Try walking around while holding your hands 12 inches from your hips. You'll notice well before lunchtime that your upper back and trapezeus muscles are quite fatigued and sore. Using handlebars that are too wide will produce similar results after a few riding miles. Alternatively, walk around for hours with your arms crossed in front of your torso. Sore shoulder joints? That's also the result of using handlebars too narrow. Many cyclists can make do with bars that are one size too wide or too narrow; many others however, cannot tolerate the discomfort of just a one size mismatch. A two size difference between shoulder and bar width is almost guaranteed to cause discomfort.
Road bicycle handlebars are typically measured in 2cm increments. Usually this is given in even numbers (34 to 48cm). The goal is to match your bar width (measured from center to center where the brake levers are mounted with your shoulder width measured from AC to AC joint. "AC" stands for acromion. That's the bony tip of the collarbone (clavicle) right above the shoulder joint. (nice anatomy diagram at right from petitebikefit.com) Matching your handlebar width to this shoulder width allows your arms to swing like pendulums (i.e. no muscle stress) out to the brake levers on your road bike. This positioning makes it easier for your arms to relax and to better act as shock absorbers over bumps. That means less teeth rattling on rough roads, less shoulder fatigue on long rides and a safer handling bicycle everywhere.
The tricky part is determining the actual width of handlebars you're considering for purchase. The above explanation talks about measuring bars "center to center" at the brake lever location. Few manufacturers use that location for labeling purposes though – they measure center to center at the ends of the bar. For bars that drop vertically from the lever to the end of the handlebar tubing (where the plugs go), that's fine. Some handlebars flare out from the brake lever to the bar end though, thus there's a difference between what the bar is labeled and what its functional width is. (diagram at left from 3TTT)
The other confusion happens when the bar maker measures and labels their bar width using an "outside to outside" method to measure. Like it sounds, that means they measure from the outside surface of the bar end (again, where the plugs go) to the outside surface of the other side. Sometimes it takes some research to determine what system a bar maker uses. Shoulder width must match the center to center measurement of a bar. If you find your favorite bar is listed using outside to outside data, just subtract 1.5-2.0cm to get the equivalent center to center width (i.e. a one bar size difference). If you're unsure about your optimal handlebar width, talk with a Bicycle Garage Indy Bike Fitter.
Road handlebars come in a bewildering variety of shapes. The "bend" of each road bar (the shape of the curve) is designed to address issues such as riding position, racing VS touring, hand width, finger length, cyclist gender, etc.. Knowing how your current bar fails to fit you perfectly and a liltle common sense will point you toward a limited number of options from which to choose a replacement. Here's a look at the range of shapes from just one manufacturer (Zipp Speed Weaponry).
Like many decisions, choosing your next road handlebar can be as simple or as in-depth as you want. A good first step is dicussing what options you have with a trained Bike Fitter. For information about the Fitting Services offered by Bicycle Garage Indy – Click Here
Frank Radaker, Certified Bicycle Fitter
Bicycle Garage Indy