# Is my rear cog a 3/32 or a 1/8?



## barbedwire (Dec 3, 2005)

How can I tell if my rear cog is a 3/32" or a 1/8"? Is there a way to measure?


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## asterisk (Oct 21, 2003)

Try and lay the cog into a road bike chain (3/32). If it meshes correctly it's a 3/32, if the teeth don't sit down in the chain, it's 1/8.


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## FatTireFred (Jan 31, 2005)

yeah, just measure the width


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## barbedwire (Dec 3, 2005)

FatTireFred said:


> yeah, just measure the width


How? You mean the width between each sprocket?


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## FatTireFred (Jan 31, 2005)

barbedwire said:


> How? You mean the width between each sprocket?



uh, 3/32 and 1/8 refer to the width or thickness of the cog...


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## StillKeen (Oct 4, 2005)

*Is a SRAM 8 speed chain a 3/32 chain?*

I have a 3/32 cog on the back, and the Sugino RD-48 Single Speed Crankset on the front ... which I think is for 3/32 tool. Do I have the right chain? SRAM PC68 ?

I just lay a 9 speed chain on it, and it seemed tight. The cog says 3/32 on it, and the cranks don't say anything (bought them six months ago, and can't remember what they were. Jenson still have them, and they say 3/32).

Thanks,

-Chris

PS just stripped the thread in my stem clamp ... so even if I get this sorted tonight, still no fixie riding tomorrow ... arrggghhh!! Was really looking forward to it.


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## StillKeen (Oct 4, 2005)

Sorted! The chain does work ... I thought I was going mad ... but not!!

Just took the bike for its first ride ... and my first ride on a fixie. Starting off is hard! A different technique to a freewheel bike.


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## tidelag (Sep 23, 2005)

I am using Surly cogs and it fits fine with 9gears chain.
Surly cogs fits 9gears chains because the cogs are thinner than "usually".

The EAI 3/32" cogs measures ~2.28mm 
The Surly 3/32" cogs measures ~2.07mm
The EAI 1/8" measures ~2.91mm


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## JCavilia (Sep 12, 2005)

*interesting numbers, tidelag*



tidelag said:


> The EAI 3/32" cogs measures ~2.28mm
> The Surly 3/32" cogs measures ~2.07mm
> The EAI 1/8" measures ~2.91mm


1/8" = 3.175mm

3/32" = 2.38125mm

So all the cogs are thinner than the nominal width.


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## barbedwire (Dec 3, 2005)

JCavilia said:


> 1/8" = 3.175mm
> 
> 3/32" = 2.38125mm
> 
> So all the cogs are thinner than the nominal width.



What's nominal width?


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## JCavilia (Sep 12, 2005)

*The width that's in the name*



barbedwire said:


> What's nominal width?


"nominal," from Latin "nomen" ("name")

The nominal width of a 1/8" cog is -- wait for it . .. . . . . .. one eighth of an inch (a quantity which can also be expressed, exactly, as 3.175mm).


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## barbedwire (Dec 3, 2005)

*Using a 1/8" cog with a 3/32" chain*



JCavilia said:


> "nominal," from Latin "nomen" ("name")
> 
> The nominal width of a 1/8" cog is -- wait for it . .. . . . . .. one eighth of an inch (a quantity which can also be expressed, exactly, as 3.175mm).



Gotcha. Thanks for that info. So, what you are saying specifically is that the nominal (by name only) widths are:

3/32" cog = .094 inches = 2.381mm
1/8" cog = .125 inches = 3.175mm

And then going off of tidelag's measurements from his post above, we have:
EAI 3/32" cogs measures ~2.28mm
Surly 3/32" cogs measures ~2.07mm
EAI 1/8" measures ~2.91mm

So, going off tidelag's measurements, can I use a EAI 1/8" cog (2.91mm) on a supposedly smaller 3/32" width chain? In theory, if the space between the plates of the chain is greater than the cog, that means I can use that cog, correct?


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## JCavilia (Sep 12, 2005)

*I don't think so*



barbedwire said:


> So, going off tidelag's measurements, can I use a EAI 1/8" cog (2.91mm) on a supposedly smaller 3/32" width chain? In theory, if the space between the plates of the chain is greater than the cog, that means I can use that cog, correct?


I mean, if the space in the chain is wider than the cog, it would work. But that isn't the case in the example you suggested. The measurement on the EAI cog is less than 1/8", but it's still considerably more than 3/32".

It's simple, really. If you use a 1/8 cog, use a 1/8 chain. If you use a 3/32 cog, use a 3/32 chain -- and use a cheaper 7 or 8-speed chain. Unless you happen to have a 9-speed chain lying around, there's no reason to use the thinner and more expensive chain -- and it might be a little tight, with some cogs.


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## barbedwire (Dec 3, 2005)

JCavilia said:


> I mean, if the space in the chain is wider than the cog, it would work. But that isn't the case in the example you suggested. The measurement on the EAI cog is less than 1/8", but it's still considerably more than 3/32".
> 
> It's simple, really. If you use a 1/8 cog, use a 1/8 chain. If you use a 3/32 cog, use a 3/32 chain -- and use a cheaper 7 or 8-speed chain. Unless you happen to have a 9-speed chain lying around, there's no reason to use the thinner and more expensive chain -- and it might be a little tight, with some cogs.



OK thanks. So basically, you can't use a 1/8" cog with a 3/32" chain since the cog is thicker than the chain. However, you can use a 3/32" cog with a 1/8" chain because the cog is thinner than the chain. Correct?

OK, so I know how to measure the cog thickness. And I know how to arrive at the nominal width for a cog, whether that be 1/8" or 3/32". It is actually supposed to be the thickness of the cog itself, but as demonstrated above sometimes actual cog thickness is slightly different from the nominal cog thickness. I was just curious to know what nominal chain thickness is. Is it the width between the outside of the chain plates, or the inside of the chain plates?


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## tidelag (Sep 23, 2005)

Sorry, I forgot this tread, so here is a small table of measurement of chains.
Width is measured inside of the (inner) chain plates, because it must mesh fine with the cogs. 

KMC 1/8 cheapo chain: ~3.62mm (pin length is 8.6mm)
KMC 3/32 K 51Z 2.44 - 2.54mm (pin length is 7.3mm)
KMC 3/32 an another one 2.44 - 2.54mm (pin length is 7.11mm)
ACS 3/32 SingleSpeed 2.30 - 2.38mm (pin length is 7.42-7.49mm)
Shimano cn-hg53 9gears 2.22 - 2.24mm
SRAM PC-58 2.35mm

(the tolerance range of my instrument is +/- 0.02mm)

When I used KMC 3/32 half-link, I found ot that the half link did not like SRAM/ACS chain, so I was forcered to use KMC chain.

Nominal does mean "normal" which is an averaged measurement.
(If I have understanded it correct)

The cogs MUST be thinner than chains, to make it mesh fine and with low friction and high smoothess to the drivetrain.


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## wim (Feb 28, 2005)

> _Nominal does mean "normal" which is an averaged measurement.
> (If I have understanded it correct)_


As others have said, nominal cames from "name." It's not an average. It's a _convenient_ way to identify something, with the understanding that the name of the object may not be its _exact_ size.

Cog widths are one example. Another example is the piece of construction lumber nominally called a 2 x 4. It's neither 2" thick nor 4" wide. But everyone understands that, so it's not a problem.


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## Kung Fu Felice (Apr 17, 2007)

According to Sheldon Brown, the rear cog at 3/32 can use either 3/32 or 1/8 chain, so it's not as big of an issue, but for some reason the front chainring must exactly match the chainwidth. Don't know the reasoning behind that.


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