Belt / chain is an operation that allows the wheels to rotate using a virtual belt. This bar is displayed in sketch form by default – so it does not contain 3D geometry.

However, you can create part of the belt, but it makes sense to enable the option first Use belt thickness.

I would like to draw your attention to a very important detail. If the wheels are belted with the inside of the belt – the problem does not occur. Because, that after turning on Use belt thickness the graphically displayed bar will move outwards by the selected thickness.

However, if we add tension wheels to the choice, things will change a bit. The tensioner is belted the outer part of the belt! Therefore, when using non-thickness in this place, the sketch should not move away as in the case of the other circles…and yet he pulls away.

However, you can fix it manually . To do this, highlight the wall/edge of the idler i reduce the diameter by the belt thickness. The changed diameter is displayed in bold (for distinction).