Ghost cog (long)

Discussion in 'Cycling Equipment' started by Doug Goncz, May 2, 2003.

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  1. Doug Goncz

    Doug Goncz Guest

    There's a PM DC motor rated at around 500 watts in the center of my Peugot diamond frame. It's
    mounted on a board and there's an 8 tooth pinion on it. There's a flip flop hub in the rear, with a
    conventional freewheel, and a modified spider with a 48 tooth cog driving the motor at 6 times the
    wheel rpm.

    This is a long run of chain and I haven't been able to set up a jockey to keep it running smoothly.
    So I tried a ghost cog.

    A ghost cog is something I read about in a web page on tandems. It goes between the captain's left
    chain ring and the stoker's left one, in the particular tandem chaining configuration the name of
    which I cannot remember.

    Anyway, whether the captain and stoker are in phase or not, these cogs are usually the same size,
    and the chain tension is adjusted with an eccentric bottom bracket on the stoker's position. A ghost
    cog fits between the two and has no pivot. It may be the same size as the two chain rings or larger,
    and it moves tooth for tooth the same along the upper side of the chain and the lower, so it stays
    where it is.

    I should have known not to get on Route 50.

    Just over the first hill, the smaller of two ghost cogs I had squeezed into the chain let go. It
    went out in traffic. I waited, retrieved it, reinstalled it, then lost it on a turn later. I
    couldn't find it.

    So I went on with just the one. It's 44 teeth. It just cleared the 48 tooth cog on the rear wheel.
    The chain tension is adjustable. The motor mount board is adjustable with a turnbuckle to pull it
    forward and I found that to keep it in place I had to run it pretty tight. I could feel the

    By the end of my ride, I was feeling worn out and reduced the tension on the turnbuckle. The cog
    promptly went for a ride on its own. So I stuck it in my trunk bag.

    Wow! What a difference! What had not been clear to me at the start of the ride became obvious. The
    return loop of chain should be pretty slack to get the least drag on the motor drive.

    So I guess I will have to pursue my jockey idea. I have a Greenfield rear kick stand and there's a
    place to mount a plate that the jockey, which I have already made up, can be mounted to. It's just
    the one link in the parallelogram that holds the jockey's pivot pin, expertly shaped on my little
    mill, with the two rivet ears bent out. It is to be riveted in position, then the plate bent to
    adjust the jockey.

    I wonder if my pinion is worn... It had to be tempered for machining but hasn't been annealed, so
    it's a little soft. It can be rehardened. I guess I will have to try hardening it. I'll check the
    chain stretch and see how well it wraps around the pinion.

    That little pinion gets a lot of action.

    Anyway, I can state with some confidence, after an afternoon of trouble, that a ghost cog only works
    betwee two chain wheels of equal size. If you run a large-to-small setup like mine, you have to
    crank down the chain tension so much that the drag becomes intolerable, just to keep the setup from
    ejecting the ghost cog in the direction of the larger chain wheel. Patooey. There it goes again....

    A lesson learned.

    With a two pound, two inch diameter rotor in the motor, spinning at six times the wheel revolution,
    um, let's see six squared is 36, and 26/2 is 13, so it's as if about SIX FRIGGIN' POUNDS have been
    added to the rear rim. Whew!

    Actually, my intent is to give the bike even more virtual mass. I hope to procure through the
    Society of Physics Students a three thousand dollar ultracapacitor. That should allow me to
    accelerate out of Greenwood Drive and reach top speed at the bottom of the hill near the 7-11. The
    ultracapacitor should then speed me to the top, in effect "flattening" the hill. Or at least, that's
    what this experiment is designed to determine. When I am done with it, the ultracapacitor will go to
    the college physics lab.

    Only by wearing myself down this way and seeing what doesn't work do I get to the place where I can
    do the math in my head like I just did and have it come out right.

    That's right, isn't it? With two pounds of steel on a two inch diameter at six times the wheel
    speed, I am feeling the same inertia as six pounds of normally unwanted mass at the 26 inch rim
    diameter? Roughly? That's certainly how it feels. It sure seems to make pedaling in circles a

    Plus the drag of the chain, and the extra weight of motor, mount, cog, and chain.... But those
    aren't inertial figures, it's just drag and weight. Much easier to understand.

    You see, if I accelerate at a reasonable effort out of Greenwood Drive with the unmodified cycle, I
    reach top speed well before the hill, wasting a lot of time moving air out of the way. And that
    pisses me off. With the extra virtual mass, I can stretch out that runup and hopefully, top the hill
    without ever hitting bottom gear.

    I guess I should try filling the tires with water to see what a few more pounds of rim weight,
    excuse me, mass, would do. I wonder if Fischer's has those filling adapters on sale...


    Doug Goncz, Replikon Research, Seven Corners, VA If a computer won't do
    what needs to be done, lie to it. Don't try this trick on people.

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