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4WD & AWD systems explained

1377 messages,  Last post on Dec 05, 2009 at 1:47 PM

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#9 of 1377 Hi Pete by drew_ Feb 07, 2001 (6:25 pm)
I would catagorise it as permanent AWD. Good choice since the system is proactive (vs reactive) and gives you additional traction in all conditions (doesn't need slippage to activate). I believe it has a torque split of 35/65 front/aft.   GM's 4WD system (called AutoTrac, I believe) would sort of be like a full-time part-time 4WD system. It's full-time in that it has an "Auto", 2WD, 4WD High, and 4WD Low mode, as well as the fact that in "Auto" mode, it is active at all times and monitoring for slippage. It's also part-time because it doesn't have a centre differential and hence cannot be used continuously on dry pavement (hence the reason why the auto mode only sends power to the opposite axle when slippage occurs). To simplify things, I refer to these systems as "Auto 4WD systems". Ford's Control Trac 4WD is another example of an auto 4WD system.   Hope I didn't confuse you! I was starting to get a little bit mixed up myself Drew Host Vans, SUVs, and Aftermarket and Accessories message boards
#10 of 1377 Drew, by bernard1 Feb 07, 2001 (7:53 pm)
While I think we may be spliting hairs, according to BMW the X5 has full time AWD w/automatic differential breaking. All wheels will not get power if one or two are slipping. Similar to the MBZ system of braking the slipping wheel and will reduce excess engine power until the traction is restored. You are correct on the 68/32 split. I would also term Porsche's AWD for their carrera as full time AWD even though it has a varible torque transfer of 5 to 40% power to the front based upon the amount of slip. While some may consider these systems permanent, BMW, Porsche and Lexus for their LX, refer to their systems as Full Time.
#11 of 1377 Subaru all wheel drive by wrxanswers Feb 07, 2001 (8:06 pm)
Alot of what was said is true, however on a Subaru made from 97 on (and an automatic) is 90% front wheel drive and 10% rear wheel drive. Not only will it transfer power upon slippage but also will transfer power on acceleration, or even going around a turn. 2000 models offer that plus an option of transferring power from side to side in the rear wheels. On the 2001 Outback VDC it will also go from corner to corner.
#12 of 1377 benard1 by drew_ Feb 08, 2001 (12:14 am)
As I mentioned, part of the reason for the confusion is the way that the manufacturers use these terms interchangably. The reason I wrote the above was to cut through their lingo and to standardise everything.   The BMW X5 has a permanent AWD system. It's not really considered "full-time" since full-time systems general transfer power only after slippage occurs. Permanent systems have fixed torque split ratios.   I agree. I too would call the Porsche AWD system a "full-time" AWD system since the front axle gets a variable % of torque.
#13 of 1377 wrxanswers by drew_ Feb 08, 2001 (12:20 am)
Subaru uses several AWD systems. The VTD AWD system in the new H6 VDC wagon is permanent AWD due to its 45/55 split. The rest of the Subaru lineup with automatic transmission gets full-time AWD. Yes, the system is proactive in certain set conditions. It can also be "locked", so to speak, in 50/50 mode with the automatic transmission lever shifted to the 1st or 2nd gear positions. The manual transmission equipped Subarus have permanent AWD with a 50/50 split (even in non-slip conditions) via the viscous coupling centre differential. The system can transfer power back and forth (due to the viscous coupling diffy), so it's not a clear cut permanent system.   The rear limited slip differential doesn't exactly transfer power side to side in the rear wheels, but it does make the rear wheels spin at approximately the same rate (using clutch packs)...up to a certain limit.   Drew Host Vans, SUVs, and Aftermarket and Accessories message boards Replies to this message: bend (Feb 18, 2005 10:57 am) russla (Feb 18, 2005 11:08 am)
#14 of 1377 GMs Auto 4WD by gpm5 Feb 08, 2001 (7:43 am)
In this mode full-time/part-time where the system reacts to slippage, does anyone know if there is damage caused by using it on dry conditions with an occasional slippery condition. Also, does it transfer on acceleration, lets say on accelerating from 30 mph, when slippage occurs and does anyone know how quickly it reacts?
#15 of 1377 Drew by bernard1 Feb 08, 2001 (5:03 pm)
The engineer in me goes with full-time/the desire to standardize terms takes you to permanent. For our purposes essentually the same. Good information. I did think that in general AWD's transfered power after slipage and if any system waited until slippage occured before transfering power it was not considered full time/permanent only AWD. And if a system has a constant front/rear torque split without slippage than the system was running the torque spilt full time as in 24/7, or permanently.
#16 of 1377 Torque Split by tincup47 Feb 09, 2001 (8:57 am)
the only way you get a permanent 50-50 torque split with no slippage is in a fully locked system. This is a part time system because you need slip on dry pavement or your system will experience damaging windup as your front and rear axles try to spin at different speeds as the wheels track on different arcs.
#17 of 1377 benard1 by drew_ Feb 09, 2001 (7:49 pm)
Yup, I think we're on the same level. As I mentioned, everything is not clearcut due to the huge variance of systems out there on the market place. The Honda CR-V's Realtime 4WD system, for example (another misnomer, IMHO), is almost like a full-time, part-time system. Zero power goes to the rear wheels unless front wheel slippage is detected. In contrast, the Chrysler AWD minivan's full-time AWD system (as well as Volvo's AWD system) sends about 10% of the rear wheels even in no slip conditions. Although the rear wheels receive some power (this is mostly so that the torque transfer delay to the rear wheels is reduced when the front wheels do slip significantly), it is not enough to consider the system as permament AWD.
#18 of 1377 tincup47 by drew_ Feb 09, 2001 (7:51 pm)
That's not necessarily true. The torque slip ratio can be configured into the transfer case/centre differential. For example, the Audi Quattro's Torsen-based permament AWD system splits the power 50/50 in no-slip conditions, as does Subaru's manual transmission viscous coupling centre differential AWD system.   Drew Host Vans, SUVs, and Aftermarket and Accessories message boards

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