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Explain topcap function and bearing preload.(6 posts)

Explain topcap function and bearing preload.Fez
Sep 30, 2003 8:50 AM
Can someone explain exactly what bearing preload is?

OK, the expansion plug puts pressure outward from inside the steer tube. The stem clamps pressure inward from the outside of the tube.

What direction does the topcap place gentle pressure in order to set the bearing preload?

My topcap does not run out of threads, but I notice I have to thread it quite a bit before it places the correct pressure for correct bearing preload.

And when all this is done, the top part of the headset and the spacers can still be physically turned by hand, correct?
When you tighten the top cap.the bull
Sep 30, 2003 8:54 AM
You are compressing all of the spacers and stem.Think about it.
re: Explain topcap function and bearing preload.TJeanloz
Sep 30, 2003 8:59 AM
The expansion plug serves only as an anchor point for the top cap - it used to be a star nut permanently mounted in the fork.

The stem clamp has nothing to do with this operation, and should be completely loose when you're fiddling with the top cap. The only purpose of the stem clamp is to keep the stem aligned with the wheel - it should not serve any up/down function.

Tightening the top cap pushes the stem down, which pushes the spacers down, which push the top race down onto the bearings - "setting" the preload. Once the preload is correctly set, you should tighten the stem clamp. If you tighten the stem clamp before the preload is set, the top cap will push down on the stem, but the stem won't move - and no adjustment will be made.

In many cases, you could turn the spacers by hand when it is adjusted properly, and this may even be true for the top race, though I've never tried it.
Thanks TJ...Fez
Sep 30, 2003 9:14 AM
You pretty much confirmed everything I know about this. I tightened everything in the correct order and to the correct torque. Steer tube is cut 3mm below as specified.

I was just a little curious as to why I had to turn the top cap a lot to set the preload. Didn't run out of threads, but I'd guess there aren't a whole lot left.
don't totally agree...C-40
Oct 1, 2003 5:12 AM
I know you're an experienced shop person, but...

"The only purpose of the stem clamp is to keep the stem aligned with the wheel - it should not serve any up/down function."

The statement above is not correct,IMO. After the top cap sets the preload, the stem clamp is tightened and serves the dual purpose of aligning the stem with the wheel and holding the headset bearings at the preloaded position. After the stem is clamped, the top cap could be removed with no ill effect. Star nuts or the expanding plugs on carbon steerers are not intended to resist substantial forces in the axial direction. That's the function of the clamped stem.

Fez also noted that he could turn the steering tube spacers by hand after the top cap was torqued. Sounds like too little preload to me. At the minimum, the bike should be ridden a few miles and the bearing adjustment rechecked. On a new installation the crown race or lower head tube race often "settle" a small amount, resulting in immeidately loose bearings.

The reason that the top cap bolt had to be turned a lot is probably just due to a longer-than-needed bolt. If the fork has a star-fangled nut, it could be set higher than necessary.
You're right,TJeanloz
Oct 1, 2003 7:32 AM
I hadn't thought that anybody would misconstrue that, but of course, you're right. I've taken the liberty of removing my top cap (every gram helps) - it is unnecessary once everything is properly adjusted. As for whether or not the spacers will turn, I struggled with that one a little. With the appropriate amount of preload, they should definitely NOT "spin", but most spacers can be turned by hand with the appropriate adjustment (though this will sometimes ruin the adjustment). If you couldn't turn the spacer, at all, I would think the adjustment was probably too tight, or you were a woman.

In thinking about the statement: "Star nuts or the expanding plugs on carbon steerers are not intended to resist substantial forces in the axial direction", I don't think there are ever substantial forces in the axial direction. By the nature of the stem, I would think that virtually all of the force on the stem is distributed along the steerer tube itself - not in an up/down direction. I just can't think of how the scenario you're describing plays out.

As for too much turning, because he's dealing with an expansion plug, the plug just isn't as far down as it could be. Some plugs will "bottom out" if you're not carefull, and you need to set them extra deep - which can be accomplished by expanding the plug without the topcap in place, removing the bolt, and re-installing the topcap.