I'm currently in the process of replacing my front forks.
The steerer tube I have is slightly too long by about a 10-20mm. No bother I thought - I'll just leave it, and place the stem clamp over the steerer and leave the excess 10-20mm of space between the top of the stem clamp and the top cap.
However, when I lift the front of the bike up, the steerer is loose inside the head tube (lateral movement), which I assume is because of the excess length on the end of the steerer.
So I guess the top cap fastens the stem clamp and steerer into the headset??although I'm not exactly sure how.
I had fastened the stem clamp just above the headset, which I thought would be sufficient securing of the steerer - if you know what I mean.
I suppose I should just go ahead and saw off 2cm from the steerer tube, but I'm hesitant.