Having a car with 4 wheels and one engine (4x4 or 4WD).

  • Two wheels in front (sharing one front axle using differnetial A).
  • Two wheels in back (sharing one back axle using differential B)
  • front-differential A and back-differential B using a centralized differntial C)

Now the engine rotates the differential C only.

If all wheels touch the ground (perfect situation, sweet-spot of force-transfer) the energy from the engine balance to all wheels the same force .

But if one wheel is not touching the ground, all the force is release on this one free spinning wheel!

You have a dead-lock, right? I am sure the modern industry solved this problem. But how?

  • 2
    $\begingroup$ This might be a better question for Motor Vehicle Maintenance & Repair. $\endgroup$ Commented Oct 11, 2022 at 20:10
  • $\begingroup$ @MichaelSeifert Stackexchange do now allow me to ask that on MVM&R $\endgroup$
    – Grim
    Commented Oct 11, 2022 at 20:15

1 Answer 1


Yes indeed, you have a problem then! Industry has solved this in two ways (at least), as follows.

The earliest solution to this problem was invented in the 1960's and was called a limited slip differential. This device contains clutches that allow slippage above a certain torque called the breakaway torque. Below this torque setting, the rear end acts as if there is no differential action: if one wheel starts slipping, the other wheel still gets torque transmitted to it. This means that if you are driving through a turn, the inside wheel is being driven at the same speed as the outside wheel until the breakaway torque is exceeded, at which point the inside wheel is allowed to turn slower than the outside wheel. This makes handling in turns a little strange, but keeps a free-spinning wheel from blocking torque transmission to the other wheel in the system.

In the case of four-wheel drive, the early solution was to furnish the driver with the option of mechanically locking out the differential action, forcing all four wheels to rotate at the same speed when driving in slippery conditions like snow and mud. On dry roads, this differential lockout made the car difficult to steer, especially at higher speeds.

Modern slip-limiting systems use sensors in each wheel to actively control the clutching action in the differential: when one wheel starts to spin freely, the system detects this and "locks out" the differential, forcing both wheels on that axle to rotate at the same speed.


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