Electric hybrid bicycle prototype operational

For the last couple of weeks I’ve been working on an electric hybrid bike.  The motivation is that my place of employment is about 12 miles from home, and while I should be man enough to bike it both ways every day, the reality is that I usually drive.  It’s about 30 mins to drive, and 50 mins to bike, and I figured if I could reclaim some of the 40 min/day difference, I would be more likely to bike it more often.  I researched electric bike conversions, but they seem to be really heavy – one kit with a Crystalyte wheel motor tipped the scales at 50 pounds, not including the bike, for 360 Wh of lead acid storage.  I figured since I was willing to pedal, I didn’t need 700W of power, and I’d really like to end up with something I could still carry, so I designed and built my own.  Here’s an image from partway through assembly:


core drivetrain components of 300W power bicycle assist system

I found a 300W 24V brushless motor with an integrated controller online, along with a freewheel and a couple of sprockets.  I got two gel lead acid 10Ah 12V bricks from digikey, and hacked up a spring-loaded potentiometer to drive the input.  The bike is a nice simple Miyata road bike that I bought from Adam for $100.  The drive is entirely in parallel; the right side human power drivetrain is unchanged, and the trickiest part of the whole thing was to arrange the left side electric drive.  I bought a flip-flop hub at Wheelworks in Somerville, which has short 1.375″ right hand threads on both sides.  Though the thread is short it accepted a standard 7 speed freewheel no problem, but on the left hand side there was nothing to keep the freewheel (which also has a standard right hand thread) from spinning off under load.  There’s a smaller left hand thread on the right side of the flipflop hub, but it was too close in diameter to the larger freewheel thread to get a jam nut on there, since the freewheel was thicker than the two threads put together.  So I ended up threading another ring into the deep internal thread on the freewheel, and jamming it against the body of the hub.  With high strength loctite I am hopeful that it will continue to hold up.  The large sprocket on the wheel is driven by a 25 pitch chain from the motor, which I mounted to a plate fixed within the main triangle of the frame by brackets to the water bottle braze-ons.  With a few hours of hacking in the machine shop over the last few days I fitted all the pieces together and arrived at a working prototype.

This afternoon I put it to test, and I am happy to report that it performs quite satisfactorily.  I hooked up the batteries, kicked off, and engaged the electric drive, and was pleased to find that despite the high gear (only about 8:1 reduction) it accelerates reasonably smartly on pure electric, and fantastically with combined electric and pedal effort.  The top speed I achieved with moderate pedal assist was was about 30mph, but I was hampered somewhat by stoplights, traffic, and a sense of self-preservation.  If I had the throttle in a position that permitted more comfortable cycling stance I could have put more muscle into it and found the real top end, but 30mph is more than sufficient for everyday use.  I took a test ride over to Holly and Becky’s house in Somerville, and was pleased to find that I could rip up the Summer Street hill in top gear, again with only moderate effort on the pedals.  The entire rig weighs in at 47 pounds.

While the prototype works well enough to convince me of the validity of the concept, there are still some things that need attending to before making it a regular commuting setup.   The two main issues are the battery attachment system (consisting of aluminum angle, hot glue, and zip ties) which needs a complete rework, and the electric drive chain tends to slap against the chainstay and could use a spring-loaded tensioner.  I need to put the throttle in a better place, and I have a feeling I’m going to wish I had those fancy built-into-the-brake-levers type shifters for maximum efficiency in accelerating.  Also, it would be good to put a fuse in the battery circuit.


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