There are two easily swappable fat o-rings, which fit both size bars. They're quite hard to stretch round the bar, but they stay put once in place. Good light, nice and neat, strong centre spot with outer halo for trail edges. Frankly, VFM is incredible, compared to Smart lights or Cateye prices. I'll be getting one of Trout's lights, but that's a whole different league, and possibly a bit too much for road use, whereas the DX is good for commuting. You honestly can't go wrong with it. I've got a DX Aurora torch with the same P7 emitter, which I also use with a home-made bracket, and helmet mounted with Velcro straps. One of those, with several spare 18650 batteries and a charger will still cost around the same as the Bike Light, but is perhaps a little more versatile. Or you could use both.