Flare: Flare is the angle of the drop, measured as if the flat part of the bar, extending out from the stem was 90 degrees, and the drop, if it extended straight down from this, was 0 degrees. So 24 degrees of flare is 24 degrees off the perpendicular.
Tops: The part of the bar that comes straight out from the stem. It goes until the drop starts to form (which is also known as the reach portion of the bar.)
Widths: Published widths of bars can be confusing. Is the measurement from center to center, outside to outside? If it’s not listed, measure for yourself (if you can, or call up a shop that’s got ’em in stock to help you), and measure center to center, across the span where the two brake hoods will mount. That is the basic bar width. We also measure ‘tip to tip’, where we take a center to center measurement from the ends of the bars, where you stick bar plugs or bar con shifters. On drop bars this is always wider than the basic width measurement taken at the brake hoods.
Handlebar clamp diameter: There are 4 clamp diameters out there worth talking about. We’ll leave out the 3T and Cinelli and the French quill stem sizes. They’re fine for restoration projects but have no application when building a reliable bike today. Bigger diameter bar clamps generally yield a stiffer handlebar. I say generally because some bars are under built, and an under built aluminum or carbon bar from maker X with a 31.8 bar clamp is gunna be more noodly than a 26mm clamp bar by Nitto . Beware of bars under 290 grams. They’re too light to be ridden in any normal circumstance. Bars are one place where you shouldn’t worry about weight. Get strong bars, period.
Old school dirt drops use a 25.4 clamp diameter, because they were made for off road riding at a time when all mountain bikes used the 25.4 diameter. This is a fine standard, and it’s adequately stiff for riders up to 190lbs with a minimal load, and riders up to 175lbs with a touring load. If you are bigger than that, no worries, just get an oversized 31.8 bar or at the least, a stiffer stem. The steering precision will be vastly improved and your bike will feel more stable.
Road bars made before 2005, and lots of current Nitto stuff use a 26mm bar clamp. 26mm was the drop bar standard for years, all thru the late 70’s on bikes that had a degree of standardization. Ideal rider size: sub 150lbs, but again, you can push this, especially with a stiffer stem.
Most road (and mountain) bars made after 2005-ish use the 31.8 bar clamp diameter. It’s the current standard, and it’s a really good one. I used to think that bar flex was a good thing, but now I know: your tires should flex and that’s about it. Stiffer bars are safer, offer better steering, and they’re more confidence inspiring on rough terrain, especially with a load. When I (185lbs or so) ride bikes with long Technomic stems and wide, small diameter bars, they feel flexy and sketchy. You can adapt to anything, but adapting to crappy steering when there is a better alternative is crazy. It’s like adapting to writing with a blunt pencil when you have a nice pencil sharpener on the desk.
35mm clamp bars are only made in ultra limited numbers. We’d like to see this diameter become more common, especially for touring and gravel bikes. Right now, we can not recommend any of the drop bars available in this size, mainly due to their shape.