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captainbrad

Posted: Wed Jul 27, 2005 8:30 pm

6ft Twenty Joined: 24 Jul 2005 Posts: 293 Location: Sea-Town, WA

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iro1751

Posted: Wed Jul 27, 2005 9:11 pm

Joined: 25 Jul 2005 Posts: 723

What have we got here? I'm not clear on what the numbers in the field represent.

lantius

Posted: Wed Jul 27, 2005 10:51 pm

1337 Joined: 22 Jul 2005 Posts: 6705 Location: right over

the top chart is the number of distinct 'skid patch' areas you have on your tire with the given ratio, assuming that like most folks you can only skid with your cranks in one orientation.

the second chart is the gear ratio, the highlighted column follows the 2.9 / 3.0 ratio.

for reference the skid math is based off relative primacy, you ideally want your chainring and cog to have no common divisors besides 1. an easy way to do this is to pick a prime chainring (43, 47, 53) and you are guaranteed to have a relatively prime relationship to the cog. the number of skid patches is the size of the cog divided by the largest common divisor between the chainring and cog.

e.g. a 46-16 has a largest common divisor of 2 between the cog and chainring; therefore it has 16 / 2 = 8 skid patches.

a 39-13 has a largest common divisor of 13, therefore it has 13 / 13 = only one skid patch.

obviously having less skid patches means you are going to wear out the tire much more quickly and you have to be dilligent about manually rotating the chain one tooth whenever you loosen the wheel.

there used to be a great site explaining the math behind this, but it seems to have gone belly-up.

futurenorth

Posted: Thu Jul 28, 2005 6:43 am

Bed Bleeder Joined: 25 Jul 2005 Posts: 839 Location: Ballard

I'd say you just did a good job.

We had talked about this once before, but thanks for the explanation.

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meat

Posted: Thu Jul 28, 2005 7:27 am

Phillip Joined: 25 Jul 2005 Posts: 191 Location: Vancouver, BC, Canada

I'd never thought about that. The math is something even I get.
I guess as a geared rider I don't have those worries, but it is a great help while I'm saving to build my fixie.

terrydean

Posted: Thu Jul 28, 2005 8:51 am

rocker boi Joined: 24 Jul 2005 Posts: 1535 Location: santa fe, i think

According to that, I currently have 18 skid points. Why is it that I wore a hole in one spot on my last tire?! I blame bad luck.

iro1751

Posted: Thu Jul 28, 2005 8:57 am

Joined: 25 Jul 2005 Posts: 723

I agree, this is a bit confusing. at 46 X 18 which I'm riding, it looks like I'd have the 9 points as well. . .

terrydean

Posted: Thu Jul 28, 2005 9:03 am

rocker boi Joined: 24 Jul 2005 Posts: 1535 Location: santa fe, i think

kingpenguin wrote:

According to that, I currently have 18 skid points. Why is it that I wore a hole in one spot on my last tire?! I blame bad luck.

I skid with my left foot forward about 60-70% of the time, right foot forward the rest of the time, so I should have a ton of skid points.. WHY DID I GET A HOOOOLE????

Rotten luck, I tell you.

j_ryde

Posted: Thu Jul 28, 2005 9:03 am

hissy fit Joined: 25 Jul 2005 Posts: 440 Location: Sea-Town

I thought ahead. I've got 17 skid spots!

Not that I don't still wear through tires.

dennyt

Posted: Wed Oct 19, 2005 2:01 pm

rocket mechanic Joined: 02 Aug 2005 Posts: 2708

I just expanded that chart:
Mo'Betta

derrickito

Posted: Wed Oct 19, 2005 2:10 pm

now with 50 percent more EVIL Joined: 22 Jul 2005 Posts: 10566

i keep sacking myself on my stem every time i skid.

joeball

Posted: Wed Oct 19, 2005 2:21 pm

Joined: 24 Jul 2005 Posts: 6037 Location: Ether

derrickito wrote:

i keep sacking myself on my stem every time i skid.

Maybe you need a pad set?

Or some junk armor

derrickito

Posted: Wed Oct 19, 2005 2:25 pm

now with 50 percent more EVIL Joined: 22 Jul 2005 Posts: 10566

yes, i need a cup!

megafauna

Posted: Wed Oct 19, 2005 2:25 pm

Joined: 06 Oct 2005 Posts: 250 Location: first hill

i shouldn't have opened that last image at work. Oh well. good test image to see if my boss is looking over my shoulder.

SofaKing

Posted: Wed Oct 19, 2005 7:04 pm

Joined: 24 Jul 2005 Posts: 109 Location: Free Ballard

I've been skidding a lot on my mtn bike lately. I completely got out of the habit when I was a kid because I learned the hard way how fast it tears through tires. Riding with you guys reminded me of how much fun it is and you know what? I can afford all the F'n tires I want now!

Bet I look kinda dumb doing it though.

Once drifted my cargo bike sideways down a wet grass hill at gasworks. Kept the bike swung out about 60 degree's from strait while sliding down the hill. Finally hit a sidewalk, I thought I had folded the rear wheel in half.

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