# how do you determine rear cog size?



## immat (Sep 22, 2008)

i have a 42t on my crank, and i need to get new wheels, so i found someone selling front and back wheels with tires, tubes, lock ring and the cog, which is 16t. would that work right with the crank i already have? i don't know if there is some sort of ratio your supposed to have or what and i haven't been able to find anything on it. just starting with fixed gears. 
thanks alot


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## Dave Hickey (Jan 27, 2002)

That ratio will give you 70 gear inches(assuming 700c wheels/tires) which many think is the ideal starting point...go for it


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## JCavilia (Sep 12, 2005)

*some reading material*

http://www.sheldonbrown.com/fixed.html#gearing


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## Colin Y (Jul 1, 2008)

I agree with Dave, 70 inches is a good starting point. I have 70 on my single speed and it seems kind of low for the terrain where I live, but my fixie is going to be 46-16 which is about 76. It just depends on the amount of hills and descents you have to deal with.

You should check out sheldon brown's site for fixies if you haven't aready.

http://www.sheldonbrown.com/fixed.html

Gear calculator (switch gain ratio to gear inches)
http://www.sheldonbrown.com/gears/

Good luck!


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## immat (Sep 22, 2008)

ok good. 
i'm getting both wheels, back is flip flop, tires, cog, lock ring, tubes, and rim strips for 155. good deal?

one more thing, does the chain thickness depend on the cog/crank, or is it just preference and 1/16 and 3/32 would both work on the same set up.
thanks again


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## Colin Y (Jul 1, 2008)

$155 sounds good for all those things.

Chain size depends on both the cog AND the chainring. It's gonna be either 3/32 or 1/8, but a 3/32 chainring can be used with a 1/8 chain but a 1/8 chainring or cog CANNOT be used with a 3/32 chain. I've read from Sheldon Brown and other sources that the 1/8 chain does not have a noticeable difference in strength but I guess it's just personal preference. Sheldon actually recommended a 3/32 over a 1/8 chain. It's also a little lighter than a 1/8


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## FatTireFred (Jan 31, 2005)

immat said:


> ok good.
> i'm getting both wheels, back is flip flop, tires, cog, lock ring, tubes, and rim strips for 155. good deal?
> 
> one more thing, does the chain thickness depend on the cog/crank, or is it just preference and 1/16 and 3/32 would both work on the same set up.
> thanks again




if those are loose ball hubs, I'd pass... but that's me. formula sealed bearing hubs are actually pretty decent and relatively inexpensive. also, it kinda pays to get a quality lockring and cog


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## Jamieshankland (Jan 8, 2005)

Ill agree that the above adice is good and correct but your gearing starting point can be specific to what your doing with your fixie.

If your like many old school trainers you spend most of your off season on a fixie pounding thru the rain, cool temps and rolling terrain to get strong and fast legs you idealy want to start somwear in the low to mid 60s. I read somwear a year ago that the guys that get the most out of training on a fixed wheel are usualy the TT riders as it can train a serious amount of speed, strenth and fittness.

39x17 61"
39x16 65"
42x18 63"
38x16 64"

The idea is that your on a gear that you would normaly cruise in on a lighter training day. Because its fixed it allows you ride it at a higher rpm that normal. Doing your off season training like this helps your develop a seriously effecient, smooth, and dangerously fast pedal stroke. Its always evident who trains either on the track or road fixie. Youll find your fittness improves because your never coasting, and your finishing sprint to the line is only faster because your legs can now go even faster. IE Mark Cavendish - clearly has the best leg speed in the peleton, his Track WC jersey has somthin to say about that too.

What alot of guys did in the past was to run a flip flop hub with a sproket one tooth smaller on the opposing side. After a serious base you could start alternating between light days and strenth days. Youd find a loop that has some steady climbs in it, aswell as some stingers and while seated almost mash your way up forcing yourself to stay in a proper seated position. Done correctly it almost feels like leg pressing. Over gearing is a technique used mostly by sprinters but its proven to be a great method of building strenth and power.

At first a half hour feels like an hour, after several months of continual work and forcing yourself to be smooth on the pedals 45 will feel like an hour. It continues to even out after a few years.

If your using the bike to commute and its fairly flat I would consider starting mid 60's and moving up towards the 70's if you have to. 

If your a messenger in a flat city go big and grow some propersized sprinters legs.

If your doing a road TT find out what the biggest gear is that your able to accelerate and get "on top of" If you have any decent fittness you should be able to sit at 115-120RPM on a fixed wheel no problem in a TT. Typicaly guys who know what their doing ride in the mid 90's if its flat and calm.


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## Andrea138 (Mar 10, 2008)

I've been riding a 42-14 since I got my crosscheck. With the addition of 35c CX tires, it could be a little steep for some, but it suits my style. I took it off-road for the first time yesterday & had a 20t freewheel cog on (flip-flop hub). It felt good on the hills, but I was spinning out a bit on the flat/downhill areas, so I night be modifying that after I try a practice CX race or two.


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## immat (Sep 22, 2008)

so with a flip flop hub, when you turn it around wouldnt the chain be either to short or to long? do you have to carry an extra chain or is there a way to make it work both sizes?


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## dunklegelb (Sep 20, 2008)

Colin Y said:


> Chain size depends on both the cog AND the chainring. It's gonna be either 3/32 or 1/8, but a 3/32 chainring can be used with a 1/8 chain but a 1/8 chainring or cog CANNOT be used with a 3/32 chain. I've read from Sheldon Brown and other sources that the 1/8 chain does not have a noticeable difference in strength but I guess it's just personal preference. Sheldon actually recommended a 3/32 over a 1/8 chain. It's also a little lighter than a 1/8


I just Changed to a Shimano SAINT 46T 104BCD ring and my freewheel Cog is a 16T 3/32 using a KMC z410 chain. It dont run as smooth as the stock chain my bike came with which I think was a KMC Z610HX 3/32. 

I wanna try a KMC z810 which is a 3/32 chain but
How do I know if the SAINT chaingring I got off ebay is 1/8 or 3/32?? 
I cant find any info on the Shimano Site.


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## roadfix (Jun 20, 2006)

*horizontal dropouts*



immat said:


> so with a flip flop hub, when you turn it around wouldnt the chain be either to short or to long? do you have to carry an extra chain or is there a way to make it work both sizes?


No worries if you have horizontal dropouts. Most horizontal dropouts should easily be able to handle a two cog teeth difference, some up to three if your dropouts are longish. What is your frame?


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## JCavilia (Sep 12, 2005)

> I wanna try a KMC z810 which is a 3/32 chain but
> How do I know if the SAINT chaingring I got off ebay is 1/8 or 3/32??
> I cant find any info on the Shimano Site.


Is that a MTB group? If so, it's 3/32. 1/8 is track stuff.

Or you could measure it ;-)


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## Andrea138 (Mar 10, 2008)

going between 14t and 20t is too much for my dropouts, so I have two different chains. It only takes a few minutes to switch between the two at home depending on whether I'm going road fixed or offroad free.


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## immat (Sep 22, 2008)

its an old reynolds 531 frame. its really cool. its beat up and un-painted right now but i'm hoping to make it really nice. 
so ya, they are horizontal and i guess i see what you mean, you just tighten the bolt when its farther back ya?

again, bare with me. new at all this stuff. no prier bike knowledge before starting this.


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## Colin Y (Jul 1, 2008)

> you just tighten the bolt when its farther back ya?


Yep, that's basically it. You just want to make sure the chain tension is correct, especially with a fixed gear. Also that the axle nuts are really tight.


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## JCavilia (Sep 12, 2005)

*Too intimate*



immat said:


> again, bare with me. new at all this stuff. no prier bike knowledge before starting this.


We haven't known you long enough to bare with you. But we'll try to bear with you.


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## FatTireFred (Jan 31, 2005)

JCavilia said:


> We haven't known you long enough to bare with you. But we'll try to bear with you.




or even beer with you


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## immat (Sep 22, 2008)

ha, how about all 3 at the same time?

thanks a lot guys. i think i've got this handled for now. i'm sure ill be back soon enough for more though. started painting today, so it should be finished in the next week or so.
thanks again!


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## SlowMo (Apr 18, 2006)

*Purchasing preference up front?*

Looking at getting a SS now; but not sure if I want to go with a 42x16, or a 48x18. Same thing? 

Also, what about tire clearance/preference. One bike comes stock with 23c, the other comes with 28c. I want to make sure I can put wider tires on for the weather conditions, but do I need to worry about clearance on a SS?


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## Dave Hickey (Jan 27, 2002)

SlowMo said:


> Looking at getting a SS now; but not sure if I want to go with a 42x16, or a 48x18. Same thing?
> 
> Also, what about tire clearance/preference. One bike comes stock with 23c, the other comes with 28c. I want to make sure I can put wider tires on for the weather conditions, but do I need to worry about clearance on a SS?


Clearance varies greatly between frame...The frame specs will usually list max tire size. If not, call or email the manufacturer


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## asterisk (Oct 21, 2003)

An SS what? It depends on the frame, it doesn't matter if it's a single speed.

Conventional(?) wisdom says go with larger cog/chainring as more teeth to spread out the wear but the important thing is to fine a combination that feels good on the bike.


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## another new rider (Dec 28, 2012)

what if my cog is 14t and the crank is a 48t


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## UrbanPrimitive (Jun 14, 2009)

Check the link posted by Colin Y above. It's not scary, I promise.

In case you missed it, here it is again:

Sheldon Brown's World Famous Gear Calculator


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