Choosing a front derailleur
I am upgrading my front derailleur. I am a little confused about what will work. I am running a 48-38-28 chainring. I have a brand new 105. I believe it is designed for a 52 tooth ring. Will it work or do I need something that will fit the 48?
The 105 triple fd is designed for optimum performance with a 52T big ring but will work fine with a 48. Install it and enjoy. Be sure to install the derailleur low enough so the chainring clearance is correct.
In fact, my son's old Trek 1220 came with an RSX crank and 46-36-26 chainrings and an RX-100 triple front derailleur (a 105 duplicate but with a bit less finish). The RX-100 was also intended for a 52T ring but shifted the 46 very nicely.
Yeah it will be fine. The only reason Shimano recommend certain chainring sizes for their mechs is to optimise the contact area to follow the line of the ring.
This has little or no effect on smaller rings than those recommended sizes when the difference is only 4 teeth or so.
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