A 1" diameter, 4" barrel 18650 flashlight. My Shiningbeam S-1 is showing signs of aging and I just purchased a Convoy S2+. It claims to be 1100 lumens (the S-1 was 400, tops) and I'll be using it on medium or low. https://www.fasttech.com/p/7875500 is a good velcro mount for helmet. Back when I used a flashlight on the bars (pre Cygolite days) this was the best handlebar mount (if your bars are sized right): https://www.fasttech.com/p/1193005.