Question: I have an aluminum single-speed frame that was marketed as a fixie. I'm wanting to convert it to a single-speed commuter. (I'm getting older and the fixie doesn't agree with my knees anymore.) It has enough clearance in the rear for 700x35 tires. It has a brake bridge but it doesn't have posts to mount v-brakes or cantilevers. Is there any kind of adapter that could make that work?

Answer: 
There used to be castings that were designed to fit over existing brake studs and stiffen frames that had too much side-to-side flex for the cantis to be effective. Don't know if they could be adapted to your purpose, though, and don't know if they're still available.
They are not cheap ($130 front & rear), but Campagnolo makes a lateral pull TT brake that mounts on the center post:

[http://www.amazon.com/Campagnolo-TT-Brake-Front-Lateral/dp/B0057P8SFM/ref=pd\_sim\_sbs\_sg\_1](http://rads.stackoverflow.com/amzn/click/B0057P8SFM)

But, wouldn't it be easier to mount side-pull caliper brakes? You can buy a pair of good-quality dual pivot side-pull calipers for $40-$60, and they will just mount on the single-bolt brake bridge you already have.