Winter driving do's and don'ts

Yes indeed. Sometimes it's necessary to engage the power divider on bone dry pavement. There's a customer in OKC I've learned to engage it BEFORE I initiate the sharp turn off a flat road to climb a steep slab of concrete to get through the gate. The tractor tilts and one set of drives will spin out. That concrete slab is almost black from tires spinning.

The closest I ever came to grenading the drive train was on a dry paved road in Pennsylvania, a narrow two lane road with steep hills and sharp turns. I was following a pilot car provided by the oil company to clear the way ahead, pulling a sand chief at about 110,000 lbs gross weight with a 12 liter 400 hp Cummins. I came down a steep hill with a curve to the right and a short straightaway to approach a WALL. That last hill to the oil well was well north of 20%. I came down it bobtail in 5th with low Jake's on and had to apply brakes!

I gunned it as I approached the hill, in 6th driving a 10 speed. RPM's started to plummet, so I tried dropping to 4th. It was a clean shift, but I start to spin out. I dropped to first while engaging the power divider and ROMPED IT. I shudder to think of that sound. I quickly eased off then got back on it. I topped out at about 800 rpm's with my brain cycling through options, like taking off the seat belt and opening the door in case the brakes wouldn't stop me from rolling backwards.

 

Not exactly. Power flows through the path of least resistance to all 4 corners. In other words, every drive wheel gets as much torque applied as it takes to spin the wheel with the least amount of traction. Once that point is reached, no additional torque can be sent to any other wheel, because the spinning wheel is the path of least resistance. If that amount of torque happens to be enough to move the truck, you roll forward. If not, that wheel with the least resistance spins and you sit stuck. This is why ANY drive wheel sitting on ice or dropped into a pothole will spin, no matter which axle or side it happens to be on...it has the least resistance. Lock in the power divider, and the wheel with the least resistance on each axle will spin. That's how open differentials work.

 

In that situation, it often helps to dump your air suspension. Especially if you're empty, the 3 corners of the drives that are on solid ground are holding the 4th corner in the air (which takes almost no torque to spin). By dumping the suspension, it'll drop that 4th corner 2-4" (depending upon suspension travel) possibly allowing it to make contact with the ground, which increases the amount of torque required to make it spin, which hopefully is enough that when applied to the other 3 corners as well will move the truck forward and out of the hole.

...but you're right. Engaging the power divider when wheels are spinning is NEVER a good thing. The massive amount of energy contained in that spinning wheel (coupled with the power from the engine if you're still in the throttle) meets the sudden resistance of the non-spinning wheels with a harsh bang. If that energy exceeds the breaking point of the gears, axles, drive shaft, or whatever the weak link is in your drive line, you're going to break something. Even if you don't break something, you still aren't getting away with anything...you have caused damage, shortening the life of those components.