Topic: Auto Tech: All-Wheel Drive

[Autos_Editor]

Technical Editor Jim Kerr explains the differences between all-wheel drive and four-wheel drive.

Read More...

[sailor723]

Not a bad intro but where is the rest of the article? Come on, less than page in total?  Awfully brief and way too basic discussion of a pretty complex topic. I would have least expected some discussion of Haldex vs Torsen systems...pretty much a waste of (very little) space.

[initial_D]

^^

Is there a difference between 4WD and AWD on today's modern techno-filled & computer control systems?

[nlm]

The related posts links provide answers to your questions. Rather than specifically identify a awd system like Haldex, Kerr explains awd applications specific to a manufacturer or a vehicle. Most use a haldex system or a variation thereof (being the slip and grip type). Not much to explain here: if front slips, send power to rear for grip.

Torsen is not a awd system but a brand (which has become the label for their mechanical method) of differential.

Yes there is a difference between 4wd and AWD today. This is a good summary from an earlier article he wrote:
"Many pickup trucks have automatic mode in their four-wheel drive systems. Automatic mode uses a computer to engage 4WD when there is a speed difference between the front and rear drive axles. When the drive axles are turning at the same rate, the system automatically changes to rear-wheel drive only. This system automatically switches between 2WD and 4WD and has many advantages of both, but tires do have to be even in tire tread depth."

Auto 4WD is still different and distinct from any AWD system.

[X-Traction]

In corners, 4WD systems can get out of control easily if you are on hard surfaces or ice, because one axle has to slip to allow the turn. AWD systems allow the front tires to turn at different speeds from the rear tires, so traction is maintained and vehicle handling is stable.

It's important to point this out.  However, there may be more to it.  To be effective, AWD systems have to have some degree of resistance to the front and back axles turning at different rates.  In the most slippery conditions, this can act the same as a "locked" 4WD system on less slippery conditions.  The result is some wheels "binding" with respect to the road surface, which contributes to loss of control.

There is another danger in many AWD vehicles.   Many of them have an option to lock the transfer case.  Many drivers of these vehicles assume that "locked" mode is exactly what they need for ice and snow or loose gravel.  In fact, once the locked mode is engaged, what they have is precisely the same as an ordinary 4WD setup.  That's why owners are advised not to use the locked mode on dry pavement. But they are not advised NOT to use the locked mode in slippery conditions.  In low-traction conditions, using the locked mode will cause loss of control just as it would in a 4WD.  The danger is greatest while cornering, and while cornering fast enough to generate g-forces. I would expect that if crashed suv's were checked for this, it would be found that many of them crash in slippery conditions due to mistaken use of the locked mode.

Something relevant that is never addressed in promotion or descriptions of AWD, is how the "slip & grip" systems decide when to revert to fwd.

People comparing AWD vehicles should also consider weight distribution, since a vehicle with an even distribution of weight between all four wheels will have a performance advantage even before the contributions of these systems are added.

[blai4754]

You mention Subaru but....

where's Mitsubishi?

[nlm]

...AWD systems have to have some degree of resistance to the front and back axles turning at different rates.  In the most slippery conditions, this can act the same as a "locked" 4WD system on less slippery conditions.  The result is some wheels "binding" with respect to the road surface, which contributes to loss of control.

There is another danger in many AWD vehicles.   Many of them have an option to lock the transfer case.  Many drivers of these vehicles assume that "locked" mode is exactly what they need for ice and snow or loose gravel.  In fact, once the locked mode is engaged, what they have is precisely the same as an ordinary 4WD setup.  That's why owners are advised not to use the locked mode on dry pavement. But they are not advised NOT to use the locked mode in slippery conditions.  In low-traction conditions, using the locked mode will cause loss of control just as it would in a 4WD.  The danger is greatest while cornering, and while cornering fast enough to generate g-forces. I would expect that if crashed suv's were checked for this, it would be found that many of them crash in slippery conditions due to mistaken use of the locked mode.

I think you are confusing 4wd systems and a transfer case with AWD systems and a centre diff. There are not many AWD systems that have an option to lock the transfer case centre diff 50:50. Of those that do, at least one does not actually lock the centre diff, it just increases rear torque distribution. The 'lock' in this case is mislabelled. Also, the open front and rear diffs will allow wheel slippage to one wheel at the front and one wheel at the back so there is no binding. This is good for cornering but bad for when you are stuck in the snow, because that one wheel on each axle that slips will get 100% of the power and the car will not go anywhere without outside intervention.

[johngenx]

More AWD systems now use the traction control system to simulate a limited slip differential by slightly braking the spinning wheel, allowing power to be transferred to the one with traction.  It works.  I prefer a mechanical/hydraulic LSD, at least in the rear, but the computer run systems are pretty effective.

There seems to be three significant systems on the market.  Traditional 4X4 systems like Jeep uses in the Wrangler, full-time AWD systems like Subaru (and other makes too...) and slip/grip part-time AWD systems that are basically FWD drivelines that can send power to the rear under some circumstances.

The problem with traditional 4X4 systems is that they need the operator to know how and when to use them.  They operate in RWD mode most of the time, and users need to understand how RWD handles differently than FWD.  This is why you see Jeeps and pick-ups and truck-based SUVs in the ditch on snowy days.  People think they're invincible, and they often can't use the system properly.

Full-time AWD, in my opinion, is superior to the slip/grip systems.  I was on a ski trip recently with a friend of mine that has a Highlander Hybrid.  It's a gorgeous vehicle that has tons of goodness, but the AWD system feels terrible compared to my Subaru.  We did get up a steep access road that had stymied all the FWD vehicles (the Highlander is on Blizzaks) but the system didn't feel anywhere near as confidence inspiring as my Forester.  The main difference was in cornering, and the switchback mountain road gave me plenty of opportunity to gauge the difference.

What about on dry roads?  Well, I again prefer the full-time AWD systems to the part-time.  The addition of power to the rear tires makes for a much more balanced feel and less understeer.

All that said, no matter what system you have, 4X4, RWD, FWD, AWD, it's usually all about tires.

[tpl]

I have had two true awd cars,
1  A4 with a Torsen centre diff. This car was set up to be 50/50 f/r and worked very well in all conditions.
2 BMW 330 xi PRE X-drive.   Like a AWD 911, always drove all 4 wheels but had fixed 38% front and 62% rear.  IMHO this worked better than the A4 in snow and about the same on ice.

Both cars with good winter tires all around. 

[Car Guy]

Ive driven many of the awd systems that are out there, and by far (really, really far) you want to have an awd system that is full time.  Slip and grip is junk.  You need a system that is always pushing power to all 4 corners.  Myself, I prefer a system that is rear biased and can put full power to front or rear axle with torque vectoring for side to side.  I currently own a 2007 328xi Coupe.  I like the rear bias and the seemless way that the system works.  You never feel it, but you know it is there.  With a slip and grip, by the time you slip, its too late!

[nlm]

...Myself, I prefer a system that is rear biased and can put full power to front or rear axle with torque vectoring for side to side....

That's my preferred set-up too. That leaves the Mitsu Evo, Mitsu Outlander and Acura TL. While the Evo is great it has a lot of compromises as a daily driver. The TL also has the benefit of a manual but it can be pricey and it's not that easy on the eyes.

I think some of the new Audi quattro torque vectors too, although it's not clear to me whether it's a proper setup via power increase to one wheel or a pseudo setup via abs braking to one wheel.

[Sir Osis of Liver]

The 3.6R gets a pretty good system too:

The 2013 Legacy 3.6R exclusively features the Variable Torque Distribution (VTD) AWD system with a 5-speed electronic automatic transmission. A planetary center differential works with an electronically controlled continuously variable hydraulic transfer clutch to manage power distribution. The VTD system normally sends more power to the rear wheels (45:55) to enhance handling agility, and it can continuously adjust power distribution in response to driving and road conditions.

[nlm]

The 3.6R gets a pretty good system too:
The 2013 Legacy 3.6R exclusively features the Variable Torque Distribution (VTD) AWD system with a 5-speed electronic automatic transmission. A planetary center differential works with an electronically controlled continuously variable hydraulic transfer clutch to manage power distribution. The VTD system normally sends more power to the rear wheels (45:55) to enhance handling agility, and it can continuously adjust power distribution in response to driving and road conditions.

Unless I read it wrong, that's not side-to-side torque vectoring though? Not meaning to imply its a bad system b/c of its absence, just clarifying is all.

[Sir Osis of Liver]

The 3.6R gets a pretty good system too:
The 2013 Legacy 3.6R exclusively features the Variable Torque Distribution (VTD) AWD system with a 5-speed electronic automatic transmission. A planetary center differential works with an electronically controlled continuously variable hydraulic transfer clutch to manage power distribution. The VTD system normally sends more power to the rear wheels (45:55) to enhance handling agility, and it can continuously adjust power distribution in response to driving and road conditions.

Unless I read it wrong, that's not side-to-side torque vectoring though? Not meaning to imply its a bad system b/c of its absence, just clarifying is all.

Yup, it doesn't do side to side torque vectoring as far as I know. A Torsen LSD comes with the Limited package, but that's about it.

[Fobroader]

Wouldnt a subaru AWD with some LSD in the front or rear take care of all that vectoring stuff??? I know a lot of the Impreza rally and road race cars use them, makes sense.

[nlm]

Wouldnt a subaru AWD with some LSD in the front or rear take care of all that vectoring stuff??? I know a lot of the Impreza rally and road race cars use them, makes sense.

LSDs are not torque vectoring. They decrease the amount of power going to a slipping wheel.

Proper torque vectoring is active application of power to a wheel. The benefits of the Evo's S-AWC vs STi's DCCD w/LSDs front and rear show how much more capable torque vectoring can be for handling.