Your headset is the pair of bearings at the front of your bike that connects the forks to the frame. It’s an ’out of sight, out of mind’ component, frequently ignored in favour of more glamorous upgrades – but it makes a huge difference to your bike. Incorrect headset adjustment and worn bearings both mean uncertain steering. A tight headset makes your steering feel heavy and wear quickly. A loose headset will rock and shudder as you brake, compromising control.
Types and styles
Aheadsets are the predominant style of headset system now for almost all types of bike. They’re simple to adjust and maintain, needing few special tools. A pair of bearings sit on the top and bottom of the bike’s head tube. These are trapped securely in place with a pair of shaped cones. The lower one is fitted tightly to the steerer tube just below the lower bearing, whilst the upper cone sits just over the upper bearing, and can slide on the steerer tube.
To adjust the play in the bearings, the stem bolts are released so that the stem can move on the steerer tube. Then, the bolt that sits on top of the stem is adjusted. This is bolted into the top of the steerer tube, so when you tighten it, it pushes the stem, the washers below the stem, and then the top bearing cone down the steerer tube, squeezing the bearings. Once the adjustment is correct, the stem is retightened, locking the adjustment. All Aheadset type systems are adjusted in a similar way, but there are a couple of variations in the way that the bearings are mounted on the frame. In a conventional set-up, a simple cup is pressed into the top and bottom of the bike’s headtube, with replaceable bearings sitting in each cup. When the bearings are worn, they can be replaced. When the cups finally wear out, they would be drifted out of the frame and a new set accurately press fitted – a job for the bike shop. There are now a couple of variations on this theme. Integrated headsets are designed with a shoulder cut into the top and bottom of the head tube, into which a cartridge bearing is placed directly by hand. This eliminates the need to press separate cups into the frame. The shoulder is cut to a very precise size, so that the correct size and shape of cartridge fits exactly, which are designed to take the cartridge bearing. When the idea first caught on, there was the usual competing flurry of sizes, however two ’standards’ quickly emerged for the cartridge size – Cane Creek (36/45), and Campagnolo (45/45). Although neater, this design does require precise machining for the cartridge seat. If this becomes damaged, the only solution may be to replace the frame.
Internal headsets are designed so that both the bearing and the headset are pressed into the head tube as a unit. Bearings can be replaced easily, but if new cups are required, again this is one for your local bike shop.
There are still a few of the older threaded headsets knocking around – you’ll recognize them by the two large nuts between the stem and the frame. The lower of the two is an adjustable bearing race, used to alter the amount of space the headset bearings have to roll about in. The upper is a locknut, used to hold the correctly adjusted bearing race securely in place. The stem height is adjusted separately, with an Allen key on the top of the stem.
Step 1: Pick the bike up by the handlebars and turn the handlebars. The bars should turn easily and smoothly, with no effort. You should not feel any notches.
Step 2: Drop the bike back to the ground and turn the bars 90°, so the wheel points to one side. Hold on the front brake to stop the wheel rolling and rock the bike gently back and forth in the direction the frame (not the wheel) is pointing. The wheel might flex and the tyre yield a bit, but there should not be any knocking or play. Turning bars sideways isolates headset play, avoiding confusion with movement you may have in your brake pivots or suspension.
Step 3: Sometimes it helps to hold around the cups, above and below, while you rock the bike – you shouldn’t feel any movement at all.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Troubleshooting : bottom bracket”