The rear idler wheels are, much like the drive wheels, the ones “in the air”, except in the rear of the tank. They need to be able to move or pivot to take up any slack in the track. I have thought about different ways to do this, each with different benefits, and pitfalls. The easiest would probably be a a sliding axle type of setup, but your hull needs an elongated hole in it, in which water and dirt can enter.
So, I am using the design that Dave Manson uses in his famous 2/5 scale Sherman (his many videos are a youtube sensation, the ultimate tank builder’s HowTo).
His design uses an eccentric shaft, which basically has 2 arms at 90 degrees, with a connecting rod going through the hull.
Looks like this:
I have begun building the idler assemblies, because I need to know where to have a hole in the hull, which effects my frame design, etc etc.
I am just using 8″ long 5/8 bolts for the idler axles. Some 5/8″ steel rod as the “peg” that goes through the hull and connects to a lever. The lever, then, when pulled down, pushes the idler wheels back, which tightens the track. Very elegant solution, thanks again to Dave Mason.