Leinster

That’s really less of a “downside” than NOT having 3 gear options in that range would be.

3 shifts is 1 throw of the downshift lever on shimano or Campag shifters (if going to the big ring) or 3 quick clicks of the thumb/upshift lever (if going small ring) and if you’re at a point of front-shifting, you’re rarely going to be making those full 3 rear shifts; most big ring shifts happen as you’re a) reaching the part of the climb where it starts to get really steep and you go to the small ring or b) going over the top of the climb starting the descent.

You make 3 shifts at the rear to stay in the same gear as before the front shift. But almost by definition, you’ll only front-shift at a point where you’re making a big shift in gear anyway. Even with a 12-25 cassette.

SoSmellyAir

Agreed. I went from an 11-32 cassette (riding almost exclusively in the big 50T ring) to a Frankenstein 12-28 cassette (with its attendant problematic downshift between the two donor cassettes) to a 12-25 cassette (using both 50T and 34T chain rings).

With a 12-25 cassette, you need to shift more than 3 rear cogs after shifting between the front 50/34 chain rings to mathematically minimize disruption to cadence. Let's calculate each gear ratio:

(a) 50:19 = 2.63, next lower gear combination on 34T chain ring is 34:14 = 2.43, from 19T to 14T is 5 upshifts (given that 34:13 = 2.62, which is almost the same rather than the next lower gear combo);
(b) 50:21 = 2.38, next lower gear combination on 34T chain ring is 34:15 = 2.27, from 21T to 15T is 5 upshifts;
(c) 50:23 = 2.17, next lower gear combination on 34T chain ring is 34:17 = 2.00, from 23T to 17T is 4 upshifts (given that 34:16 = 2.13, which is almost the same rather than the next lower gear combo);
(d) 50:25 = 2.00, next lower gear combination on 34T chain ring is 34:18 = 1.89, from 25T to 18T is 4 upshifts.

But in practice, because one also loses momentum shifting between the chain rings, shifting 3 rear cogs feels about right.

Leinster

But my point is that if you’re front-shifting, you’re not trying to minimize disruption to cadence; you front shift when you need to make a significant change to your cadence and/or when meeting a significant change in terrain that will require a change in cadence. You front-shift when you’re in 50-23 and you can see the road getting steeper. You front-shift when you’re going over the top of a climb and starting a descent. You front-shift as you exit the gravel sector onto the smooth asphalt.

Most other gear shifts can be dealt with in the rear, unless you’re already all-the-way cross-chained.

SoSmellyAir

Exactly. This is what I was trying to illustrate by numbers above, which is, when one front shifts between the chain rings, one is not even aiming for the next closest gear combo, but the one after that, hence about 3 rear shifts of the cassette. I acknowledged that, in practice, the loss of momentum from a front shift makes a disruption to cadence unavoidable. (This is perhaps the greatest issue the OP faces going from a 3x to a 2x front setup.) Which thus also means that the point of the front shift is not to "stay in the same gear" as stated in your prior post (#51).

Maelochs

Hmmm ... maybe I am doing it wrong.

I often shift down in front and way up in back at the base of a long climb because it is easier Not to shift up front under heavy load, even with modern equipment. I can look up the road (assuming it is straight enough) or if I know the route, I can prepare for the upcoming terrain. I Rarely want big changes in cadence because I have a pretty limited range of optimal output, and I don't want to go a lot slower, and faster is only good at certain points on climbs.

There are a couple places I can think of where I come downhill into a turn uphill where I need to drop from 50-midrange of 34 midrange (or just work down the cassette if I am not too tired) but as a rule I try to plan so I don't have to make big gearing jumps unless the terrain also makes big jumps ... short, steep rollers for instance. And to me the whole point of having all those ratios is that I can always find a ratio which keeps me in my peak output range---peak balance of power and revs so that my lungs and heart and legs can all keep contributing. I might spin a little faster on some hills because I feel like quicker strokes are more efficient (less power momentum lost between power pulses) but i never want to go from say, 85 revs to 65 revs on a climb. Or anywhere else, really,.

SoSmellyAir

No, you are doing it right. Shifting up front in anticipation of a change in terrain is what I do too. I think we are all trying to describe more or less the same thing, but imperfectly, so that the collective description is being refined with each subsequent post.

Gonzo Bob

I also hate 50-34 and don't really even like 53-39 but that's because my road bike has a 6-speed freewheel and the jumps between cogs are too big to make "single ring" riding convenient. But I think those ring combinations make sense with 10- or 11-speed cassettes that start with an 11t. I've put 50-34 on the bike (using the 110 BCD crank from my old MTB) but it was for a special purpose (Mt Evans Hill Climb). I've also used 53-39 for PBP. But for my home terrain, I run 53-42 and a 13-26 freewheel which gives excellent overlap and a lot of gears between 4.5m and 6.5m and gives me the gear range I need. I keep the 53 big ring because the smallest cog I can put on that freewheel is a 13t.

SpeedyBlueBiker

My three road bikes all have different gearing although two are very close. The Fuji has a 52/36 up front with a 11-28 cassette in the rear. My Specialized came stock with a 52/36 and an 11-30. After two years I replaced the rear cassette and put on an 11-32. This is an even better set up on hills. My LeMond is a triple with 52/42/30. The original cassette was a 12-25 9spd. Now it has a 12-28. If I ever were to use the 30-28 (as low as it goes) I'm probably on some hill I shouldn't be on in the first place!