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I have a FSA compact crank 50/34 with a 9-speed 12-27 cassette. I need better climbing ability for steep grades, not concerned with flat top speed. What should I do? What is the largest size cassette I can use?

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The largest cassette you can use is determined by the rear derailleur (in particular, its chain wrap capacity). For something larger than 30t in the back, you want a wide range derailleur (code SGS) - something like a Deore rear derailleur if you're running Shimano (at 9 speed, the mountain and road rear derailleurs can be swapped, and the mountain rear derailleurs can take bigger cogs in the back). Then, put on some bigger cassette which works for you - your local bike shop will probably know some cassettes which work well in your area.

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You could always go to a triple chainring at the front. That'll take you down to 30.

But it'll cost you - you're talking the rings themselves, maybe the front derailleur, likely a new BB, and certainly a new left shifter.

You can always do both what I suggest and what @Batman suggests - as long as you keep the chain width consistent you can pretty much treat the front and rear mechs as independent. Again though the limiting factor is depth of pockets.

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My 2 cents. I have tried it both ways (50/34) with MTB cassette on the back 11-34 and a triple 26/39/52 and 12-27. In the long run the triple just works much better for me.

I found that compact doubles (50/34) only work well if you can do almost all your riding in the big ring, except for the long climbs. Where I live and ride, I found that I was constantly having to double shift for the typical rolling terrain. I could never find the "right" gear for certain speeds. Also, on the very low end there is a fairly large gap in the low gears.

The range of gears is pretty similar for both setups, it's the exact distribution that is different. I think if I could get a 48/30 compact double ( Orange Velo makes one) the compact double could work for me with a 12-27 cassette.

You could try renting or borrowing a road bike with a triple to see if it's worth the extra cost for you. Unfortunately, road triples come with 30t as the lower gear, you can replace that with a 28 or 26 t gear. This makes it hard to really figure out if it's right for you, as 30/27 is not as low as 34/34.

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