Singlespeed & Fixed Gear - WTF are "skidpatches"?

the search thing won't work for some reason and ya'll keep talkin' about skid patches on the gear ration thread. please explain this to me! maybe a "skidpatches for dummies" thing.

thanks! :)

whenever you skid, it will wear your tire in certain patches depending on your gear ratio - some ratios only have like 2, some have like 17. there is a chart out there somewhere that shows the # of skid patches for your ratio.

edit -- actually, i have it. email me and i can send it to you.

skid patches are the poinst on your tire where the ground is touching when you skid. since your feet are usually in the same place (more or less) every time, its just mathematics on which points of the tire will be on the ground. more skid patches = longer tire life.

where the tire is on the ground when you skid in relation to the pedals. For example, if I always skid with my right foot back and left foot forward (running 44x16) I will have 4 spot that will wear faster then the rest of the tire. If I can skid switch I will have 8 spots. The more skid patches, the more evenly your rear tire will wear while doing skids/skips.

damn you guys type fast! beat me to it.

wait, are you pistaboy?

It's when you don't wipe your butt thoroughly and then ride your bike.

causing the need to replace not only tires but underwear as well

This guy came into the bikeshop and asked a female customer if he could skid on her fixed gear bike. She was like, "No, that will **** up my tire," and he said, "That's why I only do it on other people's bikes."

what a jackass...i hope you slapped him.

perhaps this has been sufficiently explained by the above posters, but it might help to think about it explicitly in terms of how gear ratios and skid patches are related. hopefully this is helpful to some degree and not condescending or overexplanatory. people with whom i talk in person often have a hard time understanding exactly why certain ratios result in more skid patches. hopefully this is fairly comprehensive.

okay, so, when you rotate your cranks one time, the number of times that your rear wheel rotates is determined by your gear ratio. the harder the gear, the more times your wheel will rotate per crank revolution. if, for example, you have 48x16, your ratio is 3.0. that means that your rear wheel goes around exactly three times for every one time that your cranks go around once.

when you skid, you are generally most comfortable doing so with the cranks in one position (say, with the pedals at 4 and 10 o'clock). some people can skid with either foot forward, giving them two positions (and, ultimately, double the skid patches*). this explanation assumes one position.

so, as stated above, when you pedal one revolution, your wheel goes around a certain number of times. in the example given above - 48x16 - the wheel goes around exactly three times, making it so that the same part of the tire is on the ground whenever you are in a given crank position. so, when i skid on my 48x16 setup in the 4x10'oclock position, i am always sliding the same part of my tire against the pavement. that is why 48x16 gives you one skid patch. more generally, any time you have a chainring size that can be divided evenly by your cog size, you will have one skid patch.

however, when you start to have ratios that don't divide evenly, you are not always skidding on the same part of the tire. take, for example, 52x16. every time you pedal the cranks around one time, you are moving the wheel 3.25 times. so, say you skid once, rotate the cranks once, and then skid again. unlike 48x16, you will now be skidding on a different part of the tire. if you were to map out the circumference of the tire, mark a point, and then go around 3.25 times, you would be at a different spot. 52x16 ends up giving you 4 skid spots, which, while better than 48x16, isn't too great. some ratios can give as many skid patches as there are teeth on the cog (or more, if you assume that there are positions between teeth).

so, you might ask, how do you calculate the number of skid patches? well, if you noticed, when you divided 52x16, you ended up with a remainder. that is, 16 fits into 52 three times and then some. as a matter of fact, it ends up going into 52 three and one-quarter times. when you do the long division, you end up with 3 remainder 4. in order to find the number of skid patches, you take that remainder and divide the cog size by it. so, in this case, you divide 16 by 4 and get the result of 4 skid patches. if you were to take your map of the wheel and go around it 4 times, you would see that you end up where you started.

of course, not all ratios will yield a remainder that divides evenly.

<<****tt, my work day is over and i didn't get to finish this post... i'll come back and finish it later...>>

*although, once you get above 7 or so skid patches, they will start to overlap, making additional patches less effective. this is, of course, because you don't skid on a single point, but, rather, a few inches of space on your tire.

Word up for Abstract Algebra (http://en.wikipedia.org/wiki/Abstract algebra)/Group Theory (http://en.wikipedia.org/wiki/Group theory)!

This guy came into the bikeshop and asked a female customer if he could skid on her fixed gear bike. She was like, "No, that will **** up my tire," and he said, "That's why I only do it on other people's bikes."

snare roll... CRASH. :lol:

hey shants,
that's a great explanation.

however, i want to address the "if you skid in 2 opposite positions it will double the number of skidspots."

gear ratio 3.0 (e.g. 48x16)
if you skid in 2 opposite positions, it will yield double the # of skidspots. a half revolution of the cranks will turn the rear wheel 1.5 times, which results in a new skidspot.

gear ratio 2.0 (e.g. 40x20)
if you skid in 2 opposite positions, it still yields only 1 skidspot. a half revolution of the cranks will turn the wheel once, which means you'd be skidding on the same spot.

after carefully thinking about this, i think it ends up like this: if your gear ratio is an even whole number, using 2 opposite crank positions for skidding will not double the number of skid spots.

however, this makes me wonder if this has any ramifications for other less even gear ratios. but can't quite get into thinking about that now.

It's when you don't wipe your butt thoroughly and then ride your bike.


i hate it when that happens

hey shants,
that's a great explanation.

however, i want to address the "if you skid in 2 opposite positions it will double the number of skidspots."

gear ratio 3.0 (e.g. 48x16)
if you skid in 2 opposite positions, it will yield double the # of skidspots. a half revolution of the cranks will turn the rear wheel 1.5 times, which results in a new skidspot.

gear ratio 2.0 (e.g. 40x20)
if you skid in 2 opposite positions, it still yields only 1 skidspot. a half revolution of the cranks will turn the wheel once, which means you'd be skidding on the same spot.

after carefully thinking about this, i think it ends up like this: if your gear ratio is an even whole number, using 2 opposite crank positions for skidding will not double the number of skid spots.

however, this makes me wonder if this has any ramifications for other less even gear ratios. but can't quite get into thinking about that now.



Actually, (and I haven't done any math to back this up...) I think that whether the chainring has an even or odd number of teeth determines this. An odd number of teeth will ultimately double one's skidspots, provided one is capable of skidding with either foot forwards.

I leave it as an exercise to the reader to demonstrate (or prove!) that the oddness of the cog is irrelevant. Mostly because I'm lazy.

Edit: Nevermind! The lucidness provided by a little more coffee has shown me what an idiot I am. Disregard.

http://www.optionnz.com/users/afs/i1/FixedRatio.jpg

from that chart it looks like 17t cog would be the best with respect to number of skid patches especially if you want to chang the chainring and not lose the number of skid patches available...maybe I'm just saying this because thats what size cog I run

from that chart it looks like 17t cog would be the best with respect to number of skid patches especially if you want to chang the chainring and not lose the number of skid patches available...maybe I'm just saying this because thats what size cog I run

well, really what you want are cog/chainring combos that are prime, relative to each other.
34t and 51t are outside the range of what most people ride on the street, so 17 works well.

Shants almost got it right, but here's how skidpatches (SP) are determined. If the number of cog teeth (RT) divide evenly into the number of chainwheel teeth (FT), say 51 x 17, we all agree that there is 1 and only 1 SP. If the remainder (R) of FT / RT is 1, say 52 x 17, then you go an extra 1 / RT revolution of the back wheel with each revolution of the chainwheel. So, after RT revolutions of the chainwheel you are back where you started and that means you have RT SPs, in our case 17 SPs. Things get more complicated when R is greater than 1, however. Consider 52 x 14, in which case R = 10. Every revolution of the chainwheel gives 3 of the cog plus an extra 10 cog teeth. Now 7 revs up front produces 21 in back (7 x 3) plus an extra 70 (7 x 10) cog teeth. But 70 teeth on a 14 toothed cog is 5 complete revolutions, so we are full cycle (no pun intended) with an SP of 7, only half as many as you might expect from a 14 toothed cog. The rule for remaindered ratios is:

SP = RT / gcd(RT, R) (and this is where gorn's chart came from)

where gcd is the greatest common divisor (think 8th or 9th grade here). For the 52 x 17 case we get SP = 17 / gcd(17, 1) = 17 / 1 = 17, so that works. For 52 x 14 we get SP = 14 / gcd(14, 10) = 14 / 2 = 7, and that's good, too.

So how do you maximize SPs. It's simple. Use a cog with a lot of teeth and one that yields a remainder of 1 (which always makes gcd = 1). Another way is to use a cog with a prime number of teeth, like 17, in which case gcd always = 1 for any remainder.

i've said it before--shants is my fixie guru. :beer:

all this math is making my head hurt.

http://www.optionnz.com/users/afs/i1/FixedRatio.jpg

so i see from gorn's chart that since i'm runnin' 44/16 i have only 4 skid patches. a duh observation--that means i'm gonna wear down my tire way fast, no? what with my new-found ability to skid. who cares? i got money. . . :) :)

how fast ya'll generally go through tires? what kinda mileage do ya generally get on 'em?

You all have it wrong. The skidpatch is a small peice of innertube rubber that people glue to their arm when they are trying to kick the skidding habit.

So how do you maximize SPs. It's simple. Use a cog with a lot of teeth and one that yields a remainder of 1 (which always makes gcd = 1). Another way is to use a cog with a prime number of teeth, like 17, in which case gcd always = 1 for any remainder.

Actually a cog with a prime number of teeth doesn't always work, if the chainring is a multiple of the cog (34x17, 51x17, etc). What does work is to choose a chainring and cog such that they are relatively prime, that is that they don't have any common factors (except 1 of course). For example: a 16t cog is not prime, but if you use a 49t chainring, 16 and 49 are relatively prime meaning that that the greatest common factor of the two is 1.

Oh yeah, and it's not remainder, it's GCD.

skidpatches = brown stain ground into tighty whities

one more reason I'm glad I run 52x17.

meanwhile, I have such a ****ty hub that I probably will destroy that before I skid a back tire to death

*although, once you get above 7 or so skid patches, they will start to overlap, making additional patches less effective. this is, of course, because you don't skid on a single point, but, rather, a few inches of space on your tire.

Thats exactly why I bolded 7 and above skid patches in the chart I created that gorn re-posted (thanks)
Al

****, i totally forgot to finish my post. jimshapiro covered the formula that i didn't have time to write. thanks for saving me the time and writing somewhat more succintly than i did.

so how often ya go through tires?

so how often ya go through tires?
I've got over 2k on my rear. Its time to replace soon though as its worn thin from riding, not stopping.

(my first fixed rear tire lasted less than 500mi and I decided that skidding was a luxury I didn't want to prioritize)

Al