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Monday, August 24, 2009

Pedal Freewheel

Rather than the foot pegs, I decided to add a freewheel at the pedal chainrings. Now, these things exist already, but I've never even physically seen a bike with them (only pictures). There have been DIY ones before, some of them doing something a bit like what I'm doing, but they were on the right side of the bike, which would make it easier. My pedal chain is on the left side, so it's tougher.

If I had a steel crank on the drivetrain input I'd do this back there, instead, which would again make it easier. But it's aluminum, swaged to the steel chainrings, so I'm afraid if I weld there, I'll destroy (or at least weaken) the swage to the crank which would make the whole system fail, since the motor input is also there. At least if the pedal output section fails during testing this idea, it will only leave me unable to pedal, but the motor would still work. Redundancy. :)

The pic below shows the parts used:

The crank without a chainring on it already will be used for this, so I can leave the other chainring intact in case of failure, and swap them back while on the road if I have to. I've done a little shaping of the area around the square-taper hole to better fit the freewheel/adapter, and to ensure clearance with the bottom bracket of the adapter later on. Hopefully it hasn't weakened it too much, but only time will tell.

The freewheel on the right is a fixed-gear freewheel, to which I'll bolt a chainring with as close to 43 teeth as I can find in my stuff, to match the 43-tooth ring on the other crank I'd just been using. The freewheel is threaded all the way thru from one side to the other, so it can be flipped over to use it in reverse of the normal action or be used on the other side of something.

The left half of a rear hub that's sitting near the crank is the unused part; I only need the ring with the freewheel threads on it, in the bottom part of the pic.

That bit will have the freewheel threaded onto it, then will be welded to the crank.

The other bit there is what's left of the bearing cup/race from inside the threaded section of the hub, which is in the way of doing what I need it to do.

This is a pic of just those latter two parts.

I would *prefer* to weld the former spoke-hole ring to the crank, but if I do that, the threads will be facing the wrong way, and then it will unscrew itself as I pedal. That's why I wish I could do this on the receiving end of the pedal chain; if I had more steel square-taper cranks I could.

This is pretty much what it will look like when attached:

That's a view as if you were looking *thru* the bottom bracket at the inside of the crank.

An oblique-ish view:

A "top" view, showing the spacing.

There will be about 1-2mm between the outermost part of the BB cup threads/nut/etc and the innermost edge of this adapter, once the crank is tightened back down.

The real catch is the welding on of the adapter to the crank. I'm afraid it will end up warping the threads, or damaging the freewheel. The threads might be irrelevant, because once it's welded on the freewheel can no longer be removed anyway, without cutting the adapter off.

What I might end up doing is welding it on by the spoke ring anyway, even though it could unthread. Then use a tack weld across the threads to the adapter and the freewheel to hold the freewheel in place against pedalling forces (which it probably wont' actually do if I had to pedal full force without the motor, but since I'll only be pedalling if I absolutely have to for these tests, and only minimally, it should be ok.

I can still dremel out the tack weld and unthread the freewheel if necessary (for instance, if it breaks).

But I'd rather figure out another way. Perhaps loctite or something similar. Probably insufficient.

There is a possibility that I could weld pair of small bolt to the crank itself, to either side of the square-taper hole, then use those to bolt a plate against the inner edge of the freewheel that would then prevent the freewheel from unthreading itself. It's the most repairable solution, but I am not sure I can find bolts small enough to fit.


  1. Redundancy is a great thing about these hybrid bikes. :) As a last resort, we can always push them home (Done that with that flat and no repair kit onboard). Forget about that with a car.

  2. Well, for various reasons (mostly lack of available tools and parts) this particular version isn't doable. But an accidental discovery led me to a better version, which will be posted separately later tonite. :)

  3. have you gotten anywhere with the pedal freewheel idea? i've google extensively to find something, and thought you could add a wratchet / wrench type of locket to the bicycle pedal.
    what do you think, is it feasible? my email's

  4. Yes, if you look at the Pedal Freewheel Mk2 post, it has what I actually did. I did not yet complete it since I need to slightly machine the crank itself to circularize it, and it won't fit on my lathe.

    Then I just need to complete the adapter disk from the freewheel's diameter to the actual chainring diameter. I haven't finished that part mostly because of not finishing the first part.

    If I could do that part it would work fine.

    Of course, what I'd rather do is put the freewheel on the other end of the pedal chainline, where it joins the motor, but to do that I need to build a completely custom part to hold two freewheels onto the square-taper shaft. While I could make the part itself on the lathe, and hand-file the square-taper hole, I don't have a way to thread the custom part for the freewheels to thread onto it, or to thread the inner part so I can still crank it loose with the "universal" removal tool when I need to fix or change things on the bike.

  5. As for adding a wrench-type ratchet, if you mean placing an actual wrench ratchet on a crank and using a socket adapter or extension to attach to the square-taper crankshaft, well, in theory it might work if you had a high-quality tool to start from. But buying a tool of that quality (because anything I already have would just break with the torque loads presented at the pedals when used by themselves) would cost me as much as buying an actual pre-made pedal freewheel crankset. ;)

    A large 3/4" sized ratcheting wrench would have to be used as the starting point, so that it would be able to take the strain at the square socket point.

    It would also require either welding or cotterpinning the socket/adapter to the square-taper shaft, as there would be no way to use the regular crank bolt thru the wrench as it is solid in the center. I am not sure the cotterpinning would work, even with two of them 90-degree offset and a distance apart on the shaft--it might just shear thru them, or snap the shaft or socket at the now weaker points where the holes would be drilled.

    Welding it on would prevent changing to some other method, or replacing the socket/adapter if it's square input hole was damaged (from the torque), unless the whole crankshaft was changed. I don't think the metal would last thru cutting off the socket/adapter and rewelding a new one on.

    Another problem is that there would be nothing holding the crank onto the socket/adapter except the ball-bearing socket retainer, so if there were any load pushing against that in a sideways direction, depending on how it's pedalled, it could slowly push the crank off.

    Another problem is that of mounting the chainring itself. It would need to be welded to the socket/adapter, most likely, or at least a "spider" for it would need to be made that would allow that.

    If all those things could be overcome, then yes, it might work. But it's easier to do it as I show in the Pedal Freewheel Mk2 post. :)


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