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4WD and AWD systems explained

greg116greg116 Posts: 116
edited February 9 in General
Well, its kind of a blurry line between the two, but heres the gist:

AWD is generally full-time four-wheel drive, e.g. Subaru, Jeep Grand Cherokee, Audi Quattro. All the wheels are providing power all the time, and can be used on any road surface.

4WD is basically a truck system, it locks differentials so all the wheels are spinning at the same rate i.e. Silverado, Ram, F-150. If you use this system on paved roads, you will damage your driveline.

There's all manner of "multi-wheel-drive" vehicles out there, all with their own unique system. The cheaper AWD vehicles actually only provide power for one side of the vehicle, while the more advanced systems actually do power all four wheels.
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Comments

  • drew_drew_ Posts: 3,382
    it's a lot more complicated than that. I blame it on the manufacturers and magazines who use the terms interchangably. There are variations on a theme, of course, but this is basically what it boils down to:

    4WD = Low range and High Range gearing
    AWD = Only high range gearing

    Part-time 4WD: No centre differential. Cannot be used on dry/wet, semi-slippery roads due to the lack of the centre differential. When activated, both front and rear axles are physically locked to each other and have to spin at the same rate. This becomes a problem when turning on sufficiently high friction surfaces. Examples: Suzuki SUVs, most 4WD pickup trucks, cheaper SUVs.

    Permanent 4WD:. No two wheel drive mode. System is equipped with a centre differential, and hence is safe to use on all surfaces. All four wheels are powered all of the time (usually 50/50 front and rear axles). This is arguably the best system since the torque split ratio does not change and is the most predictable. All wheels "help out" all of the time and this stabilises the vehicle + improves handling. With the extra two drive wheels, the vehicle has twice the amount of traction all of the time (even in no-slip conditions) vs. a 2WD vehicle. Examples: MB M-class SUV, the Range/Land Rovers.

    Full-time 4WD: Basically permanent 4WD but with a 2WD mode. This was born out of customer demand (for a 2WD mode). Examples: Toyota Sequoia, Mitsubishi Montero.

    Permanent AWD: Basically permanent 4WD but without low range gearing. Examples include the Audi Quattro AWD system, the MB's 4-matic AWD system, Subaru's manual transmission AWD system.

    Full-time AWD: System is active at all times, however in most cases, the one set of wheels (usually the rears) only receive 5-10% of the engine's power unless slippage occurs. At that point, power is progressively transfered to the opposite axle to help out. Some systems can transfer power to the rear upon acceleration to improve traction. However, they revert to 2WD mode when coasting.

    Hope this clarifies things for everyone!

    Drew
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  • nanuqnanuq Posts: 765
    Take a look here, it basically says the same thing Drew said, with pretty pictures.


      http://best4x4.landrover.com/index_choose.jsp

  • tonychrystonychrys Posts: 1,310
    The statement "Jeep Grand Cherokee... All the wheels are providing power all the time..." is not true unless the JGC is equipped with the Selec-trac system as opposed to the more popular Quadra-trac system.

    TC
  • The X5's have full time AWD with 65% of the engines power going to the rear and the balance to the front.
  • drew_drew_ Posts: 3,382
    No, the X5 (and the BMW 3-series AWD) has permanent AWD. Power is sent to all four wheels regardless of slippage or no slippage. However, because of the rear wheel drive bias (68% of the power is sent to the rear, and 32% to the front wheels), this makes the vehicle a lot easier to oversteer especially in slippery conditions, when compared to an AWD/4WD vehicle with a neutral 50/50 torque split ratio. BMW configured their RWD biased ratio on purpose because they wanted to give their AWD vehicles a more traditional RWD feeling, like their RWD cars.

    Drew
    Host
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  • drew_drew_ Posts: 3,382
    Thanks for the link. Good to know that I explained things consistent with Land Rover!
  • To our host: given your knowledge of drive systems, where would you categorize the new GM AWD systems in the Escalade and Denali's. They are labeled as AWD and have no low range. I know they are rear wheel biased with a limited slip rear differential per the company literature. Your comments on where it would fall in your earlier categories would be appreciated.
  • drew_drew_ Posts: 3,382
    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
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  • 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.
  • 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.
  • drew_drew_ Posts: 3,382
    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.
  • drew_drew_ Posts: 3,382
    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
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  • gpm5gpm5 Posts: 785
    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.
  • tincup47tincup47 Posts: 1,508
    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.
  • drew_drew_ Posts: 3,382
    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.
  • drew_drew_ Posts: 3,382
    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
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  • chevy4mechevy4me Posts: 203
    Is it full time autotrac?
  • drew_drew_ Posts: 3,382
    GM's AWD system in the C3 pickup is permanent AWD. GM's AutoTrac system is full-time part-time AWD. Read my previous post on this system: drew_ Feb 7, 2001 6:25pm


    Drew
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  • What do you think is better for wet and slippery roads, a FWD with traction control or a permanent AWD like MDX (if you couldn't get a 4WD with traction control)?
  • I am looking at a ford explorer (1998) with traction control and four wheel drive or a Mercury Mountoneer(2000) with all wheel drive which is better.
  • drew_drew_ Posts: 3,382
    Hmm, I don't recall the RWD Explorer being available with traction control. In any case, your vehicle will have more traction with AWD, so I would go for that.
  • drew_drew_ Posts: 3,382
    FYI, the Acura MDX has full-time AWD, not permament AWD. The reason is that it is usually a FWD (2WD) vehicle unless you're accelerating (but only up to a certain speed), if slippage is detected, or if the VTM-4 lock button is pressed (up to 18mph, only in 1st, 2nd or Reverse gears). The MDX's AWD system is not always proactive, but rather, reactive.

    Traction control only makes use of whatever traction is available at the front wheels. If those two wheels simply do not have enough grip to pull the vehicle, you will still go nowhere. With AWD on the other hand, you will have an extra set of 2 drive wheels, and hence twice the amount of available traction. Permanent AWD/4WD (ex. Audi Quattro, MB 4-matic) is still the best, but full-time AWD is better than FWD with TRACS. Which vehicles are you looking at specifically?

    Hope this helps!

    Drew
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  • wmquanwmquan Posts: 1,817
    FYI, the Acura MDX has full-time AWD, not permament AWD. The reason is that it is usually a FWD (2WD) vehicle unless you're accelerating (but only up to a certain speed), or if slippage is detected. The MDX's AWD system is not always proactive, but rather, reactive.

    Besides engaging the rear wheels during acceleration or when slippage is detected, it will also engage the rear wheels if the manual VTM-4 switch is pushed by the driver. This is only for low speeds and it will progressively disengage as the vehicle approaches 18mph. Thanks.
  • drew_drew_ Posts: 3,382
    Thanks for the addendum, William. I had already added the "VTM-4 button" to my post above. Guess you must have caught it during my editing. The VTM-4 lock is one of the more unique features of the MDX's AWD which sets it apart from many other AWD systems on the market; though the Mazda Tribute/Ford Escape has something somewhat similar with their RBC AWD system, but usable at all speeds on slippery pavement.
  • wmquanwmquan Posts: 1,817
    Drew, since this is a good board for this type of question, I've been told by (surprise!) Audi fans that their Quattro system is "better" than the permanent AWD system in the new BMW 3-series, and the 4Matic in MB's.

    Does Quattro have a number of "enhancements" beyond 4Matic and the BMW system? I know that BMW's split is not as optimal as you'd like, but I'm not sure if Quattro is really "better" than 4Matic (other than the fact that you can buy it in an under $30k vehicle in the form of the current A4 -- in fact, are there many otherPermanent/Full 4WD or Permanent AWD systems under $30k?).

    And any other vehicles out there that have the Audi Allroad's pneumatic suspension system for under $60k?
  • wmquanwmquan Posts: 1,817
    And where does VW's 4Motion system fit into as far as these classifications go?

    Thanks.
  • Host: Is GM's awd in the 2001 GMC Denali's and 2002 Cadillac Escalade's considered permanent or full-time? Their literature says permanent but I am curious how you would categorize based on your definitions in this forum. Also if you do categorize them as permanent, what other SUVs are available in the US with permanent awd?
  • drew_drew_ Posts: 3,382
    The Quattro system (but only with the Torsen - Torque Sensing - diffs) is superior...kind of, sort of, to the BMW/MB systems. The reason for this is that it is almost purely mechanical, and can react almost instantaneously to even a small amount of tire slip.

    As you know, I think, Audi's Quattro is split 50/50. Audi has also started using using traction control, but only to distribute the torque side to side. They used to have 3 Torsen diffies before (front/centre/rear) and that was complex and pricey...which brings us to the next point. The problem with the Torsen based Quattro system is its high cost, as well as fairly high weight. As such, Audi has started using a Haldex clutch based AWD system for their TT and the A3 (not sold in N.America) partly because of this. This system is actually more reactive than proactive, but it's very quick and hence that makes up for some of the differences between it and the Torsen system.

    The Passat's 4-motion system uses the A4's Torsen based Quattro system since it is basically a lengthened, but de-contented version of the A4. It shares all of the benefits, but also the disadvantages (the acceleration times are not very quick).

    Subaru's manual transmission AWD vehicles and the new H6 VDC Outback wagon have permanent AWD (50/50 split and 45/55 split for the VDC OB). Subaru's auto tranny vehicles (excluding the VDC OB) have full-time AWD systems which are partly proactive like the MDX's system. Power is transfered to the rear upon acceleration, as well as in 1st, 2nd, and reverse gears. The Toyota Highlander and the RAV4 both also have a 50/50 split in no-slip conditions. I suspect that the new Toyota Matrix hybrid small SUV will have the same system.

    FWIW, a torque split ratio of about 50/50 produces very neutral handling, but some consider it boring. This is why BMW and MB (for the cars) have reverted to a RWD biased AWD torque split ratio. To emulate some of the RWD feel of their RWD vehicles. Frankly though, I really don't think that MB 4-matic owners care too much about that 2WD feel. I certainly don't.

    As for the suspension system, no, not that I can think of in N.America anyway. The Lexus LX470 and Range Rover both have pneumatic air suspension systems (R.R has had it for decades), as does the '00+ MB S-class.


    Drew
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  • Drew/William: thanks for the clarification (I went and re-read post #2 - duh!).
    I was looking at MDX vs. Highlander FWD with traction control or Highlander with AWD (I understand the Highlander AWD is permanent unlike MDX). No one has the Highlander AWD with traction. This is one of the factors I am considering between these two. I likely won't be driving in snow (unless I take it north for the holidays) but we have lots of hard rain with slick roads in FL.
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