How to modify a BPM hub motor to drive in the reverse directionUsing 2 BPM front motors to drive a cargo trike
A friend recently asked me to modify a BPM motor to run backwards. The motor was one of a pair to be installed on the front of a Zeitbikes cargo trike (a Chinese box trike - I don't know if they are sold under that brand any more).
These box trikes are very heavy, not stable at speed or on uneven surfaces, and this one was to live at Highgate Hill (Brisbane) and would need to do a lot of climbing with loads.
An attempt had been made to drive the trike with a single rear motor, but this had proven to have inadequate torque at the low speeds the heavy trike was able to achieve safely.
|Here's the trike with a single rear hub motor: too fast, not enough torque|
The frame was not suitable for installation of a mid-drive motor, having extra tubes around the bottom bracket.
The new plan was to use 2 less powerful, higher torque, lower speed motors in the front wheels, one on each side of the cargo box.
The Zeit trike uses hydraulic disc brakes on the front wheels (which work very well), with calipers on the box side of each wheel, so making the left side wheel turn opposite to normal was necessary so that the brake disc could be on the right hand side.
Reversing the mechanics
It is relatively easy to make the internal motor of a geared hub to run backwards, just by the way it’s connected to the controller - as many of us find by accident when first wiring them up. However when the internal motor runs backward, it doesn't turn the hub backward, you just hear a whizzing inside a stationary hub. This is because geared motors use a freewheel, the main purpose of which is to allow the bike to coast along without resistance if the motor is not powered. In order to change the rotation of a geared motor, the freewheel needs to be reversed mechanically, as well as reversing the motor electrically.
If you've opened a geared hub motor, you've seen the freewheel. It's a steel disc that mounts onto the main motor shaft thru its centre (with a slot and key to lock it in place).
|Here's the freewheel straight out of the motor|
|Here's a BPM freewheel with one gear removed|
The freewheel disc carries the 3 nylon planet gears on 3 stub shafts attached near its perimeter. The "freewheel" part of the freewheel is 3 (not visible) sprung rollers that allow the circular plate of the unit to turn one way around the centre boss but not the other. To reverse the motor, we need to rearrange the freewheel so that the rollers allow movement in the opposite direction from normal.
Reversing the freewheel involves:
- removing the 3 stub shafts
- removing 3 small rivets that hold the freewheel plate together
- dismantling the freewheel body
- reassembling the body with the centre boss and the 3 stub shafts reversed to the other side of the plate.
Step by step
First the 3 nylon gears were removed from the freewheel disc. This is easily done by removing the circlips from the stub shafts, I used circlip pliers (whose tips open when you squeeze the handles) and some rag around the job to catch any flying circlips. The nylon gears, along with their sealed bearings, slide off pretty easily.
|Freewheel without gears, stub axle side|
|Freewheel other side, with centre boss sticking up in the centre|
Next job was to punch out the stub shafts, the serious part of the job. It's important to be very careful: you can't afford to have any impacts on the outside end of the stub shafts, or you risk damaging the circlip groove and making reassembly difficult.
I have ground off the short riveted end of the stub shafts in the past, to open a freewheel and repair the sprung rollers (link). But this loses a lot of metal from the rivet head, making reassembly difficult. Having recently been riveting a few chainsaw chains, it occurred to me that the stub shafts could be un-riveted the same way as a chainsaw chain is unriveted: by punching. When you punch apart a chainsaw chain rivet, you only lose a small ring of metal from the perimeter, leaving enough metal in the rivet usually to be able to peen and re-use the rivet to make a chain loop.
Here is the freewheel on the anvil, having a stub shaft punched out.
|Suitable punch on top of riveted axle end, the stub axle is inside the old nut below|
The freewheel plate is supported by large nuts, bigger than the flange in the middle of the stub shaft. I used a punch a little smaller than the rivet head, driven by a 1.2kg blacksmithing hammer.
|Here's the first stub axle punched out|
Once the stub shafts have been driven out, 3 small rivets hold the laminations of the freewheel plate together. These are punched out, like the stub shafts but with a smaller punch.
Once all the rivets are punched out, the 3 plates of the freewheel come apart - I carefully kept things together as much as possible. The centre boss (which fits onto the main motor shaft) is then lifted out and turned over to be re-inserted.
Installation requires the freewheel rollers to be pushed back into their spaces to let the centre boss in. You'll work it out.
|Axles punched out, small rivets punched out, now the first plate can come off and reveal the freewheel rollers in their tapered slots|
|Here the centre boss is lifted out. The small steel rings at rear are the old rivet head rims, left on the punch after punching the axles out|
Once the centre boss is re-installed, the freewheel plates need to be riveted back together, with both the small rivets and the stub shafts. The stub shafts are inserted into the opposite side of the freewheel plate from where they came. With the centre boss reversed and the stub shafts reversed, the whole freewheel unit looks just the same as it did before being dismantled - the only difference being the plate freewheels in the opposite direction around the centre boss.
Be careful! Don't reassemble the whole thing the same as it started. I liked having a spare freewheel unit on the bench to check I was doing things right.
|Here's the freewheel reassembled, with heads peened onto the axle ends and small rivets with a ball peen hammer|
Reversing the electrics
The internal motor is very easily reversed electrically if a sensorless controller is being used: simply swap any 2 of the phase wires and the internal motor will reverse. With a sensored controller (which I don't usually use), it takes more trouble: swap 2 phase wires, then try different arrangements of the 3 coloured sensor wires (leave the red and black alone). This has taken me some time in the past.
It works!The motor I reversed this way now works perfectly, in reverse. It and a normal non-reversed motor have been installed as the 2 front wheels of the box-trike. The motors are Bafang/8FUN BPM 500w code 13 front motors, laced into 20” rims.
|Here's the 20" wheel where a 24" was|
|The box trike on a test ride, climbing a hill with the new motors|
If you want to buy a spare freewheel unit before you start knocking the original apart (like I did), check that you get the right sort: there are 2 different freewheel types in BPM motors. Have a look at my BPM page for more info.
The customer has been very happy with the trike, using it daily in hilly Highgate Hill, Brisbane. It has loads of torque, and powers up to about 20km/h. This is about as fast as these box bikes should go: their non-independent front steering - turning the whole box to steer - is very vulnerable to any bump on one side pushing that wheel back and forcing a turn. To me, the relatively high rider position also feels unstable when the trike tilts to one side due to the road camber or crossing a slope.
The promise of 2 drive wheels
I think the 2-wheel drive system we installed on this trike has a lot of promise for cargo cycles. I know that a cantilevered axle (where the wheel is attached only on one side, like a car wheel, or a wheelchair wheel) appears a more elegant way to put wheels on each side of a vehicle. But using normal bike wheels bolted into dropouts on each side has many practical advantages. Most importantly: there are heaps of wheels like this available, in all sorts of formats: electric hubs of various types and speeds, disc braked, etc..
Imagine how much help some electric assistance would be to these hard-working charcoal makers:
|Phnom-Penh charcoal delivery trikes (sorry I have no credit for this photo)|
I quite fancy some sort of delta, recumbent cargo trike format, like this:
|(sorry, no photo credit again)|