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© 1996-2011 Harald Pietschmann
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My comment relates to the way (automatic) "all wheel drive" vehicles (usually minivans - some SUVs like Lexus RX300 or Ford Explorer V8) distribute torque to the wheels and how they manage traction. Automatic AWD is different than "true" AWD (see below).
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why I am using the term torque rather than power
Part time 4WD vehicles always distribute torque inside their transfer case 50/50 to front and rear when the shifter is in one of the 4WD positions (Hi or Lo). Most of the time each of the 4 wheels gets 25% of the torque - that minimizes wheel spin. This system is very strong and reliable - unfortunately it can't be used on pavement. Should one of the wheels or one axle lose traction - the other axle will still receive a reliable supply of torque (up to 100%) through the transfer case.
Conventional full time 4WD vehicles use a differential inside the transfer case to make 4WD use on pavement possible. This is done with a ring and pinion set or a planetary gear set. Each of the 4 wheels constantly gets 25% of the torque (as long as the ground is level and traction is equal) - that in itself prevents wheel spin. This setup is also very strong and reliable. However, this system would be handicapped for off-road use (it is much more likely to lose traction off-road than on-road) if it would not have some sort of spin control. Call it traction control, if you will. If one of the tires or one axle would lose traction - the differential in the transfer case (often called "center differential") would allow the axle with lost traction to spin (could be both tires or one). To avoid this some of the full time 4WD vehicles offer a manual center differential lock (all LandRover, Mercedes G etc.) or other means of (automatic) spin control. A mechanically locked center diff acts like a part time transfer case - and is as strong and reliable as a part time system off-road. Some vehicles do not manage or lock any of the differentials. Instead they have systems that slows down spinning wheels.
Now, all wheel drive (AWD) is a completely different "beast". In my opinion it is useless beyond pavement because it does not have low range like the other two systems. Low range provides necessary crawl speed for maximum control during difficult driving situations. And low range range provides a significant torque multiplication.Low gears in a 4WD are like the low gears in a mountain bike. Can you imagine a mountain bike without low gears? Main drawback of AWD is that the transfer case can't be manually locked. Two different systems are in use - neither one works well for use beyond pavement:
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AWD System one (true all wheel drive - or full time AWD) has a conventional differential inside the transfer case - each of the wheels gets about 25% of the torque as long as traction is equal. However, the center diff cannot be mechanically locked.
To prevent a complete loss of traction when one wheel or one axle would spin, a viscous coupling or a similar device like a Haldex coupling (see note) will try to "glue" both driveshafts together to keep enough torque flowing to the axle with traction. Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the connecting device has to kick in very infrequently. Off-road or in other situations with slow speed and high demand for torque the glue box (viscous coupling or Haldex etc.) is overstressed and fails to deliver the needed torque. High torque transfers and continous use make especially viscous couplings fail. Haldex units are much more reliable but cannot satisfy the constant high demand for torque at all wheels either.
AWD System two (automatic AWD - and in a way actually only a sophisticated 2WD system) might not have a differential in the transfer case (Volvo, Honda, Lexus, etc.) but some do (Jeep Grand Cherokee). Primary power goes only to one axle (makes spinning tires much more likely due to inefficient use of traction - as likely as in any other 2WD car). However, both drive shafts are joined by a viscous coupling or a similar device (see note) and as long as all 4 wheels turn at the same speeds the control unit remains inactive. Once the powered axle or one of the powered tires loses traction, the powered drive shaft rotates faster than the one that is just rotating along. The control unit reacts to the speed difference and kinda glues both drive shafts together. This way the previously unpowered shaft will get some of the torque and rescue the failing tires. Same story as in system one: Works kinda OK on slippery pavement when the vehicle has already sufficient momentum and the control unit has to kick in very infrequently. Fails miserably when need of high torque arises or when activated frequently. Cannot satisfy the constant high demand for torque at all wheels when off-road.
Now you may understand better why I think AWD is not suitable for use beyond pavement.
The last generation Jeep Grand Cherokees (1999 WJ and 2005 KJ) with QuadraDrive II are not to be confused with a viscous coupling system. The J GC also powers only one axle (rear) but the Gerodisc technology and E-Gerodisc used by Jeep is very strong, very reliable and provides sufficient amounts of torque to where it's needed.
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Note:
The various temporary connecting devices like viscous coupling, Haldex or Gerodisc perform different duties in full time symmetric AWD and automatic asymmetric AWD:
In full time symmetric AWD they perform traction control duty (ie keeping the rear axle from slipping - longitudinal, or front to rear, control) - in automatic asymmetric AWD they are on duty to create temporary AWD.
To make things truly confusing, these devices can also be used (inside the differentials) to keep wheels from slipping - transverse, or side to side, control.
It gets worse. The brakes can also be used for transverse traction control.
An honorable exception from the rule "AWD is almost useless beyond pavement" are VW Syncro Vangon. Yes they have viscous couplings to distribute torque front/rear - but they have a granny low first gear and they have axle diff locks (real diff locks!). So, they are much more capable than most folks think. |
Traction use of a 2WD - Traction use of a 4WD
Traction management in a turn with 2WD - Traction management in a turn with 4 WD
Need for more torque and traction management in a 2WD vs 4WD
| 4WD layouts | crawl ratio | 4WD components |
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