What wheels are powered when power divider is off?

i owned trucks for 44 years and now you guys have me confused

 

But only a few with the right answer, allan5oh is correct. The power divider is nothing more than a differential between the 2 rears. When locked, it acts as a solid shaft. There is no magic drive wheel on any truck. A axle differential allows the wheels to turn at different speeds while still having power to both of them, the power divider is an inter-axle differential to allow the front and rear drive axles to operate at different speeds (like if your rear drive tires have more wear than you fronts). In order for it to work, both both sides need traction. If you lose traction on 1 wheel, it will spin and the other will do nothing. With the power divider unlocked, all it takes is 1 wheel to lose traction and you're stuck. Lock the power divider in and both rears are now tied together, so you would need 1 side on each axle to lose traction to be stuck.

If you jack up the front axle, the front tires will spin, if you jack up the rear axle, the rear tires will spin. Lock in the power divider in either case and you'll tear it off the jack. I do not recommend trying to spin the tires on jack stands if your truck has traction control. Traction control works by applying the brake to a spinning wheel to transfer the power to the wheels with traction.

People assume that there is 1 tire that drives because 1 tire may wear faster or 1 may spin most of the time, but this is only because of the twisting torque on a rear. Under power, the twist of the drive shaft wants to twist the whole rear, which causes more weight on 1 side an less on the other. The left side pushes down toward the road and the right tries to lift, so everything being equal, the right side will spin before the left side. This does not mean that the right tire is the drive tire, it only means it had less traction and spun.

 

Jack up one corner at a time and watch the confusion set in. When all wheels have equal traction all get equal power. Power through the driveline is like electricity in a circuit it's going to take the path of least resistance. Why does it seem that the same wheel spins all the time? Because most of the time it does. Why? Because of the twisting motion of the driveshaft and suspension design. The power divider makes a physical lock to make sure equal power is sent to both rear ends.

 

Great answer! You posted it while I was pecking out mine. Yours is worded better.

 

Yes. All wheels get equal torque when all wheels have equal traction. LOL, yes, this debate seems to pop up about once a year. For those who say the rear rear axle is the main driving axle simply because the tires wear quicker, this is wrong. Tires wear quicker on the rear rear axle because when turning the truck mostly pivots on the front drive and scrubs the rear rear tires.

 

With the power divider unlocked and my truck empty my truck stops when either axle lifts up or loses traction.

 

OK, I am good with all this. But here's something funny that I don't understand. Last spring there was a greasy spot in my yard where I turn every night, and so I got "stuck" there on purpose, so I could get out and observe things with the wheels spinning. Totally safe, no way truck was going anywhere. I observed things with power divider in, and out. Of course with it out, only 1 tire spun on one axle, and with it in, 2 spun, one on each axle. This is all good. But here is the part I don't get. With the divider locked in, and the 2 wheels spinning, I observed them to be turning at only half the speed that the one had been going all by itself. I couldn't believe this, and so ran the test a couple times to verify. It seems to me there should not have been a speed drop with it locked up. There is not a 2 speed axle on this truck. Just the power divider, and a cross lock in the back diff. Anyone got any ideas on this?

 

So you need to work on a rear end. Its what spider gears (differential) do. Have you ever worked on a planetary drive?
1) You are driving the ring gear with the pinion.
2) If you stop one side, the ring gear is still going the same speed, the spinning tire will "speed" up.
3) Now you have 2 differentials working
4) multiply the rpms of the input shaft times 2

Thats why 1 wheel will spin twice as fast as 1 front and 1 rear.

The power devider is not a normal ring and pinion like a rear end, its just works the same for the explanation.

 

Exactly what cetanediesel said. Here's a visual of how each of your 3 differentials work (1 in each rear and 1 power divider):

http://www.youtube.com/watch?v=p3rGjyvNTF0

I differential is pretty simple, but hard to explain to someone who's never actually seen one.