Suggestions? 20" BMX as a power-trailer

[spinningmagnets]

I'm looking for suggestions on converting a 20" BMX frame and rear wheel into a power-trailer. They are available at the local thrift store on a regular basis for $20. I also have a few parts collected on trash days from discarded half-bikes. (not interested in regen)

The rear wheels have a stock free-wheel (replacements dirt-cheap) and I have already adapted a 47T front chainring to the rear wheels' 18T sprocket with a thick aluminum disc I made from the base of a $1 aluminum thrift-store skillet. I have a 16T BMX rear sprocket I 'think" I can adapt onto the right side of the square-shank crank. The right side 16T/47T chain-drive would give me roughly a 3:1 ratio on a 20" wheel.

By using the motor and gear-down on a frame that I'm not pedalling, it gives me a lot of engineering freedom. It also helps with the battery packaging, which I also intend to mount on the BMX frame. I plan to start with 12 ah of SLA, as its only 7 miles to work, and I can top-off the charge there.

I'm considering a toothed belt-drive from the motor to the left-side of the crank. What gear-ratio would give me about a 26-28 MPH top speed?

No freewheel is neccessary on the motor or left-crank pulley, but what are some good motors that are appropriate? I am open to 24V and 36V, but will consider 48V if there's a compelling argument.

I am familiar with the Peltzer web-page, but since I want to use a 20" wheel, I'm certain I wont need a motor as large as the 1-HP Scott. I might even be able to use a hub-motor (without spokes/rim) that is considered weak when used as a wheel (similar to "Stoke-Monkey"). I "could" weld the BMX front fork so that the hubmotor is located in the triangle of the frame or just above it. Perhaps a $200 Bafang with a 2:1 automobile toothed timing belt set? Kollmorgen?

Still many options! hope to narrow it down...voltage, motor, and belt ratio for 27-ish MPH. Thanks in advance...

 

[Grinhill]

Sounds like you're looking for a budget solution. You probably couldn't get anything cheaper or simpler than a unite motor/gearbox/sprocket, since the freewheel cranks aren't required. Plenty of options for different power ratings as well.

[Edit - link added]

http://tncscooters.com/partsdb.php?type=ES

 

[gogo]

You shouldn't need gearing on the motor because your gearing will be done via the bike's chain sprockets. If the circumference of your wheel is actually 20", then 27 MPH is 453 RPM. A 2600 RPM motor would need another ~2:1 reduction in addition to the 16/47 for the speeds you want.

BrandonB listed this source of motors in a thread:
http://www.allelectronics.com/index.php I haven't ordered from them but their prices are the best I've seen. You could also put a want add on Craigslist for junk scooters.

How do you intend to hook the push trailer to the bike?

 

[spinningmagnets]

Dear Grinhill and gogo, thank you for the responses. This is an experiment to show how cheaply it can be done. I have a welder, but I want to avoid using it on this project so others can see whats possible with just a drill and an angle-grinder with a cutting disc.

I am pleased to hear that a 2:1 belt ratio will put me in the ballpark, as salvage yard timing belts/pulleys (Geo Metro?) are dirt cheap ($15?), though, I would like to find a source for cheap/used 3:1 toothed belts/pulleys as that widens my options. Diesel alternator V-belt pulleys are often around 3:1, but they lack teeth. Although, I may try this just to evaluate it as an option.

A possible future project is lacing a 3-speed hub to the 20" drive-wheel (I have one on the shelf from a yard sale), or putting a rear gear-cluster on the right side of the crank with a derailleur. As other threads have shown, having 2 or 3 speeds available to the motor can slightly shrink the motor while increasing top-speed available (without adding voltage) or strong hill-climbing (without overheating concerns).

Some 20" BMX frames have rear suspension, and version 2.0 may have that so I could use a solid rear tire (no flats). In a power-trailer application with only one wheel, heavy batteries help add traction. For simplicity of packaging, I hope to achieve my goals with 24V using one large lead/acid battery on each side. This configuration will also allow the easy swapping-in of alternative motors to evaluate performance. I believe that the smaller the motor, the better (cost, range) but not so small that its struggling at the edge of its performance and has a risk of overheating.

I understand the benefits of brushless motors, but for this project I'd like to look at brushed because the controllers are so much cheaper and simpler for first-timers. If I can get some of my friends onto an electric bike cheaply (when the weather is nice) they will get hooked, and a year down the road they will be buying a 9-continents with 48V lithium batteries.

Any advice, links, or responses are greatly appreciated.

 

[gogo]

You're welcome. We seem to have similar ideas.

I have a rear suspension 20" with a 6-speed derailleur that I intend to make into a push-wheel. By increasing the dimension between the shock absorber mounts, the angle of the steering head bearing will be adjusted to a slight forward angle like the Bob trailers.

I haven't decided what to use for the bits that will go from the fork/handlebar clamp to the back axle of the pushed bike. I could buy Bobnuts and the Bob trailer forks if I don't improvise something else.

I'll be interested to see how you do this part.

 

[spinningmagnets]

Once I was daydreaming about the absolute cheapest way to electrify a bike, and I was inspired when I saw an $80 Schwinn Trailabout for kids in my neighborhood like this:

I immediately could visualize 2 or 4 large batteries mounted low on the sides and a cargo box on top. Cheap, with good hill-climbing and fairly long-range using cheap batteries due to motor/gearing efficiencies. I am now looking at 500W 24V brushed motors, very cheap! (thanks for the link)

I dont have an opinion yet as to the best way to make a hitch, certainly many ways to do it. Lots of good ideas when I used Google-image-search for "bicycle trailer hitch"