Okay, I posted this in another forum but no response. Any feedback welcomed.

Okay, buying a compact double crank 50-34. What rear cassette would give best climbing gear options...........a 12-23, 12-25, 12-27, 11-21, you get the picture.

It will be for a 10 spd. setup.

So the 12-27 will give you some really low gears. Which you should go with depends on the type of terrain you are on and what your cadence is while climbing. I think Carmichael recommends something like ~80rpm and to pick your gearing based on what will enable you to ride like that in the hills. I know people who are plenty strong enough to ride in a standard double, but either have a triple or compact so they can have really low gears for when they do short steep stuff of a grade similar to what you find in S.F.

Relatively steep climbs here in the NC mountains. Some grades I have ridden were around 39% for maybe a few hundred yards at a time. Main grades avg. around 7%-8.5% on typical rides.

I also have a 50 34 front, for riding in oklahoma, relatively flat with a few short steep hills i use an 11-23, but for going to colorado next month for triple bypass, long long climbs, i am switching to a 12-25. Last year i did the triple with a 53 39 front and a 12-27 rear. a 50 34 front and a 12 25 rear gives a little bit higher high gear than the 53 39 12 27. There is an excellent web site to help calculate this but i can't remember it, someone will probably post it here, but go to search engine and type in bicycle gear calculations that should get you there.

YOu might want an 11 for the descents, so maybe a custom
11-12-13-14-16-17-19-21-23-25?

that is of course only if you pedal going downhill. At the triple bypass, I was resting going downhill and holding on to the brakes for deer life- not mispelled- a buddy of mine was nearly killed by a deer which jumped across the road about 20 feet ahead of him while he was descending at 50 mph.

If you're to have one cassette, I'd probably put on the 12-27. Yes, more closely spaced gears is nice, but it's better to have gears you don't need than to need gears you don't have.

There is no way to make a recommendation without knowing your fitness level. If you are underpowered, then get a big cassette so you never have to walk up a mountain. If you have pleanty of power, then closer spacing will let you fine tune your spin on a long climb or when riding in a pace line. The main problem with compact cranks vs. triples is that you have fewer gears to work with, so you have to choose wisely.

Of course, cassettes are cheap enough that you can get a couple of different sizes and switch them around depending on your next ride.

True. But if the person's custom title is "slow but constant" and the person's really climbing short 39% grades then the choice is obvious. As low a gear as possible. Under those conditions, tighter gear spacing is a luxury. Sufficiently low gears is mandatory.

39%!

11-21 with 54/42 rings!

My wife is getting a 50/34 compact and we went with a 12-27 cassette. The gearing loss over her 30x25 triple is negligible, though you might not need anything as steep as a 34x27.

Best assumption..............avg. grades as posted I can sit in the saddle and pedal my way to an avg. speed of around 8 mph. or so. Hills are my weakness. On flats I can sustain around a 20-22 mph avg. speed with sprints upwards to 30 mph to 32 mph for hundred yards at a time or little more.

Hills just kick my ass for some reason. I have gotten better. Guess I need a combination which would not severely limit me on my flats but give me close to my current climbing gears in the new copact double like that of my 52-42-30 with a 12-25 rear cassette.

I have a ride that I do that has a small section that registers 39.2% grade over a 150-200 yard length straight up the side of a valley! It is a killer and seems like forever to reach the summit.

If best = lowest then obviously the 12-27.

If best = most low gear choices when climbing hills....then I still think it's the 12-27, especially with the 10-speed where the lower gears are "filled in" a little better than on my 9-speed 12-27.

Or some crazy custom cassette with a 29 or 31 tooth cog with a mountain derailleur...but my guess is that's not the route you need to go.

If you need the 30x25 you're using now, your only option is to get a 12-27 cassette. That gives you almost the same low gear that you have now. The wider spacing kinda sucks, but if you need it.... You need it. A 50x12 won't limit your top end at all, unless you're pedaling like hell on descents.

50x34 and 12-27 gives you a low gear 1/2 a gear higher (harder to climb) than your current setup and a high gear 1 gear lower than your current setup. By the way, why not just keep your current setup?

Oh, and I'd hardly call your hill climbing slow! I'm quite a bit slower than that.

Thanks for the compliment. I just get passed alot by others during these climbs that I must assume that I am the old slow guy.

Reason for the new makeup............I am buying a all carbon frame and doing a custom build myself and want to try and transition to a compact double. All the folks I have talked to said the compact is the newest thing to get you similar gearing to that of your triple compared to a std. double.

I want to force myself to get somewhat stronger. As long as I have the thrid ring I continue to use it in the 30-25 combo even when I don't need it. Not to mention I want to eliminate all those shifts. If I don't like it, well I can buy another triple and transition back or sell the custom built to someone who will benefit by it.

My current triple bike setup I am keeping for training rides since it will be slightly heavier.

Thanks for all the feedback. I will take everyone's advice and mold it into a build in the coming months.

Ah... a new bike. That makes sense then. I also would be tempted to go with the 50x34, 12-27 setup if I were getting a new bike. I'm not completely sure, though... my flat ground cruising speed puts me nicely in the middle of my 42 chainring at the moment. That's very convenient. (But then, by the time I buy myself another bike, I'll hopefully be faster.)

I am finishing off a CF bike build up for myself which I have set up as a climbing bike for the North Georgia Hills in the fall.
Using a Pedal Force QS2 frameset, Campy Record Compact 50T-34T with a medium cage rear DR running a Ti/Steel casette 13-26, hope that I do not have to get a 13-29 pie plate but the motor has a high number of years.
In the meantime I will train on my local ride with 6%-8%-10% grades with my 19.5lb steel Lemond Zurich in an attempt to take off an additional 10lbs off the motor. I have not charted the Six Gap ride slopes as yet using my Slage Inclinometer but I have seen pictures on various webpages of the guys zig zaging up the hills. Brings back memories of my youth when I rode my steel single speed Humber up the hills in Mo Bay Jamaica WI.
Ton.