Shifting is fine in the small ring but not when in the big r

pprendeville

Having a problem with my shifting. Using dura ace 10 speed 7900. When I'm in the small ring (39 tooth) I've no problems shifting from the big dog to the small coy. However when I shift to the big ring (53 tooth) it won't shift all the way down to the 11 tooth small cog and the indexing is all over the place.

Have checked a few things like the dropout to make sure it is securely fitted. No problems with it. Cables all look good. They're relatively new, 2 months old, Shimano. Front derailleur is correct aligned. Can't see to figure it out.

Manc33

B-tension screw might be screwed in slightly too far. How far away from the low sprocket is the upper jockey wheel when in the lowest gear? It should be as close as possible without "rumbling". If you just swapped to a cassette with a smaller big sprocket and did not adjust the B-tension screw it could cause this.

cswitch

if internal cabling check the rear cable isn't wrapped around the front cable. When the front cable has greater tension when front mech in big ring position it affects tension on the rear. Happened to me on a scott foil.

One way to check is release front mech cable and use the stops to get the chain on the big ring. then check rear shifting.

pprendeville

thanks guys, i'm going to have a look at this 1st thing in the a.m. B-tension screw is in fairly far alright. Will check sprcket/jockey wheel distances when i.m looking at it. this shouldn't be an issue though.

internal cabling alright and when i routed the cables this was something that i'd been told to be aware of so this could possibly be it. what do you mean by "using the stops to get the chain on the big ring"?

cswitch

...so if you release the front mech cable from the mech the cable is less likely to be interfering with the rear mech cable even if they are entwined with one another. To then check the rear mech shifting now works when the chain is in the big ring turn the cranks on a stand while winding out the outer front mech stop screw (probably the one closest to you as you stand to the drive side of the bike) - this moves the front mech cage as if you were changing gear. Do it until it shifts the chain on to the big ring - thats how you get the chain onto the big ring even though the cable isn't attached. Then play around with the rear shifting. If it shifts better then it is very likely the front mech cable is entwined around rear cable. Hope that makes sense...not so easy to explain without showing you!

pprendeville

cswitch wrote:

...so if you release the front mech cable from the mech the cable is less likely to be interfering with the rear mech cable even if they are entwined with one another. To then check the rear mech shifting now works when the chain is in the big ring turn the cranks on a stand while winding out the outer front mech stop screw (probably the one closest to you as you stand to the drive side of the bike) - this moves the front mech cage as if you were changing gear. Do it until it shifts the chain on to the big ring - thats how you get the chain onto the big ring even though the cable isn't attached. Then play around with the rear shifting. If it shifts better then it is very likely the front mech cable is entwined around rear cable. Hope that makes sense...not so easy to explain without showing you!

Makes perfect sense. Great idea. Thanks.

pprendeville

Well it seems you're spot on. Releasing the cable and adjusting the limit screws on the FD to put it in the big ring and it's shifting no problem so the cables must be entwined. I thought I had them installed to prevent this. Guess I'll just have to reroute them. Hopefully get this sorted for once and for all. Very frustrating when shifting is not what it should be. Thanks very much.

cswitch

yeah it is a pain, quite hard to work out whether the cables have crossed over until its set up. When you re-route the front cable, before you fix the cable you could try pulling it when you're shifting the rear to see if it is creating an issue on the rear. Other than that I guess its just trial and error.

pprendeville

Right, I've re routed the cables. Used a new RD cable as the old one was splayed (strand came loose so it wouldn't go through the outer housing).

Procedure as follows:
After releasing tension on front and rear derailleur cables (by loosening the screws that clamp the cables) I removed the little cable guide bracket under the frame BB area that guides the cables. Then I retensioned the FD cable before routing the RD cable to reduce chances of it entangling with the FD cable. I inserted a telescopic magnet into the open space under the BB area to pull the RD cable through the frame instead of just feeding it through the frame to make sure there was no crossing/entwining with the FD cable. Then fed it through the chainstay. Released tension again from FD and reinserted the cable guide. Greased it as well.

Long story short I can now shift cleanly from big to small cogs in both the small and big chainring. Shifting is still not ideal for whatever reason (using a 11-28 cassette) but it's a lot better than before. Thanks again.

pprendeville

Manc33 wrote:

B-tension screw might be screwed in slightly too far. How far away from the low sprocket is the upper jockey wheel when in the lowest gear? It should be as close as possible without "rumbling". If you just swapped to a cassette with a smaller big sprocket and did not adjust the B-tension screw it could cause this.

Sorry,I hadn't come back to you in relation to the B-tension screw. I've posted pics to show the distances of the upper jockey wheel from the low sprocket. the last picture is probably the clearest. There is a bit of rumble so I'll adjust and see if it improves things.

cswitch

you set the B screw when chain is in the big cog - I think. Either way it does look like it could be a bit closer. Shimano is about 7-8mm clearance isn't it?

dj58

"6. How to use the B-tension adjustment screw

Mount the chain on the smallest chainring and the largest
sprocket, and turn the crank arm backward. Then turn the
B-tension adjustment screw to adjust the guide pulley as close
to the sprocket as possible but not so close that it touches.
Next, set the chain to the smallest sprocket and repeat the
above to make sure that the pulley does not touch the sprocket."

Manc33

If you did not just swap to another cassette (with a smaller biggest sprocket then the last cassette was) I can't answer this.

The pic has to show it in the lowest gear to know. Even then no one can know because they aren't there to hear it rumbling (or not). :P

If you screw in the b-tension screw it will being the upper jockey wheel away from the cassette, but this will cause bad shifting on the two smallest sprockets (to the point it might not even shift to the 11t at all). Unscrewing that screw will eventually make the chain touch the biggest sprocket, aka the "rumbling". You gotta avoid that at all costs. You must be in the lowest gear doing this.

It needs to be in between so if it was unscrewed any more it would rumble and if it was screwed in any more it would affect shifting on the two smallest sprockets.

Last time I had this problem I needed to get a newer rear mech. An old M730 made it have this problem but a new M772 works like a charm. Mine changed from 11t-12t on the middle chainring, just not when on the biggest.

I don't know but I think this only happens with 11t sprockets. I wonder why most road cassettes start at 12t... well the chainrings on road bikes are bigger than on MTB and maybe its just pushing it so far its just better to have a 12t as the smallest. I hunted for ages to find a 11-23t (in 8sp) but they seem to just not exist.