Joined: Aug 2010
Posts: 12,748
Likes: 2,108
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
I assume you already know this, but just in case you don't, most lights have a vent hole in the bottom and that is one reason (besides light beam) that other posters are saying do not mount it upside down, as rain water can get into the light if it is upside down. But not yet mentioned is the potential for tire spray if you do not run fenders. So, think about where your tire spray is when you decide where to mount the light so that the tire spray does not get into the light casing.