AliExpress/E-Bike conversion - help and advice needed!


1 µW
Jul 30, 2021
Hi everyone.

Over summer, I've been looking into converting my bike to add some electrical assistance. This will be my first E-Bike conversion.

I use my bike mostly for commuting/groceries (~4 miles round trip), but also go on longer rides (~40 miles round trip) every couple weeks. The commute sees a lot of stop-start traffic and some pesky hills, so I've gravitated towards a geared hub motor - for the additional torque offered vs gearless hubs, the smaller profile, and the ability to freewheel.

I'm also not too concerned about high top-speeds, and for me, even if I can get better torque from a DD motor, the higher current draw that would be needed worries me when it comes to battery longevity - if I can get a decent torque output and keep current draw low, I'd like to go with that.

I've found a kit on AliExpress that I think fits my needs, but after doing a classic beginner thing of scouring forums for a week straight, I've generated a number of questions about E-Bikes in general, and some concerns about the kit in question too.

The kit is this one, from Eunorau:

I chose this kit because for the BPM motor. This seems to be a fairly good motor going by previous forum posts, though perhaps it's a little outdated? That said, I've played around on Looking for motors with similar torque outputs, the MXUS 4506, MAC 12T, eZee 250RPM, Shengyi DGWX, etc. all seem like equal alternatives, setting aside build quality. However, most of these seem even harder to find than the BPM, perhaps because of the pandemic?

Anyway, on to some questions.

1) BPMs from e.g. BMSBattery have differnt codes, depending on the number of windings and hence have different characteristics. Is this something worth asking the seller about? I'm not sure having a higher RPM winding would be a deal-breaker, but given my next question, I might have to eke out any extra torque I can.

2) The controller included in the kit is only rated for 20A peak. When I choose the 20A controller option in the EBikes simulator, at, the initial 'MTR Amps' is ~40A. Is this because the 20A controller is set, like the one in the kit, to have a limit current twice that of its rated current? If correct, would this mean the actual torque I could expect from the kit would be ~22Nm? That seems scary low!!

3) The controller is also unmarked/unbranded, so I don't know what kind of values to input in the simulator to simulate it correctly - can anyone advise on ballpark figures? Maybe it's just too cheap. Should I be looking to replace it with a different one after the fact - the seller also sells a 14A nominal/28A limit controller - or should I set my sights even higher? I have seen that some controllers can be adjusted/programmed, but I doubt that these can?

4) Using the trip simulator on, (I've used the eZee 250RPM because it seems fairly similar on the motor sim - though I don't know how similar the thermal properties are, which is obviously kind of important for this test!), even on a flat test trip of ~50km like this, the motor core reaches 100 Celsius. On a more hilly trip, even with freewheeling on the descents, I'm concerned about this temperature.

I've seen users here advise trying to keep motor temperature below 60, for the magnets' strength and adhesive, and I also worry about the nylon gears at these temperatures. These temperatures might be a side-effect of the controller setting in the sim as above (20A continuous), but equally if I did replace the included controller (as I would want to if it limited me to 22Nm torque output), this issue would persist.

I think I've seen that DD motors handle temperatures better because they come less thermally insulated from the outside. Is it worth completely reconsidering my geared vs. gearless choice for this?

5) Buying from AliExpress. I know it's probably not ideal, and I've seen other online stores recommended. However, I'm in the UK, which adds an extra twist. I don't have a large budget and since it is my first bike conversion, I'm not sure I'd want to spend much more even if I could. What kind of lifetime should I expect from these AliExpress kits? What kind of issues could crop up?

I think that's about everything for now. Sorry for the long post, I know it's a bit of a ramble, but I just want a sanity check before I commit to such a purchase!!



1 MW
Aug 2, 2015
Chicago area suburbs.
I also prefer geared motors for freewheeling coasting, and smaller size for the same power.

The choice of RPM will depend on your wheel size and voltage, and is something you can choose when buying motors ala carte as you do with BMSbattery. Eunoaru probably picks the rpm on their kits based on what wheel size you will order.

A 20A controller is typical for bike that maxes out at 20 mph on 36V. It will probably do 24 mph on 48V. A 25A controller might be nice to have in reserve, but you're pushing the capacity of the wires and connectors. It's something to be used in short bursts. A 500W motor would probably melt something, probably the connector first, with a sustained 25A. Bafang uses a bigger connector for their 750W motors.

It's good to think about all these parameters. I'm an engineer, but the only thing I use on the Grin site is his spoke calculator. I've looked at what some motors would do, but I rely on common sense to keep my bikes from blowing up.

dogman dan

1 PW
May 17, 2008
Las Cruces New Mexico USA
Didn't see this one while I was out of town.

you have a few basic, common worries you don't need to worry much about. Overheating the motor takes a lot of load. So you talk about hills, and hills will heat up a motor. But to melt motors, you need a hill and a 350 pound rider, or a trailer full of metal, or three kids, that kind of thing. A normal weight person can climb a mountain 10 miles long, if the grades are not crazy steep.

The other way to melt motors is the 40 amps controller, in effect feeding the motor 5 times it rated wattage.

Standard 20 or 25 amps controllers will do just fine for your needs, providing no more than 2x the motor rating at 36v, or 3x at 48v.

40 miles per ride is doable, but with a normal size battery, say 36v 20 ah, you have to slow down a lot and pedal hard all the way. But a 25 mile ride is easy, at full speeds the motor will go.

What happens with range, is at 25 mph you can't add much wattage with your pedaling. Its taking 800w, you can pedal up 100w. But at 15 mph, you need 250w, and you can pedal up 100. So slowing down lowers wind resistance, while also increasing the % of the total power your pedaling can provide.

Its quite unbelievable how much slowing down just a little can do for range. Riding 18 mph goes an amazingly longer distance than 20 mph, on the same battery. It can take time to maximize your range, at first riding faster is so fun.