Customer Nick came into the shop needing a new 135 BCD Miche front chain ring for his track bike. (Miche shares its bolt circle diameter with the classic Campagnolo BCD. You can't put a standard track chain ring, 144 BCD, on Miche track cranks.) Anyway, I ordered him a new chain ring, and we started talking about ratios and skid patches.
There are a couple of schools of thought about managing skid spots. One school is that if you keep skidding in the same place, you will wear a hole in your tire and /or tube. However, if you know where the skid spot will be, you can rotate your tire to the left or right of that spot, now and again, and manage the wear better. The other school says it is better to spread out the damage. However, spreading out the damage means that it is harder to keep track of managing the damage. And that actual spot size is a bit deceiving. In fact, there are many conversations about hows and whys, and not all of the participants agree with each other. Also, when calculating skid spots, you need to be comfortable riding on the street in the ratio that you choose for your eventual tire destruction.
If you just like to ride, and you have a decent job, you may decide to buy a new tire when yours is as bald as Kojack and save your knowledge of numbers for the daily newspaper suduko puzzle.
Screen shot taken from the Hong Kong Fixed Gear site; more info. about it and other calculators and charts after the jump.
(For good or bad, I like a big ring up front. And, I like pricey tires, so skidding is not my life. Nick likes to skid, and he rides 45Tx 15T.)
1. First, I went to Sheldon Brown's site:
"If you make a habit of doing 'skip stops' you will wear your rear tire out considerably faster than if you use your front brake. This problem is exacerbated by certain gear ratios, because you may tend to repeatedly skid on the same section of the tire.
Riders who plan to do a lot of "skip stops" should consider the ratio when selecting their chain ring and rear sprocket. The mathematics of this is actually fairly simple:
- Simplify the gear ratio to the smallest equivalent whole number ratio.
- The denominator of the resulting fraction is the number of skid patches you will have on your rear tire.
Examples:
44/16 simplifies to 11/4, so there would be 4 skid patches.
45/15 simplifies to 3/1 so there would only be 1 skid patch.
42/15 simplifies to 14/5, so there would be 5 skid patches.
43/15 can't be further simplified, so there would be 15 skid patches.
This is based on the assumption that you always skid with the same foot forward.
If you are an ambidextrous skidder, and the simplified ratio has an even numerator or denominator, your number of skid patches will be the same."
2. Then, I googled, and read the thread on Bike Forums on skid patches.
3. From there, I read the why and hows with examples on the fixed.org.au site, which has a few illustrations and walks you through the calculation if you want to try to figure out what is going on when you skid in certain cog and chain ring ratios.
4. I happily found Denny's expanded
skid spot calculator chart. Denny has done all of the work. (I have not checked the accuracy. But you can save yourself a ton of time when it is time to order a new cog or chain ring.)
5. For those who love to download and post things, here is the constantly mentioned Apple widget for calculating your skid patch.
6. And from the Hong Kong Fixed Gear site, you can download a Rabbit skid patch calculator and go crazy. *But you need to make sure that your browser has the latest Java plug-in. If you you try to use the app. and you get a screen message that you need the Java plug in, go here. Cick on the Windows Online installation, multi-language link, which as of this writing is second from the top.
Good luck, and when you wear out your tires, feel free to stop by the shop. (I am going to make this into a permanent page on the site for easy reference.)
