Originally Posted by kill.cactus
My brakes suck, as some may recall from previous posts, but when I brake hard enough down this one 15 grade incline on my commute my fork really vibrates. This vibration causes the braking to seem weird (that is how I first noticed it). What will happen is that my brakes will operate normally and then every half second or so the front of the bike will like... I can't explain... Not jump but like have a small but sharp "knock" and the brakes will resume normal operation.
At first I thought maybe my front rims were messed up (like something was knocking against the pads or some irregularity or something), but now I can tell it is the fork.
This leads me to my question - how much vibration/flex is okay for an aluminum front fork? Mine will flex maybe 3/4 of a centimeter and vibrate under the most strenuous braking.
 - the vibration's effects on the braking and weird knocks and stuff has been happening since I got the bike, and I don't think that the level of knocking actually could interfere with braking performance a lot so this isn't a life/death situation ^.^
Yes, it may well be your fork. I have just finished reading about fork design, and the Trek in your picture, assuming it it the same bike, displays a fork ripe for this symptom -- beefiness, straight through to the drop-outs, and without looking at the angles, they're also probably contributing to the vibration as well.
The reference is Cuthbertson's repair manual I quote: "Certain combinations of fork type and frame geometry can make flutter more of a problem. In my experience, round, thin-gauge, wide-diameter, straight steel fork blades with a short offset and fairly steep head angle, are more susceptible to flutter than other types of forks". His book was written in 1996 when alloy forks weren't quite the rage they are now. His advice if all else fails (toe-in, cleaning rims, checking headset preload, etc), is... buy a new fork.
Frankly, I wouldn't even look at a bike with "straight-through" forks.