In an external threadless headset if I brake hard there is about 2 mm (maybe a bit more) lateral movement of the steering tube within the headset. I've loosened the stem and tightened the headset's top bolt, but although the cap continues to move down there seems to be no effect on the headset operation. Not only can't I eliminate the lateral movement, I can't even overtighten if I want to as the handlebars continue to turn smoothly and easily. I can't quite figure out what is going on. The steering tube comes to about 5 mm below the top of stem clamp, which I take it is correct, so I don't think there are too few spacers (though it doesn't seem as if adding spacers could fix this anyway). I suspect that maybe the compression ring isn't doing its job, but since I can't figure out how the ring (or the headset itself, for that matter) works exactly, I'm not sure. So I have two questions: First, how does tightening the top bolt work (or, more accurately, how is it supposed to work) in tightening the headset? Second, inasmuch as the bike works well and steers smoothly, what am I risking by just ignoring this problem, tightening the bolt again if it loosens with use? Thanks in advance, as always.