I see most people who use the Serfas Thunderbolt lights (a strip shaped light) place them vertically on seatstays. I recently found that placing them horizontally is much better, maybe because our two eyes are horizontally next to each other, also it widens the distance covered. The only thing is to be sure to place them in such a way as not to touch the wheel (e.g. at some angle 30~45 degrees from rear). The same applies to when using them as front lights on the forks.