Friday, April 11, 2014

That's Better!

Holy cow, what a difference!

I should have done this years ago - I might actually get something posted once in a while!

The formatting controls and simple user interface are awesome! Let's try some stuff out...





Frankenbike (Frank)

Background: I am an accomplished bike rider with many thousands of miles experience with commuting and touring on many different styles and configurations of bicycles. I am also a life-long mechanic and have a keen interest in electromotive technology. I had been kicking around the idea of adding an electric assist to my commuter bike, and by chance I came across an industrial motor which, though not ideal for the application, was available for the cost of a simple welding job (I own a 250 Amp MIG welder). Once I had the motor, I set about tracking down the other parts needed, and figuring out the drive train.

Humble Beginnings: Frank started out as a 1995 GT Nomad “Cross” bike (Rigid MTB style frame with 700C wheels). Of course it’s been completely rebuilt several times over the years and has over 10,000 miles on it. The only original parts left are the frame, fork, derailleurs, and hubs. The paint is all Martin Senour spray can work; one coat primer, two coats “Leaf Green”, and one coat of clear.

Electric Assist Inspiration: The Stokemonkey electric assist from Clever Cycles, which is no longer produced. A web search should bring up a bunch of info, as it was around for quite a while. Edit: It's back! Now produced by the good folks at Grin Tech, who can be reached at ebikes.ca - check them out!

Philosophy: I ride primarily for fitness, and I wanted electric assist for increased speed, fun, and to take some of the load off after a long day. I did not want an electric motorcycle in this application (though I think the Enertia is cool http://www.brammo.com/ ). The motor assists with pedaling of the bike, but the rider must pedal while the motor is running; running the power through the bike’s drive train means the rider can’t simply sit there and let the motor do all the work, but it also means that the additional torque is multiplied through the drive train, which enables cruising two gears higher (18 vs. 16 MPH with the same effort), and near vertical hill climbing with moderate effort in the lowest gears.

Build Basics: The heart of the system is an industrial brush-type 24V Permanent Magnet Gearmotor which has its output shaft at 90 degrees to the motor axis. http://www.bodine-electric.com/Asp/ProductModel.asp?Context=14&Name=42A-5N%20Series%20DC%20Right%20Angle%20Gearmotor&Model=4636&Sort=11249

On the motor output shaft an adapter bushing is pressed into a one-way clutch bearing, and this assembly is then press-fit into a custom chain drive sprocket. The one-way clutch bearing allows the bike to be pedaled freely without motor assist, and then the clutch locks the sprocket to the motor when the motor starts.

The motor drives the timing (left) chain ring of a tandem bike “stoker” (hence, “Stokemonkey” above) crank set, which I purchased off EBay for about $50.

Power is supplied by two 12V 21AH Absorbed Glass Mat (AGM) batteries which ride in the trunk bag. The batteries are wired in series to provide 24V to the control and power electronics.

Control is via a Razor scooter trigger on/off control (another EBay purchase - $12) to a 24V power relay, which provides power to the motor. The system is on/off – no variable speed control is needed with this low power system which is geared to a comfortable 85 RPM cadence.

Technical Information: The motor is a permanent magnet brush-type rated 24VDC, ¼ HP, 8.8Amps, with a 20:1 worm gear reduction to 125 no-load RPM. Torque is rated at 74 lb-in. I have not checked the amp draw under load, but the rated draw (24V X 8.8A) is 211 Watts – a little over ¼ HP. The motor weighs 16.8 lbs. The motor is not wash-down rated, so I sealed the brush caps and wire lead with clear silicone sealant. Frank is a fair-weather bike, but living in Florida means a rain shower is always possible. I would not have selected this motor had it not been available so cheap; it is heavy, not nearly as efficient as a brushless motor, and the worm gear reduction loses more efficiency to friction. The optimum motor would be brushless direct drive like the Stokemonkey. The motor is, however, virtually silent in operation, and provides just about the perfect amount of assist I was looking for.

The motor mount is fabricated from a piece of 1/8” wall tubing with an ID matching the OD of the down tube. The tubing was cut in half along its axis and 3/16” flat stock flanges were welded on each edge of each half, and then holes were drilled in the flanges for ¼” clamp bolts. A piece of 3/16” flat stock was cut to match the size of the motor base, then plug welded to the upper half of the mount tube so that the drive sprocket is in line with the driven chain ring. The motor axis is offset of the bike center line by about ¼” to achieve chain alignment. The motor mount is clamped high enough on the down tube to provide toe clearance on the drive sprocket. I considered mounting the motor below the down tube to keep the weight down low, but there is not enough clearance behind the front tire.

The crank set is a Truvativ GXP Tandem Stoker with triple chain rings of 52T-39T-30T on the drive side and a 38T timing chain ring. I figured the motor drive ratio after I bought the crank set and calculated the size motor drive sprocket based on 85 crank RPM and 125 motor RPM. This gave a result of 26T on the motor drive.

The rear cassette is seven-speed (common in 1995) and was changed to better match the new, larger chain rings on the new crank. The cogs are now 13-15-17-19-21-23-26T, which provide close ratios and a good overall range. Top speed (52-13 @ 85 RPM) is 26 MPH, which I achieve regularly on mild downhill runs. Low gear (30-26) is a stump puller or stair climber, take your pick.

The motor drive sprocket is a 26T #40 (½” pitch) chain sprocket which I had machined to fit the clutch bearing and to thin the teeth to fit standard bicycle chain (also ½” pitch, but much thinner at 3/32”). I ordered the sprocket for a large shaft, so that it was supplied with a large amount of material in the hub to support the bearing.

Batteries are Power Sonic 12V 21AH AGM Lead Acid type. http://www.power-sonic.com/images/powersonic/sla_batteries/psh_series/PSH-12180FR_11_Jan_12.pdf  I chose these for their high rate discharge capabilities and physical size (they fit side-by-side in my Topeak MTX trunk bag). The batteries are heavy (26 lbs.) and do affect handling somewhat being mounted high on the rear rack. Of course, I would much rather have the smaller, lighter Lithium Iron Phosphate (LiFePO4) batteries, but I could get the Power Sonics for 1/3 the cost. I could get away with smaller 10AH batteries for my normal commute distance, but they would be drained almost completely each day and that would shorten their life.

Batteries are charged with a Battery Minder 24V charger I set to a 2A rate.
 
Performance/Riding Characteristics: The system works, and works well. There are a couple of design compromises which only come in to play in extreme circumstances.

Frank is heavy. Really heavy. Frank weighs about 75lbs as pictured. Keep in mind that Frank is a fully dressed, older steel bike to start with, and then I added about 44lbs of batteries, motor and control electrics. I also carry my laptop, lunch, raingear, and a change of clothes to work, so fully loaded the weight could be almost 90lbs. The only heavier bike I’ve ridden was my fully loaded cross-country tourer. I do choose my route carefully to avoid large bumps and potholes.

Much of the added weight is up high, so I keep the bike as vertical as possible when out of the saddle. The Topeak rack has held up well to the added weight of the batteries. I had considered mounting the batteries down low, but the MTX bag is just so convenient; I can slide the bag off the rack to install my son’s Schwinn Runabout trailer bike.

Because of the off-on nature of the power assist, I only turn it on after I have started rolling and have the cranks moving to reduce the amount of amp draw through the motor brushes. The power relay is rated at 50 amps, and I have a 20 amp circuit breaker installed on the motor supply. The motor would trip the 15 amp breaker I had installed previously if I let the motor stall.

As previously mentioned, the motor is smooth and quiet. It is a lot of fun to see peoples’ reactions as I cruise by at almost 20 MPH, and they have no idea the bike is assisted because even though the motor and chain are obvious if you are looking at just the bike, while I am riding it there is no extra sound to draw attention to it.

Cost: The motor can be had on EBay for about $250. Add the crank set for $50, the batteries for $120, relay/trigger/misc. electrics for $50, one-way clutch bearing for $35, sprocket for $20, and the machine work for another $25. The motor mount is dependent on skills, but if you had one made, I would guess $75-$100 tops.

So, $625 or so total to duplicate, but my cost was around $275 because I fabricated my own mount and bartered for the motor and custom machine work. I got about 1/3 the performance of a Stokemonkey for 1/5 the cost…


The story above was written for Popular Mechanics magazine in 2012, but never published. The Frank experiment ran its course, and was disassembled last year.

JP

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