Headset How-To - Part 2
Written by: Greg Kopecky
Added: Sat Oct 13 2012
When we left off, the bike looked something like this:
Use The Force
What is a headset’s job? Other than the obvious answer of “allow the bike to steer left and right”… there is a higher-level explanation. Your headset needs to deal with a lot of different forces. You’ve got the weight of the rider. Then there is the left-and-right steering. And the vertical smashing caused by riding over potholes. Perhaps the rider stands up to sprint and violently pulls on the handlebars and stem – which inputs considerable bending force on the fork’s steerer tube (and puts additional load on the headset). All of this needs to be efficiently managed by two bearings.
Let’s take a look at one of those bearings:
Headsets are one area of the bike that have remained (almost) unadultered by ceramic bearings. Most ceramic bearings are often sold for wheels and bottom brackets, on the premise that their lower friction will make you faster. First off – if you believe that all ceramic bearings inherently have lower friction than steel bearings, read my bearing primer linked at the bottom of this page (Bearing Breakdown). And even if you could have a headset that turned with slightly greater ease – it would it really make you faster?.
The key thing to understand about headset bearings is that they are angular contact in design. Since a picture is worth far more than a thousand of my words:
To show you what I mean, take a look at this illustration.
Ladies and Gentlemen, grab your wrenches
Let’s get started with our stem installation and headset adjustment. The first step for me is to install the appropriate number of headset spacers. In my case, this includes a 10mm and a 2mm spacer. If it isn’t already obvious, you want to take note of how many spacers you have under and over your stem before disassembling the front end of your bike. Your bike fitter would be disappointed to find otherwise – you wouldn’t want to spend money on an expensive fit, only to unknowingly change the height of your handlebars while doing routine headset maintenance.
No matter how much steerer tube you leave sticking out, the non-negotiable part is this: the steerer tube must sit about 3mm below the top of the tallest spacer. If there are no spacers above your stem, that means that the steerer tube must sit about 3mm below the top of the stem. In my case, I have about 2mm of steerer tube sticking out, so I put a 5mm spacer on top:
We understand that a headset has to deal with various forces. In order to do so, there must be some level of static compressive force on the system. In the diagram above, that’s the blue arrows. If there is zero force in that direction (i.e. if the stem was just loosely placed on top and nothing bolted down) – the whole system would rattle around. You need at least a little bit of force putting pressure on those angular contact bearings and hold everything together.
To achieve this, we need a way to pull the fork’s steerer tube “up and in to” the bike’s head tube, while pushing down on the stem. The top cap bolt is the means of doing this – of pulling this tube-of-parts together. The tighter that bolt is, the more “smashed together” everything gets. If the top of the steerer tube sat flush with your stem or top spacer, there would simply be no room for this to happen. The steerer tube would have nowhere to go… it would bottom out on the top cap.
You also do not want too much extra space in here. Perhaps an appropriate range is 3-4 millimeters. It’s a pretty small window. I certainly would not feel comfortable with anything more than five. Why? If the steerer tube is much too short, your stem won’t have anything to grab on to (a huge safety concern). For this reason, cutting your steerer tube down is a definite case of “measure twice (or more), cut once”.
Your top cap bolt threads in to what’s called a Star Fangled Nut. Or – star nut, for short. They look like this:
The top cap bolt threads in to the star nut. This whole mechanism is what applies that compressive force to the head tube, thus allowing you to set the headset bearing preload.
Most carbon steerer tubes do not use a star nut; the sharp edges would damage the carbon. Instead, they use some sort of compression plug. Here is a Reynolds plug from my parts bin:
My fork for this demo is somewhat unique. It is a now-discontinued Alpha Q fork. It has a carbon steerer tube AND a star nut. They pull this off by providing the user with a small alloy tube with its own star nut already installed. You use a simple 2-part epoxy to permanently bond the alloy tube to the inside of your carbon steerer tube. It takes a little bit of extra time to set up, but I always liked the system.
The Art of the Science
It’s time to set that preload, folks. This ties with wheel truing as one of the most subjective parts of bike maintenance. There simply is no way to actually know when it is perfect. Fear not, my friends – the good news is that the adjustment process is fairly simple, and due to the angular contact bearings, your headset can deal with a considerable range of adjustment.
There is one tool that you MUST have in order to check your headset bearing preload - a functioning front brake. If you don’t have that, fix the brake before going any further.
This is also the time to double check that your stem bolts are loose:
Install the top cap and bolt. Thread down the bolt just until it stops – no further.
The easy way to see if you have the headset too tight is to simply pick the bike up by the top tube with one hand, and turn the handlebars with the other hand.
If you have a carbon steerer tube and compression plug, the process is slightly different. First, put the plug in to the steerer tube. Next, you must expand the compression piece inside the steerer tube. On my Reynolds plug, this is done by using the long end of a P-handle 5mm allen wrench – going through the hole for the 6mm wrench. Expand the plug – you want it to be pretty snug. Next, you must adjust whatever type of top bolt your plug has (mine is a 6mm allen key in the top of the cap) – this is what pulls on the steerer tube assembly and adjusts the preload.
Now that the preload is set, it is finally time to tighten those stem bolts. My stem uses T25 Torx bolts; other stems may use 4, 5, or 6mm hex bolts.
What I’d like to cover below are a few common questions, conditions, or problems with headsets, along with suggested answers. Keep in mind – as I’ve said in the past, I’m not a doctor and I don’t play one on TV, so these may not cover all possible problems and solutions. When in doubt, see your local qualified mechanic.
Question: How long will my headset bearings last?
-It totally depends on how much you ride, the conditions in which you ride, how much you sweat, whether you use an aerobar-mounted hydration system (a.k.a. sports-drink-bike-soaking-system) and so on. In more harsh condition, expect to replace them annually.
As with any cartridge-style bearing, “they’re good until they’re bad”. There is no maintenance so to speak. You can remove the bearings, clean them, and put a fresh layer of grease on them – which will prolong their life-before-contamination… but there is no maintenance per se, for the bearing itself. Once it is contaminated, it should be replaced.
Problem: My headset is creaking.
-Your bearings are probably bad. Take everything apart and spin those puppies with your fingers. Feel like there’s sand inside? There probably is. Often times this creaking will happen when out of the saddle and torqueing on the handlebars.
Condition: My headset is “binding” when I set the preload. All of a sudden it goes from super loose to vice-grip tight.
-You’re probably missing parts in your headset. Or you have mismatched parts (i.e. you got the wrong size of replacement bearing).
A common problem is to be missing one or more parts in the top half of the assembly – for photo detail, see Part 1 of this How-To. Some modern internal headsets require a shim stack. They are designed to keep the top bearing cover from rubbing on the frame. If there are not enough shims to do this, the compressive force of setting the preload pulls the bearing cover down and digs in to the frame – causing the “binding” feeling when you try to turn the bars.
Condition: The front end of my bike seems to rattle over rough pavement. When I brake, I get this strange vibration.
-Your headset is probably loose. Loosen the stem bolts, set the preload, and tighten the bolts back down.
Question: How much faster will ceramic headset bearings make me over an Ironman-distance race?
-Roughly zero seconds… plus or minus zero seconds. I’ll be here all night; be sure to tip your waitress.
All images © Greg Kopecky / slowtwitch.com
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