Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!





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.
«13456772

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
    Host
    Vans, SUVs, and Aftermarket & Accessories message boards
  • 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
    Vans, SUVs, and Aftermarket and Accessories message boards
  • 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
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • 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
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • 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
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • 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
    Host
    Vans, SUVs, and Aftermarket and Accessories message boards
  • 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)?
«13456772
Sign In or Register to comment.