V-Brake making fork twitch?
 

This is my first mountain bike (but I've been riding fixed road for almost a year), and I'm not sure if this is normal: I have V-brakes, and using my brake causes the front fork to twitch forward and backwards badly (*thump* *thump* *thump*). The wheel is slightly out of true, but not terribly so. :confused: It's a Surly Karate Monkey frame + fork, so it's all steel.

Sounds like the headset is loose, that would be the first thing I would check. Braking should not make it do that.

A good braking systems can make a fork flex backwards as well as make a top tube bow up. You'll see this more with steel framesets. Twitching is a different story. You either have a problem with the rim, the surface should be even and smooth as well as true or, as sckot pointed out, you have a headset problem