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Post By d8veh
11th August 2011, 17:06
Giant Bafang BPM
I know lots of people are interested in the BPM kit, so I thought I'd do another review and show a couple of solutions to problems. The kit is very cheap - about £200 delivered to your door from BMSbattery in China. The kit comprises 350w motor (code 12) in wheel, 350w 9 FET controller, throttle, pedal sensor and brakes with switches. The quality of the motor is fantastic, but be prepared to tighten all the spokes, as mine were too loose. the rest of the stuff is standard Chinese with the odd wire in the wrong position in the connectors, so check that they all match before assembling.
I've used the optional 30 amp 12 FET controller that has useful features like cruise control, speed limit and 3 speed switch and cost about £20 extra.
Assembling was very straight forward. The wheel fits straight in, but you need to make some anti-rotation arms to resist the huge torque from this motor if you don't want to shred your drop-outs. You get the usual p[problem of trying to find room to fit the throttle. I changed to a thumb changer to make more room, but my friend Steve (Saneagle) managed to fit his on a Marin without changing anything. The overall finished weight with battery is 23KG.
I have a front hydraulic brake so the supplied brake levers were unsuitable. Instead, I fitted a reed switch in the grip and mounted a magnet in the lever, which works perfectly. The switch is necessary because I wanted to fit a cruise control.
As I still have a cable brake on the rear (but not for long), I used the supplied switched rear brake lever.
Batteries are 44v 10aH lipos, which I've been using for several months and are necessary to supply the huge current. With lipos, it's necessary to monitor the voltage, so I fitted a watt-meter with the shunts removed and located at the battery.
A 20aH Lifepo4s would be OK but heavy and expensive. On my last build with this motor, I can easily do my 30 mile commute on the one battery with pedalling.
On first testing, the motor was drawing 40 amps, which is about 1.5KW - miles more power than anybody would want. On our 1 in 4 test hill, it accelerates all the way up from a standstill. No hill will defeat this set-up.
The motor is very quiet and smooth, and now that it's done about 1000miles, it's extremely free-running. Steve and I were racing a guy on a road bike down a long steep hill and I hit 39mph, while Steve with the same motor had to use his brakes to stop running in the back of me. The guy on the road bike said he hit 41mph, so you can say that there is minimal drag from these motors when run-in. Indeed, I regularly pedal it without assistance for exercise and to save battery and it rides just like a normal bike, but a bit heavier - no different to having a bit of shopping in your panniers.
I haven't done any speed trials with the new set-up because I have the speed limit connector engaged, which limits powered speed to about 18mph. The power is still too much. With the pedal sensor, the power comes after about 1 turn and then you can't pedal fast enough to keep up with the acceleration, so I've had to make a cruise control to limit it. I fitted a 10k potentiometer to the Left handlebar with a switch to switch from normal throttle to pot throttle. The pot throttle can be set to any speed, which means that you can have a nice leisurely ride, but with all the power you need to get up hills. This controller also has electronic cruise control, but you set it by keeping the speed constant for a few secs, but our roads are too bumpy. The pot throttle should help to set it.
The standard controller (not the one I have)only gave out 14amps until we soldered about 1/3 of the shunt to get it up to 25amps, which is a nice balance between control and power.Before the mod, it wasn't enough to get up steep hills without puffing.
Overall, I would say that if it's power you want, this set-up has it in spades, but at the expense of range, unless you can find some way of taming it like I did. With the standard controller and a 15aH lipo, it would be much more sensible, but you need 25amps to get up step hills.
Total project cost:
Kit £200 (new)
RockShox Dart 3 forks £70 (new)
Frame and rear shock £100 (used)
Crank-set £30 (used)
Rack and bag £8 (new)
Battery and connectors £130 (new)
Charger £14 (new)
Watt-meter £13 (new)
City Slick tyres pair £20 (new)
Other stuff £40
You'd be lucky to get a second hand Giant normal bike for that, so that makes it a bargain in my reckoning.
The golden rule is that there are no golden rules.
30th August 2011, 12:11
Just a quick update. The bike is absolutely brilliant. I've now added Fox Series 3 Float forks, which give it a really supple ride. I don't have to look out for bumps and bad surfaces anymore.
I use the mechanical cruise nearly all the time and I can recommend it as a useful modification for any ebike with a throttle. When set to 11mph, you can pedal it just like a normal bike, but when you come to a hill, it gradually feeds in the power to maintain your speed. To me, this is the way all ebikes should work to give maximum range from the battery. Like this, or, set to a different speed, you can pedal as hard or as easily as you like: The only difference is how much battery you use.
Anyway, I thought I'd compare it with front wheel drive so I built this:
It has a code 15 (201 rpm) BPM motor for better efficiency at low speed as I like to travel at 10-15mph. I finally got the three speed switch figured out only to find that it's a complete waste of time because it works on the throttle, not the pedal sensor, which is still a bit fierce. I'm using the 22amp 450w controller on this one, which still climbs our 1 in 4 test hill with ease. One further mod is to the cruise control, which now uses an array of resistors on a five position rotating switch to give fixed cruise speeds. I'm still experimenting with the resistor values to get nice even steps, but it still works ok.
You can see it in the midle of this photo. As the three speed switch is useless, I'm going to use it to switch from throttle to cruise and eliminate the one next to the control knob.
Spec for this bike:
Giant NRS-3 frame £120 off ebay
Forks Rockshox Reba
Gearset Shimano XT hollowtech
Brakes Juicy 5 hydraulic discs.
The above parts and the rest of the cycle parts came off a ROCKhopper Pro that I got off ebay for £380
The kit was about £200
Batteries £120, which I already had for my other bike
Rack £10 ebay
Bag £1 car boot sale
Total cost less than £750
Weight with batteries 21kg
I don't believe that you can get a better ebike than this. For me, it's nearly perfect. I'm still working on taming the pedal sensor. Should have solved it in a couple of weeks. I'm working on a + and - variable speed cotrol for it with soft ramp, which will replace the mechanical cruise control.
The golden rule is that there are no golden rules.
22nd November 2013, 02:51
Here is a group of pictures of the inside of a Bafang-BPM. It also includes pics of doubling the phase wires and adding a temperature sensor to the stator. Feel free to use them in any way that is helpful.
Endless-sphere.com • View topic - Bafang/BPM geared-hub specs, teardown, and pics