jowilson

I noticed this when I had my fixie in the stand this morning. The chain and cog have ~300 miles on them,so dine so surprised if it was worn already. I have another cog and chain that are pretty new so I can switch them out if I have to. Here is a picture:

The gearing is 46x16 if that helps, KMC chain, Sugino all city 46t chainring and unknown brand 16t cog.

FBinNY 

It's hard to tell from a photo, but the chain's pitch seems to be too large. This is classic "chain stretch" but it's really early a 300 miles.
Confirm "chain stretc" measuring with a 12" ruler. The pins at both 0 and 12" should line up perfectly (24x1/2" pitch). If the 12" length of chain is more than 12", that's wear which allows the links to move apart at the pins.

Question--- were you care full to properly tension the chain?

If yes, that might be the problem. A tensioned chain is an over tensioned chain, and will lead to rapid wear. Properly "tensioning a chain is about leaving the proper amount of slack in the system. Since sprockets are often eccentric, you want to find the tightest place and leave vesitlgial slack there - about 5-8mm FREE up and down play at the center.

jowilson

I did leave the proper slack in the chain. I built this up with help from a friend and he suggested I leave at least 10mm of slack in the chain and I've always made sure to leave that much slack in it. I'll go ahead and put the other cog on and see if that fits better.

FBinNY 

OK, so take a moment to measure the chain over 12" so you know if it's a chain with long pitches, or a sprocket machined small and with short pitches.

BTW- if it is a stretched chain, then you might need to reconsider your lube.

jowilson

Chain has VERY little to no wear according to the 12"
ruler method. Probably the cog then, yes?

FBinNY 

Looks like it judging from the photo. If the pitch doesn't match spot on, then either the chain is long, or the sprocket is small, and you've eliminated the first.

clevername

Wait, is this referring to the rear gear? I've only ever compensated for eccentric chainrings/front gears, not the rear ones..

FBinNY 

Both can be slightly eccentric, though chainrings are usually more so. The allowance your leaving is probably enough, but you can make sure by turning the cranks a number of turns with a finger below the center of the lower loop to make sure it never takes up all the slack.

Since the sprocket usually doesn't divide evenly in to chainring (48/16 being a common exception, and which shouldn't be used on a fixed gear bike) the exact time that the high spots will be at opposite ends only happens once in a while. That's what you want to make sure still has slack, so turn the cranks until you have confidence that you're OK, especially if you prefer to dial out as much slack as possible. (I ride fixed, and like minimum backlash, so leave only the least vestigial slack)

prooftheory

Out of curiousity, is the cog one of @Scrodzilla's one-dollar wonders?

reptilezs

use 1/8 single speed chain

hueyhoolihan

if it was me, i'd try another cog if i had one and see where that leads me. OTOH i might, if the chain doesn't skip, just try to forget about it. after all, as long as the chain is not skipping i don't feel i have much to worry about.

Pepper Grinder

What's wrong with 48/16?

FBinNY 

This applies only to fixed wheel bikes.

If the rear sprocket divides into the front, as in 48/16,the rear wheel will turn exactly 3 times for every rotation of the pedals. That means the same part of the tire will be on the ground for each power stroke, and most importantly when reverse torquing to stop. 48/16 means that your tire wear will be uneven, and one or two spots will wear out much sooner than the rest.

Obviously, freewheels eliminate the problem by randomizing the wheel/pedal relationship.

Pepper Grinder

I understand the skid patch thing, but this isn't an issue if one doesn't skid, right? Can the "power stroke" create patches of localized wear on the tire like skidding can?

FBinNY 

Yes, depending on your power and riding style, though the effect will be much less than the skid patch wear.

dabac

+1

Strange things can happen if you run a 3/32" chain on 1/8" sprockets/rings - or if it was the other way around...

FBinNY 

It's definitely a case of the chain being too narrow since it clearly settles fully into the sprocket. When the chain is too narrow, it rides, and appears as if the pitch is too short, not too long as in this case.

As to using a 1/8" chain on a 3/32" sprocket, the tens of thousands of riders who do this prove it's OK.

fietsbob

1/8" is, after all, just 4/32" ..

jowilson

Chain is not too narrow. It's a single speed, 1/8", 4/32" chain. I switched the cog for a different one and it fits perfectly. Now I'm curious why the cog is a shorter pitch. Could it be that it's a funky 10mm pitch cog?

hueyhoolihan

i would guess that it is just defective. maybe a second that should have been caught in quality control...

BTW, as i think all know, but just for clarification, 3/32 chains don't work on 1/8 chainrings or cogs, but 1/8 chains work on 3/32 chainrings and/or cogs.

FBinNY 

No, it's definitely not 10mm, that would be day/night obvious.

The sprocket may be undersized because of manufacturing error. To see how it's possible, understand that pitch and radius are related, with the pitch changing roughly six times the radius error. (circumference = 2piR) So if the teeth are cut 0.001" too deep the pitch will be 0.006 short.

Gear sprocket machining is very exacting work, easy when dialed in, but equally easy to screw up.

BTW- a worn chain can cause a similar problem without appearing stretched. If the rollers are worn, the chain settles deeper into the sprocket, where the pitch is shorter. I didn't mention this earlier, based on the age of your chain, and I bring it up now in case other readers run into a similar issue.

If changing the sprocket solves the problem, then it was the sprocket. If not, then take another look at the chain.

gregjones

I've got some of those. I use them to try out different gear ratios. I haven't had any trouble with any of them.

FBinNY 

There are a number of ways to produce sprockets, running from stamping to generating the tooth profile on a CNC machine.

One of the ironies is that the cheapest way to produce sprockets in volume (stamping) produces the most consistent quality, decent, though not great, but always on size. Expensive sprockets produced in low volume on CNC machines are the ones most likely to be made undersized.