Topic: How Do The Small SUV AWD Systems Really Work?

[Switz]

Hi everyone,

Been a while since I posted.  I am interested in purchasing a smaller SUV (Rav4, Santa Fe, Mazda CX-5 or CX-9, etc.) for the occasional AWD benefit with our winters.  Still driving my FWD '08 Mazda5.  With winter tires it's pretty good 99% of the time. (Having said that then why am I REALLY thinking I need a AWD then?). I guess I'm just jumping on the SUV's-are-practical bandwagon.

Anyways, I was reading in a CX-9 forum that one owner driving in deep snow had a warning "4wd Heavy Load", accompanied by a "hot" smell, and then it went into FWD default mode.  What overheated?  No positive lock-up? Friction clutches slipping constantly? Bad design? What's the point then?  Are all AWD systems created equal?  What specific equipment do I need to make sure it has so that it is reliable?  Do I just only count on a actual 4X4 like a full sized truck to handle the odd rough stuff? If they won't hold up when you really need them then maybe I'll just get another FWD and drive it accordingly?

Thoughts?

[Fobroader]

There are huge differences between proper awd systems and hoakie, half a$$ed "on demand" systems. A proper awd system like on a Subaru, sends power to all the wheels, all the time, you get benefits in all conditions because of this. A SUV that has a center differential lock is also acceptable as it will get you out of deeper stuff because it will be forced to act like it has proper awd. My whole thought is, if I am paying for awd, I want awd, not front wheel drive featuring the rear wheels sometimes.

[Great_Big_Abyss]

Subaru and trucks aside, most AWD systems are 'slip and grip'.  The vehicles are powered through the front wheels 90% of the time.  Once the computer senses that the front wheels are slipping (like when accelerating away from a stoplight or stop sign) then it will direct some of the engine's torque (up to 50%, usually less) to the rear wheels, to help the vehicle accelerate faster.  This process happens within a few milliseconds, usually. 

The problem with such systems is in deep snow.  If the vehicle is stuck, or about to get stuck, that few milliseconds that it takes for the computer to shift power to the rear wheels can make the difference between getting unstuck or getting out and digging/pushing.  This is why Fobroader mentioned the 'center differential Lock'.  I believe vehicles like the Mitsubishi Outlander and Nissan Rogue (among others) have such a device.  When you have this device activated, it will send a certain percentage of torque to the rear wheels (between 30% and 50%) all the time, up to a certain speeds (usually around 40-50kph) at which point it reverts back to normal 'slip and grip' mode.  This permanent split of torque makes an AWD vehicle a lot less likely to get stuck, and a lot more likely to become unstuck when travelling through deep snow (think unplowed residential street, driveway or parking lot), especially at intersections where the windrows and ruts build up.

As mentioned before, Subaru's are different.  Their AWD is on all the time, so power is always being sent to the front and rear wheels.   The downside to this is slightly worse fuel economy, but in recent years even that difference has gotten noticeably smaller.  Subaru's have their own set of problems, however, with decidedly low-rent interiors and questionable engine reliability (although our resident Subie slappie will come on here and tell you that no, the interior on his $40 000 top of the line Forester XT is quite nice, and that the head gasket issues are blown waaay out of proportion).

Once you get into higher end luxury vehicles, as well as sporty cars, things get even more complicated, with AWD being used not only for wintry conditions, but also to improve handling and performance, with such things as torque vectoring. 

EDIT:  I may be wrong.  The Nissan Rogue may not offer a center diff lock anymore.  Certainly a vehicle like the Cherokee would. 

[Switz]

Let me see if I understand things correctly here.  Lets say you're plowing through a mile or two of deep snow and would be spinning wheels if only FWD. The system detects a differential in wheel speed between the driven front wheels and the rears so it locks up the driveshaft going to the rear wheels.  But once locked up, the system shouldn't sense any difference in wheel speed between the front and the back.  Does it then unlock the rear driveshaft only to detect a differential between the front and rear wheels once again and lock it back up? Does it keep unlocking and locking over and over again until you come to rest?  If you have a button to lock it permanently, but go over 40km/h (which I could see happening ... momentum is an asset when plowing through deeper snow) it would just kick out again and cycle through locking up and unlocking.  Is that why the system could heat up and then default to a safe unlocked mode in order to cool down?  This system seems good only for not spinning when leaving a slippery intersection, where the system doesn't have to stay locked up once underway.  This does not seem like a good system for times when you REALLY need AWD.

Or am I COMPLETELY out to lunch on this assessment?

[Fobroader]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

[Robin2]

We upgraded my wife's 2010 Hyundai Santa Fe (FWD) to 2016 Nissan Rogue (AWD) and she loves it. 

On her 2016 Rogue SV, it is primarily fwd mode and kicks in rear wheels when needed. I was concerned as I'm a Subaru guy and have owned one for nearly 20 years.... and will change my 2011 Subaru wrx for 2018 wrx later this summer.

I wondered how well this part-time system works and it surprised me so far.  is it perfect..... nope....  but it's decent to good overall.  wife loves it especially coming from a front wheel drive SUV to AWD.  Her commute to work is mainly city streets to secondary highways / back roads in around Ottawa.  With the awd lock mode, it's good feature as it will lock and distribute the power 50/50 all the time up to 40km/h.  After that, it disengages and goes into regular mode where it's front drive and distributes power to rear when needed.  this lock feature is great for side roads where they're barely plowed and helps her get out without much of an issues especially when there's lots of heavy / deep snow.

With my Subaru (manual) wrx, this 50/50 power distribution is always as such no matter what speed.....  30 or 120km/h.  I prefer it this way. 

For the majority of owners especially around any major city, the part time systems with good winter tires is all they need...  and probably wouldn't see a difference. 

The "smell"....  that was probably the driver boiling the center diff fluid.   that can happen on any awd system if you push it hard enough.

[Great_Big_Abyss]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

Nice video, but quite obviously set up by Subaru, and thus the tests are clearly geared to give Subaru the advantage.  It also appears like they don't use the 'lock' function available on some of the vehicles tested. 

However, Yes, Subaru has the best AWD system in the business.

[tpl]

Prior to the 2018 cars that are just being introduced, Audis with engines mounted longitudinally have always sent power to all wheels all the time. I had a 2000 A4 1.8t that was a 50:50 split. My Q5 is 40:60 f:r. Probably done so it can tow more.
Neither of these had lockable diffs anywhere.

Transverse engined Audis are Haldex.

The 2018 Q5 is what they call in marketing speak, Ultra Quattro,   FWD and claims it can turn on the rear wheels proactively.   I will believe that when I see a road test in deep slush layered over freezing rain!


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[sailor723]

Prior to the 2018 cars that are just being introduced, Audis with engines mounted longitudinally have always sent power to all wheels all the time. I had a 2000 A4 1.8t that was a 50:50 split. My Q5 is 40:60 f:r. Probably done so it can tow more.
Neither of these had lockable diffs anywhere.

Transverse engined Audis are Haldex.

The 2018 Q5 is what they call in marketing speak, Ultra Quattro,   FWD and claims it can turn on the rear wheels proactively.   I will believe that when I see a road test in deep slush layered over freezing rain!


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I too am a little suspicious of the new Ultra Quattro. OTOH I suspect for most drivers it will be good enough 99% of the time.

[lebowski]

I too am a little suspicious of the new Ultra Quattro. OTOH I suspect for most drivers it will be good enough 99% of the time.

+1

So for high-speed driving in the wet/and snow, is Subaru still the best? What about the Quattro system in something like an A6 - would that still be a full-time system?

[Dante]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

Nice video, but quite obviously set up by Subaru, and thus the tests are clearly geared to give Subaru the advantage. It also appears like they don't use the 'lock' function available on some of the vehicles tested. 

+1

They always pick the weakest competitors to show case how good they are. I'd like to see them side-by-side with a proper Audi Quattro, Land Rover, Mitsubishi to name the few but they never do that. The brand wouldn't even matter as what's evaluated is the AWD system not the whole vehicle.

The SUVs featured in the video are known to be some of the weakest AWD systems of that vintage.

[Fobroader]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

Nice video, but quite obviously set up by Subaru, and thus the tests are clearly geared to give Subaru the advantage. It also appears like they don't use the 'lock' function available on some of the vehicles tested. 

+1

They always pick the weakest competitors to show case how good they are. I'd like to see them side-by-side with a proper Audi Quattro, Land Rover, Mitsubishi to name the few but they never do that. The brand wouldn't even matter as what's evaluated is the AWD system not the whole vehicle.

The SUVs featured in the video are known to be some of the weakest AWD systems of that vintage.

Uhhh.....no Audi quattro or Land Rover competes with the Forester.

[Dante]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

Nice video, but quite obviously set up by Subaru, and thus the tests are clearly geared to give Subaru the advantage. It also appears like they don't use the 'lock' function available on some of the vehicles tested. 

+1

They always pick the weakest competitors to show case how good they are. I'd like to see them side-by-side with a proper Audi Quattro, Land Rover, Mitsubishi to name the few but they never do that. The brand wouldn't even matter as what's evaluated is the AWD system not the whole vehicle.

The SUVs featured in the video are known to be some of the weakest AWD systems of that vintage.

Uhhh.....no Audi quattro or Land Rover competes with the Forester.

You forgot to add the emoji to make it clear you are kidding. 

[Switz]

How could you boil the center differential fluid Ribin2?  Unless they are full-time AWD, there is no center differential!  The part-time ones have no center diff, if I understand things correctly.  They just have a center lock-up of some kind.  That's why they are only good on low traction conditions where wheel slippage can occur.  They are no good on dry pavement.  Only full-timers with a center diff will be OK. Or am I off base again?

Now here's the million dollar question: "Which vehicles have a center differential and can be run full-time?

[ArticSteve]

Subaru AWD systems are generally misunderstood and generalized particularly by Subaru owners.  Attached link identifies 2016 model variations.

http://youwheel.com/home/2016/08/07/subaru-awd-system-fully-explained/

[tpl]

How could you boil the center differential fluid Ribin2?  Unless they are full-time AWD, there is no center differential!  The part-time ones have no center diff, if I understand things correctly.  They just have a center lock-up of some kind.  That's why they are only good on low traction conditions where wheel slippage can occur.  They are no good on dry pavement.  Only full-timers with a center diff will be OK. Or am I off base again?

Now here's the million dollar question: "Which vehicles have a center differential and can be run full-time?

Real Audi Quattros ( not the Haldex nor the new ones)  have a Torsen** centre diff which may well heat up its fluid on a rally car but does not need a cooler of any sort in normal conditions.

** I have been told that mine is not a branded Torsen but an Audi version that I imagine ( cynical much?)  saves on licence fees.


The Ur-Quattro rally cars did have all 3 differentials that could be locked up by the driver.

[EV-Light]

You've basically gotten the jist of it. These on demand systems are ok for getting away from a slippery intersection, if you are faced with the mile or two of deep snow....I don't see them coping with that very well on a long term basis. Here is a practical demonstration of "on demand" vs full time awd

https://youtu.be/9cuZYTQLfA0

Nice video, but quite obviously set up by Subaru, and thus the tests are clearly geared to give Subaru the advantage. It also appears like they don't use the 'lock' function available on some of the vehicles tested. 

+1

They always pick the weakest competitors to show case how good they are. I'd like to see them side-by-side with a proper Audi Quattro, Land Rover, Mitsubishi to name the few but they never do that. The brand wouldn't even matter as what's evaluated is the AWD system not the whole vehicle.

The SUVs featured in the video are known to be some of the weakest AWD systems of that vintage.

Audi Quattro is basically FWD slip-grip system now! Gone are the Torsen days...

[tpl]

^^^ Not the pre 2018 Quattro with longitudinal engines. They are still not slip and grip.



No one has mentioned BMW Xdrive. What sort of coupling does that have front to rear?


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[Fobroader]

^^^ Not the pre 2018 Quattro with longitudinal engines. They are still not slip and grip.



No one has mentioned BMW Xdrive. What sort of coupling does that have front to rear?


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Something sketchy that will probably expensively fail .03 kilometres after your warranty is over [emoji14]

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[tpl]

I had a pre-Xdrive 330xi with a tiny transfer box that contained a triple chain drive to the drive shaft that went to the front wheels.   I don't think there was any magic coupling in there, the workshop manual didn't say but it kept on working after warranty....but X-Drive...who know I might agree with you!