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Electric Pedal Assist
Getting up those hills a bit easier
by Rich Sadler

This is a description of the electric assist motor I added to my bike to help me climb hills. I was inspired by the weedwhacker powered assist that John Tetz built for his F-40, and was moved to action when I found a nice little DC motor in a surplus store for the grand sum of $7.50.

What I hoped to accomplish
We've all had the experience of wishing we could get a little help going up hills. I'm no different. What I really wanted, though, was just a little bit of help. Not an electric scooter that would transport me effortlessly to the summit, just something to take the edge off.

Learning about the power plant
Before I could go about designing the actual drive system, I needed to find out more about the motor. I'm an engineer by trade, and got lucky: we happened to have the manufacturer’s catalog on the shelf. Here is a graph of motor power versus motor rpm:

For those more comfortable thinking in watts, 1horsepower=746 watts. That's a lot of power, far more than any but world-class riders are able to produce.

The power graph above shows that the motor’s power output changes with the speed at which it runs, and that power peaks at 1500 rpm. The next graph shows the relationship between motor speed and efficiency. Efficiency is how much of the electricity going to the motor is converted into power; the remainder becomes heat and is wasted.

This graph shows that efficiency also changes with motor speed: however, it peaks at a higher speed than motor power .I decided that It was more important for me to add as little weight as possible to the bike: if I chose instead to try to get the most power out of the motor, I would need to lug a bigger battery around or put up with a short battery life. The average human can make about 1/10 (0.1) horsepower on a long ride. I can get about 0.09 horsepower, or 0.9 human power, from the motor at 2100 rpm: this would give me a nice boost uphill, but wouldn’t turn my bike into an “electric motorcycle” by overwhelming my power output.

Designing the drivetrain
The basic idea was to leave the bike's basic drivetrain intact, and have the motor add power "upstream" of the existing crank. In other words, the motor is attached to the bike's crank by means of a chain. I decided to place the motor about in the middle of the frame, so that means the motor sends power forward to the crank.

I found one other motor characteristic from the graphs: its efficiency drops off a lot if it’s not running at its most efficient speed. To keep it spinning at its best speed, I decided to run it through the bike’s gears, so that when I shift to keep my legs at their optimum cadence, the motor will also stay in its optimum speed range.

My cadence when climbing is about 70 rpm. So, in order for the motor and manual crank to share the drive chain, I needed to reduce the motor’s 2100 rpm by 29 times. I did this in two stages: first, I mounted a 12 tooth pulley on the motor, which drives a 72 tooth pulley through a toothed timing belt. This second pulley is attached to a rear hub from a bicycle wheel.


This photo is from the right side of the bike. The chain you see in the foreground, running behind the square plate, is not part of the electric assist system. The drive freewheel and hub are hidden behind the large pulley.

The hub, spinning at 1/6 motor speed, drives a 12 tooth sprocket mounted to the hub’s freewheel. A chain runs from this small sprocket forward to a 58 tooth sprocket on the left side of the crank, further gearing down the motor to my 70 rpm cadence. This crank is a stoker crank from a tandem, so it has chainrings on both sides of the bottom bracket.

Because the drive hub is reversed (the freewheel is on the left side of the bike instead of the right), the 12 tooth sprocket freewheels when I pedal: the hub & motor don’t turn. This minimizes pedaling friction when the motor’s not running, and I don't need to disengage the motor at all.

When the motor is turned on, the hub drives the freewheel in the direction that it catches, thereby driving the cranks and boosting my human power. The motor’s power & mine are blended & flow to the back wheel through the right side chain and rear gears of the existing drivetrain.


This shows the left side of the bike. The 12-tooth sprocket is hidden behind the square plate next to the motor. It drives the chain in the foreground, which turns the chainring on the left side of the crank.

I also don't need any sort of speed control for the motor. When I shift to keep my cadence constant, I’m also keeping the motor speed mostly constant at its most efficient speed, Pretty neat, huh?

I turn the motor on from a momentary switch mounted on the left brake lever; it turns off when I release it.


The battery is a 12 amp-hour sealed lead acid type and weighs 10 pounds. I plan to upgrade to a Nickel Metal Hydride battery after I get more time on the system. That would weigh only 5 pounds, but is much more expensive than the $37 the lead acid battery cost.


The battery fits into the red bike handlebar bag shown below, mounted underneath the seat. Someday, when I run out of bike projects, I may get around to painting the garage door in the background!


The results
So does it work? Yes, it really takes the edge off of steep hills- I still have to work, but don’t get the feeling that I’m going to grind to a halt or bust a gut. I find I climb about 1 gear higher with the motor on, and that it’s difficult to resist turning it on as I tire on a climb.

It’s first outing was the 30 mile MARS ride I led from Round Valley, which has some long climbs and a particularly tough one near the end. The battery lasted the entire ride and I finished the ride fresh, not worn down by the effort of turning over the pedals on that 15% grade.

I plan to use the assist mostly when I commute to work this spring. It’s a 25 mile ride each way & the ride home has a 2 mile climb followed shortly by a granny gear grinder. I’m hoping the extra oomph will make the ride something I don’t have to get “up” for, so I can do it more often.