Old 04-15-15, 12:41 PM
  #7  
Andrew R Stewart 
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There are a number of possible issues when going to a larger ring then stock like the OP did. If too much larger the middle ring will fall into the "shadow" of the two other rings, it won't engage the chain when shifting from the granny before the chain contacts the large ring. Another problem is that with today's ft der cage s being heavily contoured to help lift/guide the chain a higher ft der position will disturb the intended chain/cage relationship WRT these contours when shifting. third is that you're asking for the der to do more work. just like climbing up a step ladder with larger distances between the rungs will require more effort to climb up, so too will the der and chain need more energy to climb up to the larger ring. The der cage might have been curved along it's lower edge to follow the large ring's curve. A larger ring has a larger curve so the cage might need to be a bit higher off the ring at it's front end so the tail of the cage doesn't contact the ring. Modern rings have had their teeth rotationally positioned WRT to each ring to allow the chain to lift off and engage the next ring's teeth at the provider shift gates best. Going to a ring with this "timing' of the teeth becoming different can upset this, hence many rings are designed to work as a set only.

Last issue is that the rider might not know good shifting technique and soft pedal during the front shift. There is another thread right now who's subject is "Front mech not shifting under load". Duh! Andy.
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