The rule is to measure your width from the knobby points on the front of the shoulder and add 2cm if centre to centre and 4 cm if measuring to the external width of the bars.
A lot of bikes come in small, medium and large frame sizes and handlebar selection would be a rough rule of thumb in fitting the potential size of the intended rider.
Additional measurements of height or rise and depth or drop can all be taken into account as an adjustment to make it more comfortable.
Narrow bars reduce the intake of oxygen when the arms push the shoulders inwards.
Too wide and there is a loss of aerodynamic advantage.
This can also cause you to have to stretch further to reach the brakes and gear shifters.
For flat bars bikes such as hybrids and mountain bikes the handle bars will more than likely be much wider than the shoulder width.
This allows for stability of riding and getting leverage for slower riding skills and handling.
Flat bars have the advantage of being able to be cut down to accommodate size.
In traditional racing bars this is not the case.
The additional advantage of flat bars is the option of adding different length “bar ends” which gives another hand position than having the hands flat pointing to the front.
The positioning of any handle bars is integral to the seat height and top bar length being set right for your height.
The head stem and handle bars are where the fine tuning for the best fit occur.
The depth of the drops for racing bars (from the top of the bars to the bottom) and the amount of rise for flat bars are as variable as there are brands.
Getting this right is very much an individual thing and can change as your skills and fitness improve.
There are a few other subtleties of safety with handle bars that are quite often not recognised.
Handle bar tape and the plugs that fill the end of the bars are a safety feature that is in everyone’s best interest to maintain.
In an accident open bar ends of any kind can produce a Swiss cheese hole in the body.
Handle bar tape is to reduce the road vibration into the hands as well as give a good surface to grip with, as do hand grips for flat bars.
Tape is best applied starting from the drops and finished off at the tops of the bars.
Look forward to seeing you on the road soon.