_DSC6803 By Aaron HershPublished Sep 21, 2012 comments Tweet Share Share Print << Back to StoryOpen full size in new window.The small dot on the roller drum is the sensor magnet. Photo: Aaron Hersh The small dot on the roller drum is the sensor magnet. comments Tweet Share Share Print Liz Hichens is the Web Producer of Triathlete.com. She is an Ironman and marathon finisher and fan of all endurance sports. Follow @lizhichensFollow on Google+All articles by Liz