LifeSpan Viper M4 User Manual page 27

NOTE:
When properly tightened,you should be able to peel the very edge of the side of the belt up
approximately 2 inches. However, this is a rough reference and not all treadmills are the same.
Some treadmills that have longer belts may give different measurements for correct belt tightness.
Simply, if the belt begins to slip during use, this is an indication that the belt still needs tightening.
Video Tutorial Available at: http://youtu.be/vllsamTSvvA
Lifespan Fitness YouTube Channel: http://www.youtube.com/user/treadmillsvideos
MAINTENANCE INSTRUCTIONS | 27