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  1. #1
    don't pedal backwards... MacG's Avatar
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    I had the idea yesterday and wasted most of a class period putzing around getting it to work right. Sheldon's calculators are nice, but they work best with geared bikes. This is more a tool for experimenting with gearing and finding combinations that end up with similar ratios. It displays pretty much all of the possible cog and ring values I have ever seen used for FG/SS applications, but you can add or remove to suit your needs on the fly.

    The main feature is that once you specify your wheel size, you also choose a target gear inchhhhh you would like to achieve, and the spreadsheet will show you which gears are higher and lower and by how much (with a variable spread) in a visual manner. Instead of reading through a chart looking for numbers close to the one you want, you can just look for the whitest boxes or know that increasingly red or increasingly blue boxes are higher or lower based on the spread.

    it sounds complicated, but it's really not. Play with it and tell me what you think. if people think it's useful and should stick around, I'll neaten it up and put it somewhere more accessible.

    http://grit.homelinux.net/gi/

    edit:
    Here are some diameters in inches for common tire sizes. I'll make a dropdown box to do this soon.
    700 x 20 = 26.1
    700 x 23 = 26.3
    700 x 25 = 26.4
    700 x 28 = 26.8
    700 x 32 = 27.0
    700 x 35 = 27.2

    27 x 1 1/8 = 27.0
    27 x 1 1/4 = 27.1

    26 x 1.5 = 24.9
    Last edited by MacG; 02-22-06 at 03:39 PM.
    from Minneapolis, with bike love

  2. #2
    Senior Member mattface's Avatar
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    awesome except I don't know the diameter for 700c x 25

    Maybe you could put in a drop-down of approximate wheel diameters

  3. #3
    hullo. drac_vamp's Avatar
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    totally gnarly and awesome indeed! much props.
    t.h.r.

  4. #4
    don't pedal backwards... MacG's Avatar
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    yeah, that's the kind of stuff I'll need to sit down at home and brew up.

    On that note, if anyone knows where I can find a list of diameters of common wheel/tire combinations, I could greatly simplify things.

    For a 700c setup, I would guess 26.5 inches or so for a starting point.
    from Minneapolis, with bike love

  5. #5
    Iguana Subsystem dolface's Avatar
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    that's pretty sweet

  6. #6
    Senior Member rvabiker's Avatar
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    I agree with Mattface...

  7. #7
    yo yo yo yo yo
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    the difference between 700c and 27inches is what, 8mm? .3 inches?
    maybe put skid patches in there too.

  8. #8
    don't pedal backwards... MacG's Avatar
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    I edited the original post with some common tire diameters I found.

    trons: what do you mean by adding in skid patches?
    from Minneapolis, with bike love

  9. #9
    true till death trial-sin's Avatar
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    very nice...consider it bookmarked!

  10. #10
    Cornucopia of Awesomeness baxtefer's Avatar
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    sweet, though as it is now you can pretty much do the same thing with sheldon's

    how about adding MPH/KMH at X RPM into it as well?
    {o,o**
    |)__)
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    O RLY?

  11. #11
    無くなった HereNT's Avatar
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    Huh? It doesn't seem to want to refresh for me... Maybe it's this stupid IE on the work computer?

  12. #12
    Accident Just Happened
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    Re: Skid patches...
    A fixed-gear drivetrain will have a limited number of skid patches, or areas on the tire that are in contact with the road during a skid. This assumes that the rider always locks the wheel with the same leg position. The number of skid patches is influenced by the same factors that determine gain (excepting crank arm length), although in a different way. The number of skid patches is important for tire wear, low numbers of skid patches will mean that just a few spots on the tire will get all the wear from skidding. Large numbers mean that the wear will be more evenly spread out.

    When I first read about it, I didn't think it was that big of a deal, until I could see the wear on my own tires after my first summer on a fixed. Now I love my 17t cog. I just keep it away from its arch-nemesis, the 51t chainring.

    Noisebeam made a chart
    Which cog/ring would you choose?

    I remember seeing a formula around somewhere as well, but I don't have it bookmarked.
    Last edited by bjorn; 02-22-06 at 04:41 PM.

  13. #13
    don't pedal backwards... MacG's Avatar
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    gotcha. So the more skit patches a gearing setup has, the more likely you are to spread the wear out over different places on the tire when you do skids based on a skid patch being selected somewhat at random based on where the drivetrain was when the rider locked it up.

    That would be really easy to add in if someone knows the formula. I think you could do it the hard way by calculating the angle that the rear wheel turns with 360 degrees of crank rotation. If you keep adding this number up until you reach an even multiple of 360, the number of times you added it together should theoretically be the number of possible skid patterns. Does this sound right?
    from Minneapolis, with bike love

  14. #14
    don't pedal backwards... MacG's Avatar
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    found it. % is the modulus operator (in PHP anyways...) which is just technojumbo for the remainder of a long division operation. cog toothcount divided by the modulus of the gear ratio.

    [cog] / %([ring] / [cog])

    See post #10 in this thread for a mathematical explanation of skid patches and how to calculate them:
    WTF are "skidpatches"?
    Last edited by MacG; 02-22-06 at 11:19 PM.
    from Minneapolis, with bike love

  15. #15
    don't pedal backwards... MacG's Avatar
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    looks like that skid patch formula only works with simple numbers or something. I found another one that nails it every time and built it into the worksheet.

    Now there is a pulldown with some common approximated tire diameters and you can still enter a custom size if you need to.

    You can specify a range of gear inches and/or gear ratios to be displayed in bold in the results.

    You can specify a cadence in rpm and/or wheel speed in mph to have the other value calculated for each combination.

    If anyone has other ideas, blurt 'em out.
    from Minneapolis, with bike love

  16. #16
    ya'll can't mush me vomitron's Avatar
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    This is awesome.

    How about speed in kmh? It's not essential, because I can google, but for figuring out good ratios to spin for target TT times, I usually use kmh.

    What I mean is, if I want to do a flying 200m TT in x amount of seconds, I calculate how many km per second I need, then I figure out at what gear ratio I can do that at if I spin at r number (generally 150-160) of rpm. Again, the only real suggestion I have is a dropdown for kmh/mph.

  17. #17
    ROBOTS... Spor's Avatar
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    This is fantastic. It's always good to see good programming.

    I really like the red/blue function, but you might want to make it optional. It would be nice to just have a non-colored version to use as a reference when no particular gear ratio is desired.

  18. #18
    don't pedal backwards... MacG's Avatar
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    Quote Originally Posted by Spor
    I really like the red/blue function, but you might want to make it optional. It would be nice to just have a non-colored version to use as a reference when no particular gear ratio is desired.
    If you leave the target GI box empty, it will show up all white.

    mph/kmh sounds like the next logical step, too. I'll put that in there sometime today.
    from Minneapolis, with bike love

  19. #19
    Senior Member mattface's Avatar
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    Coolness!!! that is a LOT of information in one little box!

  20. #20
    Accident Just Happened
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    MacG--this is awesome! NICE! A little CSS and that will be a beaut. While you're at it, you could ask for crank arm length to calculate Sheldon Brown's gain value as well:

    http://sheldonbrown.com/gain.html
    One more thing, you could gray out the gear combinations that one shouldn't use.
    Thanks!

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