Which cog/ring would you choose?
 

These are all basically the same ratio:

41x14
43x15
47x16
49x17

Al

47x16, this gives you the option of going down to 47x20 or up to 47x13, a good range IMHO...

possible selection criteria:

a) availability of parts
2) number of skid patches
iii) spreading out the wear

i would pick the combination easiest to obtain - z.b. i have a 17-tooth cog already, all i need is the chainring (although of course cogs are less expensive than chainrings).

I picked 41, 43, 47, 49 because with the exception of 49x14 these four chainrings all result in the number of skid patches equal to the number of cog teeth (at least for cogs 12-18)
I already have a 14 cog, but don't think it would be wise to go with a 41 chainring as it may be limiting.
So I guess it boils down between 43 and 47 where I could get use out of the 14 with 43.

I made this handy chart to help me figure out skid patches and equivalent ratios:
http://www.optionnz.com/users/afs/i1/FixedRatio.jpg

Al

whichever cog/ring matches the ring/cog I already own.

sweet!

just remember, big chainring = sexy

that chart is amazing

right click save!

Does this mean that I would have to change my 53/18 to a 54/18 to gain aditional sexiness with the ladies?

Actually bring your back cog down to 16 and your penis will grow;)

Seconded.

also this is for one foot only, if you skid with different foot, you can double your spots

Of course, but the chart still gives a comparision that is valid. Not to mention that one can spin tires on the rims - but the bottom line is that the higher number of skid spots the less you have to think about any of this.

I'm leaning toward a 43 ring. Right now I converted an old bike resulting in 40x14, but for a variety of reasons I (unfortunately) need to change the cranks, so I'm using this as an opportunity to pick a good size chainring.

I wonder why 48x16 is so oft recommended as it has the worst skid spot? I guess 48 chainrings are more common.

Al

i noticed that; they are all prime. nice chart! i still listed skid patches as a possible criterion because some persons might feel that 17 is a lot more than 14 (or, for the ambidextrous, 34 is even greater than 28).

What do folks who primarily skid stop think is a good minimum of actual skid patches? I bolded everything above 7 (which woudl be 14 for ambi of course)

Al

I would say that someone is conspiring with tire companies to make that claim. I am in the same boat with 14x42

yup

good work noisebeam

It would be really cool to collect really interesting/helpful things like this and jimv's post and keep them somewhere. Every so often really good, well thought out posts appear and after the thread dies it takes me forever to find them. Well, I guess that's my own problem. :) But still it would be nice.

seconded. this chart would fit nicely in a 'FAQ Sticky'.

not always, though. one case that comes to mind is a 2:1 ratio - you have only 1 skid spot even if you skid both ways.

yep. There's bound to be overlap between skidding with both feet.

umm, what's a skid patch?

All skidding aside, I have a mechanical intuition that larger chainring and sprocket pairs have lower stresses and greater efficiencies.
I say intuition because I can't prove it with numbers.
However, for a given ratio between front and rear, it seems that the smaller the number of teeth the less mechanically elegant the system.

Secondly, how much of a rear tooth spread will one chain and horizontal dropouts support?
If each tooth in the rear cog moves the axle one eighth of an inch forward, going from 13t to 21t involves one inch of travel (assuming 1/8" per tooth).
Not that anyone would do that.
Still, what tooth spread represents a reasonable spread, say on a fixed fixed hub that one wanted to flip over with the same chain?
Does the size of the gear pairs matter in all of this?

For myself, building my winter bike (with 38mm tires), a 43t chainring and 15t and 17t cog on a fixed fixed hub would serve me well, if it has the same mechanical efficiency as say a 49X17X19 combination, which has almost the same numerical ratios.
And, by the way, the chainring and cog combinations above give me the same gear inches on a 38mm tire as I presently have on my 48X16X18 with 23mm tires.

the place on your tire that's in contact with the ground when you're skidding.

I believe this has been experimentally demonstrated.

Nope. Assume the ring is x teeth, one cog is y teeth, and the other is z teeth. The first gear ratio can be expressed as x/y and the second as x/z. To compare the second gear to the first, we would look at (x/y)/(x/z). Expand this to get xz/xy. The x's cancel out and we have z/y.