A loose headset can cause serious problems. If your front end sounds especially clunky, if your steering isn’t as responsive as it should be, or if your hands feel like they’re going to rattle off the bars, a loose headset is probably the cause.
Besides making your ride unenjoyable, a loose headset can cause damage to your fork and frame. Because it’s a fast and easy task, you should check your headset adjustment once a month or so.
Check for proper adjustment by engaging the front brake and gently rocking the bike back and forth. There should be no detectable movement or clunking between the fork and frame.
On mountain bikes, squirming tire tread knobs or worn suspension fork bushings can make it feel like the headset is still loose. Start by “pinching” the lower headset race where it meets the frame while you engage the front brake and gently rock the bike. Here you’ll be able to feel if there is movement in the headset. Repeat the adjustment check after turning the bars so the front wheel is perpendicular to the frame. If you detect movement, or hear clunking during either one of these tests, you have a loose headset and need to tighten it.
To tighten any headset, first loosen the stem clamp bolts, then loosen the stem cap bolt with the appropriately sized hex wrench. Remove the stem cap and take this opportunity to make sure there is about 2mm of space between the top of steerer tube and top of stem. Or, if you’re using a fork with a carbon steerer tube or have spacers on top of your stem, about 2mm of space should be visible between the top of steerer tube and the top of the spacer.
Replace the stem cap on top of the stem or spacer. Thread the stem cap bolt into the star nut in metal steerer tubes, or into the compression expander in carbon steerer tubes. Snug the bolt with the appropriately sized hex wrench. Just snug, not tight!
Make sure stem is straight, and that there is no space between headset, spacers, stem, and stem cap.
Slowly tighten stem cap bolt. This will not require much torque; you are just tightening the bolt enough to take any play out of the headset. Don’t over-tighten! Over-tightening will make your steering feel stiff and wear out the headset bearings prematurely.
Check for proper adjustment the same way you did earlier. If you still detect movement, or hear clunking during these tests, give the stem cap bolt another quarter turn and check again.
After you’ve eliminated movement and/or clunking, check to make sure you haven’t over-tightened the headset. To do this, pick up the bike by top tube or saddle so the rear wheel is higher and front wheel is just off the ground. Turn the bars all the way to one side and release. The front end should oscillate a few times before the front wheel settles straight.
If after releasing the bars, the wheel doesn’t turn or turns stiffly, the headset is too tight. Loosen headset cap screw and start the adjustment process again until the proper adjustment is achieved.
Once the headset is properly adjusted—no movement, no clunking or banging, no stiffness when turning the handlebars—make sure the handlebars are straight and torque the stem clamp bolts to manufacturer specifications.