Problems indexing 105 rear derailleur

toontra

I have an almost new 105 6600 rear derailleur which is making bad noises. On the biggest chainring (it's a triple) it makes a nasty hunting noise on the 3rd and 4th cassette rings - every 20 or so pedal rotations it grinds for a few seconds then returns to the correct gear. There isn't a problem on the middle or small chainring.

I've fiddled endlessly with it. The high/low/b screw settings are correct, and I've tweaked the barrel shift so it goes up and down exactly as it should on all 3 chainrings.

It hasn't had any knocks to bend the hanger - the whole bike is almost new and never been out of my possession.

This is doing my head in - on club runs I get the "Can't you sort your bike out" looks!

OnTow

Ha ha - I know that feeling!
I'm going to keep an eye on this thread, as I run 105 cassette/dereilleur with a Shimano compact, and have exactly the same problem - 3rd/4th smallest sprocket.

I noticed that those sprockets have some twisted teeth - I guess this is there to aid shifting, but unfortunately causes the noise - Can anyone else with a 105 cassette confirm this?
I'm running 12-27. What are you running?

I don't suffer the grinding though - only the hunting noise whilst spinning on 3rd/4th sprocket.

I did wonder if the fiter had been a bit too handy with the chainwhip, but that doesn't make sense - now confirmed by the fact you have the same issue.

I spoke with a guy who works as mechanic in a bike shop, and he pointed out this his Campag. setup was even more noisy.
I've put it down to my new frame transmitting noise better - which probably also has something to do with it's stiffer more communicative ride.

toontra

Yes, I'm running a 12 -27!

winkywonky

I've got 105 on both my bikes and I find it a lot harder to keep them running smoothly then the old tiagra set up.

I found that one is better than the other and that 1 definitely had a issue with what probably was the 3rd and 4th cog.

I have the same problem of endless adjusting the rear mech set up from scratch and it still makes a noise.

I guess it must be constant cable stretch, chain wear, etc, etc conspiring to make our lives hell

alfablue

On an older bike I would have suggested that the cable was gunky / not running smoothly, either under the bottom bracket or more likely, within the outer on the loop to the rear mech. It could happen quite quickly on a new bike if the cables get wet. I had this a while back despite frequent cleaning and lubing, so stuck on a Mudlover sealed cable and it has run sweetly all year. The "hunting" and delayed return to the correct gear does suggest friction in the cable (if it is going down the block - to the smaller gear), can't account for why that would only be on large chainring, although the chain angle could affect this.