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Greenbank
Rear mechs have various restrictions based on "capacity". For example:-

http://www.wiggle.co.uk/shimano-105-rd-5700-rear-derailleur/ gives:-

Short cage:
```
Maximum low sprocket 30T
Maximum front difference 16T, total capacity 34T
```
Medium Cage:
```
Maximum low sprocket 32T with double chainset
Maximum front difference 22T, total capacity 40T
```
The "total capacity" is the maximum difference between smallest and biggest gears. i.e. for a 50-34 and 11-28 you've got (50-34)+(28-11) = 16+17 = 33T

So front difference is 16T and total capacity is 33T, within the specs for that short cage mech.

An 11-32 would have a total capacity of 36T, more than a short cage RD-5700 could handle safely.

So it'll depend on what front chainring setup you have too. Find out that, and the rear derailleur model number and then check the specs.

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clefty in reply to @Greenbank

@Soul It's a 5800

Thanks guys @Greenbank it's a RD-5800 short cage with a front chainring of 36/52. I'd like to change as little as possible component wise, so if I can go with a 11/28 I can live with that.

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