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Sunday, August 30, 2009

Pedal Freewheel Mk 2.5

Now I am nearly done making my pedal freewheel. Just the adapter to bolt the chainring to the freewheel is left, and deciding if I want to weld, braze, or JBWeld the freewheel adapter to the crank.

I really don't think JBWeld would hold up, based on past attempts at using it for stuff like this. It just can't take the twisting loads and shear forces like that.

So here's the basic adapter:

The rightside bottom bracket bearing cup is press-fit onto the carefully-filed (but still not perfectly round or centered) crank arm, at the shaft end. Then the freewheel is threaded onto it backwards from the way it would go on a rear wheel, since now it's on the left side of the bike, and has to ratchet in the opposite direction.

Once it's threaded fully on, then the lockring from the BB bearing cup is threaded onto it, and tightened down to ensure the freewheel will not unthread itself with the force of pedalling, since the threads are the same direction as the unscrewing forces would be from the forces on the chainring/freewheel against the crank.

I wanted to round off the crank arm's axle end on the lathe, but the arm is too long to fit above the bed, and moving the bed upwards and securing it so it would not move with the high loads placed on it by the swinging crank arm in it's jaws proved impossible with the things I have here. It just moved too much with each rotation, and would have come loose from the bed (very bad).

If I had something like a pottery wheel with a centerbolt, I could've used that to do it, but I don't, and the time it would take to create something similar is too great right now, even though I could use it for many things later.

I had some 1/8" hard aluminum (dunno what alloy) from scrapped custom-made test fixtures, so I chose that to make the chainring adapter plate since I can more precisely work with it than with steel.

I first clamped down the chainring in question to the surface of the aluminum plate, then used a drywall screw to score around the edges of it as needed to mark where I will need to cut and drill to attach it and clear the teeth, etc. Then I determined the center of it, and used the only hole saw I have (about 1/4" smaller than the freewheel's rim, unfortunately) to bore the center hole. I also drilled out the six chainring mounting holes using the plastic chainring spacer as a guide to mark them first.

The adapter will be the same diameter as the plastic spacer, partly to act as a chain guard for the sprocket (I have had two pairs of pants "eaten" by the chain on this in high winds so far, even though I had the second pair tucked into the sock!). It's also so that I can use it to bolt to larger chainrings should that be desireable later on, without making a new adapter.

The plastic spacer will be used between the adapter and the chainring, just as it was between this chainring and the larger one that actually connected to the spider/crank on the cottered cranks it came off of.

More or less, this is what it might look like if I also use the chainguard that came with it (except that the adapter plate will of course be cut out round).

This is the set of parts for it, including the cottered crank the chainring and plastic spacer came from.

Mostly what's left is to cut out the adapter plate from it's source plate, then get that plate on the lathe (probably using the larger cottered ring bolted to it, minus the crank, so that I can clamp it in the chuck jaws), and then lathe out the center hole to be perfectly round and centered, and as good a fit as I can get on the freewheel. First I have to put the lathe back together, and I was way too overheated and oversunned today to do that.

Then I drill holes near the inner edge to line up with the tooth gaps, and bolt that adapter to the freewheel's teeth. Then I can play with spacing if needed, so it will fit the space between the crank and the BB. Might require some filing away of BB bearing cup hex head on this adapter assembly, but shouldn't take much. It fits perfectly on the right side, but I need it on the left side, which has a BB cup that sticks out a tad farther.

Then all that's left is that brazing/welding decision. I don't have stuff to braze with, but I know someone that does, and I will ask if he will do it or let me use his stuff to do it. Welding I can do here very easily, but I am not confident that I can keep from destroying the threads with spatter or warping if i do it that way, even if I could mask it off somehow.

I am so close; it would be great to not have to windmill my feet to keep up with the motor driving the pedals when I choose not to pedal but want to run the motor for these tests (or later in the winter when I know my knees will hurt a lot worse than they do now).

Also so that I can then come up with a regen-braking-capable shiftable drivetrain for the motor side of things, that won't feed back into the pedals.

1 comment:

  1. I raised the seat on my assisted bike on account of knee protection.

    I think I will give some more though about a less hassle assist set up that offers the benefits simple connection to the rear wheel, preserving the natural freewheeling of the bike. Yet, it needs good gearability to the load and speed.


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