Setting the proper height for your handlebars (and adjusting other bicycle components such as the seat and cranks) can help alleviate back, wrist and arm pain while riding, according to Boulder Community Hospital’s Center for Sports Medicine. The adjustment process varies depending on whether you have a threaded or threadless headset. The headset consists of the bearings, bearing races and other components that keep the fork and handlebars aligned and rotating smoothly. Threadless headsets have pinch bolts that clamp the handlebar stem in place and a lack of locknuts just above the bike frame's head tube.
Stand over the front wheel of your bike and hold it in place with your feet. Reach over the handlebars and loosen the stem bolt with an Allen wrench (typically the 6 mm or 8 mm size) or the adjustable wrench.
Twist the handlebars from side to side, pulling up as you do so, until they turn independently from the front wheel and fork. If they do not move, loosen the locknut around the base of the stem with the adjustable wrench and drip some oil or WD-40 around the locknut and the stem bolt. Wait five minutes, strike the stem bolt three times with a steel hammer and try again.
Dislodge an especially stuck handlebar by working with a partner. Stand over the frame and twist the handlebars to your left, while your partner stands at the front of the bike and twists the fork to her left.
Remove the stem completely from the head tube. Wipe it clean with a rag and apply a thin layer of grease.
Reinsert the stem and set it to the desired height. Do not raise the stem beyond the maximum level indicated by a horizontal line on the stem, or with less than two inches of stem remaining inside the head tube. The late mechanic Sheldon Brown recommended installing a steerer extender if you need to raise your stem higher than it can go.
Tighten the stem bolt, holding the handlebars in alignment relative to the front wheel.
Use an Allen wrench (typically 5 mm) to loosen and remove the bolt on top of the steerer tube that is inserted into the bike frame’s head tube.
Loosen the pinch bolts at the top of the steerer tube. Slide the handlebars and attached stem off.
Slide additional spacers onto the steerer tube or remove some spacers to reach the desired height. Removing spacers may require you to shorten the steerer tube length, while adding too many additional spacers may make the steerer tube too short to fit into the head tube.
Install a stem extender if you need more height than spacers can give you. If you have removed too many spacers, take the bike to a bike shop to have the steerer tube shortened.
Reattach the handlebars and stem and tighten the bolt on top of the steerer tube, holding the handlebars aligned in relation to the front wheel.