Advice required on a brompton conversion

[slippa]

Hello All

I'm Long time lurker, first time poster and I would very must appreciate any advice any of you have to offer.

I've had a brompton for a couple of years now and hills are beginning to be a bit of a struggle and I'd like a bit of assistance. My initial thoughts when discovering the possibility of electrifying it were that 15mph would not be adequate and that I would want to make it do 30mph. I've now realised that 30mph may be a little hazardous on 16" wheels but think I could handle 20mph.

Based on various posts on the forums, I'm thinking of purchasing the following from bmsbattery and would like your opinion on:
1) the kit/motor itself
2) the best battery option and charger for me given an approx 6 mile total commute to and from work (with a ****** of a hill on the way home) and given that I would like to be able to achieve approx 20mph on the flats for short periods of time only.
3) what else (apart from the battery and charger) I would need to be up and running (wiring etc..)

- Bafang BPM 36V350W Front Driving Hub Motor Conversion Kit (393 rpm) - exchanging the included 450W 9Mosfets universal controller for the KU123 500W 12Mosfets High Speed Controller. (BPM 36V350W Front Driving Bike Conversion Kit - BMSBATTERY)
- A Pair of EBike Torque Arms (EBike Torque Arm)

Any advice/suggestions you have would be greatly appreciated.

Cheers
Slippa

 

[benjy_a]

Blimey another Brompton thread!

Good luck with the project... Not sure that motor will fit into the Brompton forks which are only 74mm dropout. Also there's not much room for torque arms. Whatever you do you will have issues with the Brompton fold that will need working out.

Others on here have managed to stretch the forks to 100mm but that doesn't necessarily mean that the motor you are looking at will clear the stretched forks.

Others I'm sure will be able to give you more specific advice.

Ben

Sent from my GT-I9000 using Tapatalk 2

 

[jhruk]

Hello All

I'm Long time lurker, first time poster and I would very must appreciate any advice any of you have to offer.

I've had a brompton for a couple of years now and hills are beginning to be a bit of a struggle and I'd like a bit of assistance. My initial thoughts when discovering the possibility of electrifying it were that 15mph would not be adequate and that I would want to make it do 30mph. I've now realised that 30mph may be a little hazardous on 16" wheels but think I could handle 20mph.

Based on various posts on the forums, I'm thinking of purchasing the following from bmsbattery and would like your opinion on:
1) the kit/motor itself
2) the best battery option and charger for me given an approx 6 mile total commute to and from work (with a ****** of a hill on the way home) and given that I would like to be able to achieve approx 20mph on the flats for short periods of time only.
3) what else (apart from the battery and charger) I would need to be up and running (wiring etc..)

- Bafang BPM 36V350W Front Driving Hub Motor Conversion Kit (393 rpm) - exchanging the included 450W 9Mosfets universal controller for the KU123 500W 12Mosfets High Speed Controller. (BPM 36V350W Front Driving Bike Conversion Kit - BMSBATTERY)
- A Pair of EBike Torque Arms (EBike Torque Arm)

Any advice/suggestions you have would be greatly appreciated.

Cheers
Slippa

Hi Slippa

Have you considered the 83mm Q85 from BMS. It will be a lot easier to fit to a Brompton than the BPM, as well as a lot lighter. The 36v 328rpm should give around the legal limit in a 16” wheel and if you really want more speed you could run it at 48v, which should give around 20mph. A 24v one run at 36v would give even more speed, but at the cost of hill climbing ability.

I run a 36v one in a 20” wheel with the kit supplied KU63 controller and don’t find it wanting in power at all. The lower gearing in the smaller wheel should give it even better hill climbing.

If you want to run the controller above its specified voltage you may need to modify it, or use a different one.

For the battery how about jerry simon’s A123 special(s).

You may need a box to house the controller

 

[cwah]

ahah, the brompton is invading the forum

I really don't recommend the BPM on the front wheel. The torque is too high and you may break the front fork. Especially with the brompton very light fork.

You can buy something smaller such as a Q100 or the G209. But I strongly advise against the BPM on the front.

I remember of 1 guy on endlessphere that had a front BPM and had his front fork broken twice with torque arm and a BPM. And he had a montain bike..


And I do have a Brompton running at 30 mph, so be careful

 

[slippa]

Thanks very much for the advice guys. The prospect of a breaking my forks scares the carp* out of me so I'll have a rethink and let you know what I decide.

 

[jerrysimon]

Slippa,

Firstly welcome to the forum. Do a search on Brompton conversions there are loads of options. Really depends on what you want to do in terms of keep it light (less grunt) or all out power (makes it heavy) ?

I would emphasise like others that although it is possible to fit larger width motors in the narrow forks, its not easy. The narrower 80mm wide motors make it easier but still a challenge.

Feel free to pm me if you need more info though its all in my build threads.

PS that battery post reference is really for a minimal very light assist five mile flat journey. You will need a larger one for your distance and hills.

Regards

Jerry

 

[daniel.weck]

- Bafang BPM 36V350W Front Driving Hub Motor Conversion Kit (393 rpm) - exchanging the included 450W 9Mosfets universal controller for the KU123 500W 12Mosfets High Speed Controller. (BPM 36V350W Front Driving Bike Conversion Kit - BMSBATTERY)
- A Pair of EBike Torque Arms (EBike Torque Arm)

Hi!
Beware, max legal power in the UK is 250W.
I also concur with other posters' remarks regarding the risk of permanently damaging hub motor and forks because of the excess torque.
With 16" wheels, there is ample traction even with a small 250W motor such as the Bafang (8-Fun). Depending on how hilly your terrain is, you might even consider a Tongxin (see Jerry Simon's posts). The size and weight difference is significant:

http://www.pedelecs.co.uk/forum/electric-bicycles/4682-diy-stage-1-received-bafang-tongxin-kits-photos.html#post59187

Cheers, Dan

 

[slippa]

Thanks Dan. I've actually gone ahead and ordered Q85 kit from BMS as recommended by jhruk, an additional KU123 500W controller and a 48V9.5Ah LiFePO4. I've ordered the 328rpm version and I'm now thinking I should have ordered the 201rpm for the better torque. I guess I'll just have to suck it and see.

 

[daniel.weck]

Thanks Dan. I've actually gone ahead and ordered Q85 kit from BMS as recommended by jhruk, an additional KU123 500W controller and a 48V9.5Ah LiFePO4. I've ordered the 328rpm version and I'm now thinking I should have ordered the 201rpm for the better torque. I guess I'll just have to suck it and see.

Great, let us know how it goes

Remember not to use the 48V battery with the controller supplied as part of the hub motor kit: it's rated at 36V 250W (6 Mosfets) ... so you would probably kill it with 48V!

Torque-wise: unlike direct-drive hub motors, these seemingly small geared motors tend to provide good torque (at the cost of lower top speed). If the Q85 model is anything like the my 8-Fun / Bafang (plastic-geared), you will have plenty of torque at your disposal. You could even fit a steel gear to make it more robust, although I suspect rotational energy will be wasted as wheel spin, well before any other component starts failing (assuming you have a decent torque arm fitted)

Cheers, Dan

 

[jhruk]

Thanks Dan. I've actually gone ahead and ordered Q85 kit from BMS as recommended by jhruk, an additional KU123 500W controller and a 48V9.5Ah LiFePO4. I've ordered the 328rpm version and I'm now thinking I should have ordered the 201rpm for the better torque. I guess I'll just have to suck it and see.

I’m sure the Q85 will suit the Brompton well. Is that the 24v or 36v 328rpm version?

However, I would definitely not recommend using the KU123 controller with it as I fear you’ll fry the motor. A quote from the BMS website on the KU123 :

“12Mosfets 500W High Speed Brushless Hub Motor Controller for EBike. It is suitable for all our motors. It is compatible with sensorless and hall sensor motor. But please don't use it for low power motor. Otherwise it will burn the motor.”

This controller will deliver up to 30amps and is just too much for that small motor - and probably the battery as well. The standard KU63 limits the current to around 14amps and if you’re going to run the motor at 48v I’d suggest a controller that’s limited to no more than this, possibly less. That’s already over rating the motor by 33%

You could try just reducing the current limit on the KU123 but it is quite large, several times the size of the KU63, and placing it on a Brompton could be quite difficult as well.

 

[slippa]

Hi jhruk

Its the 36v 328rpm version. You've got me worried now. I thought that controller would be ok as I totally missed the last sentence on BMS' site having probably stopped reading at 'It is suitable for all our motors'. I'm not much of an expert on the electrical side so I'd really appreciate any advice/recommendations for another controller I should use with this battery/motor combination. If BMSBattery stock a more suitable controller, I might be able to modify the order before it ships early next week. If they don't, that's fine too as they aren't very expensive anyway but i'd like to order it asap so that it arrives at the same time as the rest of the kit. It'll drive me nuts if I have everything else and no controller to run it!

How about the KU93 450W 9Mosfets High Speed Controller?

Thanks
Slippa

 

[jhruk]

Hi jhruk

Its the 36v 328rpm version. You've got me worried now. I thought that controller would be ok as I totally missed the last sentence on BMS' site having probably stopped reading at 'It is suitable for all our motors'. I'm not much of an expert on the electrical side so I'd really appreciate any advice/recommendations for another controller I should use with this battery/motor combination. If BMSBattery stock a more suitable controller, I might be able to modify the order before it ships early next week. If they don't, that's fine too as they aren't very expensive anyway but i'd like to order it asap so that it arrives at the same time as the rest of the kit. It'll drive me nuts if I have everything else and no controller to run it!

How about the KU93 450W 9Mosfets High Speed Controller?

Thanks
Slippa

I’m afraid I can’t see a suitable 48v controller for this motor on the BMS website. I haven’t tried the KU93 but it looks to be only marginally less powerful than the KU123, and still rather large. I’m out until Sunday but will have a look around for you when I return. This one, or this one, on ebay may be suitable.

Running it at 36v would be easier – lighter battery as well.

 

[banbury frank]

Hi you need this controller select 48 volts

KU123 500W 12Mosfets High Speed Controller - BMSBATTERY

Frank

 

[slippa]

Thanks jhruk. I'd prefer to get the occasional extra speed through running it at 48v.

I guess I don't understand the limits/calculations involved here but I'm hoping someone can educate me a little.
The motor is rated at 250w and 36v but it's possible to run it at 48v in order to get a little more speed. Am I right in thinking that the motor is pulling 6.94amps at 36v when at full power?
What happens when I power it at 48v? Does the current reduce or does the power increase?

Sorry for what is no doubt a stupid question to you guys who have a better grasp of electronics - I'm sure I used to understand this stuff better at school.

Thanks
Slippa

 

[benjy_a]

Current X Voltage = Watts (power)

So...if your motor is pulling 10A at 36v this is 360w.

10A at 48v = 480w.

Plus...at 48V your motor will go 33% faster than at 36v; its directly proportionate.

Ben

 

[banbury frank]

Hi Don't forget you are measuring Electrical energy the motor converts it to mechanical energy and most motors are only 80% good at doing that so remove 20% to get a true Watts reading from the shaft

and yes it is RPM per volt ( Revs per minute ) so higher volts the motor will spin faster

Frank

 

[NRG]

No, thats not correct... because the motor wont be pulling that sort of current at full speed...at full speed the current draw will be low, it will pull the greatest current whilst doing work at about half of the full on road speed when climbing a hill... IE: at 7~8mph for a motor designed to run at 15mph on 36v

At this current draw the efficiency of the motor (depends on what one) is going to be around 65%...maybe more maybe less...plus as you are pulling peak current the loss in the wiring, connectors, controller FETs and battery will be at its greatest...so for a 15amp controller running at 36v power loss in the circuit could be between 25~50w and motor efficiency at 65% = ((540w -25w) x 0.65) = 335w at the motor.

 

[slippa]

Thank you all for the explanations but I'm still a little unsure as to why the motor would get burnt with the more powerful controller. Is it because when the motor is under full load up a hill, it continues to pull as much current as it can and if the controller lets it pull too much, that's when it gets burnt?
Is it possible to somehow limit the amount of current it can pull by modifying it?

Also, how do you determine how many amps a controller will deliver?

thanks
Slippa

 

[benjy_a]

Thank you all for the explanations but I'm still a little unsure as to why the motor would get burnt with the more powerful controller. Is it because when the motor is under full load up a hill, it continues to pull as much current as it can and if the controller lets it pull too much, that's when it gets burnt?
Is it possible to somehow limit the amount of current it can pull by modifying it?

Also, how do you determine how many amps a controller will deliver?

thanks
Slippa

This is basically correct, the controller limits the Max current that the motor can pull. Many of the controllers are programmable such as those from Edward Lyen and E-Crazyman. I have a Lyen controller and to change the Max current takes a few seconds using my laptop and a supplied USB cable. I can set it from 1-30A.

Alternatively just buy a controller that has a suitable Max current for your motor which for you will likely be 15-20A.

Ben

 

[eddieo]

just came across waterloo bridge after dropping daughter back at Uni, and 2 middle aged men, one on normal bike and one on Brompton cycling side by side chatting away....what struck me was they where both cycling at more or less the same cadence? I think the Brompton must be very well geared for a small wheeled bike.