(This article first appeared in Ride UK issue No. 66 and is reproduced here by kind permission of Ride UK.)

A BMX, like any other bicycle, has two wheels. You sit in-between them and point the front one the way you want to go. All the time that you are sitting (or standing) on it you are pressing down between the two wheels trying to snap it in half.
As BMX'ers we then try that little bit harder to snap it in half by dropping on it from a height, crashing it into things and falling off it. Quite often somebody manages it and the front end tears away from the rest. Maybe the down-tube slowly cracks through at the bottom and then tears clean through in one go. If strength was the only consideration maybe we would think about fixing the wheels together more directly, run a big beam from the front wheel to the back and call it a day? Unfortunately we need to be able to point the front wheel the way we want to go so that isn't practical.
Instead we have two big bits of bike: the front wheel, forks bars and stem; which we can point wherever we like; and the back end which follows along. Joining these two bits together is one of those bike parts that people never seem to give much thought to; the headset.

The headset is one of the only bike parts still surviving in such an old form. While hubs cranks and even pedals have moved on to cartridge bearings the headset is still lumbering on with turn of the century technology. We do have the advances of threadless systems and clamp on stems but most of us still have a bunch of loose balls in simple steel cups. Most people (including me) refuse to spend more than fifteen quid on a headset and expect it to do its job without problem for years on end. Yet these two simple cups pressed into the frame have to take every single hard landing and crash, we expect them to spin perfectly with no wobble even as the same forces that run through them tear the frame apart!

Amazingly they do pretty well. Most people assemble their headset and forget about it until it goes wrong. Unfortunately when something does go wrong the problem is either very subtle and hard to spot or very very serious and expensive, or both.
Your headset doesn’t need a lot of maintenance, there isnt a lot to go wrong with it but if you ignore it when it goes wrong you could well be in big trouble.

The biggest problem is broken cups, for most riders the one to watch is the bottom one. If the headset comes even slightly loose then check the bottom cup very very carefully for cracks. A cracked cup will quickly get worse and in doing so it can kill a frame faster than you will believe. In under half an hour you could flare most head-tubes and render the frame virtually worthless.

The headset is different from most of the bearings on your bike in that it has to take large "thrust" loads. Most of the bearings on your bike, like the hubs pedals and cranks only really have to deal with big "radial" forces. You land hard straight down on the bike and the bearings are loaded by a force acting from the outside in towards the centre of the axle. Tailwhips can put some pretty big axial or thrust forces in, where the bearing is loaded by a force acting along the length of the axle, and these can lead to problems but they are nothing compared to what the headset has to take.
Whenever you land (or even when you are just sitting on your bike rolling around) you load the headset primarily along its length, the type of bearings used in hubs and cranks wouldn’t deal with this very well but the headset is specifically designed for it, the contact surfaces are aligned with the force and it works well with the combination of forces that typically get applied.

Sometimes, however, we crash. Or at least we land hard and awkwardly or the bike gets tossed and lands on its front wheel hard. If you are lucky you pick yourself up and everything feels the same and is fine, if you are unlucky maybe you hear a sharp click as the bike hits the ground and/or when you pick it up the headset feels very slightly loose. This is the crucial time. Stop, and use your eyes to LOOK at the headset. Wipe away the crap that has collected on it over the last 6 months and check very very carefully for cracks. They will be very very small and usually at the front of the cup running top to bottom.

IF you keep riding on a cup that has cracked like this you WILL flare your head-tube. Once the cup is cracked like this every landing will try to open the crack out, normally the cup acts as a continuous ring with the huge stresses running smoothly round it. Now the cup is cracked the cup is like a "C" with very little "hoop strength",  the head-tube will be trying to hold the crack closed and the cup together but that’s just not a job it can do.
Keep riding and the head-tube will begin to flare at the bottom and there is very little that you can do to fix it.

IF you really want or need to keep riding for the rest of the session, day, weekend, trip or month then go to a garage and buy a good hose clamp to go round it. Putting a hose clamp round will help restore the strength of the cup  and pre-load it in compression so increasing the load it can handle. However if the headset still seems to come loose you MUST stop riding or kill the frame. If you regularly break cups then you might want to put the hose clamp on BEFORE it cracks to give it extra strength but these days there are a lot of extra beefy cups available that are a better bet.

Flatlanders put some weird forces into a headset so they need to be particularly thorough looking for cracks, check both top and bottom cups all the way round…

Installing a new headset is fairly straightforward but it can be pretty frustrating. First you may need to get the old one out and that can have some funny problems.
Aheadsets work by having a conical "wedge" which jams the top cone of the headset onto the steerer-tube of the forks. This works very well but can be a bit tricky to get off, the key is to loosen the wedge. To loosen it, (assuming you have already removed the stem and all spacers and gyro etc) first belt the top of the steerer with a mallet a few times while supporting the frame so the front wheel is off the ground. DO NOT use a hammer unless you use a bit of wood to protect the top of the steerer-tube. If you hit the end of the steerer directly with a hammer you are very very likely to distort the top of the tube, the headset will be a nightmare to fit and the stem even worse.
Once you have a bit of slack in the headset, lower the front wheel to the ground so the slack is at the top and then use a screwdriver or punch to drive the top cone DOWN away from the wedge piece. It should suddenly let go and you can slide the parts off separately.

Getting the cups out is pretty obvious, just slide a long metal bar down inside the head-tube till you feel it catch on the lip of the bottom cup. Hit the end of the bar to drive the cup out but keep working the bar round the circumference of the cup to make it move out evenly.
Getting the "fixed cone" off the top of the forks is always either very very easy or very hard. If it has broken then it will virtually fall off when you turn the forks upside down. If not then you need to use a hammer to persuade it off. Again keep it moving straight by working round the circumference or better still hit it at both sides at once by sliding it down between the jaws of a bench vice.

The best way to do it is to press in one cup at a time using a bearing press, but not many people have access to a bearing press so what else?
Well the main thing is to do one cup at a time, some frames can be done with a mallet or a hammer and a block of wood but I have never come across one.

A bench vice is a good substitute for the bearing press. Line the jaws with wood and press the cups in one at a time. Try to keep it going in straight and keep stopping to check.

If you don’t have a bench vice then you can use an old wheel axle and some washers (old 1-piece crank parts are good for this). Just thread the axle through the head-tube and the cup (remember to do one at a time) and tighten the nuts down to pull it all together.

Once all the way in there should be no gaps between the cup and head-tube. It IS possible to distort a cup while installing it (especially with a bench vice) so don’t crank it down too hard.

If the cups go in very easily or they seem "wobbly" once in then you probably have a problem with the frame. You can check by trying another set of cups, but if they too feel wobbly then the head-tube is probably flared.

Fitting the new "fixed cone" to the forks is another pig. Again you can use the bench vice to bash both sides at once but tapping round with a hammer will probably not work. An old bit of pipe can be used to bang it on though.

Once the cups and fixed cone are in place you just need to be sure that you assemble everything the right way up. If you aren't sure you can look at the bits and work out how it should go, there are two types of bearing cage so it can be confusing. IF the flat part of the cage is on the outside edge then it goes outwards BUT if the flat part is on the inside then it goes towarss the cup. Make sure you use a lot of nice thick grease and don’t forget the gyro if you use one…

FLARED HEAD-TUBE?

If you do flare the head-tube of your frame then things look pretty grim.
Things will only get worse if you don’t take action and a slight flaring is just about fixable while a frame that has been ridden and flared very badly is a right off. The obvious solution is to get a new frame but there are some alternatives if the flare isn't too bad. Nothing will get the frame back as it was, but there are a few fixes that will do pretty well.

Some people have had success with shims, a thin bit of a tin can (preferably steel) can be fitted with the new cup to space it out a little. This can work OK for a while, but because the flaring tapers towards the bottom its not likely to work very well.

If you have access to good tools you can try cutting the bottom few millimetres off the head-tube. A good degree of skill is required but if the flaring isn't too bad and you can cut enough off and keep it square (which is the hard part), then it may well do the trick.

If you know someone who can weld fairly well then you can try my preferred method. Cut a slit up the front of the head-tube about 10mm with a hacksaw. Then, working from the top down, slowly weld the slit back up. With the saw blade you will have removed nearly a millimetre wide strip of the tube, if you weld it right the slit will close up as you weld it, so reducing the diameter slightly. If one slit doesn’t do it then you can always try again. Once it works you can grind the surface of the weld flush with the surface if you want, a lick of paint and it will look (and work) like new.

If you really love the frame then a good frame builder will be able to replace the head-tube, but this will not be cheap. However you can always try the other methods first….