At the end of the treadmill furthest away from the console you can see where the treadmill belt makes a fast turn under the treadmill to return the belt back to the front end of the treadmill. This is called th rear roller. There is also a front roller. Here is a photo or a rear roller:
How Big Should the Rollers Be?
The larger the diameter of the rollers, the fewer revolutions the roller will be required to make at any given belt speed. For example, a 1.6” roller spins 60% more than a 2.5” belt. Thus, a larger diameter roller significantly reduces wear on the roller, bearings, and belt. Also, because larger rollers spin more quickly than smaller ones, they tend to generate less heat.
Another advantage of wider rollers is that they permit the use of larger bearings, which are themselves stronger. These larger bearings generally don’t need to be lubricated as often as the smaller and hotter bearings, if they ever need lubrication at all.
The least expensive treadmills generally have rollers of around 1.6" in diameter and the most expensive have rollers of 2.5” or more.