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So my bike has Shimano Tiagra ST-4600/4603 brifters (it's a triple) but is fitted with long-drop BR-R451 calipers. In theory, these should be incompatible because of the change in cable pull ratio, which Shimano seem to call "New Super SLR" that happened in the 4500->4600 / 5600->5700 etc. transition.

However, I have found conflicting sources as to whether this is OK, including the Shimano compatibility chart at hosted at Celebrazio which firmly says "No!" (although if you're using the flat-bar lever it's OK?); whereas the Shimano Line-Up chart seems to think it's all right to use as an alternative to the short-reach in-series caliper. Also, a previous edition of the compatibility chart says that mixing the older brakes with newer levers is permissible.

I presume this is the same situation as using the short-reach brakes from a previous series of the groupset with the newer levers, in which case is there any solid advice about making the setup work?

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Presumably the brifters are ST-4600/4603? I'd be inclined to believe the old compatibility chart is right, and Shimano wants to sell you some new stuff (which will probably work slightly better mostly due to the fact that you've been told that they'd work better). In general, to match cable pulls, you need a pulley such as a Problem Solvers Travel Agent, but this introduces other problems such as cable wear and expense (which would probably be better spent just getting the right brake calpier!). –  Batman Feb 11 at 5:17
    
Yep, that was a typo on my part. The issue with buying the right brakes is that my frame needs the longer reach. Hopefully Shimano will release a replacement for the BR-R451/BR-R650 family designed for the new cable pull. –  jbuk Feb 11 at 8:48
    
Some of the dealers (e.g. chainreactioncycles.com/us/en/…) seem to state that "Must be used in conjunction with ST-4600, ST-4603, ST-5700, ST-5703, ST-6700 or ST-7900 STIs for correct leverage ratio". I'm inclined to believe this is from the old compatibility chart making it seem like a money grab at slightly worse braking. –  Batman Feb 11 at 12:38
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