What is the reason, that all 4x4 and AWD vehicles use front-to-back driveshafts with lateral semi-axes, vs lateral driveshafts with front-to-back semi-axes? How would the later affect vehicle dynamics during turns and acceleration?
It's an interesting idea. Some potential advantages I could see:
- You could get away with only one differential. On loose ground, the front/back difference isn't much of an issue, and on tarmac you could just switch to FF mode entirely. (That mode-switch would of course require two clutches, though!)
- Even if that differential is not lockable or limited-slip, you would avoid some of the typical traction-loss scenarios. In particular, a 4×4 with open differentials crawling on rough ground easily ends up with e.g. the left-front and right-rear wheel slipping, because of a diagonal-loading situation. By contrast, at least one wheel on each side will generally be loaded, so if both left wheels and both right wheel are rigidly locked together you should have quite reliable traction.
At higher speed this might not be as useful. Left cornering actually distributes the load away from both left wheels, so then you might need a limited slip differential anyway for best performance.
- A trailing-arm style rear suspension could be used, which would offer good ground clearance and might actually be somewhat easier to implement as other kinds of independent suspension.
Of course, these are offset by several disadvantages, already mentioned in the other answers. Apparently, these have proven too strong to make this design viable for any vehicle in practice.
I can't understand how your alternative suggestion would work.
Wheel mounts accept rotational drive, delivered via a shaft. For the front steering wheels, this drive also has to pass through the wheel turning mechanism as well, whether that be a CV joint or a large ball with wiper seals.
If the drive was coming from a shaft running parallel to the side of the vehicle, then it would need translation by a right-angle in order to power wheels. This would be extra hard at the front, but all wheels need to respond to suspension movements as well.
Traditional 4WD designs use an additional gearbox called a Transfer Case, which is after the main gearbox and splits the drive between front and rear. Since its aft of the gearbox, the TC is about halfway between the front and rear axles, and needs a short prop-shaft to get to the front and rear differentials.
More modern AWD setups tend to package all that into the gearbox, and are more similar to a front wheel drive and transverse engine, where the rear drive is sent via a longer shaft from the TC/gearbox combo mounted under the engine. Its a smaller package, and integrates the front diff with the TC, allowing more controlled distribution of torque between front and rear. Some AWD vehicles turn off the rear power while cruising, for efficiency and are just FWD for a while.
In short Some reduction has already been incorporated into the designs, biased toward the front wheels.