Handlebars change depending on what sort of riding you are doing. Road bike handlebars are called drop bars. They vary in materials used, which affects the overall weight of the bike, as well as the width of the bars which changes with the size of the rider (shoulder width should be approximately the same as handle bars). Alloy is the most commonly used material and in higher end bikes carbon is used. Flat bars are used on a variety of bikes. Almost all mountain bikes use flat bars as do flat bar road bikes. Like road drop bars they vary in materials used and width. Alloy and carbon are the most commonly used materials. BMX handlebars have a design which differs from road and mountain bikes, BMX handle bars are reinforced by having a brace bar which gives them increased strength and rigidity. This is necessary because BMX riders put a lot of pressure on their handlebars while performing tricks/stunts or dirt/race riding.