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4WD & AWD systems explained

1376 messages,  Last post on Nov 23, 2009 at 9:11 AM

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What is this discussion about? Chevrolet Suburban, Jeep Grand Cherokee, Subaru Outback, Ford Explorer, Nissan Pathfinder, GMC Envoy, Performance Mods, Tires, Suspension, Transmission, Truck, SUV

#2 of 1376 Actually... by drew_ Feb 05, 2001 (8:24 pm)
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 Replies to this message: one_post_guy (Nov 07, 2004 2:41 pm) nsisk (Jul 09, 2005 8:50 pm)
#3 of 1376 A good link... by nanuq Feb 06, 2001 (7:35 am)
Take a look here, it basically says the same thing Drew said, with pretty pictures.   http://best4x4.landrover.com/index_choose.jsp Replies to this message: ccaywood (Dec 08, 2005 11:30 pm)
#4 of 1376 And another thing... by tonychrys Feb 06, 2001 (7:43 am)
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
#5 of 1376 Actually by bernard1 Feb 06, 2001 (9:47 am)
The X5's have full time AWD with 65% of the engines power going to the rear and the balance to the front.
#6 of 1376 benard1 by drew_ Feb 06, 2001 (9:53 am)
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
#7 of 1376 nanuq by drew_ Feb 06, 2001 (9:58 am)
Thanks for the link. Good to know that I explained things consistent with Land Rover!
#8 of 1376 Denali/Escalade AWD by heatwave3 Feb 07, 2001 (6:30 am)
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.
#9 of 1376 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 1376 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 1376 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.

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