There is a correlation between modern "long, low and slack" geometry and wide handlebars, but if your guess was more leverage, you'd be mostly wrong. You may need 800-millimeter handlebars to tame a 280-pound, 60-horsepower off-road motorcycle, but few riders need that much leverage to control a vehicle that weighs only one fourth of its rider. Beyond offering enough mechanical advantage to show the boulders who's the boss, the more important role that wide bars played was to transform riding styles to adapt to much slacker head angles, lengthening reach, and the trail bike market's wholesale switch to short stems.
To set the record straight, personal preference is reason enough to defend your favorite width. I'm not going to dictate which bar you should ride. This is an attempt to frame the handlebar-width/handling equation in a historical perspective that explains why at one time, exaggerated handlebar widths were beneficial (perhaps, essential), and to support why the average handlebar width is now progressing in the opposite direction among the sport's most notable bike handlers.Wider is Better: Why that was true
Not so long ago, frame geometry encouraged riders to stay well back over the bike while descending and cornering. Both downhill and trail bikes had shorter reaches, steeper head tube angles, and more compact wheelbases. On the trail bike side, 50-millimeter stems had become mandatory, (down from 90mm), but bike makers had not yet compensated for the missing 40-millimeters by lengthening their cockpits. Centered over the bike, the combined effect of those factors put more than enough pressure on the front wheel to balance cornering and braking traction on level terrain and down modestly steep trails.
Why Down-Sizing Makes Sense: Balance and flow
That was fine and dandy until you needed to pressurize the rear wheel at the apex of a berm-turn, lower your center of mass to prepare for hard braking, or balance your weight between the wheels to maximize control on a steep descent. The solution was outstretched arms, butt behind the saddle, and heels down. It worked well, because bikes were more compact, and that's where you needed to be to keep just enough pressure on the front tire's contact patch to maintain traction and steering precision, while being in command of the rear tire at all times (or simply to prevent being pitched over the bars).
Try that on a modern DH or trail bike and the front tire will push so badly you'll probably hit the ground before you figure out what is going on. Reaches are longer, head-tube angles are much slacker and, in the case of trail bikes, additional travel and bigger wheels have lengthened forks. The combination of those factors has moved the front axle much farther forward, which means that riders must put a lot more pressure on the front of the bike to maintain traction. This is where wide bars come in to play.
Wide handlebars literally forced riders to shift their weight forward, because that's where you needed to be in order to bend your arms enough to control the bike. Get back too far and your arms quickly straighten, so you can't decouple your steering inputs from the bouncing and bashing your bike is taking. Wide handlebars were an organic, stylish and transparent solution to make the jump from old-school to new-school geometry. Shopping through before-and-after images shows a pronounced improvement in riding posture after bike makers started spec'ing bar-widths north of 760-millimeters - even Joe Blows were looking aggressive, elbows out, head forward, with their bodies low over the bike.
Once you know something, you can't un-know it. Most of us have adapted to rider-forward geometry by now, and it's doubtful that we'll forget to stay over the front end. So, unless you prefer the widest bar you can ride, downsizing can further improve your handling. The attack position that exaggerated bar widths create naturally tightens up your body, which isn't always helpful. Also consider that wider bars require faster and broader steering inputs to make minor corrections. Slicing off as little as ten millimeters can improve steering precision, and give you more freedom to move the bike around while negotiating technical sections. Plus, you'll be better balanced when your upper body is decoupled from lateral forces.
Stable geometry is another factor that has reduced the need for massive widths. Custom offsets, mid-stroke suspension support, and slack head tube angles have reduced the workload necessary to pilot a bike. DH bikes can straight-line sections that competitors once picked their way down and trail bikes are not far behind them. Corrected geometry makes it feel like modern bikes have power steering.So, What's the Bottom Line on Bar Width?
What feels best is always the bottom line for choosing the right handlebar. That said, if you subscribe to "wide as you can ride" and you don't have an ape index like PB's Paul Aston or Greg Minnaar, you may be in for a surprise. Just for fun, grab a couple of grips, shut your eyes and place your hands on a table where you believe the width is just right. Compare that measurement against your current bar (I was 20mm off).
Curious? Experiment with a pair of lock-on grips. Slide your grips and controls inboard five millimeters at a time. Wait for a few rides before you pass judgment. You should be searching for the point where your arms and shoulders begin to relax noticeably while you are riding at pace. You'll know you've gone too far, when steering inputs get wiggly in the corners. When you think you've found your sweet spot, ride it there for a month and then return to full width to make a fair comparison. Only then should you consider the hacksaw.
Compare Your Handlebar Setup With the Pros
Height vs width for 24 of the sport's best riders
PB Even Made a Video on the Subject