AWD vs. 4WD

By Brandon Baldwin

              Many people wonder why we see an increase in All Wheel Drive vehicles now-a-days.  AWD cars and light utility vehicles simply get better traction.  Of course a statement like this is completely dependent upon good tires, but then ALL traction is.  AWD has all 4 tires driving the car.  4 Wheel Drive on a vehicle with open differentials is actually 2 wheel drive where 1 tire in front and 1 in the rear drive the vehicle.  

What’s an open differential you say?  The “pumpkin” on the axle houses the differential which allows the “outside tire” of a turn to spin faster than the one on the inside of a turn.  For example, if you are turning hard left, the left rear tire will turn very little compared to the right rear tire.  This is great for not tearing up the  lawn  on  turns, plus tire wear, and  it makes  turns  easier so that the car will not push through the turn.

Pushing through the  turn  means the car wants to go straight even as you turn.   This is also called understeer.  (The fun steering is the opposite: oversteer.  This where you can “hang” the rear of the car to steer with the rear end.)  Now, if you place the 4WD vehicle with both tires of one side on ice and the others are on pavement, the vehicle will have great difficulty moving.  One way of overcoming this in the past was to install a limited  slip or  locking  rear  differential.  These would engage the other rear tire on  the  pavement.

You wouldn’t put this in the front of the vehicle if you intended to be on road as it would bind.  In addition, because the front and rear are mechanically connected, a 4WD vehicle binds on drive pavement.  

AWD uses a few different methods, but typically, the front and rear are NOT mechanically connected.  Between the front and rear driveshafts will be either a locking clutch or a viscous coupling.  Both are a method of being a differential between the front and rear wheels.  AWD is driving all the tires, most of the time.  If we take the same situation of one side of the car being on ice and the other being on pavement, the car will be able to move.   You  may hear electronics involved, but it will move.  Some AWD systems still employ an open front differential, but still call themselves AWD.  Yes, this is really 3WD, but is still better than 4WD as you won’t find the drivetrain binding.

I can’t help  but mention Subaru as they employ Symmetrical AWD which is the only one on the market.  The advantage to this is that all the axles are the same length, therefore, there will be no torque steer in slippery situations and only 1 tire (doesn’t matter which one) requires traction to move the car.  The Jeep Grand Cherokee Limited, I believe with Select Track, is the one that is close to the same, but still has the torque steer due to the different front shaft lengths.  Today’s Electronic Stability Control, which has Traction Control under it, will apply brake to the spinning wheel or reduce power regardless of AWD or 4WD to provide you with better traction.  But, I’ll say it again, all of this is completely dependent on good tires.  Look back at an old issue of Ask-a-tech for the tire article.

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