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Recently a few people following a thread I started in the
Jeeps Unlimited Grand Cherokee forum have stated that locking the transfer case
in a WJ,
which is the same as locking the centre diff, will cause 50% of total engine
torque to go to each axle. It isn’t that simple. Certainly not in the context of the discussion on the failure of the QuadraDrive system to work as advertised. Torque is a force that tends to rotate or turn things. You generate torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut. When the nut is loose, very little torque, or force, is required to turn it. As it gets tighter, or loads up, more and more torque, or rotational force, is required to turn it. On a 4x4 vehicle where one wheel is in the air, very little torque (force) is required to turn that wheel. When a wheel is on the ground and has good traction a considerable amount of torque is required to turn that wheel because there has to be enough force to move the entire vehicle before the wheel will turn. Let’s look at an example: A 4x4 vehicle with fully locked front, centre and rear diffs finds itself in a situation where two of its wheels are up in the air and two are on solid ground. Because all the diffs are locked, all four wheels are forced to rotate at exactly the same speed. This is not the same thing as being forced to deliver the same amount of torque. The amount of torque delivered to each wheel depends on the load on each wheel, not its rotational speed. Now, our intrepid driver tries to drive the vehicle forward. All four wheels are going to turn at exactly the same speed (because all the diffs are locked) but the two wheels that are in the air are going to take very little torque (force) to get them turning while the two wheels on the ground are going to require lots of torque to get them turning. So, here we have four wheels, all turning at exactly the same rotational speed, all driven from the same engine but receiving different amounts of torque (force) to turn them. Remember, all diffs are locked. If we assumed for the sake of the argument that the drivetrain was perfect and exhibited no frictional or other losses, then 100% of the torque output by the engine would arrive at the wheels, split in some proportion between the four wheels depending on the load (traction or grip) at each wheel. Theoretically – remember our perfect drivetrain now – as near as makes no difference to zero torque would go the two airborn wheels and almost exactly 50% of the torque would go to each grounded wheel, adding up to a total of 100%. If we put this hypothetical 4x4 onto a specially made test bed, where three of its four wheels were on zero friction rollers, like you see in factories for moving goods around on a kind of conveyor made of rollers, then we’d find that, again, when we tried to drive forward, all four wheels would rotate at exactly the same speed but those on the rollers would require almost zero torque to turn them while the single wheel with traction would get almost 100% of the engine’s output. If you took our 4x4 and put it on a level road where all four wheels had equal grip then, yes, under those circumstances, each wheel would demand and would receive approximately 25% of total engine output but if, and only if they all have equal traction. But, cause any wheel to spin and that wheel’s 25% share doesn’t mysteriously vaporise just because all the diffs are locked, it goes instead to the remaining wheels that do have traction. Locking the diffs does not force an equal torque
distribution, it merely forces equal rotational speed. The actual amount of
torque is dependent on the load (grip / traction) at each wheel. On a nice paved even surface you would see pretty much 25% of total torque going to each wheel. On a muddy slope with all manner of stones and stuff in it and undulations and unevenness in the levels you would see all the gauges bobbing about all over the show as different wheels got different amounts of torque depending on their respective levels of grip at any instant. Put the left two wheels on ice and the right two wheels on dry pavement and watch how the left two wheels get very little torque and the right two wheels get the lion's share. Put three wheels on smooth rollers to simulate loss of traction and watch how the fourth wheel gets the vast bulk of the developed engine output. All this happens without varying the state of having all diffs locked. The amount of torque applied to each wheel is dependent on the load (grip / traction) at that wheel. Locking all the diffs does not force 50% of torque to the front and 50% to the rear. Neither does it force 25% of torque to each wheel. All it does is force the wheels to rotate at the same speed. The amount of torque, and the percentage of total torque output by the engine that goes to each wheel is determined by the load on the wheel. |
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