Has anyone tried this "poor mans" homemade electric bike idea? I have a wiper motor from some car or van that has a shaft on it.  When I apply 12 volts to it, with no load on it, it seems to spin at about 125 RPMs.  I vaguely remember that when I load it, it can draw up to about 10 amps (10A).  I suspect it can output maybe 100 watts of power or so.  It also has 3 mounting holes.  I suspect someone mechanically inclined could perhaps connect this with 1:1 ratio to the crank/chainring assembly.  The 125 RPMs would likely drop under load but under low load it should maintain close to that I would think.  It would be fun to try it with some type of freewheel mechanism or centrifugal clutch so when the motor is turned off, it would not load the cranks.  Since the rotational speed mimics a normal cadence (probably will drop to 60 to 100 RPMs under load), it can possibly be used to run a multispeed bike and if the rider pedals too, they would help each other out (like 100 watt motor plus 100 watt rider = 200 watts total).

So has anyone ever tried this and do you think it could work and if you think so, do you think it would work well?  What about pitfalls?

UPDATE: I tried the motor with a 12V battery and a short crescent wrench and I was able to hold the wrench still but there was a fair amount of tug on it, meaning it might be ok as a helper motor but not strong enough to move the bike on its own.  It would be interesting to see if after getting the bike going say 10 MPH, if this motor would at maintain that speed or at least help the bike to coast down longer.

Also since it spins at about 125 RPMs, it can be geared down even more such as thru a 16T drive sprocket and a 20T driven sprocket which is a a 4:5 reduction so then about 100 RPMs, giving it more torque.  I think if this motor spun at half the speed, it would be harder for me to hold that wrench still.  Maybe some other wiper motors spin slower like 60-100 RPMs.

Thank you.
No I've not done that, and it would likely be a bad choice for an ebike conversion due to lifespan of motor.

Electric motors have an optimum rotational speed which depends on their design and the number of coils on the armature.

Your wiper motor will have a speed of around 2000 rpm, and it will drive a worm gear - see the excellent photo in @Frisbee's answer.  That worm gear will drop the output RPM to 1/50th  or about 40 RPM.  The output will be a low speed and high torque.  This means the motor wants to spin at a fixed speed, not faster and not slower.

A wiper is also driving a cable or rod system that has half a kilogram of wiper blades attached, compared to the weight of a bike plus rider.  So there's the duty cycle to consider too-how hard the motor has to work.  Remember its a brush based permanent magnet motor intended for forward-backward-stop duty in a relatively cold environment, so running it hard out is not a good idea.

Finally, you'll need to fab up some way to mount the motor and apply drive to the bike.  Your options are


Front or rear hub - not suitable for home job, need to build motor into wheel hub
Tyre drive - possible.  The motor has a roller and pushes that against the tyre creating drive.  Wears out your tyre in triple-quick time.
Mid drive - Attach the motor to the drive chain by a cog and mounting plates.  Downside this messes with your legroom, but may allow the use of rear gears still. 
Offside drive - Some of the liquid motor kits add another drive cog on the left-side of the rear wheel, and run a second chain.  These can't pull away from stopped because they lack a clutch, so you need to pedal up to speed before engaging the liquid engine.    This technique would leave the freewheel clicking when pedals are not providing drive.


You also need to consider batteries for an ebike.  Mine had 3x 12AH 12V batteries mounted in the frame, and all up the bike was about 28 kilograms.
More details at http://www.electric-bike-kit-forum.com/post2249.html 

If you wish to electrify a bike, consider a kitset from somewhere like  http://www.electric-bike-kit.com/hill-topper.aspx   The $800 USD ranger seems decent, and you can revert your bike to normal by taking the kit off.

It looks like you're not the first to have this idea though.... http://www.autospeed.com/cms/article.html?&amp;title=Building-an-Electric-Bike-Part-2&amp;A=111538 
Note he had to build an airscoop to cool the motor, and a powered fan for when stopped.  That's a friction drive too, so uses up tyres fast.

Finally - your locality will have some restriction on the maximum power motor you can fit before your bike becomes classified as a motorbike.  In New Zealand its something like 250-300 watts which is good for about 20 km/h maximum.

Now I can exceed my electric bike speeds by riding my road bike, so I sold the ebike parts and use the old MTB for towing my bike trailer.  Consider upgrading your own motor by biking more for health and fitness.