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So in the shop they asked me "what chain do you need" and I said 7. They said "you could try this 6-speed, it is half the price of a 7, and we put it on 7-speeds all the time". I said "ok", only to determine later that my bike is 8-speed. (Excuse: I never use the several highest cogs).

I know "higher-speed" means just that the chain is thinner, fitting the recommended chainring, plus any other with less cogs (because there is more space between the cogs).

And, as expected, my bike now makes all kinds of clicking noises and skips. However, upon inspection, the chain does not rub the chainrings. It fits tight, but does not touch the other chainrings.

I will probably replace the chain for a correct one, but before that I would like to know what is the cause of the problem (the skipping).

EDIT: some more detail
We are talking a $300, used regularly for three years. Of it, only the chain has been replaced - twice.

If needed, I will post a photo tonight. Just ask in the comments!

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Chains widths are confusing, since there have historically been several different configuration for the same given number of sprockets. I see even Sheldon Brown doesn't attempt to address the topic in any detail. – Daniel R Hicks Oct 2 '12 at 11:19
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I think the answers from Jahaziel and Jimirings contain the real deal: most probably your sprockets are a bit worn, and it's possible that the derailer is not properly aligned (looking from above is not enough). Also, very cheap chains might not be so "indexed-shifting-friendly" and might touch neighbour sprockets more easily / more often than a more dedicated and expensive chain. – heltonbiker Oct 2 '12 at 16:15

5 Answers

up vote 4 down vote accepted

The clicking and skipping could be because of the mismatched chain/cog combo as you suspect.

However, it could also be that your rear sprockets are worn. Chains "stretch." As they do, they also stretch the teeth on the rear sprockets, especially the highest gears (smallest sprockets). The old stretched chain will work just fine since the sprockets have stretched to match. However, if you replace it with a new chain, the teeth and the chain links no longer match up. The result is a chain that skips and makes a bunch of noise.

You can read more on Sheldon Brown's site.

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See my edit to the question. So, how long does it take for a new chain to wear-out to match the cogs. Also note that the cogs are not homogeniously worn - I ride at the faster half of the cogs. – Vorac Oct 3 '12 at 5:05
First off, the cogs will never be homogenously worn. The smaller (faster) cogs wear faster. This is because they have less teeth gripping the chain which puts more pressure on each individual tooth. Second, I don't know how long it would take for the new chain to match the cog wear. I'm not sure if anybody does. Most people would just replace the rear cassette or freewheel, whichever the case may be for your particular bike. I'm sure that if you put it to the test, it would take long enough that the skipping would drive you completely insane while you're waiting. – jimirings Oct 3 '12 at 17:07
Ok, couple of months passed and now I can assert that your answer was correct. My two smallest cogs were worn out and clicked. Now, after the chin is a little bit worn out also, the clicking has stopped. – Vorac Jan 25 at 7:16
up vote 8 down vote

The skipping will always be because of the chain touching on the other cogs, or trying to slip back down from the pull of the rear derailleur. The chain is too wide and, as you already know, is doing one or the other. It might not be evident from looking at the cogs while riding as the chain is fed from underneath when you pedal, so the clicking from the chain moving may only be visible when the bike is upside down.

If you have a reasonable quality shifters/cables/mech then you should be able to just about get away with it with some really fine adjustment.

The difference between 6, 7, 8 and 9 speed is actually the spacing between the cogs, and not the width of the cogs themselves. Hence the need for thinner chains.

Edit: I haven't got comment everywhere ability yet, but as suggested below, you should make sure your chain is the correct length. The recognised method for this is to wrap the chain around the largest cog at the front, the largest cog at the back, pull tight and then add two links (or 3 if it ended up with two inner links).

This generally ends up being shorter than what you had originally, but is correct.

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Now you've given more info it sounds more than likely your rear sprockets are worn and need replacing. Your most commonly used front chainring is also most likely starting to wear and may 'slip' when you're doing some powerful pedalling with the new chain. To reduce wear on the sprockets, keep the chain clean and replace it before 12 outer links (which should measure exactly 12 inches) grows by more than 1/8th of an inch. – mikezs Oct 4 '12 at 13:01
up vote 3 down vote

As far as I know, 6, 7 and 8 speed cog groups, all share the same cog spacing and can use the same chain size. If you lay flat on a table a sample of each, you'll see each one is "taller" than the one with less cogs.

However, they stop growing after that, as a 9 speed cassete has the same overall width as a 8 speed, thus, they had to reduce the cog spacing and need a narrower chain. The same happens when you go up to 10 speed, the overall cassette width is the same as 9 speed, having even a smaller cog spacing and a narrower chain.

As 6-8 cog groups share the same spacing, it is usually possible to use, say a 7 speed indexed shifter on a 6 speed cog group, since the derailleur has to move the same distance to perform a gearshift.

It seems to me that your problem is due to derailleur fine tuning. To check that visually, examine the bike from its rearview. In each shift position, the guide pulley should line almost perfectly with its correspondant cog. It is easier to check this with the chain removed, but it's not mandatory. Good lighting helps.

Inproper chain length can also be the cause, as other answers state.

Note: I speak for my experience with several mountain bikes, including "formal bikes" and crazy experiments. I don't know if this is also valid for road or other types of bike.

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It's not quite that simple, especially if you go back historically. From vague memory 6 and some 7 & 8 speed clusters used 5-speed chains. Then narrower chains were invented for 9-speed and retrofitted to 7 and 8. Or something along those lines. Especially if you're dealing with an older (pre-indexing) bike there's no telling what width chain should be used -- you need to measure sprocket pitch and chose the chain appropriately (if you don't have a used chain to measure). – Daniel R Hicks Oct 2 '12 at 16:43
Your first paragraph contradicts everything that I have heard and read. If it is correct, this fact has to be publicised! – Vorac Oct 3 '12 at 5:07
up vote 1 down vote

Dumb question - I'm assuming you lined it up with the old one and got a rough length before you put it on? Yes, width is usually the major issue when doing the 6 -> 7 -> 8 etc. move, but it could be that it's too short (pulling the rear derailleur up too high) or too long (they will often include a couple of extra links in these chains).

I usually lay the new one out next to the old one and make sure that the length is approximate (the old chain will, of course, have stretched from use to you want to keep that in mind). If I'm feeling particularly anal retentive I'll count the links out (which is, I believe, the recommended method).

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I lay them out side-by-side, line up one end, and then work along the length of the two, so I'm sure I get the same number of links. A stretched chain can be well over one link longer than a new one of the same "length". – Daniel R Hicks Oct 2 '12 at 16:45
Yes, I counted the links. Twice. Both on the old and the new chain. 110. – Vorac Oct 3 '12 at 5:08
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I prefer to use the Sheldon Brown method. That way, if the old one was incorrect because some yahoo put the wrong length on last time (the yahoo could certainly be me) I know that the new one is right. sheldonbrown.com/derailer-adjustment.html#chain – jimirings Oct 3 '12 at 17:12
up vote 0 down vote

http://www.ctc.org.uk/desktopdefault.aspx?tabid=3946 has a table of common sprocket spacings (and no, 6, 7, and 8 aren't all the same). http://sheldonbrown.com/cribsheet-spacing.html has more extensive details.

(As other people have said, the sprockets having worn to the old chain is also a likely problem, but the chain being slightly too wide won't help shifting.)

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