Singlespeed & Fixed Gear - 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.

it's been done already. ****ing wish i could remember what thread it's in.

http://www.bikeforums.net/showthread.php?t=166218&highlight=skid+patches

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.

i have it... hold on

http://img.photobucket.com/albums/v41/iamjacobpriest/FixedRatio.jpg

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

damn! 60x30 only has one. i was gonna switch to that until now!

yeah, 64x4 only has one, so i've changed my mind about switching to that.

damn! 60x30 only has one. i was gonna switch to that until now!


yeah, 64x4 only has one, so i've changed my mind about switching to that.

glad i could be of service

ha ha...while i was joking about it, i think that the wide range of combinations that chart affords is really nice.

this does ratios, gear inches, skid patches, mph@rpm, rpm@mph, etc:

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

Does it make sense to anyone else that the 30x30 combination has 5 skidpatches? Why wouldn't that have just 1?

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...

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.

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.

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.

http://img.photobucket.com/albums/v41/iamjacobpriest/FixedRatio.jpg
Thanks for posting it!

Al

you can get a few more in most situations if you can learn to skid with your other foot too

Whenever I see this great table, I've always wondered what the yellow highlighting is supposed to indicate. Any ideas?

he was highlighting the combinations that were 3.0 or slightly below, and then checking the skid patches

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.

Al

thanks for the chart, its pretty useful

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...

get a 17t cog

DA doesn't make 17t cogs, do they?

I think they only go up to 16... Do you need DA in particular? (I have a phil on my 7600s)

i thought they did but it doesnt look like it.

yeah, 17t cogs are the way to go for most gear ratios, it seems.

yeah, 17t cogs are the way to go for most gear ratios, it seems.
I am very happy with 48x17, but want to go up a bit, which would lead to 48x16, which I want to avoid, so I am looking at going 49x17, then later to x16 since going up one chainring tooth is too tiny a difference. But since I don't skid 48x16 should be ok for me.

But I wonder if wear patches are only caused by skidding? Since I accererate from stops with the same crank position and same foot forward, doesn't this wear the tire out in patches too, although slower than skidding?

Said differently: Rear tires on all bikes wear much faster than front, primarily because they transfer power. If that power transfer is not even across the crank rotation, won't that cause uneven tire wear in patches?

Al

I am very happy with 48x17, but want to go up a bit, which would lead to 48x16, which I want to avoid, so I am looking at going 49x17, then later to x16 since going up one chainring tooth is too tiny a difference. But since I don't skid 48x16 should be ok for me.

But I wonder if wear patches are only caused by skidding? Since I accererate from stops with the same crank position and same foot forward, doesn't this wear the tire out in patches too, although slower than skidding?

Said differently: Rear tires on all bikes wear much faster than front, primarily because they transfer power. If that power transfer is not even across the crank rotation, won't that cause uneven tire wear in patches?

Al

i've been on 48x15 and 49x15 for a while and both are nice. skip over 16...

But I wonder if wear patches are only caused by skidding? Since I accererate... doesn't this wear the tire out in patches too, although slower than skidding?

I think you're looking at wearing your tires down from acceleration in extremely small rates as opposed to skidding. Some of the people that really seem to be concerned with this stuff, like mikearena who wore through his $35 tubular in one parking garage skid, acceleration doesn't even have a chance to play a role in this.

As far as your other questtion, the rule of odd numbers is good advice. 49 is a great number also cause it's only divisible by 7. And 49 x 14 is just huge for the street. I ride 49x 15 right now, but 49 x 16 and 49 x 17 are only slightly better as far as patches go.

Correct me if I'm wrong, but isn't the rule about switching feet (skidding with your opposite forward) to double your skid patches only practical if you have an odd number of patches from the chart?

I think you're looking at wearing your tires down from acceleration in extremely small rates as opposed to skidding. Some of the people that really seem to be concerned with this stuff, like mikearena who wore through his $35 tubular in one parking garage skid, acceleration doesn't even have a chance to play a role in this.

As far as your other questtion, the rule of odd numbers is good advice. 49 is a great number also cause it's only divisible by 7. And 49 x 14 is just huge for the street. I ride 49x 15 right now, but 49 x 16 and 49 x 17 are only slightly better as far as patches go.

Correct me if I'm wrong, but isn't the rule about switching feet (skidding with your opposite forward) to double your skid patches only practical if you have an odd number of patches from the chart?
Sure, but as a non-skidder I only wear tires out thru mileage. I really wonder (out of curiosity, not practical matters) if after 1500mi on a 48x16 if one would have a tire flat spot that varies around the circumference. I am not suggesting this would be bad, just a result.
I too have blown thru (not new) tires with a skid. Thats why I don't skid anymore, just a personal preference to save money on tires. But it is fun and I do it once in a while just to keep the technique up for emergency needs.
I only have 48t as it came with my cranks. Based on the chart I put together I decided on a 49t before I bought the cranks. Someday I'll need a new chainring and then I can get the 49.

Don't hurt my brain with the ambi question ;)

Al

dura ace stops at 16. those bastards.

I ran into that very problem when I ordered my RDs with the 48t ring and wanted to keep my ~75 gear inches. I ordered a Surly 17t cog and we'll have to see how it fares.

my surly 17t is fine.