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Jack

I will check but I think both FWD and Nash were full time four wheel drive back in WW1. Aren't the WW2 (if not forever) FWD trucks full time four wheel drive?


Well, we had to have some rules somewhere! This puts the kybosh on motoring writers flights of fancy about different systems - they are all "All Wheel Drive" - this also includes one we have not mentioned IWD (Individual Wheel Drive ie hydraulic or electric motors on each wheel such as Tesla)

SAE Recommended Practices
Per the SAE International standard J1952, AWD is the preferred term for all the systems described. The standard subdivides AWD systems into three categories

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 a 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
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