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Tuesday, January 12, 2010

Trike Motor Mount Thoughts

A little redrawing needed. I found the article by Julian Edgar I had been thinking of that explained this stuff, which reminded me of a few things.

The "steering inclination angle" for scrub radius is indeed pointing at the center*line* of the tire, but not the contact patch itself. I still need to have caster (trail) so it needs to point ahead of the contact patch. ;)

I also came up with a couple of possible ways to add the motors to the front wheels, but I am thinking I might want to have shiftable gears, at least two of them, since I end up in stop-and-go traffic often enough to make the power usage horrible, since it will use a lot more current at slower speeds if it's geared high enough for 20MPH.

Apologies for the incoherence of the sketches; I did them while eating dinner since I had the ideas in the middle of that. So they're not pretty. Later they'll be translated into Sketchup if worthwhile.

One issue I have is that i need to get about 5:1 gearing if I use only 24V to drive each motor, because they were originally designed to drive a 10" wheel at 8MPH. To get a 20" wheel up to 20MPH, I need 5 times the output RPM from the motor. On CrazyBike2, that's partly acheived by running it at 36V (now 48V), and partly by the bike drivetrain. Plus, the motor on there runs about 200RPM at 24V IIRC, and these only run about 135RPM or less. So it is a challenge either way, if I use a series wiring setup as the electrical equivalent to a "differential".

One of the ideas uses rear triangles on the front, with the old style one-piece cranks for the triple, and a 5-speed cassette on the back. Only the back gets a derailer, because the triple isn't used for shifting, just as a jackshaft. The left side triangle keeps it's triple. Only a single large ring will be used on the right side's triangle instead of a triple, with a granny ring welded on to the left end of the shaft.

The motor would bolt to a bracket fixed to the triangle in a way that leaves it below the chainstays (for COG reasons) and on the "inside" of whichever side it's on. On the left side wheel it's on the right side of the stays, so that the chainring mounted on the axle hub will be lined up with the granny ring of the triple. The right side wheel will have the motor on the left side of the stays, running to the welded-on granny. The large ring of either side will run to the wheel, shiftable to each of the cassette rings by the derailer. Both derailers would be operated at the same time, by a ganged shifter setup.

Has to be a ganged setup so I can properly adjust each derailer for shifting, while still shifting at as close to the same time as possible. Will also have to shift while not under load, cutting throttle way back during the shift, so that I don't end up being pulled back and forth on the road by the difference in speed while one wheel is shifted but the other is not.

I fully expect this to be too complex to pull off well, so an alternative is to use the regular front fork, and set up the wheelchair motors themselves as if they were the bottom bracket with two rings and a front derailer. This would require putting the U of the fork in front of the wheel instead of behind, and might require putting the righthand motor on the outside of the wheel, which I don't want to do.

Now, I know the reports on the internally-geared hubs under motor power aren't good, but if I used one for each wheel, and ensured no shifting under load and no sudden starts, I might get away with using them. This would make it a LOT simpler to use these wheelchair motors and gearboxes to drive the front wheels. Ganging their shifters would present the only real challenge. Besides actually acquiring two identical ones as part of scrapped bikes, that is. ;)

If I don't use a shiftable transmission, then I can go with a simple chain and ring, but I will probably have to use a two-stage to do it, due to the large ratio required, with the largest ring on the motor, and the smallest on the wheel, with an intermediate jackshaft making the second stage. I could do a 48 to 21 first stage, and another 48 to 21 second stage. Alternately, a 48 to 16 first stage, and 34 or 32 to 16 second stage, to keep stuff a little smaller with a little less chain weight.

Any way I do it, I will need to widen the U forks to accomodate a regular freewheeling cassette, even if I only use one of the rings on it, with no shifter.

The original thought before i realized the ratio needed was just to use a belt and pulleys:

The tank style steering:

More later as I think of it.


  1. I have to ask. Why are you so dead set on driving the front wheels? It's so much simpler to tie the motor drives into the rear one!

  2. First and foremost, because I have already had enough problems with integrating the pedal and motor drives onto one wheel causing me problems getting home when something fails that I would like to keep them separated. As long as a wheel is not actually destroyed, I would be able to disconnect even the entire motor system and still limp home on pedals, or use only the motor should even the entire pedal drivetrain fail.

    Plus, pulling thru powered turns is likely to be more stable than pushing, with the forces then acting to drag the bike around the turn instead of the turn dragging the power source direction around.

    It also lets me divide my power drive easily among two independent ways to get it to the road. Three if you count the pedals to the rear wheel.

    Another issue is dished wheels. They're not as strong, and I have already shown to myself quite well that the rear dished side can break a lot easier than the non dished side, and both sides easier than the front wheel which is not dished.

    Part of that problem is the motor torque being applied thru the spokes, for which I have a different possible solution I haven't fully worked out to post yet, of applying the power to the rim directly rather than pushing it thru the spokes first.

    It's also another challenge. :) I like challenges, as they are what keep what little sanity I still have. It'd be fairly easy to copy the drive design I already used in CrazyBike2, with a couple of minor changes that would make it easier to align/etc, but that is not challenging. I can make those minor changes to CB2 itself, once I get the SLA batteries out of the way.

    Doing things differently is more interesting, if also more troublesome.

  3. I'll have to agree with you on doing things differently can make it a challenge.

    I guess it just confuses me because I have never had the problems you have with the rear wheels. I've dished a few front ones but all but one was my own fault doing donuts. A road grate got the other one.

    The only thing I have ever had happen on the rear (besides a broke chain now and then and it's always the one from the pedals that breaks) is a freewheel quit (wore out) on the rear wheel and that can happen without a motor.

    I think you have just been overloading the wheels with weight more than anything else.
    Using 3 on this one should help there.

    The current chain setup I use now works well and was easy to do. I like having all the gears for the motor too.

    I can perfectly understand your want to have separate drive capabilities though. My little 2wheel drive e-scooter proved how handy that was a couple of weeks ago when the rear chain snapped on me. I got home just fine using just the front motor drive.

    By the way, how are the new battery packs coming along?

  4. I know I'm overloading the wheels, and that the potholes are doing them in because they're already so close to or at their limits. Suspension will help that, in that it keeps the potholes from impacting so hard.

    All the chain and sprocket problems I have had are from my own inability to create an aligned chain system.

    The dishing I refer to in this post is the pre-dished state of most rear wheels. The wheels designed for multi-speed cassettes must be dished in on the right side to allow the rear wheel to still be centered in the dropouts while still having space on the right for the cassette.

    Wheels with only a single freewheel or sprocket on the right don't need dishing, or at least not as severe. That makes them stronger wheels to start with, at least for sideways forces. It also puts less pre-tension on the right side spokes relative to the left side.

    I also like having the gears on the motor, and that shifting is simple as all power sources are shifted the same. But it's definitely harder on the drivetrain, and so far I haven't been happy with the problems created by having it this way.

    There are not many problems with it, or very often, but when they happen they are all disaster-class, usually stopping me in my tracks and sometimes leaving me unable to even pedal it home. That last is the deal-killer.

    I need reliability, and since the stuff I use is used, sometimes worn out, and my inexpert ways of using it don't help, I need to compensate for that as well as I can. :)


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