Slipping gear on one sprocket when changing: big cog > sm

jeepie

Hi Peeps,

Any ideas on what could be the problem here? I had a problem with my freehub which was replaced. My freehub problems are over - yippie - but ever since then I've had this specific problem with my 105 set-up:

On one particular rear sprocket on the rear, when I change down from the big cog to the small cog at the front - the chain seems to come out of alignment by a tiny bit and therefore doesn't engage properly and so the cranks just spin. Tres dangeours in traffic as you can get rear ended! This is most likely to happen under load. If you get off the bike and spin the pedals you'll hear a click as the chain engages correctly.

I have tried the following:

1) Released the front and back gear cables and retightened them.
2) Re-indexed the rear derallieur. Every single change on every rear sprocket is silky smooth.
3) Readjusted the lower adjustment screw of the front derallieur so the chain can come across more

But I'm still getting the problem, and my LBS is not taking in any more jobs at the mo and I'd like to keep commuting.

It's almost like I need a barrel adjuster like you have on the rear derallieur to tighten the cable at the front. Is there a way to re-index the front derallieur ?? I have released and re-tightened the front derallieur and it's very taught.

Any ideas what the next step would be? Do you think my problem is with the front derallieur?

Cheers

J

whyamihere

How worn are the cogs and chain?

jeepie

Thanks for your reply

I've done 650 miles on the cassette and chain which were changed together so I'd have expected it to last a bit longer.....

[Edit] - could it be that my inner chain ring at the front is worn? In this case I don't quite understand why only one rear sprocket at the back has the issue with the down change?

My hunch is that I need to chain to move over a bit more towards the bottom bracket by a small adjustment of the front derailleur, but I don't know what to adjust....

whyamihere

Hmmm, my thought was that that particular cassette cog was worn, and that was causing the skip. Unlikely after 650 miles though.

Which cog is it on when the skipping occurs? I'd guess it's one of the extremes, either the largest or smallest? Also, if you put it in the workstand and watch closely: Does it, when engaging, drop slightly to one side, or drop straight down. Basically, is it not moving far enough to engage immediately, or is it skating on top of the ring before dropping down to mesh with the teeth properly?

This may be completely wrong, but it's an idea I've just had. If it's the latter, with the chain skating then dropping, it could be that the chainrings aren't aligned correctly. The tooth patterns are milled to avoid these problems, so if one of the chainrings is rotated relative to the other, this can occur. Quick way to check is that on the outside of the outer ring there's a peg to catch the chain, and on the inner ring there is a small bit which sticks out, you can see it here on the top right:



Both the peg and the sticking out bit should be aligned with the crank arm. It's possible that the chain length is just right to cause the chain to not engage properly when on that cassette cog.

Alternatively I could be completely wrong, in which case the answers to my previous questions would be handy.

jeepie

Which cog is it on when the skipping occurs?

It occurs on the 5th cog of a ten speed rear cassette.

Workstand analysis:

With regard to the workstand, I can't actually replicate the problem on a workstand as it seems to need to have my full 13 stone weight going through the pedals e.g. hill climb. When it does happen I can change back onto the big cog and carry on, but the transistion to the small cog seems to knock out the alignment on the previously fine rear sprocket.

Does it, when engaging, drop slightly to one side, or drop straight down. Basically, is it not moving far enough to engage immediately, or is it skating on top of the ring before dropping down to mesh with the teeth properly?

I will check this tonight...sometimes I have to ride for 20 mins before it happens. At this point I'm not sure exactly whether the chain is skating on the top or not moving far enough but will check and post back. If think it's not moving far enough, but I will check to be sure. Thanks for your help whyamihere.

jeepie

Chain and cassette have done 533 miles. Checked the exact figure now I have my logbook.

Will ride the bike in to work tomorrow to replicate the problem and see if the chain is not moving far enough or skidding over the top of the sprocket.

Looking at this > http://techdocs.shimano.com/media/techd ... 603724.pdf I wonder if I should fiddle with the outer casing adjustment. In my case I have two rings at the front not three but it's happening right in the middle of the cassette.

Will let you know how I get on.

Cheers

J

jeepie

Tightened the outer casing adjustment and had no bad changes on the way in, but this is not conclusive...

Out of interest could anyone with 105 on their bike check their outer casing adjustment as shown by:

http://techdocs.shimano.com/media/techd ... 603724.pdf

My outer casing adjustment bolt for the front derailleur is very very tight. It says in the doc tighten it by 2 turns, but I can only move it two clicks (which is about a 1/4 turn in total)....this maybe equals 2 turns, but will have a fiddle with it and post back.

jeepie

Hi,

Ok. The problem seems to be that on changing from the big cog to the small one, as whyamihere mentioned the chain:

"is it skating on top of the ring before dropping down to mesh with the teeth properly"

As you can see in the attached picture it seems like the chain needs to move 1mm or so across to the left to mesh with the teeth.



Any ideas. Do you think the gear cable might need to be replaced? I have tried pulling the cable as tight as possible...any thoughts?

Cheers

J

jeepie

HI there - I realise I might not have posted the piccie at the best time to catch peeps. Does the pic help at all? The chain is resting on the teeth rather than engaged with it? Going to phone the LBS to replace the cable to start off with.

Cheers

J

whyamihere

Not a cable problem. The chainrings aren't aligned properly. Have a look in my previous post about aligning the rings properly, and that should sort it.

Alternatively, it's not a great picture, but that inner ring looks to be worn out. The troughs where the rollers lie seem to be quite elongated. Replacing the inner ring may well help.

jeepie

Cheers whyamihere. I know whyyouarehere!

I'll check the chain ring alignment following your advice.

Thanks

J

jeepie

In the end took my bike to the LBS. Full service. New gear cables and chain. New BB. Seems sorted. Fingers crossed.