Dura Ace 7900 Rear Derailleur shifting issues

Ginger_beer99

Hoping someone can help me out because this issue is driving me crazy!

I picked up a used, but great condition 2012 Giant TCR Advanced SL with full Dura Ace. I never rode it at the time, and stripped it down to clean all components, fit new cables, a new chain, and a new cassette.

Upon rebuilding I'm having issues with the rear derailleur indexing correctly. I've set it up countless times using the Shimano/Park Tool/Sheldon Brown guides (and have set up a lot of bikes in my time), but to no avail. The issue is that I can get it set up perfectly when in the large chainring, shifting is crisp up and down the cassette, but when I drop it onto the small chainring it won't index properly and I can't shift down through all the gears, or get a lot of chattering.

I've tried it the other way round, setting up in the small chainring, but then I can't get the higher gears when in the large chainring. Indexing on a single chainring is fine and it works perfectly, until I change chainring!

So far I've changed out the cables (again), checked the hanger isn't bent (it isn't), lubed the cables, checked the derrailleur functions with no cable (i.e. moved it by hand), degreased and relubed all RD pivots, checked cassette spacer for 10 speed is in place (it is), checked cassette lockring tightness, changed jockey wheels, checked chain tension/length, and ensured cable bolt is located correctly. I've also ruled out cable kinks or issues as I get perfect up and down shifting in a single chainring. All in all I've spent two full days trying to work it out, but still haven't solved the problem.

When looking behind the bike I can see that for any given cassette sprocket, the derailleur moves ever so slightly outwards when changing from the large to the small chainring which appears to be causing the issue. I'm running a 53/39 with an 11-28 at present.

If anyone has any ideas it would be much appreciated. At the minute the only other thing i can think of is a weak return spring or an inbalance in the RD springs that is causing the issue. I did strip the mech, but couldn't find any issues, so rebuilt it, but that hasn't helped.

chrisvanderkaap

looks like you're working thoroughly but might have missed the b-screw adjustment.
This adjustment positions the angle/height of the derailleur. Adjust it so the jockeywheel
runs 2-3 mm under the largest sprocket. My guess is that it might be too low.

good luck!

C

binkybike

Are you sure that the chain isn't too long? just a few extra links can mess up the shifting on the small chain ring, while all will be fine on the large.

keef66

Is the mech hanger securely attached to the frame? And is it straight??

Ginger_beer99

chrisvanderkaap wrote:

looks like you're working thoroughly but might have missed the b-screw adjustment.
This adjustment positions the angle/height of the derailleur. Adjust it so the jockeywheel
runs 2-3 mm under the largest sprocket. My guess is that it might be too low.

good luck!

C

B adjustment screw is about right, I've tried it in a variety of different positions to check, but it seems to make no difference at all, shifting is just as bad either a few mm away or a cm away.

Ginger_beer99

binkybike wrote:

Are you sure that the chain isn't too long? just a few extra links can mess up the shifting on the small chain ring, while all will be fine on the large.

I thought that myself, took out a couple of links and purposely made it "snug" but this didn't change a thing unfortunately.

Ginger_beer99

keef66 wrote:

Is the mech hanger securely attached to the frame? And is it straight??

Yep, hanger is secure. On the TCR SL the hanger is inboard of the frame dropout, so in addition to being screwed in ace the skewer/axle assembly of the freehub clamps it in place as well. The hanger was straight, but I'll check it again today to make doubly sure.
I just tried a DA mech off my other bike, and the problem is still there, so it's not the RD that's the issue. I'm going to do a bit more head scratching!!

rubbernekker

It must then be the chain and cassette. Are they used? If so change them. I had this at work a while back and I had never seen worn components present like this. The explanation for the big/small discrepancy could be attributed to the added tension the mech spring apportions to the chain in the large ring.

Lookyhere

rubbernekker wrote:

It must then be the chain and cassette. Are they used? If so change them. I had this at work a while back and I had never seen worn components present like this. The explanation for the big/small discrepancy could be attributed to the added tension the mech spring apportions to the chain in the large ring.

Read the post :idea:

you seem to have covered all the bases.
Its internal cable routing - yes? assume you ve fitted outer and inner cables? what brand? Shimano DA worked best for me.
On my 595 with 7900 - its very sensitive to any cable stiction, meaning larger loops of cable from under the bar tape, to reduce this - but once set up is totally trouble free.
Limit screws set? Chain line is ok? no weird spacers? could you try another chain set? 53 links would be correct for this combination.
When you clamp up the rear wheel, does the RD move in board at all? ie is there any damage around that frame area? or else where? chain stay/BB ?
Other than that, (surely could nt be a shifter fault?) its got to be an index setting - follow the shimano tech docs to the letter, including FD setup as well.

desweller

Lookyhere wrote:

rubbernekker wrote:

It must then be the chain and cassette. Are they used? If so change them. I had this at work a while back and I had never seen worn components present like this. The explanation for the big/small discrepancy could be attributed to the added tension the mech spring apportions to the chain in the large ring.

Read the post :idea:

you seem to have covered all the bases.
Its internal cable routing - yes? assume you ve fitted outer and inner cables? what brand? Shimano DA worked best for me.
On my 595 with 7900 - its very sensitive to any cable stiction, meaning larger loops of cable from under the bar tape, to reduce this - but once set up is totally trouble free.
Limit screws set? Chain line is ok? no weird spacers? could you try another chain set? 53 links would be correct for this combination.
When you clamp up the rear wheel, does the RD move in board at all? ie is there any damage around that frame area? or else where? chain stay/BB ?
Other than that, (surely could nt be a shifter fault?) its got to be an index setting - follow the shimano tech docs to the letter, including FD setup as well.

Try adding an inch or two onto the rear mech loop to widen the radius; perhaps it's too tight.

Ginger_beer99

Lookyhere wrote:

rubbernekker wrote:

It must then be the chain and cassette. Are they used? If so change them. I had this at work a while back and I had never seen worn components present like this. The explanation for the big/small discrepancy could be attributed to the added tension the mech spring apportions to the chain in the large ring.

Read the post :idea:

you seem to have covered all the bases.
Its internal cable routing - yes? assume you ve fitted outer and inner cables? what brand? Shimano DA worked best for me.
On my 595 with 7900 - its very sensitive to any cable stiction, meaning larger loops of cable from under the bar tape, to reduce this - but once set up is totally trouble free.
Limit screws set? Chain line is ok? no weird spacers? could you try another chain set? 53 links would be correct for this combination.
When you clamp up the rear wheel, does the RD move in board at all? ie is there any damage around that frame area? or else where? chain stay/BB ?
Other than that, (surely could nt be a shifter fault?) its got to be an index setting - follow the shimano tech docs to the letter, including FD setup as well.

Yes, it's internal. I'm using SRAM Professional Gore cables with liners, and they're silky smooth. I've got decent loops at head tube and actually increased the loop at the RD when I changed cables to see whether it made a difference. It didn't.
BB is press fit and in correctly, no damage on the frame, limit screws are correct. Tried a compact crank from my other bike, but still had the same problem. Shifters seem ok as it indexes correctly in either small or big chainring, but not across both.
I'm going to have another look at the cables.

Ginger_beer99

Ok, so apologies if I've wasted people's time with the diagnostics!!

I checked the RD again and found that when I moved to the small chainring, the RD moves slightly out towards the lowest gear as if some of the tension in the cable is lost. This prompted me to disconnect the FD and jam it (with a perfectly sized piece of rod) int he large chainring location. After a spot of indexing I removed the rod and hey presto, the RD stayed indexed perfectly. Hence the conclusion was the cables must be crossed inside the frame!! I had replaced them both carefully, but obviously in the two attempts I'd had I had crossed them both times.

I ended up removing the cables and sheaths, and then using a straightened metal coat hanger through each head tube port to ensure that there was no crossing. I then used these steel rods to pull through the cabling, ensuring I routed the RD cable all the way to the RD prior to pulling through the FD, therefore ensuring I couldn't accidentally cross them at the hole on the underside of the BB when fiddling around with the routing which is where I though I may had gone wrong before. One other thing I did was to remove the bottle cage bolts as it seemed like their penetration into the downtube was affecting the routing and could've led to both cables being on one side of the tube.

RD now indexes perfectly in both chainrings and I'm happy again! About 18 hours of diagnostics and checking, $70 of new cables, $50 for a new chain (over shortened one to try it), and nearly $200 for a new RD! Still, I'm a happy man knowing I've got a full compliment of gears and no annoying rubbing on the cassette.

Thanks for the replies.