Edmunds.com | Inside Line | Auto Observer Your Account | Help | Directory
You are here:
![[F]](http://a332.g.akamai.net/f/332/936/12h/www.edmunds.com/media/townhall/webxicons/site.gif){kind=small}
Forums
SUVs 4WD & AWD systems explained
1377 messages, Last post on Dec 05, 2009 at 1:47 PM
You are in the SUVs Forum. Your Hosts are steve_ & tidester
|
|
|---|---|
| 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. | |
|
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. |
|
|
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
|
|
| 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? | |
|
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. |
|
| 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. | |
| 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. | |
|
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 |
|
| Is it full time autotrac? | |
New? Join Now!
Forum Tools
Search Forums
Browse by Vehicle
2010 Chevrolet Suburban
2011 Jeep Grand Cherokee
2010 Subaru Outback
2010 Ford Explorer
2010 Nissan Pathfinder
2009 GMC Envoy
Browse by Board
Browse by Topic
Today's Chats
