Front mech/chainline problems

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simonrt
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Front mech/chainline problems

Postby simonrt » 31 Dec 2015, 5:39pm

My front mech wont move far enough inwards to avoid chain rub when the chain is on the large rear sprocket (+small chain ring). The only way I can get it to work is by angling the front mech inwards [towards the rear] which causes poor shifting and problems in the higher gears. It's not cable tension or inner adjusting screw and I've tried various heights between about 1 - 5mm above the large chainring. The set up us Ultegra 10 spd: compact SL 6650 chainset (50/34), all the rest [shifters, bb, front mech, rear mech, rear hub] is Ultegra 6700. Cassette is 12-30. BB shell is 68mm and chainline is about 43-44mm. Downtube is 34.9mm. So it seems either I need a different front mech [that can travel inwards a couple more mm] or modify the chainline for example by adding a spacer to the drive side of the bb. Is that possible or would it shorten the axel on the left side so the left crank wouldn't fit? There doesn't seem to be any scope to add a spacer to the cassette. Any ideas? Thanks

LWaB
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Re: Front mech/chainline problems

Postby LWaB » 1 Jan 2016, 8:28am

Try running the front mech with about 2mm clearance to the chainring teeth.

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DaveP
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Re: Front mech/chainline problems

Postby DaveP » 1 Jan 2016, 9:26am

Obvious question (not necessarily to the afflicted!): What was the last change before the problem appeared?

simonrt wrote: modify the chainline for example by adding a spacer to the drive side of the bb. Is that possible or would it shorten the axel on the left side so the left crank wouldn't fit?

If the clearances are so tight that this appears likely then you might be better off looking for a longer bb spindle.
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mattsccm
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Re: Front mech/chainline problems

Postby mattsccm » 1 Jan 2016, 10:45am

Is 6650 comparable with 6700? If not then just maybe the c/s and mech have different chain lines. I bet you could checvk this on line. Assuming they are then getting the mech set damn close to the outer ring as suggested would be a good start and check that it is in line with the chainrings. That can be tricky as often the cage does not have parallel sides.
I am assuming that you started by setting it up with no cable and the chain on the biggest rear sprocket. Use the stop screws to set it so that is silent. Remember that this may change when load is applied.
In the good old days tweaking them mech slightly was normal.
I guess you cannot change the axle as its integral to the c/s.

pwa
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Re: Front mech/chainline problems

Postby pwa » 1 Jan 2016, 10:59am

How much extra movement to the left would you need for no chain rub to occur? If it were only 1 mm, frankly, I would gently bend the mech cage to make it just a tad wider in the area where the rub happens. It sounds like bodging and probably is, but it can work a treat. It is what I do when I know I have done everything else right.

simonrt
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Re: Front mech/chainline problems

Postby simonrt » 1 Jan 2016, 12:09pm

Thanks

mattsccm - ive tried searches and cant find the answer to the specific question of 6650/6700 chain line though most seem to think they are generally compatible. however I think you are probably right that is where the problem is. Yes I disconnected the cable and completely loosened the stop screw. And I cannot change the axle as you say.

I'll try pwa's suggestion of bending the mech. An alternative might be a 9 speed front mech

simonrt
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Re: Front mech/chainline problems

Postby simonrt » 1 Jan 2016, 12:14pm

DaveP Problem occurred with 'upgrade' from 9 to 10 spd [necessitated by broken lever ]. i suspect as pwa suggested it may be a 6650/6700 compatibility/chainline issue

Valbrona
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Re: Front mech/chainline problems

Postby Valbrona » 1 Jan 2016, 1:51pm

What frame and what FD clamp? Every seat tube clamp does not always put the FD in exactly the right spot. Suggest you try the exact same clamp as mentioned in Shimano tech docs.

I am not a Shimano person, but the assumption is is that you can fine tune chainline by fitting spacers to the BB. Check out tech docs.
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Brucey
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Re: Front mech/chainline problems

Postby Brucey » 1 Jan 2016, 2:21pm

I don't understand why you are having this problem to start with (it should just all work OK.... is your frame out of track leftwards at the back by any chance? Are the chainstays unusually short?....) but I do have a suggestion of how to fix it.

If you fit a 'MTN' BB unit instead of a 'Road' BB one, you ought to find that the MTN OBB cups have shoulders that are slightly thinner. This can allow you to adjust the chainline (using the spacers in the kit) more than with a 'road' OBB unit.

BTW if you have an oversized seat tube this can restrict the movement of the front mech; occasionally this can be adjusted with a little strategic filing to the front mech. Stupid questions; you are not using clicks 2 and 3 of a triple LH STI? The cable goes full slack and you've adjusted the limit screws?

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mattsccm
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Re: Front mech/chainline problems

Postby mattsccm » 1 Jan 2016, 10:04pm

Many years ago I threw out the stop screw as it wasn't needed. Next stop was to file the mech away so that it all moved in a bit more. Can't remember why . :D

simonrt
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Re: Front mech/chainline problems

Postby simonrt » 1 Jan 2016, 11:25pm

Thanks for all the suggestions.
I don't think the frame is out of track but I'll get it checked. I'd have thought there would be other shifting problems if it was. i am also puzzled because the chain line for 6600 series is the same as for 6700 at 43.5mm. I can see that the the 34.9 seat tube will permit less inward travel than a 31.8 or less but I think they are supposed to work with a 34.9 seat tube.

The mtb bb sounds like its worth a try. filing the mech also sounds as though it would work - though fiddly.

it is a double front lever [6700] and i have disconnected the cable and screwed the limit screw right out.

Thanks

JohnW
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Re: Front mech/chainline problems

Postby JohnW » 1 Jan 2016, 11:37pm

I know that this is going to precipitate mocking and derision, but with friction down-tube lever shifting, you set the travel of your cage, and then YOU decide how far it travels when you change.

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RickH
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Re: Front mech/chainline problems

Postby RickH » 1 Jan 2016, 11:50pm

JohnW wrote:I know that this is going to precipitate mocking and derision, but with friction down-tube lever shifting, you set the travel of your cage, and then YOU decide how far it travels when you change.

Whatever benefits or otherwise of DT levers, it isn't going to help when the OP has disconnected the cable from the shifter & backed out the adjustment screw. I don't think even DT shifters can't help to magic some extra movement in this case! (Is that sufficient mocking & derision? :wink: )

Rick.

JohnW
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Re: Front mech/chainline problems

Postby JohnW » 2 Jan 2016, 12:17am

RickH wrote:
JohnW wrote:I know that this is going to precipitate mocking and derision, but with friction down-tube lever shifting, you set the travel of your cage, and then YOU decide how far it travels when you change.

Whatever benefits or otherwise of DT levers, it isn't going to help when the OP has disconnected the cable from the shifter & backed out the adjustment screw. I don't think even DT shifters can't help to magic some extra movement in this case! (Is that sufficient mocking & derision? :wink: )

Rick.


Well - yours wasn't mocking derision, was it Rick? - it was a rational response to my flippancy.

I think that the only way any of us can really respond is by having a look ourselves. It could be so many things, and may be resolved by some minimal adjustment. You're right of course, but I can't resist the temptation to bang my drum...............

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cycleruk
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Re: Front mech/chainline problems

Postby cycleruk » 2 Jan 2016, 4:59pm

JohnW wrote:
RickH wrote:
JohnW wrote:I know that this is going to precipitate mocking and derision, but with friction down-tube lever shifting, you set the travel of your cage, and then YOU decide how far it travels when you change.

Whatever benefits or otherwise of DT levers, it isn't going to help when the OP has disconnected the cable from the shifter & backed out the adjustment screw. I don't think even DT shifters can't help to magic some extra movement in this case! (Is that sufficient mocking & derision? :wink: )
Rick.

Well - yours wasn't mocking derision, was it Rick? - it was a rational response to my flippancy.
I think that the only way any of us can really respond is by having a look ourselves. It could be so many things, and may be resolved by some minimal adjustment. You're right of course, but I can't resist the temptation to bang my drum...............


with respect JW your drum is worn out by now and no matter how many times you bang it doesn't help answer the problem :roll: .

Regarding the OP, if there is a spacer on the non-drive side then swap it over to the chainring side and try it.
This will of course move the chainset over a couple of mm' but still keep the same overall width and not compromise the left crank fitting.
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