4WD and AWD systems explained
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.
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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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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http://best4x4.landrover.com/index_choose.jsp
TC
Drew
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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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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.
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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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.
Drew
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Drew
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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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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.
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?
Thanks.
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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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.
Drew
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As far as mechanical reliability, which is sometimes a function of complexity, is today's Quattro any more difficult / expensive / problem-prone than other systems?
And is it issues (cost?) in integrating automatic transmissions that causes Subaru to not have permanent AWD in their automatic tranny vehicles, except for the H6 VDC?
To see a major reason why VSC is beneficial, watch this video clip. FYI, skids usually occur before rollovers, so if you can prevent a skid from occuring and going out of control, the probability of rolling over will be significantly reduced as well.
Drew
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MB's and BMW's AWD systems are actually relatively simple compared to Audi's system (hence the significant cost and weight savings). As you know, much of the work is electronically controlled. I would think that they would be cheaper to maintain in the long run. Then again, Audi has been in the AWD business for many years as well.
VW did initially want a different (called Syncro) AWD system for the Passat. However, it wouldn't be very cost effective to design a whole new system when the sister company has a (proven) system right there and then. FYI, Skoda and Seat (also VW owned, I think) use the Torsen & Haldex Quattro systems as well.
I'm currently watching a segment on the restyled VW Passat and I have to admit that it looks pretty good. Those new HID Xenon headlamps are impressive, and no doubt borrowed from the Audi division. Like just about all factory Xenon headlamps, lighting performance is improved by 250%. Now, if only the standard warranty was longer than 2 years...the price is also pretty close to a basic ML320. Wow...the new C230K coupe is also being covered in this MotorVision segment.
Drew
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As I mentioned, perhaps Acura will decide to add VSA in future versions of the MDX.
By the way, you said with authority that the next-gen M-class won't arrive until 2004 (2005 model year). Any news on when the MB 4Matic minivans will be on the scene?
The next generation V-class (if they decide to carry over the name) based on the W164 next generation M-class platform is supposed to arrive - based on what I've read (unconfirmed officially with MB, of course)- when baby Justin is about 10 years old...in other words 2009.
What does the above mean Drew? can it or can it NOT be used on dry conditions where slippage could occur, i.e. dry with occasional ice spots for example?
How reactive is the auto mode. Is it reactive under acceleration or only from a stop. Any help here would be appreciated. Thanks.
Hope this helps!
Drew
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The GM Autotrac equipped vehicles are basically rear wheel drive rigs. When the rear wheels slip, the AutoTrac activates an electronic control module, which uses a wet clutch pack, activated by an electric stepper motor, to send torque to the front wheels that still have traction. It is a "4WD on demand" system (or auto 4WD system), activating 4WD when needed. It has part-time 4WD and 2WD options, as well as a low range in the T-case.
The Isuzu Trooper's TOD 4WD system is considered full-time 4WD. It is similar to the Acura MDX's VTM-4 full-time AWD system, except that it is RWD biased, instead of the FWD bias of the MDX's system. Heck, the Isuzu TOD system is even made by the same company as the MDX's VTM-4 system...Borg Warner. It is considered 4WD and not AWD because it has low range gearing; AWD does not.
Drew
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AWD with the skid control/traction or AWD with the limited slip differential? Can't have it both ways.
Also, on the AWD version does the TC only control the front tires or all? Obviously the 2WD version only controls the front. Thanks.
drew_ Feb 11, 2001 7:06pm
drew_ Feb 11, 2001 8:00pm
VSC incorporates 4 wheel traction control, which is a component of stability control. Watch the video clip in my 2nd message (#35) linked above. Since 4 wheel TRACS simulates a locking differentials for the front and rear axles, a mechanical rear LSD would be redundant.
Drew
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Thanks