
 Originally Posted by 
isuzurover
					 
				 
				.........
There are 2 different scenarios (assuming open diffs front and rear).
(a) You lift 1 wheel in 4x4, but have sufficient momentum/traction to maintain forward velocity/motion.
(b) you lift 1 wheel, and at least 1 wheel on the other axle has insufficient traction to maintain forward motion.
Case b is easier. The wheel that lifts off the ground will start to spin at DOUBLE the rotational velocity (rotate twice as fast). e.g. for a 4.7:1 diff, it will suddenly rotate like a 2.35:1 diff. This exacerbates the problem. Since 2 wheels are now spinning at 2.35:1 (quite fast) - the shock loading when the wheel returns to the ground can break something - as the wheel must come to a dead stop. A wheel on the 2nd axle doesn't need to lift, just have sufficiently low traction to spin (compared to the alternate wheel).
case a - the wheel that lifts will still turn twice as fast as the other 3 - since the other wheel on the same axle will be effectively receiving NO DRIVE. However, when it comes back to earth, the shock loading won't be as high.
I don't think you could ever break anything (in a 2.25P/D) by driving dirt roads in high range 2wd.
.........
			
		 
	 
 
Actually it is worse than you picture.
1. In 2wd - a wheel lifts, and the engine, now unloaded, but with the accelerator still in the same position, accelerates, with all the increased speed going at half the diff ratio to the unloaded wheel. When it grips again, the engine has to slow immediately to match the vehicle speed, putting a shock load on the entire drive train, including both half axles. 
3. In four wheel drive, if one wheel loses grip, the engine cannot speed up, since it is constrained by the other axle, so the unloaded wheel does not speed up, and there is no shock load when it regains grip (there may be a little shock as slack is taken up.
4. I am quite prepared to believe that severe potholes or corrugations could damage axles on the 2/2a with the 2.25, especially if shock absorbers are not too good, because of the very large number of shocks.
5. A locked diff also prevents this shock load problem, but has the alternative axle breaking scenario that it is possible to put all the torque to one side when the other loses grip - suddenly increasing and decreasing this load as the other wheel loses grip with similar effect, and of course in low gear, even in high range, it is probably easy to get close to axle breaking loads without invoking fatigue failure due to repeated load cycling.
John
				
			 
			
		 
			
				
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
Bookmarks