Purpose of top cap on fork

[EJensen]

I am planning on swapping out my fork soon, and I have been checking various sources for how-to articles.

As I understand it, installing the star nut in the steerer and threading the top cap bolt into the star nut through the top cap creates a compression force between all of the parts stacked above the headset and all of the parts below. Ideally, this creates just enough pressure on the upper and lower bearings to allow them to spin freely without any play. With the proper compression force established, you tighten the bolts on the stem, which maintains the force from that point forward.

Once that is done, and assuming you don't have any spacers above the stem, does the top cap do anything other than keep goo out of the steerer and headset? Not that there would be any particular advantage of doing so, but couldn't you replace the top cap with a plug at that point?

Mind you, I'm not planning on doing that, but I am interested in knowing whether I am missing some other crucial function of the top cap and bolt.

Thanks for any insights,
Eric

[saxman]

The top cap secures the star nut with enough torque to cause the star nut to press out against the sides of the fork and eliminate the play from the headset. Even with the stem bolts cranked down, if there was no top cap, or even if there was insufficient torque on the bolt, given the pounding a mountain bike fork takes, the star nut would eventually lose its compression on the steerer tube. Eventually, the fork would have both lateral and fore & aft play in it. If done long enough, it would ruin the headset and eventually the head tube...so keep that top cap in place. This actually happened to me on my Trek Fuel 70 where the headset loosened up and I had to put additional torque on the bolt to tighten things up again.

The bike would be rideable without a top cap as the stem bolts would hold the fork in place, but I certainly wouldn't advise doing this.

[drevil]

The top cap secures the star nut with enough torque to cause the star nut to press out against the sides of the fork and eliminate the play from the headset. Even with the stem bolts cranked down, if there was no top cap, or even if there was insufficient torque on the bolt, given the pounding a mountain bike fork takes, the star nut would eventually lose its compression on the steerer tube. Eventually, the fork would have both lateral and fore & aft play in it. If done long enough, it would ruin the headset and eventually the head tube...so keep that top cap in place. This actually happened to me on my Trek Fuel 70 where the headset loosened up and I had to put additional torque on the bolt to tighten things up again.

The bike would be rideable without a top cap as the stem bolts would hold the fork in place, but I certainly wouldn't advise doing this.
I respectfully disagree with most of what you said Sax. If a stem is tightened (and designed) properly, then it should stay in place with or without the top cap.

The star nut & top cap assembly should have no part in holding your stem to your fork. Your stem should be the only thing that does.

A slightly loose headset shouldn't destroy your headset or frame. It will probably just clunk. There is a point where the headset being loose can mess up the whole assembly, but it'd have to be so loose that the races don't contact the bearings any more. If it got to that point, you'd know.

On the weight weenie board, you'll see guys asking about removing the star nut after compressing their headset and tightening their stem. The problem with doing this is that what if your headset does get loose? Some stems work better than others, but there are some poorly designed ones that eventually get loose. Also, what if you crash, and your stem turns sideways? You may be able to strongarm the thing straight, but if not, you'll need to loosen the bolts, and once you do that, you'll lose proper compression. Basically, I think it's a bad idea to run without one because you save such little weight, and it's a pain if things get loose.

If you absolutely have to run without one, you can use one of the top caps that either have a bolt (http://www.chainreactioncycles.com/Models.aspx?ModelID=567) that runs from the top to the bottom, or you can use one that has its own compression plug (http://www.chainreactioncycles.com/Models.aspx?ModelID=911). After tightening, pop it out...then carry it in your bag. :rolleyes:

[saxman]

I respectfully disagree with most of what you said Sax. If a stem is tightened (and designed) properly, then it should stay in place with or without the top cap.

I have no doubt that the stem would stay in place. What I found was that I started getting play in the headset when the top cap bolt was loose. My observations were based on my experiences on my own bikes. It is possible that my stem/steerer tubes, etc, weren't designed or tightened properly as most of my stuff is entry level or at best mid-grade items. I imagine higher quality componentry (particularly in the headset and stem) would be more secure.

[drevil]

I have no doubt that the stem would stay in place. What I found was that I started getting play in the headset when the top cap bolt was loose. My observations were based on my experiences on my own bikes. It is possible that my stem/steerer tubes, etc, weren't designed or tightened properly as most of my stuff is entry level or at best mid-grade items. I imagine higher quality componentry (particularly in the headset and stem) would be more secure.
You're right. My bad.

BTW, this loosening isn't limited to the less expensive stuff. On two different bikes I ran a King headset with a Thomson Elite stem and I'd get loosening and clunking, even though the headtube was faced. I don't really dig the wedge clamping mechanisms much anymore.