Handlebar bags are a convenient place to carry a load. On a bike with a suitable front-end geometry, they affect the handling less than a rear load. You balance by moving the front wheel from side to side, so a front load is easy to balance, whereas the rear wheel only follows the input from the front wheel with a lag.
The contents of a handlebar bag are easily accessible, either while riding or during a stop, without dismounting the bike. The bag also shields the rider’s legs from rain and provides a convenient place for a map or cue sheet.
Handlebar bags should sit as low as possible on top of the front wheel. They are designed to be supported by a small front rack and attached to the handlebars with the provided straps or, better, with a decaleur.
Gilles Berthoud continues to make the classic Sologne handlebar bags. With more than 50 years of experience, their bags are sewn in France by hand from waterproof cotton, and edged with leather. These traditional materials work well, keeping the contents dry even during a rainy 50-hour ride in Paris-Brest-Paris, and they are lighter than most “modern” materials, too.
Berthoud bags come with stiffeners, which can be useful if you carry very heavy and irregularly shaped objects. For most riding, remove the stiffener to make your bag much lighter and have it sit more firmly on the rack.