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Thursday, November 8, 2007

Minerva sprang from the head of Jove...thus 1.1.0 is born

While on the phone talking with Steven about other things, while marking out the metal pieces to make the mounting brackets I spoke of before, I had one of those "Minerva" ideas, the kind your brain has worked out in a fair amount of detail, then springs it upon you fully formed.

I was already going to use a part of the Murray's rear frame on the *front* wheel to hold the motor, gears, shifters, etc, but wasn't doing that on the back because it wouldn't really fit the way I wanted it to, and wouldn't mount right. I'd also found that that particular frame part was too bent up at the wheel mounts to work properly with a full wheel on it anyway (wobbled too much, and the metal was soft from the wheel bolt U cutout to the rear derailer mount).

However, I have a stack of old bikes/frames now, and at that moment I was looking at the wrecked Schwinn Ranger, which is a 24" bike, not a 26", but could be modified to work, and is a sturdy frame on the mostly-undamaged back half. It's derailer mount is bent a bit, but the U-cutout for the wheel bolt is not. I wanted to think about it for a bit, so I waited until I went with Steven to Bike Den, where I got one tool I needed (freewheel remover) and though they were out of the crank removal tools for sale, they had the one they use in their shop, and removed the cranks for me, from that Schwinn frame.

After I got home, it was a bit late for power tools, so out comes the hacksaw, and I got everything from the seatpost down to the crank/pedal bearings on back off the frame in one section (just like I did off the Murray before). Now, the way I want to mount it is to the wheel bolt itself, the same way the wheel is already mounted to the bike itself, but in reverse--the seatpost will be *behind* the tire instead of in front of it.

However, to do that, the wheel end of the frame piece I'm adding has to be expanded to fit *outside* the actual bike frame at the same point (because they're the same width, naturally, before modification).

I've got a 2-ton hydraulic pump jack from when I had to fix my mom's car a fair bit, so I dug it out and wedged it between the wheel mount points on the frame piece I'm adding, and jacked it apart about 1/2", just enough to slip snugly over the existing frame. This still leaves me enough wheel bolt to tighten the nuts down on securely, which holds both frames together as well. The seatpost top has a welded-on clamp for the seat itself to slide down into the tube and be clamped by, and I took the bolt out of it and am making a ring that will go thru that and around the tubing my rear cargo rack is made of, to secure the frame section from moving up or down.

Now the motor will be mounted along the right side of the frame section, just between the back of my normal rear derailer and the crank mount of the frame section. The 15/16" socket I needed I could never find in the right kind that would not be too loose, and the 7/8" sockets are all too small, and I couldn't find anywhere that had the size between them in 12-point star (only in hex). So I ended up filing the gear teeth in a couple of places just a teeny bit (less than a millimeter) on the motor's gearbox output gear, and now it fits very snugly in my original 7/8" socket, but comes out if I need it to. I bolted the socket into the crank mount on the right side, using the same bolt that held the crank on, and it tightens in the direction it has to turn to drive the wheel, so it should never come loose in operation even without loctite and such.

I *would* like to find a 7/8" socket that has a larger drive square hole in it; I think the one I have is 3/8"; if I could use something a hair bigger than a 1/2" drive (which they don't make) it would fit onto the square crank mount and guarantee not to spin in place as the motor torques against it (in "lab" tests so far, it hasn't been a problem, but no road tests have been done yet since the bike's not finished enough). I'd just need to get some washers to stack onto the crank mount to hold the socket against the bolt's underside to keep it tensioned against the motor's drive gear (otherwise it would end up sliding down the crank mount away from the motor and no longer engage the gear, as the crank mount's shaft is about as long as the socket's depth).

The 3 crank-chainrings are mounted on the left side, to line up with the sprocket that's been mounted on the left side of the rear wheel. It's still little-to-big, but closer to a 1:1.2 ratio instead of 1:3. I have a modification to fix that and add the absolutely necessary freewheeler to the left side, but I have some other work to do before I can build that. It will require cutting the axle-shaft (not the bolt, just the wheel portion) of two identical rear wheels, and taking the freewheel half including the spokes and such (not the rim) from one, and replacing the left (non-freewheel) half of the other wheel with it. I'd *like* to re-weld the two halves of the axle-shaft back together, but that's not something I have equipment for yet, so the best I can do is a small ring of pipe (or something) JBWelded across the two pieces to keep all torque forces as equal as possible from one side to the other. Once that's done, I *also* have to modify the freewheeler on the left side so that it works in reverse (which may require machining a new ratchet ring), otherwise it won't do me any good (since it will freewheel in the forward direction, not the reverse, as it must do).

There is no place the wheel's derailer can be mounted on for the right side currently, because of the size of the wheel's sprocket/chainring, so for the current tests the chain has been shortened to exactly fit the distance between the wheel's chainring and the motor's chainring (since without the wheel's derailer, there's also no tensioner). I'll re-add the chain back into it once I have a freewheeler in there, since it will then be small enough for the derailer to work normally. Derailer must be mounted "upside down" relative to the way they normally would, so it works in the right direction of chain path to perform it's job properly. For that reason, I may only use a wheel derailer, and leave the motor derailer off entirely (since I would need to weld on a few inches of post *below* the crank mounts in order to have a place to mount it). There are other "little issues" I have to work out, too, mostly clearance of parts to each other.

One more thing this frame section gives me is posts for a second set of rear brakes, which I will likely need to add due to the extra mass from all the motor/battery/frame/etc so I can have the same stopping power I did before. But those will wait until I need them, since they add even more complexity to the system (more cabling, figuring out where to put the Y-cable splice to the rear brakes, etc). Normally, I'd want better front brakes, but since almost all the weight is added at the rear wheel itself, or behind it, it will force the balance to shift less to the front during braking, especially on level surfaces (where I usually ride). So more rear braking is ok in this case, though I will eventually probably improve the front braking as well, in some other way.

So, at the moment, it's mostly bolted together, with some various mounts in the process of marking and making, again hoping by this Saturday I have a testable v1.1.0.

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