Discussion in 'Motorbike Technical Discussion' started by Bill Miller., Aug 24, 2008.

  1. Bill Miller.

    Bill Miller. Guest

    Here's my original post and two of the specific questions I asked.

    Let's try these several more times and see if anyone in this group can
    wandering off into the wilderness and yakking about master links and
    any other subject to avoid simply providing the SPECIFIC ANSWERS TO

    On the TW200 I thinking of buying, I understand it comes with a stock
    14 front sprocket and a stock 50 or 52 rear. And since I've clearly
    stated many times that its use will primarily be ON-ROAD, I intend to
    switch out the tires to the TW203 and TW204 plus perhaps some sprocket

    1--What will be the performance results of changing the front to a 15
    or even a 12?

    2--What will be the performance results of changing the rear to a 47
    or a 42 or even a 55?

    By performance results I mean mpg, shift points, higher speed and
    quieter engine operation, and any other aspects of ON-ROAD RIDING.
    Bill Miller., Aug 24, 2008
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  2. Bill Miller.

    Guest Guest

    The real answer is,

    - you may get slightly better mileage if you gear the bike up
    - gearing changes will probably not increase the top speed
    - you need to look at gearing changes as percentages (or just
    fractions) and do your own math.

    Lets say the sprockets are 15:45, or look at it as 45:15.

    That's 3:1.

    If you make the front a 16 tooth, that 45:16 or (45/16:1) = 2.8125:1

    now do (2.8125 - 3)/3 and you get -0.0625, or a -6.25% RPM difference
    for any gear at a given speed as compared before changing the front

    This means that for a given speed of the bike, call it 50mph, the
    engine will be spinning 6.25% slower than it was before.

    If at 50 it spun at 3000rpm in a given gear, at 50mph the engine will
    now spin at 3000 - (3000 x 0.0625) = 2812 rpm.

    This approach works for any gear.

    You can also look at the mph this way- your shift points are going to
    be at a 6% higher speed (mph) than they were before.

    Lower engine RPM will usually mean better mileage and quieter
    operation, but there is no way of predicting what gearing will give
    you the best operation vis-a-vis the operating characteristics you
    desire. You really have to ride the bike, change the gearing, and ride
    the damn thing again to see what the result is.

    For on-road operation I'm sure you will want to restrict yourself to
    larger front sprockets, and/or smaller rear sprockets.
    'Gearing it up' in this fashion will decrease the acceleration, but
    may indeed change the manners of the bike to better suit your intended
    use. Going the other way (gearing it down) would be done if you wanted
    to be able to ride the bike over more difficult off-road terrain.

    It is suggested you try a 1-tooth larger sprocket on the front 1st, as
    this is a cheap mod, and probably can be done with the stock chain.

    Please note that most bikes are geared to pull their maximum possible
    top speed just about at redline. This is why re-gearing rarely nets a
    better top speed, but can frequently successfully lower the engine
    revs at the desired cruising speed.

    I predict you would not at all increase the bikes best MPG (at the
    speed at which the bike gets its best MPG), which is probably at about
    45mph or so.

    If you want a very nice small streetbike, just get a used super-sherpa
    250 and enjoy its 90-100 mpg out of the box. It will be more street
    capable than the TW, no matter how you mod the 200.

    Guest, Aug 29, 2008
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