Build spec by committee, might that include you?

[FastFreddy2]

So you pick a motor with the rpm to get you that speed (+ about 10%), in your sized wheel, under no load conditions. That way, when loaded, it should give the legal requirement but still give enough torque to climb.
If you can maintain 50%+ of top rpm when climbing then the motor remains efficient enough not to overheat. So that means climbing at 7-8mph.
My own Ezee motor has a top speed of 19mph on the stand (if the cycle computer is anywhere near accurate). It has done many years up and down the hills of west Wales without issue. I have had the controller thermally cut out on hot days though.

Like this? (Thank goodness I didn't need to calculate it myself.)



The rpm figures seem a bit low given others I've seen referenced elsewhere, though it does tie up, or is pretty close to what you have at 19mph. It's also in the ball park of what Woosh has mentioned at 250rpm. That said, if your wheel diameter is smaller, your wheels will be rotating faster to achieve the same smph figures. Mine is a 47x622 Road Tour with a diameter of circa 70cm. Which is the same diameter as my 650b bike with its fatter tyres.

Seems unusually, I'm not interested in 20-30mph speeds where rpm's might be up in the mid 300's.

I've no doubt at all, 250w at 36v would be perfectly adequate for long 50 mile journeys on the flat. Possibly I would use some help on the outboard journey, but I certainly would want some during the return leg and would not be contributing much for the last 15 or so miles. What I'd be really happy with, is the full 15mph assistance going up nasty hills, or the 8th or 9th hill and all those after. That's the time when 48v seems attractive. Not for speed in my case, but the "grunt" I might hope to get from it.

Windings, can affect all this, but I can't remember where I read up about 'windings for torque', not speed.

 

[soundwave]

a bosch motor is rated 250w, its not because under this speed a motor can pull as many amps as it wants and gets you up the hill.

you want a motor with a 20a controller and set it to the speed you want and a bafang 750w mid drive will give the same power i have to climb any hill if i remove my dongle both wont be much different bar my bike is a money pit

 

[FastFreddy2]

you want a motor with a 20a controller and set it to the speed you want and a bafang 750w mid drive will give the same power i have to climb any hill if i remove my dongle both wont be much different bar my bike is a money pit

I'm not going the mid-drive route. Apart from anything else, I may want to use the hub motor on different bikes. I've got about 4. "Technically" 5 if you count the one I'm building for someone else.

Are you saying if I can get enough amps into a 250w motor, it would climb hills?

 

[soundwave]

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www.alibaba.com

you can limit it to 15mph but at 110 amps the motor will rip the frame in half and over take you.

a controller with 20-25a is what you want

 

[soundwave]

no one will let me review any ones bikes

 

[lenny]

12 Ebikes that use a hubmotor as a non-hub

A direct drive hubmotor is affordable, simple, and robust. However, they turn at the same low speed as the drive-wheel. Electric motors perform best at higher RPM’s and that is why the most c…

www.electricbike.com

 

[Benjahmin]

Are you saying if I can get enough amps into a 250w motor, it would climb hills?

Yes !
A 48v controller in the max assist setting (let's call it 17A) will give a notional 816w. Lets allow a massive 20% for various inefficiencies. That still leaves you at 652w.
Having watched the Tour de France, they sometimes show a riders power output and this is the sort of wattage a fit, young professional develops when out of the saddle launching an attack on a mountain.
I don't think you're going to have any problems !

 

[chris_n]

Yes !
A 48v controller in the max assist setting (let's call it 17A) will give a notional 816w. Lets allow a massive 20% for various inefficiencies. That still leaves you at 652w.
Having watched the Tour de France, they sometimes show a riders power output and this is the sort of wattage a fit, young professional develops when out of the saddle launching an attack on a mountain.
I don't think you're going to have any problems !

Bear in mind the all up weight for a young fit professional cyclist on a road bike is probably less than half the weight of the average ebiker on an ebike. They also aren't always going as fast as 15mph uphill. I can hold either of the ebikes I have had (one Yamaha, one Shimano) at max power output so about 600W plus my input and can't hold anywhere near 15mph on a steep hill. Your idea of hills is probably completely different to my normal riding though.

 

[FastFreddy2]

So, after hours of searching (mostly with head banging a wall) I've discovered a motor that probably many here will be familiar with the Bafang 36V 250W RM G020.250

At first I thought this table referred to the maximum rpm, but I've worked out, they will be the controller shut-off speeds at reaching maximum legal assistance.




The motor also seems pretty tolerant to 48v too. Is it any good though?

 

[Peter.Bridge]

So, after hours of searching (mostly with head banging a wall) I've discovered a motor that probably many here will be familiar with the Bafang 36V 250W RM G020.250

At first I thought this table referred to the maximum rpm, but I've worked out, they will be the controller shut-off speeds at reaching maximum legal assistance.

View attachment 53196


The motor also seems pretty tolerant to 48v too. Is it any good though?

Is it not identical to the RM G020.500 - they just stamp 500 on it when they are marketing it as part of a 500w kit ? (with 48v battery rather than 36V) e.g. here (I just ordered the kit and it had RM G020.500 stamped on it , without the battery for a second hand 26" wheel Carrera Vulcan - it came with winding code 12 - which I think is what Woosh sold as the 36V SWX02 - but was stamped with G020.250)

Review here and here (take it with a pinch of salt - but think it's fine - bog standard rear hub)

 

[FastFreddy2]

Is it not identical to the RM G020.500 - they just stamp 500 on it when they are marketing it as part of a 500w kit ? (with 48v battery rather than 36V) e.g. here (I just ordered the kit, without the battery for a second hand 26" wheel Carrera Vulcan - it came with winding code 12 - which I think is what Woosh sold as the 36V SWX02 - but was stamped with G020.500)

At the start of this enquiry (I think) I asked about 250w x 36v motors becoming 350w motors if used with a 48v battery. I don't recall getting an answer, and in hindsight, possibly I can understand why.

The "500" hub package may well be the same as the "250", but I'd be surprised if the windings were identical if the ratings are actually different. I'm sure there are others here who are better able to specifically identify the differences. I'm very new at this e-motor stuff, and I'm finding it a bit like swimming around in treacle. I wouldn't be surprised to find a 500 case was used on a 250 motor, or vice versa.

Could be as you suggest, what's stamped on the outside may well be to do with what it's being sold as? Meaning the only difference between a 250w motor and 500w motor, is what is written on the casing?

 

[Woosh]

the copper wires inside the 48V version is upgraded to take on 30% more power. You also have more spires per Volt, giving you better torque.

 

[Peter.Bridge]

the copper wires inside the 48V version is upgraded to take on 30% more power. You also have more spires per Volt, giving you better torque.

what's a spire per volt !

 

[Woosh]

what's a spire per volt !

turns per volt.
More turns / volt = stronger magnetic field for the same voltage = more torque