I'm currently planning and training to ride the C2C next month. Having done a practise ride this weekend with a rucksack loaded up with the weight I'd use on the C2C I have come to realise it would be nice to shift some stuff from my back to the bike. An under seat bag is one obvious choice for tools and bike gubbins, but it would be nice to use a bar bag as well to carry stuff as well as have someplace to stick a map. Obviously bar bags are designed for road bikes which have totally different cable routings at the front. But has anybody managed to bodge a smaller bar bag onto a MTB, as if so which one? Granted I could just get panniers as well, but I'm only looking for a bit of extra capacity on the bike to get load off my back, and I fear if I took panniers I'd only go and take more stuff I don't need...
Any bar bag should fit no problem onto a straight bar. if you have risers you may have a problem, but its possible to get small extensions that clamp to the bar to overcome this. The best bar bags by far are made by Ortlieb. The main problem I've found with a barbag on an mtb is that the extra weight affects handling quite badly with suspension forks - if you go for one I'd advise getting the smallest type you can - they are very handy for valuables, maps, mobile, camera, etc., but I wouldn't recommend them for significant amounts of kit. Ortlieb do a very compact one. Bikefix in north London (www.bikefix.co.uk)do a full Ortlieb range.
An alternative, which I think would result in better handling and much better load carrying is a specialist carrier for sus forks. Unfortunately, they are expensive - the best I've heard of are only available from the US - www.oldmanmountain.com. They have a very full range of luggage carrying options specifically for mtbs. There are clamps available for small bags to sus forks, but they are hard to source.
Also, Carradice make a range of small bags that clamp to the seatpost (SQR mounts) - so long as you keep the weight in them low, they are reliable and don't affect handling too much. I think SJS cycles have a full range = www.sjscycles.co.uk
problem with bar bags is that you load the front so your forks might need a bit more air in along with extra cair when using the front brake as it does silly things to your C.O.G. possibly lobing you over the bars best bet is a set of panniers on the rear or if your flush with cash a bob trailer
We had one on my wifes bike but it made it to heavy at the front, basics like energy bars tubes and a few tools, also a bottle carrier at the side. No problem fitting bag came frome Halfords.