Watch the video-

https://www.facebook.com/video.php?v=835434113180760

 

Wicked

 

Amazing, I didn't think they would move that much.

 

Probably not on you average street driven vehicle. Not some 1000hp dragster. Lol

 

It's all about the forces on the belt, you would be surprised. The belt's length doesn't change, but the way the forces act on it does. Kinda like when you watch top fuel and see the tires take a pentagonal shape. Rigid belt tensioners are actually better, which is why most blower cars run the rigid adjuster. In cases the video here, the potential belt route is much shorter than the actual belt route as dictated by the tensioner. When the crank turns, it is pulling on one side of the belt and pushing on the other side. However, there are accessories that create drag and slow the belt down. As a result, there it a tension on the pulled side (pulling against the accessories) and compression on the pushed side (pushing into the accessories). While the belt length doesn't change, the tension between accessories does. Because of this, with a flexible tensioner, the tensioner can actually get pulled in one direction or another.

A part of tensioner placement is putting it where these forces are minimized. On the 3.9/5.9/5.2, the tensioner is under the alternator on the passenger side. As the engine turns, this is the PUSHED side. Think about if the engine turned the other way, and you locked up an accessory (like the power steering pump). The belt would be getting tugged, which would have the ability to pull the tensioner in, resulting in the belt coming off. This is why tensioners are supposed to always be on the pushed side.