Pedal-Electric Hybrid Bicycle: parts list and fragmentary plan
So that others may take better advantage of the hacking that I report in this blog, I’m going to make an effort to document things better.
Most of the key components came from electricscooterparts.com, as follows:
- Rear wheel: 700c “Flip-flop” wheel, intended for combination fixed gear use
- Right side freewheel: standard 7 speed Shimano freewheel
- Motor: 300W brushless DC with integrated controller, 8mm flatted shaft – # MOT-K24300
- Motor sprocket: 25 pitch, 11 tooth, 8mm bore w set screws – # SPR-2511A
- Wheel sprocket: 25 pitch, 80 tooth, 4 bolt flange pattern - # SPR-2580
- Left side freewheel: 1.375″ threaded ID, 4 bolt flange pattern – # FWM-125
- Chain: 25 pitch steel chain, available from http://www.mcmaster.com
- Idler pulley: mcmaster PN 60425K181
- Idler bearing: standard 608 skate bearing (purchased from mcmaster, but sporting goods stores sell them)
- Batteries: 2@ 10Ah, 12V lead acid, Digikey PN 522-1030-ND or similar
- Throttle potentiometer: 5K pot, Digikey PN CT2158-ND or similar
Rear wheel assembly
The rear wheel is probably the trickiest part of the whole project. The design calls for an uncompromised pedal drive on the right side, with a completely independent electric drive on the left. So the right side is straightforward, though as it happened I couldn’t reuse the freewheel off the beat up old wheel on the bike since it was cassette style, so I bought a new one.
In order to avoid backdriving of the motor when pedalling eg. with a dead battery, the left side needs a freewheel too. The flip-flop hub I bought had a short standard thread (it’s 1.375″ diameter with a fairly fine pitch) on the right, intended for a single speed freewheel, and an even shorter standard thread on the left, with a short reverse-threaded portion outboard of that for a lock ring. While short, the thread on the right side is sufficient to take the standard freewheel. I had to add a few spacer rings to get the outermost nuts on the axle bolt out past the hardware on each side, and spring the frame a bit to accept the wider-than-usual assembly, maybe 6mm or so.
The wheel is not intended for use as a left-side drive, as it has a right hand thread, rather it would typically be swapped to provide fixed or free single speed operation. The lock ring is necessary since fixed-gear zealots apply torque in both directions, but I could not use it directly since it did not protrude out beyond the end of the freewheel. The key issue is that since the freewheel I got was right hand threaded (as was the left side of the hub), the motor torque would simply spin it off. To avoid this, I spun it on using high strength Loctite, and since the internal thread on the freewheel was substantially longer than the thread on the shaft, I could jam it against the hub with an externally-threaded ring. I made this ring out of a bottom bracket component, which by custom has the same thread as the standard freewheels (and fixed gear cogs). I had to modify it substantially, first cutting the outboard half off with a cutoff wheel on a die grinder, then turning the resulting edge flat (with carbide tools, the bottom bracket part is bloody hard since it is also a bearing race), then flipping the part in the lathe and cutting away some of the unthreaded end of the ring and cutting a chamfer on the ID to allow it to make good contact with the end of the hub. In order to avoid munging the thread in the jaws of the lathe, I turned a thin-walled aluminum tube with a step on the ID to accept the bottom bracket part and clamp it firmly in the 3 jaw chuck. Finally, I cut two slots with a die grinder on opposite sides to accept a custom spanner, which I made by pressing two hard pins into a scrap of aluminum. I doped the lock ring with loctite and jammed it as hard as I dared given the few thin threads on the hub.
[It might have been a better idea to engage the smaller left-handed lock ring thread with a custom turned part which in turn would have a standard right hand thread 1.375" OD with an outboard flange to keep the freewheel from spinning off. However, this would have required figuring out what the lock ring thread is, and turning flawless custom OD and ID threads in a thin metal part. This should be doable, however I haven't had great luck turning fine threads; generally I've had better luck using a thread mill on a CNC, but I'm not familiar enough with the CNC that's available to pull this off. BTW, a tap can be carefully ground into a single-point thread mill; this is a lot cheaper than buying a custom thread mill for whatever size you need. Perhaps the best way to deal with the left side freewheel problem is to machine away the threads on the hub and make a custom adapter that simply bolts on to the hub in the spaces between the spokes. Of course the holy grail is to find a left hand freewheel and a hub with left hand threads on the left side. I'm told that BMX tricksters sometimes set up left-side drive to streamline the right side of the bike for tricks. If anybody has other ideas I'd love to hear about them, since a bunch of folks I know are toying with the idea of building these.]
So, that got the freewheel securely fastened to the hub (or at least it hasn’t come off yet, knock on wood). The large rear sprocket bolts to the flange on the freewheel, though to get more clearance between the chain and the inner surface of the chainstay I put it on the inside of the freewheel flange, and machined a 1/8″ spacer to get more clearance still (washers would have been fine as well, though not as sexy). That puts the sprocket pretty close to the spokes, so I used flat head socket cap fasteners and countersunk the holes in the sprocket – the parts are set up for 6mm but since I had only 1/4″ I had to grind the holes out a bit with a Dremel (the freewheel is also made of seemingly very hard metal, since it has bearings in it). As it happened the 1/4″ nuts fit snugly against the body of the freewheel at the outside, so it wasn’t necessary to use a wrench when tightening them. The bolts have to be just the right length since there’s not a huge amount of space between the freewheel assembly and the dropout.
The rear wheel assembly was the most involved part of the conversion; the motor mount and battery box are more or less simple fabrication. The motor and battery box mount to a chassis plate of 1/8″ aluminum which sits inside the front triangle of the frame; they attach to the frame via the water bottle braze-ons on the seat tube and down tubes, with the help of right-angle brackets made from 1/8″ x 1″ aluminum angle. The motor has a raised concentric flange on the mating end, with a three bolt pattern. The holes in the aluminum motor housing happen to be the right size to tap for 5/16″ bolts; this saves the weight of three full length motor mounting bolts. I CNC’d a pattern to allow the motor to arc around one of the bolts for chain tension; that level of precision was gratuitous, and a perfectly good result could be achieved with careful use of a drill press and jigsaw. The motor bolts to the chassis plate, and a loop of 25 pitch chain is fitted. I don’t have a 25 pitch chain tool, but I made a little jig consisting of a scrap of aluminum with a hole bored the right diameter and depth to press a pin almost all the way out on the arbor press, using a small piece of drill rod as a press pin. At least with the geometry of my bike, the chain runs entirely inboard of the stays; it is not necessary to form the loop of chain around the stay as is the case with the main drive chain of a bike.
I made a box to support the batteries. It started with a piece of aluminum, 1/16″ thick and 12.75″ square. I folded it into a U shape with a flat bottom, and machined three spacers out of 1/4″ aluminum to hold the two edges together. One went on each end, and the other went in the center to separate the two batteries. The sheet metal was fastened to the spacers with #6 button head fasteners, and four holes were drilled through the chassis side of the battery box and into the spacers. These holes were tapped 10-32 to accept fasteners to hold the battery box to the chassis plate. 1/8″ of adhesive-backed rubber was used to pad the interface between the battery box and the chassis plate, and to provide clearance for the screw heads. Flat head fasteners might have been substituted for the button heads, but this would have required a greater degree of precision than was readily attained working with the sheet metal.
The motor power connection is a standard “flat 2″ connector that looks like a 2-position version of a trailer hitch plug. I bought the mating connector at an auto parts store. The throttle interface requires a 5k pot; I bought one at Digikey and fitted a hardware store tension spring around the shaft as a torsion spring to force it to return to “off” when released, and I bought a crufty-looking black knob at the hardware store to go on the shaft of the pot. It’s not pretty (especially since it’s zip-tied to the handlebars) but it works.