How to Assemble a Stem on a Threadless Headset (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Mon, 18 Mar 2024

How to Assemble a Stem on a Threadless Headset

(This is a followup to Upgrading an Old 1-1/4" Cannondale to a Modern 1-1/8" Fork.)

After installing a new headset and fork on my mountain bike, I slid the stem onto the new steerer tube, cinched it up, tightened the top bolt and went for a test ride.

It tested out fine, but the way I'd installed it was exactly wrong, and I didn't realize it until I did some more reading. It turns out cinching down a threadless headset requires a step that's not at all obvious if you don't know any better. Or at least it wasn't obvious to me.

[diagram of threaded headset, from Wikipedia] On the old threaded setup, there was no question how to install the quill stem. You slide the stem into the steerer tube, make sure it's straight with the handlebars, then tighten the top bolt that makes the wedge slide sideways against the inside of the steerer tube — really cranking down since you don't want it coming loose while you're riding. (This image and the next one courtesy of Wikipedia: Headset (bicycle part) and Wikibooks: Bicycles/Maintenance and Repair/Headsets/Headset Overhaul.)

[diagram of threadless aheadset, from Wikipedia] On bikes with threadless stems, it seemed equally straightforward. (This paragraph describes the wrong way: don't do this.) Slide the stem down over the steerer tube (with spacers above or below to reach the stem height you want). I only had one spacer from the donor bike, so I left the stem pretty far below the top of the steer tube. Get the stem straight with the front wheel and tighten the pinch bolts (really crank down, etc.). Then there's that bolt in the top center, which apparently has something to do with headset tension. Crank down, or just tighten a little? It didn't seem to make any difference, so I just got it reasonably tight and figured I was good to go.

Turns out that's backwards. The right way: use enough spacers that the top of the stem — or the topmost spacer above the stem, if you want the stem a bit lower — is a few millimeters above the top of the steerer. (Some say you should always put at least one spacer above the stem, but most people say that's only important if the steerer is carbon fiber rather than metal). Don't tighten the stem's pinch bolts until after you've set the tension via the top bolt.

When you tighten the top bolt, it pulls up on the steerer tube (via the star nut inside), tensioning it against the stem and spacer stack and preloading the headset bearings. So the stem needs to be able to slide on the steerer tube.

If the steerer tube protrudes above the stem, as mine initially did, then when you tighten the center bolt, you're just compressing the top of the steerer against the star nut wedged into that same steerer, accomplishing exactly nothing. If the stem protrudes as it should, but you tighten the stem bolts first, you get the same result: you're just putting tension on the star nut, not the bearings. But if the stem is free to slide on the steerer tube, that center bolt will put some tension on the headset bearings. Get it to the right tension, and only then should you get the stem straight and tighten its pinch bolts.

Most pages don't make this super clear, and I bet a lot of people are making the same mistake I was. I have no idea how bad that is: the fork isn't going to slide off or anything like that, because the stem will prevent that once it's tightened down. It just means that there's no tension on the headset bearings, so the bearings are tensioned only by gravity. Is that bad? Honestly, I don't know. I couldn't find anyone discussing it. I've been riding around town on a different mountain bike where I'd added a stem riser and I never tensioned that bearing, so I hope I didn't do much damage (of course I've fixed it now that I know better). The steering didn't feel loose on either of the bikes after assembling the wrong way.

I ordered some spacers and have now reinstalled the stems on both bikes the way they're supposed to be done. I couldn't tell much difference in the way the steering felt with the center bolt cranked down versus barely making contact. But maybe the bearings have more preload on them now, and maybe that's a good thing. Plus I have a higher stem with some cool-looking carbon fiber spacers underneath, so that's good.

A curious side effect: once the stem is tightened, the top bolt and cap no longer do anything and can be removed. Not that I intend to do that; my new reducer headset came with its own cap, so the one from the donor bike will stay with the donor bike. But apparently some people put compasses or other gear in place of the stem cap.

[ 10:52 Mar 18, 2024    More bike | permalink to this entry | ]

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