When bike manufacturers are deciding on the specification of each bike model – basically the bits they put on it – the width of the handlebar and the length of the stem are typically dependent on the size of the frame.
The thinking behind is that a taller rider will need a longer reach and will generally be wider across the shoulders. For example, a 56cm bike may have a 100m stem and a 42cm handlebar, while a 58cm bike may have a 110mm stem and 44cm handlebar. If everyone was the same build for any given height this wouldn’t be an issue but we are and it is.
If the bars are too wide this will cause your shoulder blades to move inward creating a sagging sensation that in turn can cause neck pain and tension in your shoulders. Conversely, go too narrow and while comfort won’t be as much of a concern it could restrict your breathing. A narrow bar could also cause twitchy steering which, at best, will be tiring and worst could cause you to crash. On the other hand, some riders in the pro peloton have been downsizing to a narrower handlebar, as it allows them to be nimbler and squeeze through tighter gaps in the bunch.
In order to know the width best suited to you, you need to get the tape measure out and find a friend to measure you. To get the correct width, you need to get the measurement between your two acromioclavicular (or AC) joints. These are the outermost parts of your shoulder bones and you should be able to feel the bumps where they protrude easily.
When it comes to actually buying the handlebar, you may need to round the measurement up or down a few millimetres as most manufacturers offer them in widths from 380mm to 460mm in 20mm increments. Handlebars are typically measured ‘centre to centre’. That means the width measurement is taken from the centre of the drops, but some still measure from the outside, so watch out when buying a bar if that’s the case.
There’s one other width measurement to consider, and that’s the diameter of the clamping area. Traditionally, it was 25.4mm, but that has been superseded by the modern standard of 31.8mm. Of course, there are exceptions to this rule.