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Most interesting - More 4WD ramblings

Illustration for article titled Most interesting - More 4WD ramblings

So I’ve got a thing about 4WD...did you guys know that? Anyway, I was looking for the history of the systems and terms (because fun) and I found something interesting - The SAE actually has a paper on this topic. J1952A.


It’s a recommendation for classification of All-Wheel Drive systems through class III (1 ton trucks).Anyway in my mind there is no difference between AWD and 4WD and that there are only 3 overarching systems for 4 wheel drive systems that are as follows:

  • Part time
  • Full time
  • On demand

I had no idea until just now. I can’t actually read the paper without parting with $80 or so of my real monies but wikipedia has a summary:


Part-Time AWD systems require driver intervention to couple and decouple the secondary axle from the primarily driven axle and these systems do not have a center differential (or similar device). The definition notes that part-time systems may have a low range.

Full-Time AWD systems drive both front and rear axles at all times via a center (inter-axle) differential. The torque split of that differential may be fixed or variable depending on the type of center differential. This system can be used on any surface at any speed. The definition does not address inclusion or exclusion of a low range gear.

On-Demand AWD systems drive the secondary axle via an active or passive coupling device or “by an independently powered drive system”. The standard notes that in some cases the secondary drive system may also provide the primary vehicle propulsion. An example is a hybrid AWD vehicle where the primary axle is driven by an internal combustion engine and secondary axle is driven by an electric motor. When the internal combustion engine is shut off the secondary, electrically driven axle is the only driven axle. On-demand systems function primarily with only one powered axle until torque is required by the second axle. At that point either a passive or active coupling sends torque to the secondary axle.

In addition to the above primary classifications the J1952 standard notes secondary classifications resulting in a total of eight system designations:

  • Part-Time Non Synchro System
  • Part-Time Synchro System
  • Full-Time Fixed Torque System
  • Full-Time Variable Torque Passive System
  • Full-Time Variable Torque Active System
  • On-Demand Synchro Variable Torque Passive System
  • On-Demand Synchro Variable Torque Active System
  • On-Demand Independently Powered Variable Torque Active System

Boy that sounds familiar. Whats funny is I came to this conclusion seperatly.

Of course the difference is I say there is no AWD and they say there is no 4WD. Tomato/potatoh. The moral of the story is that they refuse to play the “4WD vs AWD” game.


Good Job SAE.

p.s. if any of you engineering types have access to the SAE site and want to slip me a copy of that report I would LOVE to read it. You can reach me at

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