If your treadmill belt starts running closer to either foot rail -- either right after you buy the treadmill or after steady use -- it's usually not a big deal. On almost all treadmills, you can fix the problem yourself by using an Allen wrench to adjust the tension on the rear rollers. If adjusting the rollers doesn't fix the belt, though, it's time to call a technician. That off-center belt is signaling that you have a bigger problem.
Which Way Did It Go
There should be even space between your treadmill belt and the foot rails to either side of it. Which side the belt slips to will determine how you make the fix. On almost all treadmills, including machines from Horizon, ProForm and Smooth Fitness, you can re-center the belt by tightening the roller bolt on the side the belt is slipping toward, or loosening the roller bolt on the side the belt is sliding away from.
Finding the Roller Bolts
On almost all treadmills, it's impossible to mistake the roller-adjustment bolts for anything else. They're the two hex-head bolts in the very trail end of the treadmill, embedded in the foot rails to either side of the walking belt.
Which bolt you adjust -- and whether you default to tightening or loosening it -- depends on your treadmill brand. For example, on treadmills by Horizon and Smooth Fitness, you can tighten or loosen either bolt. On treadmills by ProForm and True Fitness, you're supposed to make adjustments via the left bolt only. With the ProForm treadmill, you rotate the Allen wrench in the direction you want the belt to move. With True Fitness, you do the exact opposite. If you're not sure which bolt to adjust, check your owner's manual.
How Much to Adjust
The Allen wrench you need to adjust the bolts came with your treadmill. If you can't find it, any Allen wrench of the proper size -- which varies between treadmill brands -- will do. Once you locate the wrench or find a substitute, adjust the designated bolt one-quarter turn at a time. Between adjustments, let the treadmill belt run slowly for several minutes, giving it a chance to re-center itself.
In many cases, manufacturers want you to leave the belt moving slowly as you make adjustments, although every so often, one recommends that you make adjustments with the belt at a standstill. If you're not sure whether to run the belt as you adjust the roller bolts, check your treadmill's instruction manual.
Don't Overdo It
If a few adjustments don't solve the problem or if it keeps recurring, something else is going on with the treadmill. Don't keep tightening the rear rollers in an attempt to fix it. The problem might lay with the front rollers, deck springs or other parts. Some treadmill manufacturers aren't very specific about when you should give up and ask for help, but True Fitness clearly says that if three quarter-turns haven't solved the problem, it's time to call a technician.