It is a small wheel supporting the top run of the track.
The top run of track of a tracked vehicle could simply be left to hang in a catenary between the idler and sprocket; indeed this is the practice on a number of tracked vehicles. One objection to this is that the arrangement will limit the travel available before the wheels hit the top run, with consequent adverse effect on the ride. Another concerns the dynamics of the track itself, which results in lateral vibration resonances taking the form of a travelling wave moving from front to rear. These may reach violent proportions, and the resultant fluctuations in tension can cause an increase of as much as 20-30% in rolling resistance at certain critical speeds.
By making the road wheels sufficiently large, the top run can be allowed to rest n the tops of the wheels. The consequent crease in unsprung mass makes this an unsatisfactory solution for the higher speed vehicles, especially those with heavy tracks, and the modern tanks all use separate top rollers (also called track support rollers, or track return rollers) to support the track and to keep it clear of the wheels, a price has to be paid for these in the form of added weight, inertia and friction.
Sergyi Way
23.11.2006