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I have a compact 50/36 crankset with a 12-25 cassette. It's flat where I ride and it seems that I'm often shifting between the 36/12 combo and 50/25. Would I benefit from getting a smaller large ring (a 48 or 46) and a bigger small ring (maybe a 38)? I know it's recommended that there be no more than 14 teeth between chainrings, but are there any recommdations regarding minimum number of teeth between rings?
 

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The 50/25 or 36/12 really shouldn't ever be used due to the extreme chainlines. If you get to the 50/23 and find it not low enough, the next step in a uniform progression is a 36/18. Your 12-25 does not have an 18T cog, so a tighter 12-23 cassette might be the ticket.

If your terrain is really flat, that 36/23 low gear is the basically the same as a 39/25 and a lot lower than I ever used on flat terrain.

I rode the rolling terrain around Kansas City for many years with nothing lower than a 39/21. A 12-23, even with a bigger 53/39 would be a common choice.

My other advice is to pay attention to the transition speeds. A 36/13 should get you up to 20 miles per hour quite easily. Over that, and you better be in the big ring. At the opposite extreme, a 50/21 can be used at 15 mph and higher speeds. A 50/23 could go down even lower.
 

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C-40 said:
The 50/25 or 36/12 really shouldn't ever be used due to the extreme chainlines. If you get to the 50/23 and find it not low enough, the next step in a uniform progression is a 36/18. Your 12-25 does not have an 18T cog, so a tighter 12-23 cassette might be the ticket.

If your terrain is really flat, that 36/23 low gear is the basically the same as a 39/25 and a lot lower than I ever used on flat terrain.

I rode the rolling terrain around Kansas City for many years with nothing lower than a 39/21. A 12-23, even with a bigger 53/39 would be a common choice.

My other advice is to pay attention to the transition speeds. A 36/13 should get you up to 20 miles per hour quite easily. Over that, and you better be in the big ring. At the opposite extreme, a 50/21 can be used at 15 mph and higher speeds. A 50/23 could go down even lower.
That is excellent advice!! I read it twice... the thing about transition speeds is really helpful.
 

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pdxtim said:
I have a compact 50/36 crankset with a 12-25 cassette. It's flat where I ride and it seems that I'm often shifting between the 36/12 combo and 50/25. Would I benefit from getting a smaller large ring (a 48 or 46) and a bigger small ring (maybe a 38)? I know it's recommended that there be no more than 14 teeth between chainrings, but are there any recommdations regarding minimum number of teeth between rings?
I don't understand your shifting patterns. There's no reason you should ever be using either of those gear combinations, let alone "shifting between" them. The 36/12 is almost exactly the same as the 50/17, a nice middle-range gear that allows plenty of smooth shifts in both directions. The 50/25 is also redundant: 36/17 is a bit higher, and 36/18 is a bit lower, so when you need that big a drop it's easily done with a quick front shift.

Make yourself a gear chart and study it and think about it a little. You've been using your gears illogically.

Edit: Looking back at your post, I think you may have a fundamental misunderstanding of the math of these gear combinations. Shifting between the small-small and the big-big combo suggests you may be under the impression that the progression of gears is to from biggest to smallest cogs in the small ring, then shift to the big ring and start back at the small cog. That's not how it works. The ranges overlap a great deal, and have duplications and near-duplications.

If you make a chart you can see it more easily. You can use a spreadsheet if you want, but the math is simple, so you can do it manually or with a simple calculator. Put your cog numbers across the top, the chainring numbers down the side, and for each combination calculate the "gear inches" by dividing ring teeth by cog teeth, then multiplying by outside wheel diameter in inches (for the typical 700/23 tire, that's about 26.3 inches, but 26 or 27 is close enough -- it's the comparisons that matter). You'll quickly see where the overlaps and duplications are, and shifting will immediately make more sense.

My apologies if I've falsely accused you of a misunderstanding you don't suffer from.
 

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I ride a 52/39 on a 23-11 DA cassette, on flat Florida land. I rarely use the little ring, unless it's like 20mph headwind, or I'm climbing up one of the causeways. I usually drop in the little ring at a light and ride the first few gears, once I hit 4th I usually click up into the big ring and cruise the 12t, sometimes the 11t. I hang in the 15t to 11t when riding straight and flat.
 
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