I just replaced the chain on a used Trek Cobia. I didn’t have any problems with the old chain in terms of shifting.

I took off the old chain and counted links, to my surprise it was the length of a brand new KMC chain that I used to replace it. I put on the new chain, uncut and found that out of the saddle pedaling when in the middle chainring caused the chain to move around enough that it jumped onto another chainring.

I took off the chain, used the big chainring/big rear cog method (not through the derailler) and this warranted cutting off only about 5 links. So I did that. Now, when I back pedal, the chain is slack enough that it hits the chainstay. Overall the chain just looks too long.

What am I doing wrong here?

Thanks

What you're doing wrong is confusing causes and effects.

I don't know for sure why the new chain tended to autoshift, which could be related to worn sprockets, the shape of the chain's inner plates, FD adjustment, BB play, frame flex, or warped chainringds (for starters), but assuredly was not caused by the chain being too long. The length of the chain in the upper loop is fixed, and the tension is proportional to haw hard you pedal, having nothing to do with resting tension.

As for the "slack" when back pedaling. This again has nothing directly to do with chain length or tension. You get slack in the upper loop because of friction in the freehub or the chain running through the cassette and RD pulley system. The slack or sag happens because the crank is spooling the chain back faster than the rear drive will take it, and the chain is therefore transferring from the RD cage to the upper loop. The specific causes vary, and the source of the friction can be any or a combination.

So, stop fretting about the chain length. It was OK before, and as long as you measured big/big+1" minimum, it's OK now.

Deal with your backpedaling issue by observing and detecting the source of friction in those items I described earlier.

Thanks, I’ll check the rear pulleys first as I took them off at one point.

Some Shimano rear derailleurs have a small tab extending across the pulley cage just below the top pulley. It's real easy to install the chain on the wrong side of the tab, and when it is installed that way things don't work right.

Thanks, I did notice that tab and that is in the right position.

OKay, I am not trying to be contrarian, truly, but I notice on my road bikes the chain is not nearly as slack over the chain stays. Sitting here, the mtn bike chain is in the front middle ring and middle-ish rear cog and the chain is nearly touching the chain stay, maybe two cm away at the closest point of contact.

I agree in principle with this, and especially with the last statement that if it worked OK at the same length before, the length should be ok now. However, if the chain should be way too long, there will be slack somewhere, and it will be on the run from the driving to the driven sprocket (the chainrings are sprockets, after all, at least for the purposes of this discussion). When you are pedaling, the slack will be on the bottom, when back pedaling, it will be on the top, as no system is frictionless. In my limited experience with this, the chain has to be quite a bit too long to cause skipping or jumping, though.

You probably have smaller chainrings on your MTB and thus less space between the chain and chainstays.

You describe the basics fairly well, but need to refer back to the OP. He had chainring autoshifting, not skipping when the chain was long, and backpedal top loop slack when back pedaling when it was short. Theory is sound, but you then have to apply it to the correct situation.

Here is a pic of the chain when in the front middle chainring (cog/sprocket/whatever) and the rear is on the 16 tooth or so.

Something's wrong. Derailleur spring is sprung or too much friction somewhere.

The friction issue makes sense if I was backpedaling say, but there is no tension in the system and the chain moves easily right now pushing back on the pedals.

How do I check the derailleur spring? Again, I have not noticed any shifting issues other than major cross-chaining, i.e. big - big doesn’t really happen without a lot of fuss if at all.

(thanks)

The nRd spring is w=easy enough to check. You'd know at a glance of the sagging lower loop if it were bad. Or you can shift to any mid or high range combination. Pull the lower loop down at the center, and the RD should rebound and pull it back up when you let go. That's all the RD spring does, so it's yes it takes up slack, or no the chain sags.

Update: still not fixed!

Something is definitely wrong as my road bike (Ultegra) does not exhibit this behavior. I have backed out the pulley screws (previously quick cleaned them) to make sure they weren't overtightened to no avail. The spring seems to operate correctly as far as I can tell. Im starting to wonder about the derailleur though. I double checked the big big length method of the chain and so what else could it be?

I am starting to get like another poster on BF: simply swapping chains should be a 10 min job, which for me takes well now hours/days and still not resolved! To think, I used to rebuild engines but now I simply don't have the patience/time.

The bike needs a new mechanic, not new parts.

You're failing diagnostics 101 and listening to too many people. Chainring auto-shifting which is what you originally described is NEVER caused by the RD (except sometimes when backpedaling) because the upper loop runs directly from the top of the rear sprocket to the chainring.

So, go back to my my first post (no.2) and review, ft derailleur adjustment, or look for a loose BB, or a damaged tooth on the chainring for starters.

One thing you might look at is how the chain meshes with the teeth. Look down from the top through the FD cage and the chain to see if the tips of the about to engage teeth are coming up into the center of the plate gap, or touching the sides of the inner plates. Think of a ferryboat coming smoothly into the slip vs. coming in at an angle and bumping down the sides of the slip, and think of the sprocket's teeth the same way.

Another possible factor, is chain lube. Well oiled chains will slide smoothly down the sides of the teeth even when there's some rubbing, but poorer lube can make a chain not engage smoothly, and once it begins to ride up on the teeth it's derail.

So my advice is simple, look for the problem where it is (at the chainrings) and stop fretting about unrelated stuff at the RD.

Haha...love it. I should have mentioned that the autoshift issue occurred when out of the saddle rocking the bike back and forth with some gusto.

Ft. derailleur puts chain dead center, part of the derailleur even covers the larger chainring so that the chain shouldn't jump there. Ferryboat looks to be coming in dead center captain.

Okay, some teeth on each chainring are looking as if they got a chunk taken out of them. They look like they need to be replaced.

The chain is basically new.

Even with the chainrings needing replacement, the sag in the chain is not correct. None of my other bikes exhibit this. It is as if the rear derailleur is not pulling on the chain enough to take up the slack even though the spring seems to be okay as per the post above.

Yeah, I think you are right with your opening statement.

That's normal. Those are shift gates to allow for smoother front shifting. Chainrings that are toast look very sharp and pointy, and you frequently see deformed metal near the end of the tooth.

thanks [MENTION=341677]gsa103[/MENTION]

OK this points directly to a short list of likely suspects. It's possibly a case of "raise the bridge or lower the water".

Normally, shifting would happen if the FD cage moved over a bit, but instead the crank is probably moving out a bit at the top (compared to the FD cage).


Next time you hammer look down at the chain, ring and front derailleur. You notice movement of the ring compared to the FD cage. If so, that's the problem. The cause is a bit harder. It could be frame flex, crank flex or a loose BB, so you still have some detective work to do.

Thanks again [MENTION=158672]FBinNY[/MENTION]

From the photo it appears to me that the rear derailleur spring is not working at all. There is slack chain in the upper run of chain as well as the lower run of chain.

Final Update:

The rear derailleur is toast. I got fed up and took it to Fairwheel. They immediately noticed that a tab on the derailleur which is involved with spring "windup" was broken/missing. So I decided to upgrade from the x5 to x9.

Cheers to all the help.

X9 installed and it now works properly.