simply a location of the tranny. A tranny in front adds a bit of protection, but a well placed HE can mess up the gears. (or sprocket) ....tranny at the rear allows more armor in front....then the old argument.....is it better with a sprocket pulling in front or pushing from the back....? A sprocket is less vunerable to enemy fire at the rear..

With most tank power packs being built with the engine-tranny as a single unit, it's a lot easier and faster for maintenance without a drive(propeller) shaft, differential, couplings etc. The Merkava is unique....it's the only tank ever designed as a defensive weapon, as a tank, by design is offensive....the Merkava, engine and tranny are in front with the driver to the left rear of the power pack....with all crew members having access-exit thru a door at the rear..reasoning is that a tank is easier to replace than a trained crew.