM in first gear.

 

[Nosweat]

So it sounds like commuting along a canal towpath I'm better off with a hub motor but for commuting on a hilly road the mid motor comes into its own? As I've never actually ridden either type of motor up a steep (say 1 in 10) incline are you suggesting I would be disappointed if I got a hub motor if my reason for electrifying my bike was to give me a bit of a push up the hills so I didn't have to choose between arriving at work in a sweat or walking up them?

Just a curiosity question to traders on pricing - how much of the differential between hub and mid power is down to the type of sensor? Review complain that torque sensors add a lot to the cost. Yet the cost of a Woosh torque front motor conversion (mentioned only because Woosh offer cadence hubs, torque hubs, cadence mids and torque mids) is only £50 less than a Woosh torque mid conversation with similar battery and if that's the case then the type of sensor shouldn't greatly affect the price of an ebike. But most hub motor bikes seem to use cadence sensors and most mid motor bikes use torque sensors. Annoyingly for the consumer they rarely if ever state which type of sensor they use, yet it's clear from various other posts on these fora that people who have tried both often have a distinct preference.

 

[Deleted member 25121]

I have already answered that question in post #5, 72NM in first gear.

Sorry, we were talking at cross purposes, I thought you were questioning my conclusion that with the new Wispers bikes:

In hilly areas the crank driven bike is able to provide almost 50% more torque ie substantially higher assistance.
On flat roads the hub driven bike can provide higher assistance.

 

[Sturmey]

A hub-motor gives much more measurable and meaningful torque figure because torque is more or less proportional to current and the controller holds the current more or less constant in the lower half of the speed range.

Re the above, I think there is a need to distinguish between battery current and motor (phase ) current. I agree that torque is proportional to motor or phase current. But the controller may only limit the battery current. At low rpm, the battery current is limited but the phase current continues to increase as the controller acts like a buck converter or stepdown transformer due to the increased flyback current caused by the inductance of the motor coils and the pwm (pulse width modulation). This increased circulating (flyback) motor current (through the mosfet body diodes) does not pass the shunt and is the cause of heat in the bottom 3 (non pwm driven) mosfets.

 

[Woosh]

So it sounds like commuting along a canal towpath I'm better off with a hub motor but for commuting on a hilly road the mid motor comes into its own? As I've never actually ridden either type of motor up a steep (say 1 in 10) incline are you suggesting I would be disappointed if I got a hub motor if my reason for electrifying my bike was to give me a bit of a push up the hills so I didn't have to choose between arriving at work in a sweat or walking up them?

Just a curiosity question to traders on pricing - how much of the differential between hub and mid power is down to the type of sensor? Review complain that torque sensors add a lot to the cost. Yet the cost of a Woosh torque front motor conversion (mentioned only because Woosh offer cadence hubs, torque hubs, cadence mids and torque mids) is only £50 less than a Woosh torque mid conversation with similar battery and if that's the case then the type of sensor shouldn't greatly affect the price of an ebike. But most hub motor bikes seem to use cadence sensors and most mid motor bikes use torque sensors. Annoyingly for the consumer they rarely if ever state which type of sensor they use, yet it's clear from various other posts on these fora that people who have tried both often have a distinct preference.

Let me apologise to David firtly, this is not the place to talk about what we do .

 

[Amoto65]

Not so simple to compare as you have to put effort in with a torque sensor crank motor where as you can ghost pedal with a cadence sensor hub motor.

 

[Woosh]

On flat roads the hub driven bike can provide higher assistance.

no, the same, you are limited by law.

 

[Deleted member 25121]

no, the same, you are limited by law.

Good point, limited by the law of the land rather than the laws of physics

 

[Deleted member 25121]

Not so simple to compare as you have to put effort in with a torque sensor crank motor where as you can ghost pedal with a cadence sensor hub motor.

The hub vs crank debate is somewhat independent of the sensor employed since cadence and torque sensors can be used with both hub and crank motors.

 

[Amoto65]

Ok my mistake, did not realize you could use a cadence sensor with a crank motor, having only rode the bosch crank motor I assumed they were all torque.

 

[EddiePJ]

If I were looking for a daily ride with the overall use biased towards road and even just low non technical off road terrain, and could only choose from between these two bikes, then I'd opt for the hub drive every time.

Whilst I love mid drive for my predominately off road biased riding, I do miss the relaxed, physically un-taxing feel that a hub drive offers.

Dave has summed this one up pretty well.

 

[vfr400]

So it sounds like commuting along a canal towpath I'm better off with a hub motor but for commuting on a hilly road the mid motor comes into its own? As I've never actually ridden either type of motor up a steep (say 1 in 10) incline are you suggesting I would be disappointed if I got a hub motor if my reason for electrifying my bike was to give me a bit of a push up the hills so I didn't have to choose between arriving at work in a sweat or walking up them?

You can't generalise like that. I have hills as steep as 30%, and my hub motor is perfect for them (my weight 85kg). I've done the same hills on crank-drive bikes, which also managed them fine. It's not so much the motor, but exactly which motor it is and the voltage and current that you apply to it. An old Cyclamatic with 24v and a 12 amp controller is going to struggle on a 15% hill, but stick a 48v battery and a 17 amp controller in it, and the same motor will power you up the hill pretty fast without pedalling.

If you don't weigh too much, just about any bike can get you up a 10% hill without much effort.

 

[vfr400]

Good point, limited by the law of the land rather than the laws of physics

There is no law on how much power you put through the motor in the UK! There's only a limit on the type of motor.

 

[vfr400]

Re the above, I think there is a need to distinguish between battery current and motor (phase ) current. I agree that torque is proportional to motor or phase current. But the controller may only limit the battery current. At low rpm, the battery current is limited but the phase current continues to increase as the controller acts like a buck converter or stepdown transformer due to the increased flyback current caused by the inductance of the motor coils and the pwm (pulse width modulation). This increased circulating (flyback) motor current (through the mosfet body diodes) does not pass the shunt and is the cause of heat in the bottom 3 (non pwm driven) mosfets.

Do you really think that's going to help people choose between the hub motor and crank-drive version? I try to keep things simple where necessary. Take any bike with say a 15A controller. Solder the shunt to get 18 amps. You get 20% more current and approximately 20% torque. Simples!

 

[Woosh]

There is no law on how much power you put through the motor in the UK! There's only a limit on the type of motor.

the context for that question was on a flat road. You don't need more than 200W to ride at 15mph, even ghost pedalling, unless you have a gale wind blowing in your face.