[SailDrive]

Over in the Snow thread @lost1750GTV mentioned that AWD doesn't help with stopping in snow. This is certainly the standard wisdom, and also a common mistake drivers make thinking that because you can go doesn't mean you can stop.

I'm going to go ahead and get started and say that Lost is in fact WRONG, and that AWD WILL in fact improve braking and stopping distance in the snow, but I want others to chime in and tell what they believe. Feel free to include anecdotes and personal experiences.

Now I'll be clear I'm starting this thread to create a stir and possibly ruffle some feathers, but I plan to end the discussion with evidence and proof of the true answer to eliminate ALL question on the subject.

 

[Knightofargh]

In theory the extra drag from engine braking on all four corners helps. Empirically I’ve seen it demonstrated that AWD stops shorter in snow. In my experience sipe pattern has a lot more to do with winter performance than anything.

Yokohama Avid Ascends for example are terrible tires. They have a lot of sipes, tiny sipes which clog with even normal road grit in the rain. Meanwhile competent tires like Michelin Pilot Sport AS have deep and wide sipes. The stock (in the USA) Contis are in the middle for sipe design.

 

[karlfevans]

Zero interest in trying to be right

 

[Scalewoodman]

Here's my opinion just from personal experience: AWD engine braking can be helpful in SLOWING the vehicle and maintaining control however when it comes to actually STOPPING the only thing in your favor is tires, ABS, any potential for friction on the surface and prayer. It's simply a matter of physics- specifically momentum and gravity forces heading down hill (or helping going up hill).


AWD like Q4 (being one of the better ones) is helpful in maintaining stability and control at speed in bad weather as well.


Losing the electronic Low Range and electronic slip diff's (X2) from the previous Quadra DRIVE II (using Jeep as the benchmark) was a concern however Stella has proven she is probably 90% or better as good in my experience and I'm fully satisfied because of the other benefits (better mileage, fewer parts, more 'car like' ride, handling bias).


I have used the diff lock on previous SelectTrac Jeeps in horrible conditions on-road to minimize the effect of rutting and frozen slush and improve stability and control. I'm pretty sure the electronics in Q4 are good enough to compensate and had a pretty good experience this winter.


We have a long, steep driveway so I have much experience over the years. With some discipline I am able to keep it cleared off enough but we are fortunate with a Southern exposure so the sun hits it first thing and typically cooks it off enough to give some friction. If the weather involves sleet, crusty icing, packed snow, or even partially exposed asphalt we are good to go in the morning using Hill Descent or at minimum CREEPING with judicial use of the brakes. IF the situation is ice such as freezing rain or frozen over packed snow (I didn't get to it fast enough) we are SOL and not going anywhere.


There is no worse feeling than sliding out of control down a hill in your beloved vehicle and relying on prayer to stop you.


Fortunately me and our pups always walk down 1st thing in the morning to collect the newspaper so I have a pretty good feel for what we are dealing with. The older I get staying home is more of an option!

 

[lost1750GTV]

I will concede that IF you are in a situation so slick that you cannot use your brakes and are instead relying solely on engine braking, and that is then spread between 4 wheels instead of two, it MIGHT work better than the anti-lock brake system NOT installed in the vehicle the empirical data is based on.

 

[WanderingDane]

Here's my opinion just from personal experience: AWD engine braking can be helpful in SLOWING the vehicle and maintaining control however when it comes to actually STOPPING the only things in your favor are tires, ABS, any potential for friction on the surface and prayer. It's simply a matter of physics- specifically momentum and gravity forces heading down hill (or helping going up hill).


AWD like Q4 (being one of the better ones) is helpful in maintaining stability and control at speed in bad weather as well.

I agree with Scalewoodman. By far the biggest factor is tires.

WD

 

[lost1750GTV]

"the biggest factor is tires"
sure is, but then there's a whole ball of wax, depending on average ambient seasonal temperatures, amount and depth of snow, if you have to travel on untreated or unplowed roads or can wait a day etc.
I will happily defer to anyones personal preferences based on their experience and needs on that one, and they are welcome to change their opinion based on whatever causes them to.

but I will say it is quite possible to drive in northern winters with summer tires, no ALB, TC, YAW, ETC, heck no roof or side curtains if you take them off because you can't see through frosted up plastic and can only scrape the windshield while driving - but then we would be talking about the last century, rather than current knowledge and technology, and I would seem very old.

 

[Scalewoodman]

We've all seen these recent videos and related to the feeling of terrible anguish when you know things are out of control and there's nothing you can do about it (attached recently circulated from Iowa)- everybody's worst nightmare and all the hooning skill in the world wouldn't make a darned bit of difference. Traction smaction. Humility is best served cold. Being in the wrong place at the wrong time and anything's possible.

 

[lost1750GTV]

forget hooning, the best winter tires in the world count for nothing if you stop - then and get slammed into by a dozen other vehicles

 

[1265]

The braking strategy in this new cars with abs vcd esp esc ecc. are very selective and follow one master target, stop as fast as possible.
The awd, 4motion, quattro q4 and so on are controlled by the braking computer too and impact not so much on stopping the car.
Only for information the q4 is the same 4 wheels drive of BMW, they use the same magna actimax

 

[CalgaryStelvio]

A part-time 4WD that can be switched from 2WD to 4WD will stop shorter on snow. A full time 4WD or an AWD vehicle will not make a difference.

 

[Scalewoodman]

A part-time 4WD that can be switched from 2WD to 4WD will stop shorter on snow. A full time 4WD or an AWD vehicle will not make a difference.

Just to understand, what you are saying is that 'AWD' doesn't 'bite' in deceleration mode? Interesting point. Braking modulation in skid control and ABS are the tools at hand.

Thinking... and don't know... is Hill Descent brake activated or gear activated or both?

 

[CalgaryStelvio]

the difference is a part-time 4WD locks the front and rear drive shafts when switched into 4WD and will not allow them to operate at different speeds, unlike a full-time 4WD or AWD. For this reason that type of system is never recommended to be driven with 4WD engaged on dry pavement.

I honestly don't know about Alfa, but as far as I know other manufacturers (like Range Rover) use each wheels ABS sensor and brakes to control wheel speed so I dont imagine theirs would be any different tech.

 

[SailDrive]

ok well I had hoped to chime in more often today but work got in the way... I wasn't very clear but of course the original comment was when compared to a 2WD (either FWD or RWD) vehicle.

I believe @CalgaryStelvio got closest to the truth behind the answer with the idea about 4WD systems locking the wheels together. The real answer is that any 4WD or AWD system which allows for a partial or complete lockup / torque transfer between the front and the rear axles will ALWAYS out stop a pure 2WD system on snow, sand, rocks, or mud. The greater the degree of lockup between the 4 axels the better the stopping distance will be.

I could go on into more detail, but these videos go into perfect detail already.
Demonstration

Description

Hope everyone learned something new

 

[CalgaryStelvio]

This thread was started with the intent to prove that AWD stops better in snow but the videos are strictly about part-time 4WD being used.
Sorry but this has no relevance to an AWD vehicle.

Instead of just the front wheels locking up the transfer case won't let any of the 4 wheels turn at a different rate so it locks up all 4 and they act like snowplows. AWD systems do not lock the wheels, they are specifically designed to move power for and aft based on traction.

 

[CalgaryStelvio]

The same rally school made another video after those 2 were made:

In the description on their YouTube channel:

TeamONeilRally
Published on Jan 31, 2018
Our (probably) final look at the braking difference in 4WD and 2WD with part-time 4wd vehicles and normal transfer cases. The advantage that 4WD has over 2WD on snow and ice is again apparent. This will only work in vehicles with standard transfer cases, "full-time" and "on-demand" 4WD systems and AWD systems will all perform differently.

 

[Scalewoodman]

Fascinating and makes a good point. I'll be lying awake tonight trying to think this through and while I get the benefit of the diff lock (back to SelecTrac I used it for stability on slushy snowy rutted roads) - QT II on Jeep was all electronic all ways - so the issue to me is if the benefit of electronic wheel control (by brake actuation as Marzio noted) provides superior vehicle stability or is it simply another way of accomplishing the same thing?

Of course I need to obsess about the weight of the plow in the first video as it would provide benefit to the larger front brakes as described in video 2... and none of them had skid control rollover mitigation etc. And the Jeep had no electronics whatsoever.

I think we can all agree that Stelvio is state of the art and even the processors are much faster than same on my 2008 Jeep. I am comfortable that except for off road and extreme conditions it is nearly as good as the Jeep. Coming through some bad weather upstate NY down through the Poconos and 33 to Allentown terrible conditions Stella felt very secure and composed through icy slushy ruts. At one point I passed the plow vehicle to get her ahead of the messy spray and friction/salt material.

 

[CalgaryStelvio]

I think one of the biggest differences is the rise of the ABS vehicles. ABS won't decrease your stopping distances, and in some cases will actually increase them, but it's primary reason is to prevent wheel lock up so you can maintain control. I think for this reason they aren't going to allow the differentials to lock up the wheels and lose steering control.

We had enough butt-pucker moments in an old International Scout with locking differentials driving up narrow snow covered mountain roads with a drop off on the side. Sure, all 4 wheels would power but any side slope on the road or ice underneath the snow would suddenly make the whole truck crab towards the edge. The primarily rear wheel drive Stelvio with its slip-then-grip AWD system will move power around and maybe won't have the ultimate grip of locking diff's but I think it's a much safer system to aid you in maintaining your lane.

 

[SailDrive]

The same rally school made another video after those 2 were made:

In the description on their YouTube channel:

TeamONeilRally
Published on Jan 31, 2018
Our (probably) final look at the braking difference in 4WD and 2WD with part-time 4wd vehicles and normal transfer cases. The advantage that 4WD has over 2WD on snow and ice is again apparent. This will only work in vehicles with standard transfer cases, "full-time" and "on-demand" 4WD systems and AWD systems will all perform differently.

Oh I was never trying to say AWD was identical to 4WD here, or our AWD system had any specific level of attributes, or even all AWD systems had the same benefit, this was entirely meant to be a conceptual discussion. The video is correct that to gain the full benefit you need the standard transfer case, and that some setups will show little to no appreciable gain.

Generally with most AWD setups, downshifting and engine braking prior to or in conjunction with pedal braking will result in the transfer case being under some / heavy load, which also means that some torque transfer between the front and rear will be taking place. The exact % of transfer relative to a 4WD truck locking transfer case will vary from one vehicle to another so it's difficult to say, but the truth is that there will always be some torque transfer taking place which will serve to improve braking in snow. While the improvement may be almost immeasurably tiny in some AWD setups, it can be significant in others. This variation is due predominantly to the variations on how the torque split and torque vectoring is controlled and managed.

Interesting note, There is one AWD system to which no torque transfer will likely ever take place, and that's the Twin-Transmission 4WD System in the Ferrari FF

 

[lost1750GTV]

I'm not much for technology, but independent 4 wheel ABS is what will stop you fast.
"won't decrease your stopping distances"
well, you coulda made that argument, and plenty did, when ABS first appeared, and was applied to the front axle or back axle - and there was argument over which was preferable, or neither, and motorcycles certainly didn't need it because they already had seperate front and rear brakes - and were single track vehicles.
but sorry, that horse left the barn a long time ago.
being able to apply maximum braking force to each tire for whatever it is more effective than being limited by left right or front back traction.

but, anyone is free to tell anyone anything they want, and I would like a proper handbrake and the ability to disengage AWD on my stelvio Q - but apparently that is backwards thinking.

plows? yeah, you could stop real good with a snow plow down - and if you want to use your tires as plows, that works better if the vehicle is sideways.