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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.

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  • I'm a little skeptical as to whether the wiper motor is up to the challenge, but it sounds like a fun thing to try, if you have the time and resources. Don't forget that you will need some sort of speed controller.
    – Daniel R Hicks
    Commented Jan 23, 2016 at 3:19
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    Poor mans speed controller... an on/off switch.
    – David
    Commented Jan 23, 2016 at 3:35
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    Unlikely to be sufficient, but maybe will work for testing.
    – Daniel R Hicks
    Commented Jan 23, 2016 at 3:44
  • Maybe to get this to work, you could get one of those mountain tamer adapters with something like 20 teeth and put a same size or slightly smaller gear on the wiper motor's shaft. For example, if you wanted it (under no load) to spin the cranks at 100 RPM, that would need a 5:4 reduction so 16 on the motor gear. (16/20 reduction). I think the power would be too little go get any performance with the motor alone but as an assist to pedaling, it might do something beneficial.
    – David
    Commented Jan 23, 2016 at 3:52
  • Getting a slower motor (of the same power) will do nothing different from what simply changing your external gear ratio would do.
    – Daniel R Hicks
    Commented Jan 23, 2016 at 20:17

2 Answers 2

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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?&title=Building-an-Electric-Bike-Part-2&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.

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    Since when do motors "want" to spin at a fixed speed? I counted the revolutions (rotations) of my wiper motor shaft in 1 minute and I got 125. It seems like it is the perfect speed to use as an assist motor (the rider would still pedal). It would be nice if a centrifugal clutch could be attached to it like the kind they use on go carts and mini bikes. If the motor draws up to 100 watts then it might be able to make a difference while pedaling. Many places it is not legal to have electric bikes so pedaling with the motor might save your butt. Who cares if it fries it sounds like fun to try.
    – David
    Commented Jan 23, 2016 at 2:08
  • I am talking about the clutches where no matter how fast the outer ring spins, it will still freewheel if there is no input to the center portion of the clutch. Only when the motor drives that center portion would it engage the outer spindle (whatever it is called) and provide power to the bikes transmission. It would be interesting with a weak motor what would happen. It might just feel like you are running a gear or 2 lower when the motor kicks in. Sure would be interesting to try.
    – David
    Commented Jan 23, 2016 at 2:23
  • The right way to make an electric bike... hundreds of $ or possibly $1000+. My way, use an old junkyard spare wiper motor and some old (but good) batteries lying around (12V 7.2Ah each) and "get 'er dun". What I can do sometime is put it on an ammeter and watch the reading as I put a load on the wiper motor. It is geared down so much that it takes a reasonable load to even make it slow down. I wonder if I attach a crescent wrench to the shaft and secure the body of the wiper motor then juice it up with 12V if it will rip the wrench right out of my hand or if I can hold it still.
    – David
    Commented Jan 23, 2016 at 2:36
  • If it rips it out of my hand then that tells me it may have enough power to drive a multispeed bike. Just start it out in the low gear and work your way up. When it starts to burden then pedal to assist it. If it starts to overheat then turn it off. I think it would be fun to try in the different gears such as when going up a large hill in the granny gear for an extra boost but also if you are chugging along in your favorite gear on level pavement then you encounter a slight headwind for which you would normally downshift 1 or 2 hears but instead you stay in the gear and turn the motor on.
    – David
    Commented Jan 23, 2016 at 2:56
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    compared to the weight of a bike plus rider - plus batteries.
    – jqning
    Commented Jan 23, 2016 at 4:11
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Could always try running 2 in series, and turn 1 off as you gain momentum! But even the lowest commercially available motor kits are more than double a wiper motors power

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