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I just bought a used 2009 Kona Dew and it comes with regular rim brakes. I'm perfectly happy with them so far, but have been wondering if disc brakes would come handy when it rains.
What is involved in converting a bike from rim brakes to disc brakes (mechanical or hydraulic)?
Is this something that is feasible, or am I better off buying another bike that already has disc brakes?
Your frame and fork already have disc tabs, which is the first step. The second is a disc wheelset, which it does not appear that the stock wheels are. Disc hubs come in two varieties: the common six-bolt ISO and the proprietary Shimano Centerlock, the former are identifiable by a large six-bolt-hole protrusion on the left (non-drive) side of the bike. The latter have a rubber cover over a smaller splined mount.
Once a new wheelset is selected, you'll need a set of calipers and disc rotors. The Avid BB7 is a popular mechanical disc caliper, and will work with your existing linear-pull brake levers.
Installation is fairly straightforward, but since it involves braking it's worth involving a professional if you're at all in doubt of your mechanical aptitude. The disc rotors bolt to the wheel, the calipers to the frame. The old v-brakes are removed. New cables and housing are routed from the levers to the calipers and the calipers are adjusted.
Cost-wise, if you're happy with the bicycle otherwise it's not a poor investment - good wheels and brakes can be moved to future bikes if you later decide to upgrade the frame. The ideal time would be to consider when the wheels already need to be replaced due to wear, since the new wheelset will represent the majority of the upgrade cost.
I love disc brakes. I took a 2002 Trek Alpha 4500 with the goal of making a raceworthy hardtail bike, and I knew that disc brakes were a must. I love making wheels, so I laced my own front and rear wheels to my own specifications. Making your own wheels is a big undertaking if you're new at it, and since you want to make sure the disc wheels are strong, your best bet is to buy them.
Now, there are two main kinds of disc brake hubs. The standard Six-Bolt Hub and the Center Lock Hub. The Six-Bolt Hub is the one I recommend. Brake calipers often are packaged with six-bolt rotors (the disc part of the disc brake). Based on the size of the rotors (I recommend 160mm for both for your style of riding), you will likely have to get a mounting bracket or an adapter so the calipers will attach properly to your frame.
I rebuilt my Trek with an Avid BB7 mechanical rear disc brake and a Hayes HFX hydraulic front disc brake. Based on my experiences, I advise against getting Hayes.
The Avid BB7s are great as far as mechanical disc brakes go: You can adjust the brake pads on either side of the rotor and get the braking power you need, and since it's not absolutely necessary to have full housing on mechanical disc brakes, you do not have to alter the cable stops on your bike frame in order to convert to disc brakes. You would have to alter the cable stops if you were to switch to hydros.
Disc brakes are far better than rim brakes in wet conditions and dusty conditions. The drawback is that your bike will look flashier, and if you don't bring it indoors with you or lock it up in a safe place, it may disappear when you leave it unattended.
