Jim Turner is an electric bike designer and founder of https://optibike.com/ Optibike in Longmont, Colorado, just north of Boulder. I just happened upon his tutorial about electric bike motors. Our initial design of the Electric Handcycle contemplated using a Bionx hub motor in the front 20 inch wheel. Everyone said it would work fine. Just one problem. Watch the tutorial, and you’ll realize that as soon as the hub motor design hits a hill, the motor will become more stressed. It will have to put out more torque at a lower rpm than its sweet spot. This will increase current draw, increase heat, and heat up the battery, all leading to a big drop in range.
In an handcycle, it’s even worse. If you start to go too slow, the electric motor will stop helping at all. The Bionx motor requires a certain amount of wheel rpm in order to add power. So you could quickly get to a point where you’re pedaling up the hill with no electric assist at all.
Watch the full tutorial here:
This is a kid’s toy called the Huffy Green Machine. The pedals directly drive the 20” wheel. This is the same condition we have with our Bionx front wheel hub motor. It is built into the wheel, and and it has no mechanical advantage — it just drives the wheel directly.
A measure of bike gearing is called “gear inches.” It goes back to the old big wheel bikes. Anatomically, a person couldn’t ride a bike much bigger than a 60” wheel diameter. Later, when geared bikes came along, they used the gear inches term to say what virtual wheel size the rider was pedaling. Usually bigger would be better, at least for level ground speed.
So our Electric Handcycle starts out by offering the Bionx the equivalent of a direct drive 20” wheel. (um, wait, its ACTUALLY a direct drive of a 20” wheel)
|60 rpm||80 rpm||100 rpm||120 rpm|
|Medium||70||5.6||5.2||53/19 or 39/14||12.5||20||16.6||26.7||21||33.6||25||40|
This chart from Wikipedia shows a range of gear inches for regular bikes with 700c (almost 27”) wheels. Note that the very high range is 125 gear inches, or a the equivalent of a penny farthing bike with a massive 10 foot high wheel.
At the very low end in this chart we have 20 gear inches. This is very low for a rider using their legs. But when you pedal with your arms, you have about 1/4 the available power. This usually means you need a lower gear ratio.
|Chainring||Int Small||Int Big||Int Drive||Front Small||Front Big||Full Low Gear In||Full High Gear In|
|New England Handcycle||25||13||38||28||13||38||9.7||82.8|
Here we see that the New England Handcycle and the Travel Bike have a low gear of 9.7 inches.
This John Deere kids tractor has a front wheel that’s about 12 inches. In my case we have a 220 lb adult and a 50 lb bike, or 270 pounds to climb the hill. The Electric bike gets a 8.3 inch equivalent wheel to give you a really low gear.
With two derailleurs - one on the front wheel, and one half way up the to the pedals, we get a wide range of gearing. But its interesting — the overall ratio of 12.5 is used to make 20” wheel become an 8.3 inch wheel for climbing. And then for speed, those gears turn the 20” wheel into a 103” wheel - equivalent of a 8.6 foot penny farthing direct drive wheel. So, most of gearing is used for speed, and just a bit to make the hill climbing easier.
To use Jim Turner’s mid drive suggestion — which certainly does seem like a good idea — we need to mount a 7.9 lb electric motor somewhere along the drive train. We have two main options. One is to mount it at the pedals. This gives us the benefit of both derailleurs and will let the electric motor see that tiny 8.3 in front wheel on the hills, and then see a nice big 103” wheel for speed on level ground and down hill. I’ve simulated the rough size of the motor in the model above. Gosh, it seems really big there, and a shame to have a really heavy device perched at the end of a long tube.
A better alternative is to mount the motor as part of the intermediate derailleur. This would mean that the electric mid drive motor would only see the benefit of the front derailleur, which in our current design is an 11-42 large range cassette.
Drive Gear: 24 t (mounted to mid drive)
Low Gear: 42 t (24/42 * 20 = 11.4 gear inches)
High Gear: 11 t (24/11 * 20 = 43.6 gear inches)
Low Ratio: 0.57
High Ratio: 2.18
I can’t imagine mounting this big motor up by the pedals.
So this begs these questions:
Hmmmm… Maybe Jim Turner himself will provide some expert guidance. Stay tuned.Question 1: How big a deal is it to give the motor a 11.4” (virtual) front wheel vs a 20” front wheel for Bionx?Question 2: How big a deal is it to give the motor an equivalent 43” front wheel for speed, vs a fixed 20” wheel?
Sketchup Models (Sketchup 15 or higher)
Here is the gearing spreadsheet used in this post: