How 4x4 delivers torque to your wheels

Here at EZ Wheeler we get a lot of questions about 4x4s and how torque is delivered to the wheels, so we figured it would make a great blog post that could help a lot of people.

All four tires are driven equally by the 4x4 system and all four pull equally if they all have equal traction. If all four wheels have equally good traction, then all four will pull equally well. So even if your truck or Jeep does not have a locker or limited slip differential, each of the four wheels will receive approximately 25% each of the torque from the engine, when traction is equal under each of the four tires.

A standard open differential splits the torque 50:50 between both sides. The problem is that when one tire starts spinning due to poor traction, that reduces the amount of torque seen by either side by an exactly equal amount. So if one side starts slipping, the other side's torque is reduced by an equal amount which usually means insufficient power to keep you moving.

This means, the engine can develop no more torque than the tire with the least amount of traction can accept before spinning.

So, all four tires pull in 4wd. But the moment one side starts slipping, neither side gets enough power to keep you moving even if the non-slipping side still has traction.

Why does the torque get reduced to both sides and not just the side that is slipping? Again, because the differential always splits whatever torque it receives 50:50 to both sides. Why does the amount of torque get reduced at all? Because the engine only develops torque when it is working into resistance. Run an engine with a torque meter connected and watch what happens to the amount of torque produced as resistance to the engine is varied. When the engine is working into zero resistance and just running free, it produces nearly zero torque. Apply a braking force to the engine and the amount of torque will increase in direct proportion to the amount of resistance the engine is working into. The more braking force applied to the engine's output shaft, the more torque the engine produces.

So because the engine only produces torque when working into resistance, a spinning tire reduces the amount of torque the engine produces because the engine is working into less resistance caused by the spinning tire. This works the same way with 2wd and 4wd, it's just that with 4wd, you have more of a chance that at least one of the two axles will have enough traction to allow the engine to produce enough torque to keep you moving. Which is why you get stuck in the first place... when one side is spinning, there is insufficient power being delivered to the other side to get or keep you moving.


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