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Can I replace an FD 4403 3x9 with a FD 4503 3x9?

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Maybe, but it depends on the chainrings you run. The two are separated by Shimano's switch from 30-42-52 to 30-39-50 for their road triples.

4403 has a nominal max range spec of 22t and a minimum difference between top and middle of 10t. 4503 is 20t and 11t respectively.

If you have 30-42-52, it usually goes badly to replace the FD with one of the later Shimano road triple front derailleurs without changing anything else. The max range being wrong is somewhat overlookable if you stay out of the small/small combinations, which may allow the chain to drag on the cage (chainstay length and BB drop are also factors in this, so how bad the problem is is not universal). However, the minimum difference spec being wrong by even the 1 tooth does in fact matter and will make it so the inner cage wants to hit the middle ring unless you make the clamp height higher than optimal, which in turn can create problems with poor performance and/or chain drop. It is the kind of thing where getting it to shift in the stand when everything is new is easy enough, but will tend to work badly in practice.

A good solution is to switch to the much more sensible 30-39-50. Otherwise there's a currently-produced Microshift part called the FD-R732 that's basically made to be a legacy option for 30-42-52 road triples.

The actuation ratio is the same between them and also with the Microshift one, so all of them work with all Shimano road triple shifters.

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