This will repeat some issues others have mentioned, but might add a few more, and put it all together.
As I understand it, the common reasons that tanks had sprockets at the front were:
- Locating the final drive/transmission close to the driver makes shifting gears more manageable.
Others have mentioned this. Give some thought to the stories about how hard it was to shift gears in T-34s or KV-1s, with transmissions in the rear. Mallets were provided as an aid to driving.
- Track tensioning.
The track under the roadwheels doesn't care about tension. But the track along the upper run (the return run) does. Drive sprockets in the back push the track along the return run. That encourages slack. Drive sprockets in the front pull the track along the return run. That encourages tension. Doesn't mean you can't make a working tensioning system with rear sprockets, just that it takes more attention and exprience. T-34s and British cruisers, with sprockets in the rear, were notorious for shedding their tracks.
- Debris in the sprockets.
The tracks will be cleaner when reach the sprocket in front sprocket designs. In rear sprockets everything the tracks pick up goes straight into the sprockets. In front sprockets the tracks have the entire length of the return run to shed the crud they've picked up. That means less wear, less friction, and less likelyhood of shedding a track when driving cross country in a front sprocket design.
- More even weight distribution.
Both engine and transmission are big hulking metal things. Lots of weight. Splitting them front-and-back in the tank is more balanced than putting them all in the back. Balance helps gunnery -- tanks rock less when they stop driving, or when they fire the main gun and suffer recoil effects. First shots from short-halt, and second shots, can be on-target faster if the tank doesn't rock too much.
All of that said, though, most tank producing countries came to realize that the best solution was having the drive sprockets in the rear. Main reasons were:
- It's easier to make a faster tank.
This is what drove Christie to it, IIRC. If you pull the tracks up from under the tank (with rear sprockets) then gravity is on your side. You nose the tank down a bit, and the tank "falls" forward over the tracks. Not a big thing, but even 5% makes a difference when you're trying to move something that weighs many tons. Less of a concern today, as engines produce more than enough power and are more often governed back to efficient component life. But made a difference in the past.
- Drive sprockets are prone to damage. They require stiff straight axles to work properly, and those axles are just the outer-most extension of all kinds of expensive moving mechanical parts. Any wobble in a drive axle can damage the final drive and tranny -- muy bad! Tensioning wheels, on the other hand, are just free-spinning, and their axles are spring-loaded, and they can often tolerate a bit of wobble (yes, too much and you risk shedding your track, but at least you won't wreck your final drive), and are far faster and easier to replace. The front of the tank tends to bump into stuff (walls, trees, HE rounds). Put the sprockets in the rear and you have fewer tanks in repair depots.
- All the weight of engine and tranny in the rear can be balanced by lots of thick armor on the front. Armor provides WAY better protection than sticking the tranny up front, and helps just as much to keep the tank from rocking. Seriously, the experience-driven realization that you need to put disproportionately more armor on the front helped drive the tranny and final drive to the rear.
- More compact designs are possible when you put engine/tranny/final drive together. Smaller power train means less interior volume means less area to armor means less weight for the same thickness of armor means more weight to thickness than area means better protection. Smaller tanks with more armor and it's easier to pull and replace your power pack. What's not to like?
These factors tend to out-weight any concern about tensioning and debris. Make a tougher track, put better return rollers, whatever, as long as your tanks are smaller, better protected, and don't wind up in the depot every time they bump into a log.
At least that's my understanding.
-Mark
(aka: Mk 1)
