I have recently been evaluating these two similar motors in preparation for making a BB-drive. I've given each motor its own thread including a complete teardown with lots of pics. I will populate those threads with more data as the info and links become available. If you cut off the spoke flanges, both these motors are roughly 6-1/2 inches in diameter.
BPM: http://endless-sphere.com/forums/viewtopic.php?f=16&t=51237
MAC: http://endless-sphere.com/forums/viewtopic.php?f=28&t=45245&start=450#p719603
The short story (this is just my opinion), the BPM is about $100 cheaper (approximately $150, vs $250 for bare un-spoked motors), but the MAC is capable of about 25% more power due to the magnets and stator-teeth being 25% wider.
I measured the space between the flats from one side to the other, and the BPM is about 4.5mm wider. Clearly...at a glance, the edges of the motor cases show that the BPM EDGE is narrower, but the BPMs planet-gear side "bulge" is also much more prominent. The MAC is wider at the spoke flanges, and the BPM is wider at the housing next to the axle For a mid-drive, the MAC is 4.5mm thinner at the axle area of the housing..
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BPM
PROs:
If you occasionally abuse the motor you're using, saving $100 each time you need a replacement can add up. Also, it's a cheap experiment if you are unsure whether you want a geared hub or not.
I initially thought that since the planet gears are a little wider than the BPMs, that this was a benefit (14mm wide gears on the BPM, vs 10mm on the MAC). But, they are thicker because they are still using a softer grade of plastic. This is not to save $2 per motor, the planet gears are a mechanical fuse that will strip when the motor is being overheated. The plastic planet-gears are cheap, so keep an extra set on the shelf if you have a BPM.
In spite of the affordable and common 0.50mm thick laminations, the low pole-count means this motor can still be run up to 2,000 motor RPMs without significant eddy-current losses. However, the low pole-count also means it may occasionally stutter from a dead stop if you are using a sensorless controller. Clearly it has hall sensors, so this shouldn't be an issue for most.
The magnet bowl is already skeleton-ized with 6 large holes to save weight.
CONs:
If you truly limit this motor to the advertised 500W, this is a great motor at a very affordable price. But...nobody here is interested in 500W (36V X 14A = 504W / 48V X 11A = 528W). The stock 16-Ga phase wires are very small (.060"/1.54mm copper diameter on the BPM for each wire, vs .068"/1.74mm on the MAC) and the small axle-bore limits how much you can upgrade the wires.
The stator teeth are 17mm wide, vs the 22mm wide stator teeth on the MAC. The stator-teeth become an electromagnet when energized, so a roughly 25% larger magnet, combined with more copper mass means the MAC starts out with more power, and has more hot-rodding potential.
When running higher amps, a frequent upgrade is to swap-in one steel gear so the motor can take more heat. This is noisier than having all the three planet-gears be made of the stock plastic.
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MAC
PROs
cell_man has started ordering all new MACs with the thinner 0.35mm laminations. So, in spite of the high pole-count, it can run to the higher motor RPMs typically found by using 72V. The older MAC was good up to 48V, but suffered eddy-current issues at the high-RPMs provided by 72V.
For only $15, cell_man will use thicker phase wires, and add a temperature sensor to the stator from the factory, so you don't have to disassemble the motor to add these features.
The MAC stator is a hair larger in diameter, but the difference is negligible. However, the wider stator and wider magnets are a significant benefit whether you will use the stock motor, or hot rod it with higher amps. This is the major difference between these two motors.
The high pole-count means the MAC works well with sensorless controllers at 36V/48V. At 72V the electrical frequency may be too fast for some controllers to work well in sensorless mode.
CONs
It costs $250 for the bare motor, instead of $150 like the BPM.
The back-iron on the MAC is only 2mm thick, so a small amount of un-shorted magnetic field is enough to hold a steel paper-clip onto its outside without falling. The BPM's 6mm back-iron is excessively thick and heavy. 4mm would have been better for both of them, but it should not be a performance issue.
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When I was researching the BPM (the largest geared hub made by Bafang), I found the Q-BPM, which is the same motor, but made for the front wheel. If you are considering this motor for a non-hub build, the Q-BPM is an intriguing option. The axle is solid instead of hollow, and the phase wires exit the motor-case through the side-plate under a larger-than-average shaft bearing, and the axle is already shorter with threads up to the case already.
http://szbaf.com/uploadpic/201110171547322.pdf
http://endless-sphere.com/forums/viewtopic.php?f=2&t=41375
The Q-BPM does not have a freewheel on the right side, but it does have the 6-hole disc brake flange on the left. There are fixed sprockets (15T-22T) that bolt right up to the disc brake flange. Also, SBP has an adapter that attaches to the disc brake flange that has freewheel threads. You may have to run the Q-BPM to a jackshaft to get its benefits.
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7192&start=25#p401162
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7192&start=25#p746386
http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2012-59-QCN9.1A2VC
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Any input (pro or con) is welcome. Especially experience on how much these motors can take before the weak link in it snaps. By giving the motor some gears to downshift into, the motor RPMs can stay up higher on a slow uphill climb, which helps keeps the amps and heat down, so either one of these motors should perform better as a non-hub compared to how they are known to perform in the wheel.
Also, oil-cooling either one of these motors will definitely help it shed heat. Stock geared hubs are known to have a poor heat-shedding path. Without oil-cooling, but using these motors as non-hubs, I would be very comfortable using 25A on the BPM, and 30A on the MAC. If they both had temp sensors, you could slowly raise the amps even more.
BPM: http://endless-sphere.com/forums/viewtopic.php?f=16&t=51237
MAC: http://endless-sphere.com/forums/viewtopic.php?f=28&t=45245&start=450#p719603
The short story (this is just my opinion), the BPM is about $100 cheaper (approximately $150, vs $250 for bare un-spoked motors), but the MAC is capable of about 25% more power due to the magnets and stator-teeth being 25% wider.
I measured the space between the flats from one side to the other, and the BPM is about 4.5mm wider. Clearly...at a glance, the edges of the motor cases show that the BPM EDGE is narrower, but the BPMs planet-gear side "bulge" is also much more prominent. The MAC is wider at the spoke flanges, and the BPM is wider at the housing next to the axle For a mid-drive, the MAC is 4.5mm thinner at the axle area of the housing..
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BPM
PROs:
If you occasionally abuse the motor you're using, saving $100 each time you need a replacement can add up. Also, it's a cheap experiment if you are unsure whether you want a geared hub or not.
I initially thought that since the planet gears are a little wider than the BPMs, that this was a benefit (14mm wide gears on the BPM, vs 10mm on the MAC). But, they are thicker because they are still using a softer grade of plastic. This is not to save $2 per motor, the planet gears are a mechanical fuse that will strip when the motor is being overheated. The plastic planet-gears are cheap, so keep an extra set on the shelf if you have a BPM.
In spite of the affordable and common 0.50mm thick laminations, the low pole-count means this motor can still be run up to 2,000 motor RPMs without significant eddy-current losses. However, the low pole-count also means it may occasionally stutter from a dead stop if you are using a sensorless controller. Clearly it has hall sensors, so this shouldn't be an issue for most.
The magnet bowl is already skeleton-ized with 6 large holes to save weight.
CONs:
If you truly limit this motor to the advertised 500W, this is a great motor at a very affordable price. But...nobody here is interested in 500W (36V X 14A = 504W / 48V X 11A = 528W). The stock 16-Ga phase wires are very small (.060"/1.54mm copper diameter on the BPM for each wire, vs .068"/1.74mm on the MAC) and the small axle-bore limits how much you can upgrade the wires.
The stator teeth are 17mm wide, vs the 22mm wide stator teeth on the MAC. The stator-teeth become an electromagnet when energized, so a roughly 25% larger magnet, combined with more copper mass means the MAC starts out with more power, and has more hot-rodding potential.
When running higher amps, a frequent upgrade is to swap-in one steel gear so the motor can take more heat. This is noisier than having all the three planet-gears be made of the stock plastic.
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
MAC
PROs
cell_man has started ordering all new MACs with the thinner 0.35mm laminations. So, in spite of the high pole-count, it can run to the higher motor RPMs typically found by using 72V. The older MAC was good up to 48V, but suffered eddy-current issues at the high-RPMs provided by 72V.
For only $15, cell_man will use thicker phase wires, and add a temperature sensor to the stator from the factory, so you don't have to disassemble the motor to add these features.
The MAC stator is a hair larger in diameter, but the difference is negligible. However, the wider stator and wider magnets are a significant benefit whether you will use the stock motor, or hot rod it with higher amps. This is the major difference between these two motors.
The high pole-count means the MAC works well with sensorless controllers at 36V/48V. At 72V the electrical frequency may be too fast for some controllers to work well in sensorless mode.
CONs
It costs $250 for the bare motor, instead of $150 like the BPM.
The back-iron on the MAC is only 2mm thick, so a small amount of un-shorted magnetic field is enough to hold a steel paper-clip onto its outside without falling. The BPM's 6mm back-iron is excessively thick and heavy. 4mm would have been better for both of them, but it should not be a performance issue.
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
When I was researching the BPM (the largest geared hub made by Bafang), I found the Q-BPM, which is the same motor, but made for the front wheel. If you are considering this motor for a non-hub build, the Q-BPM is an intriguing option. The axle is solid instead of hollow, and the phase wires exit the motor-case through the side-plate under a larger-than-average shaft bearing, and the axle is already shorter with threads up to the case already.
http://szbaf.com/uploadpic/201110171547322.pdf
http://endless-sphere.com/forums/viewtopic.php?f=2&t=41375
The Q-BPM does not have a freewheel on the right side, but it does have the 6-hole disc brake flange on the left. There are fixed sprockets (15T-22T) that bolt right up to the disc brake flange. Also, SBP has an adapter that attaches to the disc brake flange that has freewheel threads. You may have to run the Q-BPM to a jackshaft to get its benefits.
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7192&start=25#p401162
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=7192&start=25#p746386
http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2012-59-QCN9.1A2VC
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Any input (pro or con) is welcome. Especially experience on how much these motors can take before the weak link in it snaps. By giving the motor some gears to downshift into, the motor RPMs can stay up higher on a slow uphill climb, which helps keeps the amps and heat down, so either one of these motors should perform better as a non-hub compared to how they are known to perform in the wheel.
Also, oil-cooling either one of these motors will definitely help it shed heat. Stock geared hubs are known to have a poor heat-shedding path. Without oil-cooling, but using these motors as non-hubs, I would be very comfortable using 25A on the BPM, and 30A on the MAC. If they both had temp sensors, you could slowly raise the amps even more.
Is this really a con? Thx for all your pics! I will definitely go for the QBPM. Still more powerful and lighter motor than a GNG Gen1 
