According to Rivendell Bicycle Works, "the components that matter most are those that affect riding position and safety. Those are stems and handlebars." Choosing quality handlebars, stems and headset parts and attaching them securely to your frame and fork ensures you excellent control of steering and turning when you turn the handlebars.
Basic Parts
Bicycle handlebars clamp into the handlebar holding part of the stem. The vertical part of the stem inserts into the head tube, or front vertical tube, of the frame. The headset, an assembly of parts including nuts, bearings and bearing races, attach the stem to the fork through the head tube and allow it to rotate smoothly. The fork in turn holds the front wheel.
Headsets
Bikes built since the 1990s more commonly have a threadless headset; earlier bikes have threaded headsets. The "thread" refers to whether the headset screws on or is pressed on with bolts and nuts. Threaded headsets have four races, one on the bottom fork, one on either end of the head tube and one that attaches near the top of the steerer post that is inserted into the head tube. The races hold the ball bearings that enable smooth rotation of the fork and handlebars. The adjustable race is held in place with a nut. There are three types of threadless headsets which attach to the steerer; these use bolted collars and handlebar stems instead of threads, but have races just like threaded headsets.
Steering
When you turn the handlebars, the stem and fork rotate. The ball bearings inside the races rotate smoothly to facilitate the turn. Well greased ball bearings are essential to smooth turning. If the parts are adjusted properly, turning the handlebars causes the fork to rotate the same amount as the handlebars.
Physics
To steer a bicycle into a turn, you must first lean your bike in the direction you are trying to go. According to Joel Fajans at University of California Berkeley, contrary to most people's intuition, to turn right, you lean your bike to the right by turning the handlebars to the left. This is called countersteering, and it is necessary to allow gravity to cancel out the centrifugal forces that act on you and your bike during a turn. Because only the front wheel of the bicycle can change its direction relative to the frame, the rider has control over the steering.
Problems
Stiff steering or grinding sounds during steering may be caused by a number of different issues, including dry or damaged bearings, overtightened components such as the stem, incorrectly sized components and an incorrectly aligned front wheel. Loose or unresponsive steering can also cause grinding sounds and may be due to a stem that has not been tightened down properly onto the steerer. This in turn causes play, or looseness in the rotation of the bearings.
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