gear ratio question
 

what's the difference between having, say, 49:16 (3.0625) versus 46:15 (3.0666) versus 40:13 (3.07). obviously there are differences in the numbers, but does anyone really use a 40:13? if you were looking for a ratio around 3.0, wouldn't 40:13 be better than 49:16 because it's lighter?

sure, the chain will be shorter too. But a 40t track ring is right up there with a 10t track cog.
Most of the track chainrings run higher baseline tooth counts (that sounds odd...don't it?). You can find smaller tooth count chainring in road/MTB applications. Hence teeth ratios common from track like 46-53:12-17, maybe even higher in the front ring, but most track racers do not run 18-23t cogs, at least the ones I know of.

they'll all take about the same effort to push.
However, people think that fewer teeth on the chainring and cog tend to lead to a higher rate of wear. Mind you, the 40:13 will be lighter, but not light enough for you to notice or care. Then again, a 40 tooth chainring should cost less than a 49 toother.

also, with small gears and rear facing horizontal dropouts, some gear combos make it quite difficult to pop the chain off the sprocket to remove the rear wheel.

What? I don't follow. Why would one ratio make it harder to remove the wheel from the dropouts? If you talking about the position of the axle/wheel in the dropouts, thats a different story....

Big rings feel good. Those who know, know.

well crap. i ordered a 42 up front... oh well, it was cheap.

yes. some gear ratios will force the wheel to be slammed forward in the dropouts, so you can't get much slack to get the chain off the front sprocket. in addition, it is harder to get a chain off of a smaller sprocket when there is little slack in the chain.

yeah but this can be independent of rear cog size.
its more dependent on chainstay lenght.


then again, if you have 2 different gear ratios that somehow manage to put the rear axle at the same position in the dropout, wouldn't it be easier to remove the chain from the smaller rear cog?

Actually the opposite is true. A small cog means that it takes less slack to be able to get the necessary play to clear the teeth and move the chain laterally off the cog. Note that this applies not only in the stand but on the street, so a drivetrain with smaller cogs and chainrings will be easier to derail.

you know you could just add a link on the chain

i'm not trying to be rude, but that is wrong. removal from smaller sprockets takes more slack because there is a more severe angle difference needed to allow the chain to climb over the leading teeth of the sprocket for removal. the same goes for belts on pulleys on a car. it's easier to get a somewhat tight belt on/off a large pulley than a small pulley as you rotate them. try it.

yes, this does depend on chainstay length and dropout length and sometimes rear wheel setup preference. sometimes a bikes dropouts may be too short to ad a link. when dealing with smaller gears, taking a link out or adding a link make a much bigger difference in wheel position in the dropout so _sometimes_ the dropout can not accomodate. also, remember i'm talking about horizontal rear facing dropouts. so, you can always run smaller gears, but you may have to change frames to be able to run the exact gearing/wheel position setup you want.

peace.



*edit* ok ok you could use a halflink, but i really do not care for those things so i normally don't even consider them.

Anyone care to do the math? I'm not convinced. Yes, angular separation is higher the smaller you go. At the same time, so too are the number of teeth that must actually be cleared in order to actually remove the chain reduced. Imagine if you will a cog that's so small that it has only 4 teeth. If I can get the chain off of 2 of them, I have cleared an entire semicircle. On the other hand if we consider instead a 68 tooth ring as at

http://www.bikecult.com/works/chainr...sinTTcrank.jpg

it takes far more than 2 teeth worth of slack.

I don't think it's reasonable to compare with belts and pulleys because a chain drive flexes a great deal less than a rubber belt. So while the increase stretch available along the length of a large belt is very appreciable that's much less true for a strong steel chain.

What is required is that a certain amount of play be available to sufficiently separate the chain from the chainwheel or cog. If I have 1/2" of slack in my chain, that equates to 4 teeth worth of play (what that actually amounts to in terms of clearing the teeth and moving laterally off the wheel is a harder question to answer since it requires knowing things about the lateral rigidity of the chain). Obviously on larger rings, as cited above, that amount of slack will clear less of the drivetrain than on smaller rings.

i'm just speaking from a few decades of experience/obsession with ss. *shrug*

bottom line...go for it. if it works for you, great.

42 is a man's chainring. Hold your head up high and rejoice in the awesomeness you have just thrown yourself into. Now get a 14 t cog in the back and we'll be twins!

Luciano wrote:

"Big rings feel good. Those who know, know."

This brings us back to the original question.

One can acheive the same ratio with both larger and smaller chainring/sprocket combinations.
If one goes with a smaller pair, he gets a shorter chain and lighter weight all around.
If one goes with a larger pair, according to luciano, it feels good.
To me, a larger pair would seem to impose less stress and wear and tear on the sprocket, chainring and chain, but I arrive at this intuitively and I can't substantiate it.

On the surface, though, it seems like a trade off between less weight on the small side, and less wear and tear (and perhaps better feel) on the large side.
Any comments about how it feels?
How it feels matters.

Oh, and should I have done all the size matters jokes just to get them out of the way? :)

You are right about bigger ring/cog combos having less wear, but more weight. Bigger ring/cog combos also have a better 'feel' if you know what i'm sayin. It's hard to describe, but its like a smoother feeling. You have to ride it to know....

the weight difference would be negligible.

actually, i am! but i haven't put the bike together yet, and after all this i might as well just order a 49:17 or a 49:16 and forget (i.e. put them on ebay for $1) the stuff i have now.

another bad thing about 42:14 is that it gives you only one part of the tire to skid on (as explained here: http://www.dartmouth.edu/~ccatalan/skid.html )

wellp, good thing i orderd the drivetrain before doing the research

the skid-patch thing is easy to fix, just rotate your tire periodically.
(just make sure you don't rotate it the distance between your skid-patches).

no. when you hit at least 46, then we'll talk.

Nuts! Balls! Hair!

Am I in the club?

If they're rear facing, don't they cease to be dropouts?

yes

Three of mine use a 52t front ring with a 16t, 18t and a 19t cog in the rear.
The 52x19t combo is 75" which is about the same as my Van Dessel's
44x16t or 74". It is the best comprise for the hills and rollers we have
in North Carolina. The 52x16t combo is pushing 88" and makes the hills
a bit tougher with more standing. It can hit close to 4o mph on the way
down though. :D My commuters both run a 42x16t.

It's true, but then again I have a suicide hub so I don't skid at all. Yeah I know I'm a weenie, but at least I'll admit it.