Reading around the topic there seems to be a debate about using one at all. Some say that a barbag interfers with steering, balance and that it can crowd out handlebar space.
I've had 4 handlebar bags over the years, but never really liked them.
3 of them were small, the best of these for waterproofing and ease of access to contents was a small Carradice cotton duck fabric over a plastic shell. On this one I didn't like the mounting system, and the bag tended to bounce on rough surfaces. I have now converted it to use as minimalist luggage for my Brompton - on the proprietary Brompton bracket fixed to the head tube, so much lower and doesn't affect handling at all.
I had a small (less than 5 litres) commercially made bag that simply strapped to the handlebars (centre of stem and over the brake hoods). Used this for a few years, but it wasn't convenient for quick removal to take valuables with me when leaving the bike locked up.
I made myself a minimalist handlebar bag (about 3 litres) that straps with Velcro to the same 3 points (centre of stem and brake hoods). This is small enough not to make any handling difference, and removes fairly quickly, leaving no mounting hardware. I use this bag very occasionally on my lightweight derailleur bike when I need a little more space than is available in my Carradice saddlebag.
I also made myself a large handlebar bag, but quickly abandoned it as I didn't like the interference with access to the handlebars and the effect on steering when loaded. I converted this to a rear rack bag, but never used it much.
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To solve the problem of quickly removing valuables to take with me when parking the bike I have two methods:
- for day trips and other lightly-loaded riding I use a Carradice saddlebag with a quick release mount. Downside is that the mount is more or less permanently fixed to the Brooks saddle.
- when carrying more luggage I use a rucksack. Mostly a very light folding rucksack (about 15 litres), but sometimes a more substantial one, depending on whether I intend to walk far or just explore a town. The small rucksack generally goes into the top of one of my rear panniers, so protected from rain and easily accessed with the snaplock and drawcord closure on my Ortlieb Bikepacker panniers. Exceptionally, if I fill the pannier completely (food stop, etc.), I strap the rucksack on top of the rear rack. If I have more than 4 panniers (rare) I strap the extra luggage on top of the rear rack and the small rucksack goes in one of the front panniers.
If using a bigger rucksack, I just strap it over the rear panniers. I use bin liners inside to keep stuff dry.