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Friday, January 30, 2009

CrazyBike2.0 Drivetrain

Today I didn't have much time to work on the bike, so all I really got done was trying out some various mockups of chainlines and whatnot for the pedal part of the drivetrain, and getting a final method for attaching the motor chainwheel without wasting a freewheel doing it.

This is the chainline I will probably end up with, using a derailer fixed in one position to tension the chain and hold it up out of the frame/etc.

I chose small chainrings for both ends of the pedal drivetrain mostly because it keeps the chain in a smaller line, making less objects for me to have to keep it out of the way of. I may yet change the chainrings, but right now the front will be 18T, and the rear 16T, not because I wanted them different, but because I don't have a 16T that fits right on the pedal axle, but I do have an 18T off a kid's bike. The 16T rear is because that's what the fixed-gear freewheel has built into it. I might be able to bolt on an 18T chainring to the freewheel, or figure out a mounting method for a 16T cassette chainring to the pedal axle, to make them 1:1 ratio again, but 1.25:1 is not far from that, and it gives me a bit more power in the low-end range of pedalling than the 1:1 would.

I might end up flipping the derailer vertically, and using a mounting point on the rear triangle of the Magna instead of where it currently is clamped to. This will let the tensioner actually do it's job automatically, instead of having to be manually set by the angle of the derailer.

If it can work automatically, then it means I can attach a lever arm to it to rotate a potentiometer, or a magnet for a hall sensor, etc, and use it to measure the tension on the chain as I pedal. That will then be able to control the throttle of the motor--if I'm having to pedal too hard, it'll give the motor more power, so I can stay in my comfortable range without hurting my knees or other joints. (I suspect just having the different seat and pedal position will help that anyway).

This is the freewheel/receiver chainring for the pedals. It's just hanging by the chain in front of where it will be mounted, as a mockup, but you get the idea. The freewheel allows the pedals to not be turned by the motor's input to the wheel, if it's faster than my pedalling. (If the throttle system works correctly, that shouldn't happen very often, except when I use the override throttle on the handlebars).

The orange chainring is the motor's receiver chainring, which is directly mounted to the power-transfer axle, so the motor turning that chainring will drive the rightside normal rear-end chainline/chainrings/wheel.

The adapter plate (silver) behind the orange chainring is off the exercycle, formerly to adapt *it's* huge chainring to the pedals. Unfortunately it's centerhole was too small to fit the shaft here, so initially I was not going to use it. But I finally just got out the big Unibit and drilled out the hole to one size larger. The four mounting holes for the chainring itself don't line up, though, so I had to drill out a new set of holes in the adapter plate just inside the circumference of the original holes. Now I just need to find thinner-headed bolts and nuts to use, without just grinding down the ones I have on it (which I'll do if I have to), so I can more easily fit the pedal receiver chainring's freewheel on here.

The black hex nut you see will not be needed later, because the adapter plate will be welded to a retainer/bearing nut from a similar one-piece crank set, so that threading the chainring on will tighten it as the motor runs, rather than loosen it or potentially spin it.

I will do the same thing for the pedal freewheel, but first I have to extend the threaded portion of the crank's shaft--this means welding a second crankshaft onto the end of the first, which is going to be difficult. I can't just bolt one on, because I have no way to make the counter-clockwise threads needed to keep it all from unscrewing itself as it is used. There's a few steps to that.

First, I took the cut crankshaft I already had and chucked it up in the lathe, machining away at the face of the shaft's end until it was perfectly flat and perpendicular to the shaft. That gives me a much better chance of getting the shafts lined up right. Next I will do the same thing to the shorter cut piece of a second one-piece shaft, which will only be the smaller threaded portion from the left side of the crankshaft. After they're both finished properly, they should mate to give a straight shaft. I'll then clamp them together end-to-end, and weld them together between the threaded sections. This weld will be critical to get right, as it will be taking the load from all the pedalling I do. It won't have to support the motor load, too, fortunately, which means that even if the pedal weld fails at some point, I could in theory limp home on motor only if I had to. I might (for testing stages) make up a second of these shafts, and keep it with me just in case. :)

Once that's done, I'll be able to thread on the pedal freewheel with it's retainer/bearing-nut, which again will just tighten with use rather than unscrew.

More soon, once I get all this drivetrain stuff finished--then I can put the chains on it, setup cabling and brakes and shifters, adjust them all, then try riding it. Then I'll know if it really sucks or if it's actually rideable.

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