Bicycle Mechanics - Threadless headset question

Is the function of the star nut in the steering tube to reinforce the tube against the external force of the stem clamp, or is it to allow one to tighten the headset by torqueing down the cap, thus pulling the stem into the headset?

Thanks.

Is the function of the star nut in the steering tube to reinforce the tube against the external force of the stem clamp, or is it to allow one to tighten the headset by torqueing down the cap, thus pulling the stem into the headset?

Thanks.

Door #2.

The stem clamp screw(s) secure the stem to the steering tube; the top cap/screw/star nut is for adjustment purposes only (and the cap keeps crap out of the steering tube).

I think that it depends on the fork. If you have a metal steerer tube, I don't think that the star nut is necessary for anything but setting the pre load on the headset bearings. On some of the lighter carbon steerer forks, a compression plug is used which also prevents crushing the top of steerer tube with the fork clamp.

If you have a metal steerer tube, I don't think that the star nut is necessary for anything but setting the pre load on the headset bearings.

Thanks, Retro! For a Cane Creek style headset and a metal steerer tube, what should my preload torque be on that star?

For a Cane Creek style headset and a metal steerer tube, what should my preload torque be on that star?
You tighten up the star nut until you can turn the headset with neither binding nor play (no tightness and no wobbliness either). There's no exact formula, you're adjusting a bearing and you have to do it by feel. It's a bit tricky to learn, but at least threadless headsets are easier than threaded headsets.

What I would do is tighten up the star nut until the fork can't turn perfectly freely, then loosen it slightly, and check for play in he bearings. Continue making small adjustments until it's just right.

By the way, the star nut can actually be removed from a metal steerer once the stem is clamped in place. Racers do this to save a few grams...

You tighten up the star nut until you can turn the headset with neither binding nor play

Thanks - I did it exactly right, then.

Thanks - I did it exactly right, then.
Nice work! The same principle of bearing adjustment applies to hubs as well, and for adjustable bottom brackets (should you ever encounter one).

It's a nice system once you know how it works :)