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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 01-23-05, 11:20 AM   #1
bac
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Same ratio, different sized rings cogs, which to choose?

Are there any advantages, or disadvantages to running a large (53) vs a smaller (39) chainring given the same overall gear ratio? In other words, what's the difference between say a 53/20 vs. a 39/15 with all other variables being equal? You'd get more chainwrap with the larger ring, but I don't know if that's good, or bad.

Any help is GREATLY appreciated. ThanX!
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Old 01-23-05, 11:33 AM   #2
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afaik there is no real difference. some folks say that smaller cogs == less weight, which is true, but i don't think it makes any difference in practical terms.

others say that larger cogs == less wear, again, in real terms i'm not sure it makes a difference.

my $0.02
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Old 01-23-05, 01:09 PM   #3
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I've heard that you;re also less likely to get chain skip or jump on bigger cogs/rings because there's more chain wrap, and you're less likely to break teeth off because there is less force on each tooth.

As well, the smaller cogs and rings force the chain to bend / wrap more tightly, which supposedly increases friction in the drivetrain, although i would imagine that's not very noticeable.
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Old 01-23-05, 03:15 PM   #4
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you don't have to roll your pant leg up as far with the smaller chainring. I don't know about you but I think this is important.
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Old 01-23-05, 03:20 PM   #5
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whichever is cheaper.
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Old 01-23-05, 03:24 PM   #6
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I don't know whether it makes a difference to you, but I think that bigger chainrings look cooler..
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Old 01-23-05, 04:12 PM   #7
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Cogs are cheaper than chainrings - unless your budget is unlimited...
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Old 01-24-05, 08:20 AM   #8
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ThanX for the replies! I'll probably go with the 53, but it's good to know that there is no real difference between set-ups.
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Old 01-24-05, 08:35 AM   #9
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Large chainring = less wear on chainring (lower number of revolutions to turn a given length of chain).
Chainrings are more expensive.
Enjoy
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Old 01-24-05, 09:44 AM   #10
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Quote:
Originally Posted by jasonyates
I don't know whether it makes a difference to you, but I think that bigger chainrings look cooler..
Definitely! When you look at a bike, it's hard to tell what size cog is on there (unless it's really enormous) because the cog is mostly hidden by the frame. But the chainring is quite visible. When you rock a bigger chainring, people with think you are tougher, and that's good.
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Old 01-24-05, 09:52 AM   #11
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Quote:
Originally Posted by thechamp
you don't have to roll your pant leg up as far with the smaller chainring. I don't know about you but I think this is important.
Ha! I never thought about it, but that is a very good point.
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Old 01-24-05, 10:48 AM   #12
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Originally Posted by Ya Tu Sabes
When you rock a bigger chainring
Too cool for school..........
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Old 01-24-05, 11:43 AM   #13
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You bring a smaller chainring in closer to the chainstays for chainline purposes. Something I am dealing with on a 50T chainring that I have.
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Old 01-24-05, 01:05 PM   #14
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Quote:
Originally Posted by ofofhy
You bring a smaller chainring in closer to the chainstays for chainline purposes. Something I am dealing with on a 50T chainring that I have.
Good point! Has anyone else had a problem with proper chainline when using a big (more outside) chainring??? ThanX!
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Old 01-24-05, 01:29 PM   #15
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if you can space the ring, the problem lies in the frame, specifically the bend or crimp in the chainstays,
i've had frames that i could'nt go bigger than a 48 tooth and keep a straight chainline on a flip flop, but if you space the hub uneven you can usually go 52 and up within reason on a problem frame
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Old 01-24-05, 01:38 PM   #16
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Quote:
Originally Posted by [165]
Cogs are cheaper than chainrings - unless your budget is unlimited...
not if you know where to look. ;D
i paid 99cents for the last chainring i actually bought.
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Old 01-24-05, 05:44 PM   #17
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I got my last chainring (47t, 110mm) for $4.99 from danscomp.com - they are $5-14 or something like that. Of course, those are cheap chainrings. If you are talking track chainrings, I would be blown away to see one for less than a decent cog.
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Old 01-25-05, 07:57 AM   #18
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Quote:
Originally Posted by ofofhy
You bring a smaller chainring in closer to the chainstays for chainline purposes. Something I am dealing with on a 50T chainring that I have.
Yea, thats why I run a 40x15 on my road conversion. The chain ring is about .5 mm from the stay and the chain line is perfect. Prior to that I was ruinning a 46 on the outside of the road crank spider and the line was off. It really was too big a gear to push for a commuter anyway.
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Old 01-25-05, 10:01 AM   #19
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Quote:
Originally Posted by jfmckenna
Yea, thats why I run a 40x15 on my road conversion. The chain ring is about .5 mm from the stay and the chain line is perfect. Prior to that I was ruinning a 46 on the outside of the road crank spider and the line was off. It really was too big a gear to push for a commuter anyway.

So how close to the stay is too close?
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Old 01-25-05, 11:09 AM   #20
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Quote:
Originally Posted by ofofhy
So how close to the stay is too close?
As long as it doesn't touch. I mean you could probably stick a credit card between mine but thats about it. It never hits the stay. I guess there could be some flex there but with the smaller chin ring it's marginal.
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Old 01-25-05, 11:33 AM   #21
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show me a track chainring (sugino, etc - track specific) that is brand new and less expensive than a rear cog.
I should have been more specific.
Quote:
Originally Posted by pgringo69
not if you know where to look. ;D
i paid 99cents for the last chainring i actually bought.
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Old 01-25-05, 06:15 PM   #22
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Quote:
Originally Posted by [165]
show me a track chainring (sugino, etc - track specific) that is brand new and less expensive than a rear cog.
I should have been more specific.
who said anything about brand new? there is stuff out there that literally only has a touch of grease on it that can be had for cheap or free. imo, unless you bust a couple of high quality 3/32 chains (do your research), you're throwing your money away on 1/8 stuff and adding big rotating mass.
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Old 01-26-05, 12:57 PM   #23
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Quote:
Originally Posted by jfmckenna
As long as it doesn't touch. I mean you could probably stick a credit card between mine but thats about it. It never hits the stay. I guess there could be some flex there but with the smaller chin ring it's marginal.
Too close and the crap that accumulates on the ring and frame will act like an abrasive, slowly but methodically chewing away at the frame.

Somebody posted some pic recently showing the result of running tires that were quite a tight fit. Did some nice damage to the frame over time.
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Old 01-26-05, 01:05 PM   #24
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I'm currently reading "Bicycling Science", and there's some research in it showing that the efficiency of a 53/15 drivetrain is about 93%, less than a 53/20 drivetrain, about 96%. I can find the exact figures and references if anyone's interested. This is including a rear derailer and non-straight chainline. The principle is that the chain has to bend more around smaller sprockets, causing greater frictional losses.

All the mechanical research in the book fascinates me, it's a really good read.
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Old 01-26-05, 01:33 PM   #25
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Originally Posted by moxfyre
The principle is that the chain has to bend more around smaller sprockets, causing greater frictional losses.
This 'principle' which you are talking about is the smoothness of a ratio that you feel. A ratio such as 44:22 will feel smoother than 30:15, due to the fact that the chain has to bend more, and engage less teeth at any certain time in the crank rotation.
The derailleur and off chainline caused by derailleurs suck a lot of efficiency. You'd be surprised at how much efficiency a derailleur system loses over a SS/fixed system.
I know, because when I converted my 80's touring bike to SS, and I could feel the difference immediatley.
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