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Kickass.

They're almost unaffected by funky weather, mud, snow, etc. Better modulation than rim brakes.

Kind of a PITA to set up. Not really worse than V or cantilever brakes, just different.

If you get the pads or rotors contaminated, it's really bad. On a rigid, that's less of a problem. If you're using mechanicals and not hydraulics, it's less of a problem.

People really like Avid BB7s for mechanicals. Which hydraulics are better is a question that will incite sectarian violence.

Some people like a 180mm rotor on the front for a 29er even if it's just going to be used on the street or for light off-roading. For downhilling, bigass rotors are better.

They are good, but there are a few issues to consider.

Rigid forks: Most do not work well with disc. Ideally, you want forward facing fork dropouts so the brake effort does not try to force the wheel out of the drops. The other option is to have huge lawyer lips, which is what most folks do. Just be aware of it.
Hydra versus Mech: Mostly moot actually. Unless you are doing downhill, heavy cargo loading, or pedicab work, either is fine. The only real reason hydraulic brakes make any sense, is if you need to address heat expansion, brake fade resulting from it, or you already have a set of hydraulic brake laying about. Mechs are nice in the no new tools, any lever works, regular brake cable front, but use compressionless housing. Regular brake housing is too squishy to get good results.
IS versus Post Mountings: Do you want to use an adapter, add extra weight, and unneeded parts? Get IS mounts. Do you want to bolt directly to the frame and fork out of the box? Get post mounts. You can still use an adapter to change rotor sizes on post mounts if you like.
Rotor Size: 180mm. Smaller is fine, bigger is fine. 180mm is what nearly everything is by default, and the easiest to get parts for. It has plenty of stopping power. You will probably need an adapter if you go with anything other than 180mm to position the caliper correctly.
Torx?: Sorry, you are stuck with this one. It is a torque rating concern for the rotor. You have an indexing torque wrench, right?

I have had 3 pairs of Avid BB7s
Some 2006 vintage 160mm on the 1x1
Some ~2007 vintage 203mm on my former xtracycle
Some 2008 vintage on my new to me still forkless mtn bike.

For power and feel I like the ones on the 1x1 best. In part I think the older rotors were better, the ones they ship with now are pretty cheap. I do like the new calipers as the adjustment dials are easier to turn.

The problem with bigger like 185 or 203mm rotors is that if they are cheap and not flat they rub and you either have a draging brake or one where you run out of pad travel before you get the stopping power you want. Alex hacked a pretty cool travel agent mod to get better power out of his 203's.

You might want to compare Lee's set up to my 1x1. Levers and cables make a difference.

In short I'd say get these F&R set of these


These levers


And a cable set like this

Very cool. Well these are the culprits. I already know I'll have to get a different front wheel/hub if I go this road. Right now I'm just collecting information.

<img src="http://sweetbike.org/gallery/d/22339-4/IMG_0372.JPG">
<img src="http://sweetbike.org/gallery/d/22342-4/IMG_0374.JPG">
<img src="http://sweetbike.org/gallery/d/22296-2/IMG_0345.JPG">

I have a set of midge bars I'm thinking of dropping on her. I dunno - I'm at that stage of things. I know. 3 Surlys in my stable now, total of 4 bikes.

Nice.

What are you planning to use it for?

Mountain and winter biking I'd guess - trail and logging roads. No trail blazing or anything. It's got big apples on it now because dude had it set up as a commuter. Still working it out I guess. Lots of ideas, lots of directions to go. I also have a 9sp cassette I'm throwing back there because I am fat, hills are hard, and a single speed in a city filled with hills continues to sound silly to me - and because sometimes you have to cover distance in this city and 36/25 isn't really going to cut it. It makes sense if you're tooling around the Colonnade I guess, or are up in the mountains, but here in the city I need a bit more diversity.

I'm messing around in today, from around 3:45 until I get bored, or my friend gets bored, or one of us gets tired, or something like that. It's not too technical, with a combination of paved roads and rolling singletrack. Also sand pits. And not a lot of elevation change, so it's friendly to single speeders who aren't Ethan, who can kick my ass on a climb on his. There are some neat flow lines at the south end of the park for anyone who's into that. I've got a crappy pickup truck, so I'll be parking in the north lot. Unless the gates are magically open. People with nicer cars should consider the library on the south side.

Anyway, it would be a good prequel to the Tuesday night ride if anyone's interested. If you come, call me. 9one7-6nine7-6nine85.

scott, you have a problem.

I'm fucking done! I swear! No more bikes for me - this is the last one!

Fire. You. Innit.

If you're going to use road-style levers that are levers only (no integrated shifters) I'd go with BB7 mtn calipers and a road-lever specifically made for that cable pull (will work with V brakes, too). I'm not sure if bar-end shifters will work on the Midge bars, though.

If you're using integrated road levers/shifters then I think it's a toss-up between BB7 road calipers or BB7 mtn calipers+travel agent.

Make sure and have a barrel adjuster in the equation somewhere. Mountain levers usually already have them at the lever, but for road levers get the in-line kind that splice into the housing. Clean cable routing and very secure under-bar-wrap taping helps, too.

If you're not going to use drop-bars, just get hydraulics. Juicy 5s are fantastic.

I'm using BB7 roads on a road/cross bike with 160/140 rotors. That's plenty for my 210# self. On my 29er I've got hydraulics with 180/160 rotors. On long, very steep switchbacky singletrack descents I really like the hydraulics and that rotor combo, my forearms are much less tired than when I had BB7 mtn calipers and 203/160 rotors.

That's been my experience.