Autocar
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If the slider at the top is in high gear, the 22 tooth gear acts as an idler so the "high" rear shaft ratio is 1:1(33T/33T) and the "high" front shaft is 33T/22T x 21T/34T=.9265 underdrive or 1.079 gear ratio(1/.9265). In "low" the rear shaft is 25T/33T x 22T/33T=.5051 underdrive or 1.98 gear ratio. The front "low" is 25T/33T x 21T/34T=.4679 underdrive or 2.137 gear ratio. All this means is that the front shaft is being driven by the TC(transfer case) at a lower rpm than the rear shaft. Since the front and rear drive shafts spin at the same rpm on hard pavement, the front drive shaft is spinning faster than it would be driven by the TC, so the sprag freewheels. When the rear wheels slip, they speed up until the front driveshaft TC driven speed matches the rear wheel speed and then the front sprag locks up and starts to TC drive the front drive shaft. In reverse, the wheel speeds and the TC driven speeds are the same as in forward except that the forward sprag would lock up, so the slider is moved to release the forward sprag and engage the rear sprag. This is why you just can't remove the air lines or lock the sprag slider in one position.
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