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forum
Stem spacer safety?
The stem spacer above my steerer tube is only in contact with the steerer bung, not the steerer tube, is this dangerous?
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.
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11 comments
You need a 2-3mm airgap between the steerer and the bung cap to allow for the compression when you tighten the cap bolt to take the slack out. Most frame manufacturers show how it should be setup for your bike in the manual. Many manufacturers actually say not to have a spacer above the stem but I've always ignored it.
This sounds absolutely fine to me. Crack on.
My stems all have proprietary top caps these days, so after installation, preload and clamping, I have no spacers at all above the stem. This also means my steerer tube does not reach the top of the stem. Whilst this made me feel very uneasy at first, I've followed the manufacturers guidelines to the letter so am assuming all is good.
The main issue to swerve when it comes to steerers and stem clamps is stress risers. These occure when the clamping pressure from the stem is not evenly applied to the steerer. The biggest offender for this is clamping the top of the stem against the top cap and not the steerer, this increases pressure at the bottom of the stem clamping area, which is already a point of high stress. This is why logic dictates that you run a spacer between the stem and the top cap. It avoids you clamping down on anything apart from the steerer / bung.
That's all the spacer above the stem does, so it really doesn't matter if its not perfectly lined up etc. Indeed once the stem is clamped, the top cap is more or less redundant anyway.
With the bike standing on its wheels (not in a workstand) the top of the spacer (above the stem) needs to be proud of the top of the steerer. This is so that when you tighten down the top cap (prior to tightening the stem clamps) it tightens down against the spacer itself and not against the top of the steerer. Otherwise you will not be applying any pre-load to the headset bearings.
The top cap bolt is threaded through the steerer bung. When you tighten it one of two things will happen, depending on whether the top spacer is or is not proud of the steerer tube.
If the spacer is not proud of the steerer then you will simply be tightening the top cap against the steerer, like screwing the lid on a jar, achieving nothing.
If the spacer is proud of the steerer then the top cap will bear against the spacer, and hence against the top of the headtube bearing, instead of against the steerer. Tightening bolt will be pulling the whole steerer upwards, gripping by the bung inside, through the whole stack of spacers, stem and head tube.
Edit - sorry, I had written stem instead of steerer more or less throughout now corrected. Also, whereas you could simply set the spacers all below the stem such that the stem itself sat proud of the steerer, whilst that would still allow for preload, it would also tend to mean the stem was clamping around the open end of the steerer tube where it is less able to withstand tbr clamping force - think squeezing the open end of a loo roll inner.
Absolutely how it should be done but I'm confused by the OP's words: "stem spacer above my steerer tube is only in contact with the steerer bung, not the steerer tube" (my bold and italics).
It can't only be in contact with one thing - the top cap on one side (assuming that's what he means by "steerer bung"), yes - but what else is it in contact with? It could be sitting on another spacer or the stem which itself reaches above the top of the steerer tube - but if so, why would you add another spacer?
Others suggested a photo, which I agree would be helpful.
A lot of steerer bungs for carbon forks have a lip that sits on the top of the steerer; stops the bung disappearing down the steerer when you undo it, and also provides protection against crushing the steerer (arguably).
My assumption is that the OP is saying the steerer itself does not sit proud of the stem, however the lip at the top of the steerer bung does... which is what the spacer is then sitting over.
As long as the stem is clamped against the steerer tube (ideally all of the stem but minimally all the clamping bolt positions) then I would say it's probably OK, the spacer is just allowing you to pre-load the headset bearings eliminating play and to stop the top-cap contacting the steerer tube. If you are still concerned then a picture with the stem in place and top-cap removed would help.
If the top of the stem is above the top of the steerer tube then that is incorrect and you should move spacers from below the stem to above it so that the top spacer is a couple of mm above the top of the steerer tube as Rendel says, if there is already a spacer above the stem that is in contact with the steerer tube and sticks up slightly above its top then you can remove any additional spacers above that.
That doesn't sound good, though you should post a picture here (it's easier to upload a picture on the comments than in the original forum post).
It may also depend on the manufacturer and the material of the steerer. There can be an issue with the stem clamp bolts deforming the steerer and causing a nasty failure if there's not sufficient spacers above the stem.
By steerer bung guessing you mean top cap? The spacer should be slightly (about 3mm) proud of the steerer tube to allow for headset adjutment, if more, or if it's completely above the top of the steerer tube, could be at risk of movement, fit a smaller spacer.
And expander bungs should be located at the level where you want the stem to be, for carbon steerers. You want the stem to provide reinforcement of the carbon steerer against the outward force of the bung.
Maybe a photo or two would help to explain the setup.