By Scott Wilson
A headset is a relatively simple set of bearings resting in cups that connect your fork to your frame. It’s one of the most overlooked components on a bicycle, but it’s absolutely vital, without it there would be no bikes, and we’d all have to walk everywhere -awful.
How Headsets Work:
The most obvious use of the headset is that it enables the handlebars to gyrate, although you don’t need a lot of refinement to do that. Plenty of machines turn just fine without fancy ball-bearing interfaces. The earliest precursors to the bike, hobbyhorses, were given steering by sticking a wooden dowel through a hole in the frame and adding animal grease to the joint for smooth operation. But those early, gimmicky devices had relatively light performance demands compared to our modern machines. The reason every bicycle needs a clean, precision-manufactured, properly adjusted headset is not just to turn, but also to go straight.
That might seem counterintuitive; so let me explain from the ground up. To understand how a headset keeps you going in a straight line, we must first understand how the wheels keep you upright. Bear with me here; I’m a bike mechanic, not a physicist. Bicycles work like levers. When a bike is rolling along, and you lean to the side to turn, the wheels are effectively falling down. The total mass of the bike will naturally want to take the quickest possible route to the ground, which necessitates the front wheel turning. However, once the wheel starts to turn the forward momentum of the bike and the friction of the tire against the pavement will put force on the outer side of the wheel, making it resist the turn, flicking it back upright. Think of it like this: the mass of the bike falling to one side of the wheel with gravity is on one end of the lever, the forward inertia is on the other end, and where the tire touches the road is the fulcrum. According to Newton’s first law of motion, the wheels of the bike will always try to take the path of least resistance, so the front will be compelled by the forward momentum to resist the falling/turning momentum so that it can straighten out. The mass of the bike and the inertia of the forward-spinning rear wheel are what give the front wheel energy to resist gravity. It’s a complicated game where all sorts of constantly changing vectors pull the front wheel towards their own path of uniform motion, requiring it to oscillate back and forth constantly, often at an imperceptible level, to keep you from falling over. The headset assembly is the nexus of communication and translation for all these forces.
To see this in action, take two Pinarello Dogmas, with carbon Zipp 404 wheels and a full Campy Super Record grouppo, and line them up at the top of a hill. On one, lock down the headset so the front wheel is perfectly inline with the back, but can’t move; leave the other bike normal, and push them both down a hill at the same time. The Pinarello with the locked headset will always fall over after just a few feet, but the normal Pinarello will go until it runs out of momentum. You can also try this experiment with a single, unconnected wheel, and you’ll notice it travels even further than the normal bike, proving that unicycles are better than bicycles. As if there was any doubt.
There’s a common misconception that the gyroscopic forces created by the wheels spinning are what keep the bike upright, but researchers at Cornell disproved that theory by building a bike with counter gyroscopes to resist the forces created by the spinning wheels. Their anti-gyro bike was able to self-correct and stay upright by the process described above. Although gyroscopic force might still be a vector in the forward momentum vs. gravity equation, they aren’t necessary to keep the bike from falling over.
How Headsets Wear Out:
Now that we know what headsets do, we can get into how they undo. Companies like Chris King exist to serve the precision headset market. Every aspect of their headsets are machined to very tight parameters using the best possible materials to create a bearing assembly that lasts as long, or longer, than the frame –assuming it’s lubed and cleaned from time to time. I’ve seen headsets so contaminated by sweat water and corrosion that they expanded and broke the frame. This is rare, but headsets, like all bearing assemblies, eventually fail.
The most common indicator of a dead headset is that it becomes “indexed.” This means that if you pick up the bike at its saddle and spin the handlebars the fork will naturally want to settle in the straightforward position. You’ll be able to feel a little hiccup when you turn the bars at the point where it hits that notch. This is bad because if the headset has a bias towards one position it will have a hard time transmitting the oscillations of the front wheel, meaning you’ll have a harder time balancing.
This happens because headsets, like all ball bearing assemblies on your bike, are composed of little metal balls surrounded by lubrication and sandwiched by an inner and outer race. Headset bearings, unlike the other bearings on a bike, don’t spin around much; they stay in one area, wiggling back and fourth as the wheels make their minute course corrections. Unlike on a hub bearing assembly, where no single area of the race is subjected to pressure and friction from the bearings for more than a moment, the same little millimeter-wide area on the headset’s bearing races are subjected to constant friction in the same spot. The problem is compounded by the fact that the little movements of the bearing tend to push lubrication away from the micron-thick contact point where the ball touches the race, and temperatures at that itty-bitty pinhead spot can get hot enough to deform steel. Miniscule fractures appear when the ball bearing micro-welds to the race, then pulls away, creating a notch. As the little divot increases in size, the contact point becomes larger, which makes more friction, which makes more heat, which leads to more deformation. What starts as an imperceptible nick quickly becomes a dent, and then the headset is ruined. This phenomenon is known as “false brinelling.”
Quick aside: “brinelling,” as opposed to “false brinelling,” is what happens when a ball gets squished into a race to cause a dent. This doesn’t happen very often with modern headsets, but I’m sure it can, especially if the bearing is adjusted too loosely and you’re riding on a bumpy road. The majority of dead headsets, however, are caused by the bearings digging material out of the race, a la “false brinelling.” This process is also referred to as “fretting.”
Oh, but there are more ways to kill a headset: dirt contamination, water contamination, using the wrong type of lube, and even the flexing of the fork and frame can damage a headset in more varied and interesting ways than I have space to write about or you have patience to read about.
What To Do with a Worn Out Headset:
If you have an indexed headset, and are using open ball bearings contained in races, you will probably need to replace the whole assembly. If you’re desperate, you can take the bearings out of their cages and add some more and that will give you a few extra miles of smooth-ish operation. If you have an old French bike, like a Peugeot, you’ll need a special “French Standard” headset. Velo Orange makes one. If you’re replacing the fork, stem, and handlebars too then you can switch to an English headset. French head tubes usually have an inner diameter of 30.2, same as the old English, Italian, and Japanese bikes.
If you have a headset with sealed bearings, just buy a new set and plop them in with plenty of grease; it’ll cost you about $40 to do it yourself. Just keep in mind that there are at least four internal diameters that correspond to steerer tube sizes: 1”, 1 1/8”, 1 1/5”, and the rare 1 3/8”. There are also several outer diameters: 38mm, 41mm, 42mm, 52mm, and some others that were developed before the “Standardized Headset Identification System”, or S.H.I.S. There are also internal and external tapers to consider, usually 36 or 45 degrees. And of course, the weirdos at Campagnolo don’t follow the S.H.I.S. rules and prefer their own sizes. Take note that some companies, like Specialized, use Campy bearings for some of their integrated headsets, even though they’re spec’d with SRAM or Shimano shifty bits. Most sealed bearings are 6.5mm tall, but some are different. The specifications of your headset bearings are usually indicated on the side of the bearing case, written in light paint that wears off long before you need to read it. So, good luck.
What separates the crappy headsets most bikes come with from an excellent headset, like a Chris King or a Cane Creek 100, is the application of smart engineering. By using articulating sleeves and appropriate race angles, sophisticated materials, and exacting quality control standards manufacturers are coming out with headsets that last longer and oscillate more smoothly all the time. Checking your headset should be part of your annual tune-up, and if you trust your mechanic and s/he says it’s time for a new headset, buy quality, keep it clean, and never worry about it again.
Scott Wilson is an MFA writing student at Columbia College as well as a seasoned professional bike mechanic. Scott’s “wrenching” experience includes bike shops, racing teams, and professional triathletes across the US. The aim of Scott’s technical articles is to explain in detail how bicycles and their individual components work...and in doing so, help you keep your own bikes running better and lasting longer.