A friend of mine suggested lowering my handlebars to help with uphills. So I took out the spacers, flipped the headtube, put the spacers on top, and then put it all back together. However, I think I must have done something wrong, because I took it out (just on a road) to make sure that I didn't put them too low, but the handlebars rattle whenever I hit the front brake. They're fine for everything else, but I don't really want to take it out on a trail if my handlebars are going to act weird every time I hit the front brake. It kind of makes a weird sound and vibrates slightly. They're riser bars if that matters at all. Any ideas on what I did wrong and how to fix it? Am I supposed to have at least one spacer between the frame and the headtube? Should I crank on the screws tighter? Thanks in advance.

First, remove the top cap by loosening the bolt on the top of the stem. Make sure there are a couple millimeters of space between the top of the steerer tube and the upper edge of the top spacer. (Edit: If you kept the same amount of spacers and you had no problem before, this is probably not going to be the problem now, but it is worth checking.)

A common mistake is not pre-loading the top cap bolt before tightening the steer tube clamp bolts on the stem. Loosen the bolts that clamp the stem around the steer tube (enough so you can just rotate the stem around). Tighten the top cap bolt--you want it just tight enough that if you hold the front brake lever and rock the bike forward and back there is no play, but not so tight that the fork/wheel can't rotate left and right easily. Align the stem. Re-tighten the steerer clamp bolts on the stem.

Also re-check the bolts on the front of the stem that clamp the handlebars.