Powercranks for Less Than $ 6.00





Introduction: Powercranks for Less Than $ 6.00

The other day I found some videos on the internet about powercranks and they seem quite interesting until you get to know the price, more than a thousand dollars in the US plus the smuggler fee and bribes to import those things to my third world country, we will be talking about $3,000, I could buy a car for that, and considering that my bike cost less than $100 this things are just wild dreams.

But that doesn't stop me so I decided I would construct my own, so after seeing some more videos I figured out how they work, but before I could design and manufacture an equivalent product I need to make a fast prototype for testing. This instructable is the way i made that prototype which really exceeded my expectations and you could use it to make your own and save a lot of cash.

But what exactly are this things, well to my understanding they have a freewheel mechanism between the axle of the crankset and the cranks which make each crank independent from each other so you need to actively maintain the 180 degree separation of the cranks.

This supposedly makes you go faster (hence the price tag), but I really don't care about that, they just seem fun to me.

Step 1: Materials

You will need the following things

1.- Cheap chinese steel crankset. I used the one that came with my bike but a new one cost about $11.00. You could buy one new or salvage one from another bike if you don't want to destroy your actual crankset.

2.- 2 cheap chinese frewheels. One speed freewheel BMX type, I got these for $0.90 each at the chinese store

3.- Mighty Putty (HI BILLY MAYS HERE!!!)

4.- A handsaw, a file, a grinding stone and welding equipment.

5.- Patience. (Yoda: "The boy has no patience." Obi-Wan: "He will learn patience.")

Step 2: Cut the Cranks

Before you cut the crank you want to measure your crankset so you could rebuild it with the same dimensions specifically crank length and pedal offset.

Just cut the crank arm and file the part that attach into the bottom bracket so it can hold the freewheel. I was very lucky that the inside diameter of the freewheel was almost the same diameter of the round part of the crank arm.

Cut enough material to make sure that the axis of rotation of the two pieces is concentric and parallel mine is misaligned by 0.4 mm, which I considered it was good enough. The front part of the freewheel (the one with the brand on it) need to point to the right side of the bicycle in the two arms.

Cut the crank arms to the correct length and file some of the teeth of the freewheel so you can connect the crank arm to the exterior race of the freewheel.

This is a good opportunity If you want to change the lengh of your cranks if you are too tall or too short. I suggest you read this http://www.nettally.com/palmk/crankset.html

Step 3: Assembly

Assemble the cranks using the epoxy putty or if you have an arc welding machine just weld the three pieces together.

I used the epoxy because that way when I go to the welder he will not misalign my cranks, What he did was to put a piece of metal between the crank arm and the exterior of the freewheel an weld the center of the crank arm to the interior of the freewheel. He charged me $4.00 for that.

Just be careful not to weld the freewheel mechanism together.

Step 4: Enjoy

Put some paint in your cranks and back into your bicycle and enjoy the ride which will be much more difficult than ever.

From the little testing I have done so far I draw the following conclusions

1.- You feel which one of your legs is the weakest because you lose your coordination. In my case the right leg is the weak one
2.- You feel muscle pain from other muscles that probably you never exercised before. In short you feel like you when stop riding for the winter and start again.
3.- Is a little bit difficult to maintain coordination but I think I'm getting better at it.
4.- You could drive both of your legs in parallel like a kangaroo, is funny as hell and very hard.
5.- I haven't got outside to test this in traffic. I will get out as soon as I feel more confident.

Now I would expose the disadvantages of this design:

1.- Is ugly as a neimodian. Well yes, but if you are or have access to a good welder you could skip the epoxy and just weld the components and it will look better, also you could file all the teeth of the freewheel, but as I said this was the first prototype maybe I made another one.
2.- I don't know how much radial force and torque the freewheels are designed to withstand. but seem they are holding up well.
3.- The freewheels have a backlash of 16 degrees. I suppose that is desirable to have this backlash as minimum as possible but I don't know if this is small enough.

I will test this design for at least 2000 km or until I get bored to see what else comes up and probably will make another design a little bit more refined, but  all things considered I think the results are good for just an afternoon of work and less than six dollars of total expenditure.

Here is a sample of how they work.



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    91 Discussions

    I've never seen this idea before. Thanks for posting this, it's a fantastic hack you've done there.

    I've just had a total hip replacement after 27 years of having a very weak and oft painful right hip. I think, in terms of range of movement, it was about 40% before the surgery, now it's 60% flexible after 3 months, but I can only motor it to 40% range with still little power. I've used a bike for mobility for the past 15 years, as my good left leg can compensate for all the weakness in my right. I can see instantly that something like this may be beneficial for my rehab, but I'm not sure that I'd be able to ride it at all. I wonder if I could ride something like this, that some form of lock on the cranks wouldn't be a bad idea, so I could lock it out if I was too tired to operate it properly. I guess it would take a couple of sliding lock pins.

    7 replies

    I came across your old post on the power cranks.this is a great hack,but power cranks can be a work out,if your not use to them.Their are a few ways to get around limited knee flexion,,the best I have seen is a modified the crank arm,through some measurements,you cut the crank into two pieces,the lower part with the pedal,pivots around the upper part.This is for experienced DIY,if the crank does not pivot, or fails you could be seriously hurt.Their are a few places you can buy them,or have them built.Google or Bing, Limited knee flexion bike cranks or similar word phasing

    Dewex, independent cranks are truly a most powerful hip rehab device. If you have already tried this I would be interested in how it went. If you haven't tried this and still are having trouble please give me a call. We will see if we can help you out.


    If you were planning on using just your left leg if the right was too tired, all you would have to do is just not move your right foot. As you can see in the video between :30 and :40, this system allows you to use just one foot to pedal.

    I suppose they'd have to be toe-clipped or clip-ons. I reckon if I did build something, I'd probably only do rolling road with it indoors, as handling in traffic with all the stop/starts would be too tricksy.

    This whole system doesn't work unless they have a retention system. In this video he's using special shoes that clip onto the pedal.

    I understand now. Thanks. You'd have to be able to back pedal the cranks independently, so they'd have to be connected to your feet to enable you to lift them. I still think this would be OK for me on a rolling road. Thanks for your reply.

    Hey men!!! U R GREAT!!!! Super cool...

    Thanks. I also made the rolling road (rollers) about 8 months ago since the one at the bike shop was too expensive for me. Soon I will post an instructable of how I made it.

    hi, I didnt find any on www.benotto.com.mx, plus they dont sell in my area... I'm interested on those chinese freewheels, know any webpage or something?
    I live in Spain

    1 reply

    Sorry I don't know anything in your area but you can get in contact directly with the manufacturer in http://www.made-in-china.com just search for freewheel or any other bike part and it will display a list of companies, you could ask if they have an importer in your area or even you could become one, but I assume you need to place a large order.

    that's pretty awesome! I thought it said PowerTap for 6.00 and I was jumping for joy before I noticed this will work well though :P, good 'ible

    1 reply

    Yes they will look very silly riding like a kangaroo, but maybe it could be a big thing in Australia...