Problems with front shifting

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al78

Guru
Location
Horsham
This week I seemed to develop issues with changing from the small to the big ring on my road bike (50-34 10-spd). At first, I thought it was because my ankle band had failed and I had snagged my trousers in the front mech, possibly knocking it out of position, as after I did this I couldn't shift from the big to the small ring. I did try to adjust the low limit screw which helped but then had problems with overshifting and dropping the chain when changing up to the big ring. I also had problems changing up from the bottom gear on the cassette which I resolved by putting a new chain on, as I found out the chain was worn. I can now change gear smoothly at the back but am still having problems at the front. I have managed to adjust things so that I can change down but I am still throwing the chain when changing up at the front. Trouble is I can get it to work when I am testing indoors, but doesn't work out on the road (very annoying). My thought is that the big ring is worn however it would surprise me that this would wear quite this quick as I understand the wear rate of a cog should decrease with increasing number of teeth. I have attached a couple of side-on photos of the front ring, one of which shows the meshing of the chain and chainring part way through a change, and was wondering if anyone could confirm whether or not the big ring is worn from these photos. Any advice would be much appreciated.

PICT0355.jpeg PICT0359.jpeg
 

mrandmrspoves

Middle aged bald git.
Location
Narfuk
Front chainring looks moderately worn - but not enough to be causing the problem you're having
. I suspect from the 2nd photo that your derailleur is not straight and also it looks like it could be raised a few mm. Loosen the clamp bolt lift it sightly ( is that an outline of where it used to be in picture one?) Then with it set on the large chain wheel ensure that the cage is exactly parallel to the chain wheel. Tighten the clamp..readjust stop screw and try again. Good luck!
 

Biker Joe

Über Member
It sounds as if the front mech shifted when your trousers got caught up in it.
First check the following:-
The spacing between the top of the large chain ring teeth and the lower edge of the derailleur cage should be 1-3mm (regardless of design).
Make sure the cage is parallel with the chain ring.
Once this is OK then carry out the usual fine adjustments.
I hope this is of some help.
 

Smurfy

Naturist Smurf
Shimano 105.

The exact model number should be stamped on the back of the cage. It should be something like FD-5500.

Next, go to the Shimano website and download the appropriate technical documentation for installation.

Shimano tech docs, Road Bike, 105, then scroll down and select your model number of front derailleur

For example, document SI-59G0C is the sheet for the FD-5500 model of the 105 front derailleur.

Study the installation notes. In particular, note the specified clearance between the front derailleur and the big ring. In document SI-59G0C, it is labelled as 'Clearance 1-3 mm'. Adjust this clearance by moving the derailleur up or down the seat-tube. If the clearance is too large, the chain can 'escape' through the gap between the derailleur cage and the chain ring, and end up wrapped around the pedal spindle, instead of on the chain ring.

Another point to note is that not all chains have the same amount of lateral flexibility. Apart from hub-gears, single-speed and fixed-wheel, all chains need some lateral flexibility or the derailleur wouldn't work. The amount of lateral flexibility can affect how easily the chain shifts from one sprocket or chain ring to the next. If you have fitted a new chain of a different make or model, this could cause problems that appear to be due to derailleur adjustment, but are actually caused by too much or too little lateral flexibility in the chain.
 
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