Tuesday, 10 May 2016

Bafang BPM Review

Bafang BPM Review - Intro

I you are looking for a large heavy duty hub motor at a relatively low price then look no further then the bafang BPM. At nearly 20cm (8") in diameter it is large. But is is also powerful, with lots of torque.

The Bafang BPM comes in 48v and 36v offering at 350w and 500w power setting, although in reality, these motor can be run at all sorts of wattages and voltages.

In a 20" wheel, the spokes will be short -  110mm approx. 

If you are reading this having noticed that your ready built ebike has a Bafang BPM installed then well done, you worries are effectively over. Spares (yes spares) are available for this motor, including gears and clutch assemblies, so it should last along time. Your main worry would be overheating the motor on a long up hill.

Bafang BPM Review - In Use

In use the most noticeable thing about the BPM is the torque, and how quite it is.

There is a little lag between applying the throttle and starting to go, but the acceleration is rapid, and hill climbing is strong.
Disc Brake Mounts are available.

Bafang BPM Review - RPM Variants

The revs per minute of the motor will ultimately dictate the speed at which you bicycle travels. I have opted for a 230 rpm variant, which with a 20" wheel will give a top speed of approx 14 miles per hour. 15mph is the legal maximum in the UK. So if you are using this motor in a 26" wheel, then you will require a a lower rpm version.

A simple excel sheet can be used to give speed vs rpm.
This RPM vs Speed Spread Sheet can be down loaded here.

Bafang BPM Review - Choosing Voltage

Typically a higher voltage battery pack is more expensive. But if you live in a hilly area it is probably worth it. Here is why . . .

A motors wattage, is obtained by multiplying the volts by the amps it uses, so for example, my 48v motor will use approx 10A of current to produce 500W, a 24 volt motor would have to use 20A of current to produce the same power output.

So if you use lower voltage you will require:
  1. A higher performance battery.
  2. Thicker wires to connect everything up.
Add to this that fact that a motor normally produces far more power than advertised, and you end up with a powerful 24 volt system drawing 30 Amps sometimes.

Bafang BPM Review - Choosing Wattage

Always choose the 500w variant. If you are legally required to use less power then, use a smaller controller. I have read there is no difference between the 350w and 500w versions. I have also read that the 500w version has wider stators (part of the motor). . .  either way the 500w version is normally the same price.

Clearance for mounting brakes etc. is limited.

Bafang BPM Review - Batteries

I have tried to assemble my own battery pack, and wish I have never started. It is not easy. Read my guide on buying a ebike battery. But ignore the last chapter, building a battery is waaaaaaaaay hard.

Again clearance is limited for forks with wide stanchions. 

Bafang BPM Review - Fitting

Forks. Do not use aluminium. There is a guy on youtube who did this, the torque will split the dropout.

The axle on the BPM is a funny size, it is 13 - 14mm on outer edges, but 10mm on the "flats". I presume this is to stop the axle turning in the fork. The BPM comes with torque washers, and various other wasters.

Steel forks are best. But try and choose forks with narrow stanchions. I had to squash by stanchions in a vice to provide clearance for the hub body and disc rotor, which was not very nice.

So chunky BMX forks, are not a good choice, especially if you choose to use disc brakes. 

Bafang BPM Review - Conclusion

There are better hub motors out there than the Bafang BPM (MAC for example), but I do not think it will be beaten on value. 

It offers bags of power. 

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