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PostPosted: Sat Oct 26, 2013 4:55 pm 
Dirt Disciple

Joined: Wed Apr 10, 2013 4:52 pm
Posts: 79
Folks, I'm having some issues with a current bike rebuild.
The front mech is not shifting too well up into the large ring.
The mech has been positioned and the limits set correctly and it shifts to all the rings cleanly when on the work stand, as soon as there is any load on the chainset, i.e. riding the bike it won't shift up to the large ring.
Releasing the cable and manually pulling the mech shows there is enough travel to move over the large ring, once the cable is clamped there is still enough travel however when operating the gear shift lever it feels as if there is not enough travel in the shift leaver to get the mech to shift out to the large ring.

The levers are Shimano 8 speed stx, front mech is stx RC
Any suggestions?

ta
Andy


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PostPosted: Sat Oct 26, 2013 5:18 pm 
King of the Skip Monkeys
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Dial in more cable tension? New cables sometimes stretch when first fitted along with cable outer/ ferrules bedding in. This can add a good 4mm of extra flappy cable.

You could also clamp the cable in a way so it adds more leverage per 'click' on the shifters.

As an aside - and for others reading with similar problems, how far out from the frame does the chainset sit? if its too far sometimes the front mech wont be able to shift far enough.


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PostPosted: Sat Oct 26, 2013 6:06 pm 
National & North West AEC
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Check the cage isn't bent. The outer cage should be parallel to the chainrings while the inner cage will have the front edge slightly closer than the rear end.
Also see how much play there is in the mech pivots.

Older front mechs can suffer from both poorly aligned cages and sloppy pivots. Both can contribute to poor shifting, especially under load.


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PostPosted: Sat Oct 26, 2013 6:20 pm 
Retro Guru
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Location: It's not easy being a dolphin.
As said above, but will add to make sure the outer cage clears the big ring by about 3mm in height, and when in big ring small sprocket a good 1mm from the inner side of the outer cage to the outer chain plates.


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PostPosted: Sat Oct 26, 2013 6:34 pm 
BoTM Winner / Gold Trader
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Location: Burlington, ON Canada
Check your chain line of the cranks too. it should be 47.5 to 50mm (47.4 is optimum for 28.6 - upwards of 50 for 31.8/34.9). Any more than 50 and the cages start moving upwards in their arc, meaning you have less outward throw to the outer ring in respect to cable travel. Not a huge problem with friction shifters but it can wreak havoc on the last shift with indexed.


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PostPosted: Sat Oct 26, 2013 6:42 pm 
Dirt Disciple

Joined: Wed Apr 10, 2013 4:52 pm
Posts: 79
legrandefromage wrote:
Dial in more cable tension? New cables sometimes stretch when first fitted along with cable outer/ ferrules bedding in. This can add a good 4mm of extra flappy cable.

You could also clamp the cable in a way so it adds more leverage per 'click' on the shifters.

As an aside - and for others reading with similar problems, how far out from the frame does the chainset sit? if its too far sometimes the front mech wont be able to shift far enough.



Cheers fellas, some things to check/try in the morning......how does one clamp the cable in a way so it adds more leverage per 'click'?

thanks again

andy


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PostPosted: Sat Oct 26, 2013 6:55 pm 
King of the Skip Monkeys
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Location: Moomin Valley
something like:

Image


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PostPosted: Sat Oct 26, 2013 8:21 pm 
Retro Guru
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Location: the Netherlands
Probably not applicable, but I had this once: Make sure your bike stand grip does not interfere the cabling. If the clamp touches the cable in some way, there might be more cable tension. This could explain why the mech functions on the stand but not on the road.


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PostPosted: Sat Oct 26, 2013 10:32 pm 
retrobike rider
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Location: Yorkshire, England
Do the shifters feel nice, they should be smooth a clean shifting, no signs of drag.

But try a tighten up of the cable, it may need to over shift a bit to pick up. Under tension the cable may not hit the pickup teeth to well as it doesn't bend as easily. If you shift up while coasting does it work?
Basically you could have the pickup teeth making the shift and not correct mech to ring shifting.


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PostPosted: Sun Oct 27, 2013 1:04 am 
retrobike rider
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Joined: Thu Apr 17, 2008 1:24 am
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Location: Manchester
What do you mean by "under any load" just riding slowly along a road or thrusting the cranks up a steep climb?


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