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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 02-28-06, 02:17 AM   #1
Fugazi Dave
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Skid patch comparison chart...

So....someone should make a chart in Excel or something with cog sizes on the x axis, chainring sizes on the Y axis, and # of resulting skid patches for corresponding ratios.

I could do this myself, but I've already fallen asleep twice at my desk tonight. Time for bed.
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Old 02-28-06, 02:28 AM   #2
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it's been done already. ****ing wish i could remember what thread it's in.
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Old 02-28-06, 02:31 AM   #3
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WTF are "skidpatches"?

okay, down the page halfway. some nerdy math about skid patches courtesy of shants and someone else who was talking in math terms waaaaaaaaay above this dude's head.
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Old 02-28-06, 02:31 AM   #4
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i have it... hold on
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Old 02-28-06, 02:35 AM   #5
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Old 02-28-06, 02:39 AM   #6
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Code:

function skidpatchchart(),

gears = [30:66];
cogs = 4:30;
fprintf('    cogs');
for k = 1:length(cogs);
    fprintf('%4d',cogs(k));
end
fprintf('\ngears\n');
for j = 1:length(gears),
    fprintf('%4d    ',gears(j));
    for k = 1:length(cogs),
        fprintf('%4d',skidpatch(gears(j),cogs(k)));
    end
    fprintf('\n');
end

function sp = skidpatch(g,c),
% this is totally non optimal code, but i had fun writing it

primes = setdiff(1:100,unique(repmat(2:50,49,1).*repmat([2:50]',1,49)));
primes = primes(2:end);

sp = c;
while (g>primes(1)),
    if g/primes(1) == floor(g/primes(1)) & sp/primes(1) == floor(sp/primes(1)),
        sp = sp/primes(1);
        g = g/primes(1);
    else
        primes = primes(2:end);
    end
end
        


    cogs   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
gears
  30       2   1   1   7   4   3   1  11   2  13   7   1   8  17   3  19   2   7  11  23   4   5  13   9  14  29   5
  31       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  32       1   5   3   7   1   9   5  11   3  13   7  15   1  17   9  19   5  21  11  23   3  25  13  27   7  29  15
  33       4   5   2   7   8   3  10   1   4  13  14   5  16  17   6  19  20   7   2  23   8  25  26   9  28  29  10
  34       2   5   3   7   4   9   5  11   6  13   7  15   8   1   9  19  10  21  11  23  12  25  13  27  14  29  15
  35       4   1   6   1   8   9   2  11  12  13   2   3  16  17  18  19   4   3  22  23  24   5  26  27   4  29   6
  36       1   5   1   7   2   1   5  11   1  13   7   5   4  17   1  19   5   7  11  23   2  25  13   3   7  29   5
  37       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  38       2   5   3   7   4   9   5  11   6  13   7  15   8  17   9   1  10  21  11  23  12  25  13  27  14  29  15
  39       4   5   2   7   8   3  10  11   4   1  14   5  16  17   6  19  20   7  22  23   8  25   2   9  28  29  10
  40       1   1   3   7   1   9   1  11   3  13   7   3   2  17   9  19   1  21  11  23   3   5  13  27   7  29   3
  41       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  42       2   5   1   1   4   3   5  11   2  13   1   5   8  17   3  19  10   1  11  23   4  25  13   9   2  29   5
  43       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  44       1   5   3   7   2   9   5   1   3  13   7  15   4  17   9  19   5  21   1  23   6  25  13  27   7  29  15
  45       4   1   2   7   8   1   2  11   4  13  14   1  16  17   2  19   4   7  22  23   8   5  26   3  28  29   2
  46       2   5   3   7   4   9   5  11   6  13   7  15   8  17   9  19  10  21  11   1  12  25  13  27  14  29  15
  47       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  48       1   5   1   7   1   3   5  11   1  13   7   5   1  17   3  19   5   7  11  23   1  25  13   9   7  29   5
  49       4   5   6   1   8   9  10  11  12  13   2  15  16  17  18  19  20   3  22  23  24  25  26  27   4  29  30
  50       2   1   3   7   4   9   1  11   6  13   7   3   8  17   9  19   2  21  11  23  12   1  13  27  14  29   3
  51       4   5   2   7   8   3  10  11   4  13  14   5  16   1   6  19  20   7  22  23   8  25  26   9  28  29  10
  52       1   5   3   7   2   9   5  11   3   1   7  15   4  17   9  19   5  21  11  23   6  25   1  27   7  29  15
  53       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  54       2   5   1   7   4   1   5  11   2  13   7   5   8  17   1  19  10   7  11  23   4  25  13   1  14  29   5
  55       4   1   6   7   8   9   2   1  12  13  14   3  16  17  18  19   4  21   2  23  24   5  26  27  28  29   6
  56       1   5   3   1   1   9   5  11   3  13   1  15   2  17   9  19   5   3  11  23   3  25  13  27   1  29  15
  57       4   5   2   7   8   3  10  11   4  13  14   5  16  17   6   1  20   7  22  23   8  25  26   9  28  29  10
  58       2   5   3   7   4   9   5  11   6  13   7  15   8  17   9  19  10  21  11  23  12  25  13  27  14   1  15
  59       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  60       1   1   1   7   2   3   1  11   1  13   7   1   4  17   3  19   1   7  11  23   2   5  13   9   7  29   1
  61       4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
  62       2   5   3   7   4   9   5  11   6  13   7  15   8  17   9  19  10  21  11  23  12  25  13  27  14  29  15
  63       4   5   2   1   8   1  10  11   4  13   2   5  16  17   2  19  20   1  22  23   8  25  26   3   4  29  10
  64       1   5   3   7   1   9   5  11   3  13   7  15   1  17   9  19   5  21  11  23   3  25  13  27   7  29  15
  65       4   1   6   7   8   9   2  11  12   1  14   3  16  17  18  19   4  21  22  23  24   5   2  27  28  29   6
  66       2   5   1   7   4   3   5   1   2  13   7   5   8  17   3  19  10   7   1  23   4  25  13   9  14  29   5
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Old 02-28-06, 02:42 AM   #7
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damn! 60x30 only has one. i was gonna switch to that until now!
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Old 02-28-06, 03:43 AM   #8
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yeah, 64x4 only has one, so i've changed my mind about switching to that.
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Old 02-28-06, 03:51 AM   #9
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Quote:
Originally Posted by jacobpriest
damn! 60x30 only has one. i was gonna switch to that until now!
Quote:
Originally Posted by humancongereel
yeah, 64x4 only has one, so i've changed my mind about switching to that.
glad i could be of service
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Old 02-28-06, 04:00 AM   #10
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ha ha...while i was joking about it, i think that the wide range of combinations that chart affords is really nice.
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Old 02-28-06, 07:11 AM   #11
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this does ratios, gear inches, skid patches, mph@rpm, rpm@mph, etc:

http://grit.homelinux.net/gi/
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Old 02-28-06, 09:05 AM   #12
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Does it make sense to anyone else that the 30x30 combination has 5 skidpatches? Why wouldn't that have just 1?
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Old 02-28-06, 09:12 AM   #13
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I think if you're running 30x30, you could pretty much skid anywhere in the rotation. It should be 8 skid patches, but laid down on it's side...
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Old 02-28-06, 09:18 AM   #14
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I was just wondering if the math was right. If that combination wasn't true, are there others that don't work?
Mind you, if you're going to ride a 1:1 ratio, 47 tooth cogs are really the way to go.
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Old 02-28-06, 09:31 AM   #15
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Quote:
Originally Posted by Grimlock
Does it make sense to anyone else that the 30x30 combination has 5 skidpatches? Why wouldn't that have just 1?
Yeah 30x30 would have one skid patch.

Basically you you take your gear ratio as a fraction, reduce it and take the reduced cog - that's your number skid patches. For example, I ride 46x16.

As a fraction: 46/16
Reduced: 23/8

I have 8 skid patches.
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Old 02-28-06, 09:52 AM   #16
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any time you run a gear ratio that comes out to an even integer (1:1, 2:1, 3:1, 4:1, etc.) you will get exactly one skid patch. With a 1:1, when the pedals are in the position used to skid, the tire will always be in exactly the same position. With a 2:1, the same thing happens, but the tire went around an extra time in the mean time. #:1 gives you two extra rotations before it stops at the same spot again.

The most skid patches you can ever have is the same as the number of teeth on your cog. The best combinations are ones in which the cog tooth count and ring tooth count are relatively prime with regards to eachother. Using Yoshi's example, this would be any instance where you cannot reduce the fraction at all.
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Old 02-28-06, 10:03 AM   #17
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Quote:
Originally Posted by jacobpriest
Thanks for posting it!

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Old 02-28-06, 10:08 AM   #18
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you can get a few more in most situations if you can learn to skid with your other foot too
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Old 02-28-06, 10:14 AM   #19
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Whenever I see this great table, I've always wondered what the yellow highlighting is supposed to indicate. Any ideas?
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Old 02-28-06, 10:15 AM   #20
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he was highlighting the combinations that were 3.0 or slightly below, and then checking the skid patches
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Old 02-28-06, 10:31 AM   #21
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Quote:
Originally Posted by spud
he was highlighting the combinations that were 3.0 or slightly below, and then checking the skid patches
That was basically the intent. To mark which ratios are close to 3.0, which is a good starting ratio for most flatlanders who use 700 wheels. The yellow on both charts just makes it easier to quickly x-reference ratios with skid patches.

I also bolded skid patches that were 7 or above as this roughly keeps the wear evenly distributed. (A skid patch often nds up being about a 1/8th circumference on the tire as most folks are not static in the skid, but of course the wear is concentrated in the center of that patch.)

Yes, this assumes singled sided skidding only. If your ambi then double the number of skid patches.

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Old 02-28-06, 10:38 AM   #22
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thanks for the chart, its pretty useful
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Old 02-28-06, 12:01 PM   #23
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Huh, I just went from 8 skid patches to, um 8 skid patches. I guess not much is different from 46x16 to 42x16, except that I'm going to be spinning a lot more...
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Old 02-28-06, 12:16 PM   #24
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get a 17t cog
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Old 02-28-06, 12:19 PM   #25
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DA doesn't make 17t cogs, do they?
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