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Friday, October 31, 2008

Swivel Joint Works; Cable Steering and Drivetrain Thoughts

Well, I still haven't found another of those nylon rings or anything I can make one from, but I did get the bracket-mounting plate attached to the front frame, and test-fitted it together.

Oh, and I painted it all. :-) Only a temporary paintjob, as I already know the crappy RustOleum would come off no matter how I paint it, so I didnt' bother with primer/etc, and just did this to protect the many areas of bare metal on the bike, where I've had to remove large areas of the original paint to weld the parts together. Should at least keep it from rusting up. Be my luck that the only areas the paint won't come off of are the ones already a little rusty. ;-)

I dont' know that this will be the final paint scheme; I think there is too much orange and not enough yellow up front. The red on the rear triangle also looks just like the orange in these images (perhaps a shade darker). Hopefully once I get the Krylon version of these colors and repaint it all after it's a usable bike (or it's MarkII if this one has too many flaws to fix), the Krylon red will be very red compared to the orange. If not, perhaps I'll dust it with pink.

Here's some pics of the joint itself, without the chainrings in the way:

The first pic shows it from slightly below, behind, and to the right. The second is from the bottom. The third is a simple side view from the right. You can clearly see the black nylon ring and how it fits flush to the bottom bracket tube, and the gap the U-bolt has on the left side without one. The only modification I might make to the ring is to notch the inner part where it meets the chainstay, to allow it to fit a little straighter on both sides.

The reason the metal plate is not symmetrical side-to-side is because the chainring side needs more clearance, so the chain itself doesn't catch the corners of the plate. Even as it is, it will need a bit of rounding at the corners themselves to prevent any touching of the inner edge of the chain on the plate when shifted onto the grannyring--it's a VERY close fit.

The plate itself is actually two identical 1/8" plates welded together along the edges. I still need to trim the angled piece off, and weld that edge together, assuming I can't think of anything to bolt to it (left it on just in case of that). The plates came off a washing machine, for the tub and motor supports. They appear to have been designed to take more mass (given a full tub of water's weight) than I will be putting on that pivot. I tested one of them (there were four) by clamping it to the bench vise and trying to bend it, and could not. I even hit it with a largish hammer, and only bent it a little at the edge where the hammer struck. So it should work for at least the testing stages, and if it shows signs of stress I'll find something else to replace it with.

Drilling the holes was pretty tough, as I could not press hard enough with the hand drill while keeping the drill straight to even begin the holes. After an hour of trying that, I was about ready to go find a phaser and shoot the holes in it. Then I realized we don't have those yet. :-)

So the lathe came to the rescue, by placing the drillpress chuck in the lathe's chuck, and clamping the plate to the toolholder, so I could use the lathe's own cranks and adjustments to press the metal against the bit much more precisely and powerfully than I could possibly do by hand. Since I ran the lathe at it's slowest speed so as not to overheat the bit or the plate, it did take a while, perhaps 10 minutes per hole, but it worked perfectly.

Here's the problem this whole joint was designed to solve:

The motor's chainline would change length if I had to have the swivel point anywhere other than the center of the bottom bracket's axle, which is the center of one end of the motor's chainline. The chainline of the rear drivetrain (where all the power from both motor and pedals goes to the rear wheel) also has to have this point as the center of one end.

Of course, I could always run that motor to a point earlier in the front chainline than this, but I really didn't want to, partly because it would add more complexity and partly because it would likely force me to put the motor farther forward--I'd rather have it in the center with me (basically it's directly under the line between my head and the road). I'm trying to distribute the weight as evenly as possible about the center of the bike, and also keep it all as low to the road as I can.

The swivel point I chose is the most elegant solution to the chainline problem and the rear shock problem I could come up with. I've looked at quite a lot of rear-shocked bikes, and none have anything that would allow the chainlines to not have to change length as the shock operates. Most of them actually cause the rear chainline to change length during operation, which seems silly to me, as there is no need for that to happen. But most have the swivel point behind the bottom bracket, instead of in front of it, and none have it *at* the bottom bracket's axis. I guess for most bikes it doesn't really affect the operation significantly, as the rear derailer already has a tensioner that takes up the slack easily enough. But it prevents any bike that doesn't have a tensioner in the rear chainline from having a rear shock that pivots (or at least, it runs the risk of jumping the chain off a sprocket if something were to jolt the chain in the wrong direction during a shock that loosened the chain because of the pivoting).

I wonder if something like this idea is patentable. :-) Probably not. Just in case it really is a new idea, and someone tries to patent it for themselves, I'd like to offer the idea for free use, under the license of share and share alike, where improvements to the idea must be made freely available under the same license. :-)

The bottom bracket's left side is going to be a bit complex to make, as it is the point where both the pedal drivetrain and the motor drivetrain come together to drive the bottom bracket's axle, which then drives the rear chain/drivetrain on the right side. The bottom bracket axle is of the square mounting point type with a threaded hole. The hope is that I can use a long bolt I already have that fits the threads in that hole, secure it to the axle (by welding if need be) so that counter-clockwise rotation isnt' going to unscrew it. Then I will need to mount two freewheels to it, one in line with the motor's sprocket, and one in line with the pedal's second sprocket.

The motor has to be geared down as far as possible so that I can run it at a higher speed and still keep the bottom bracket's rotation at about the same speed that pedalling it will give. I don't know exactly what that speed is yet, as I have not determined the gearing from the front pedal sprocket to the rightside second pedal sprocket, and then from the leftside of that to this bottom bracket assembly. What I will most likely have to do (because of the severe limitations in which parts I have available to fit the motor's shaft that also have a very high ratio between them and their matching larger sprockets for the bottom bracket axle) is to calculate the rate at which the bottom bracket will need to spin if the wheel is to go a particular speed, for the range of speeds the bike is designed to operate at, for each of the combinations of chainrings on that drivetrain.

Then I will have to calculate how fast the motor will need to spin (based on whatever ratio I can get in the motor's chainrings) to get that speed at the bottom bracket, and setup the controller so it delivers that voltage as needed.

I then have to calculate out appropriate ratios for the chainrings in the pedal drivetrain so that I will be pedalling around 80-90RPM or so (my comfortable base cadence) and still get the appropriate speed at the bottom bracket. I'm sure I won't be able to find exact parts to do this in the stuff I have laying around, but I'll get as close as I can. :-)

Now here's where the steering is going to complicate things for me.

As you can see, a simple chain all the way won't work, because the angle of the front fork I had to use is too steep to allow a straight line back to the USS crossbar. That means the chain will not properly engage the sprocket on the steering column of the fork, and will not turn it very far (about 20-something degrees) before the chain begins to just come off of it. I already knew this would be a problem, and had planned on a cable attached to each end of just enough chain to be able to fully rotate the front fork for full steering angle, which is less than 180°.

The cable bits I was going to use were brake or shifter cables, but I found it is very difficult to attach and bind them together properly around the ends of the chain's links, in a way that will not come loose. They don't bend easily enough to simply run a full loop from bar to chainlink and back to the same bar, without crimping and then sometimes breaking some of the individual strands in the cable at the bend point (at the chainlink end).

Then I was taking apart what someone had left behind of a large copier, and found these pulleys and cables in the document scanning section on top:

The pulleys are better than the ones I had originally intended to use, although their "steel" mounting brackets are of so soft a metal I can bend it in one hand. And I'm a wimp, so.... I'll have to move them to a better bracket if I use them. Anyway, the cables are teflon-coated, and a little thinner than brake cable, but quite flexible. I'm sure they're nowhere near as strong as brake/shifter cable, though. But they'll make it easier to play around with various cable steering systems without messing up and wasting a bunch of otherwise-usable bike-type cables. Once I have a working solution, I can then replace them with tougher cables. The pulleys will handle either kind.

They would be mounted on a post that intersects the line the chain would make if it continued rearward and downward at the same angle the sprocket on the fork is at.

Imagine in the pic above that the chain only goes about 1/3 of the way back from the sprocket to the post, with the rest of the distance covered by cable. The cable loops from there around the pulley (one on each side of the post, or stacked in the center on a single pivot axis, whichever is easier and/or works better), and on to the adjusting clamps on the USS crossbar. Thus the flexible cable will make the angle change at the pulley, and should not cause a problem of jumping tracks like the chain would, and the chain is all at the straight line angle of the fork's sprocket.

I'd like to use a simpler steering mechanism, but I haven't got the parts needed to make either a U-jointed shaft-style or a rod-pushed/pulled system, and buying either of those sets of components would be too much of a break in the goal of using only recycled parts on this bike. I still hope to find some recyclable U-bolts I can use for the swivel joint, instead of the cheap purchased ones I have now.

Tomorrow I am going to try to get the steering working, and if there is time, the pedal portion of the drivetrain. Might not be possible on the latter, as I still have to make the axis extension and freewheel system on the left side of the bottom bracket. Maybe I'll just do the steering and the seat's covering (which I just have to cut, sew edges together on, pop-in the lacing rings, and then lace to the seat frame). Then I can at least do downhill test runs of the steering and shock systems.

Oh, I guess I should at least cable up the brakes, too. :-)

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