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I’m looking at buying a used Orbe Orca complete with power meters and Ultegra Di2. The front rings are 50/34. I asked if the Di2 could handle 50/30 and was told yes but the shifting would just be a bit slower. A “bit” is ok, but first of all, is this really advisable and second of all, aside from “a bit slower” would I find myself in chain drop issues or catching between the rings? Buying a new front crank is obviously an option but expensive.

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    What's the cassette on it now? What is your ultimate goal - more low/climbing gears ?
    – Criggie
    Commented Jun 15, 2023 at 22:09

2 Answers 2

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Presuming it's got one of the very few cranks that has any ability in terms of chainring mounting to go from 50/34 to 50/30, the first challenge is the ramping on the 50 won't be located with a 30 in mind. Di2 needs its shift aids because it doesn't have a human's ability to sense when the shift is complete. It doesn't have an equivalent to finessing the lever, holding it down a little longer or harder, etc. (I'm not sure whether the FD can sense resistance and do some version of this - it might, but I don't think it's quite the same). It will likely perform badly if at all.

Wanting to go to 30 in front implies the back end is already set up with the largest cog the RD can clear. That mean's you'll be going past the RD total capacity by 4 teeth. That in and of itself isn't necessarily a dealbreaker, but it bears asking whether you want a bike set up that way.

The FD will likely work fine with a 46/30 crank, as long as it can be moved low enough. So one question could be whether 46/11 is high enough.

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The other issue is that if you have a bike with Ultegra Di2, the crankset is probably an Ultegra crank. Standard road cranks physically cannot accommodate 30t small rings. By this, I mean that a 30t ring would sit inside the circle inscribed by the bolt holes on the crank. So, you would need a different front crank anyway. 33t is the usual minimum for chainrings on a 110mm BCD crankset. Some cranks may be able to accommodate 32t small rings (e.g. Praxis makes one). You may be able to find oval chainrings that give effective gearing smaller than 34t (I think Absolute Black made these at some point).

You could get a GRX crankset with 46/30 rings. However, you would need an aftermarket big ring. I am not sure that anybody is making one.

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