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Discussion Starter #1
Yesterday I backed up a snow-covered driveway and had my front left wheel spin while the other wheels weren't rotating. I was surprised. Is it the case that SH-AWD doesn't work in reverse? That would be really bad for snow country. But maybe it wasn't put back together correctly after the 9-sp transmission replacement? I'd like to know.
 

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Dunno about SH-AWD 3.0 but in SH-AWD 2.0 it does work on reverse.
 

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I have traveled a lot in northern parts of Canada with rental cars in winter. They all have had A/S tires and did just fine. One trip went to bed, woke to 4 inches of snow snowed all day and I had to drive 50 miles back to the main highway. Three ruts in 10 inches of snow. Never had a problem with a front wheel drive rental car getting me back to town.
IMHO its not what you got but your ability driving it.
 

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I have traveled a lot in northern parts of Canada with rental cars in winter. They all have had A/S tires and did just fine. One trip went to bed, woke to 4 inches of snow snowed all day and I had to drive 50 miles back to the main highway. Three ruts in 10 inches of snow. Never had a problem with a front wheel drive rental car getting me back to town.
IMHO its not what you got but your ability driving it.
Yup! I have never had a front wheel drive that got stuck before I hung it up in snow deeper than the clearance and with 4X4 you can generally scrape at least a couple of inches and that is with all-season tires. Yes, there are specific scenarios such as a steep driveway or hilly areas where snow or studded tires can be the difference between getting where you want to go or not, but many times it just comes down to driving skill. One key is to not letting the tires break traction, which is why many newer cars have traction control, but if you are old school with a feel for driving on snow/ice the traction control is no more useful than training wheels on a bicycle for someone who knows how to ride.
 

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You should have snow tires installed.
What has snow tires got to do with the OPs question?

Getting stuck (or not getting stuck) and losing traction in snowy conditions has nothing to do with decreased braking capability and a decrease in a safety when the rubber compound of A/S tires gets hard in very cold temperatures. Just because the awd was good enough to get you through, does not mean that winter tires aren't significantly safer and highly recommended. They are not just for getting you through some snow.
 

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Discussion Starter #10
There are lots of threads on tires, including snows: can we please not hijack this thread? Also, I never said I experienced any difficulty in maneuvering in this situation. I just noted that one wheel spun when reversing into a snow bank, which AWD shouldn't do IMHO. Can any of you snow "experts" try reversing on snow/ice with a 2016+ MDX SH-AWD and confirm this observation?
 

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Keep in mind that the front wheels only have a standard differential. One wheel can spin while the other has traction. The rear differential action can be augmented by the SHAWD system by sending more or less power to each rear wheel, based on the amount of slip.
 

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Discussion Starter #12
I would have thought that the SH-AWD system would have braked the spinning front left wheel, allowing the right wheel to turn. And the rear wheels should have provided drive power. Very strange...
 

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I would have thought that the SH-AWD system would have braked the spinning front left wheel, allowing the right wheel to turn. And the rear wheels should have provided drive power. Very strange...
No it does not as SH-AWD requires the front differential to turn in order to turn the rear wheels. Remember that the PTU (Power Transfer Unit) takes power directly from the front differential into the Prop Shaft into the rear SH-AWD diff.. So if the MDX tried to stop the front wheels from spinning it would actually stop the rear wheels too. That is why the front wheels must turn at all times in order to have AWD for Ex: If you locked the front wheels completely? the rear tires would not move at all, So in winter snow you should see at least 1 front tire spin in order for the 2 rear to move.

This is why SH-AWD cannot engine brake either, When you let go off the gas the MDX becomes 100% FWD... You can feel this while cornering thus why we recommend to always press the gas pedal in a corner on a car with SH-AWD even if for a little bit to keep the car in AWD mode and not disrupt the chassis balance.
 

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Discussion Starter #14
I was thinking of slowing, not stopping, the one spinning front wheel - which should allow power transmission to the rear IIUC. Even if the spinning wheel were stopped I would expect the other front wheel to rotate, and hence the differential. I thought that using the brakes to force power to the other wheels was the whole point of SH-AWD. Anyway I am learning a bit about how it should work, at least in forward. I still think it should do better than allow one front wheel to spin and take all of the power. Thanks for the insights.
 

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I was thinking of slowing, not stopping, the one spinning front wheel - which should allow power transmission to the rear IIUC. Even if the spinning wheel were stopped I would expect the other front wheel to rotate, and hence the differential. I thought that using the brakes to force power to the other wheels was the whole point of SH-AWD. Anyway I am learning a bit about how it should work, at least in forward. I still think it should do better than allow one front wheel to spin and take all of the power. Thanks for the insights.

No in fact you would want the front wheel to spin as fast as it can because that will allow the rear axle to do more, In snow you basically will have the least traction naturally so by stopping or reducing the wheel spin in the front it will reduce rear bias in SH-AWD... So if you see a front wheel spinning? that means the rear is getting power as well so it should in theory make you move.. GRANTED you have snow tires of course, SH-AWD is not a weather AWD System after all it is designed to enhance driving capabilities so it wont do miracles on inferior tires.

As for brakes forcing power to the other wheel? Not in SH-AWD... SH-AWD is fully 100% Active Torque Vector, It does not need to stop a rear wheel to send power to the other because there is literally no differential in the rear so by braking one wheel it wont send power to the other as SH-AWD can fully disengage 1 axle to send 100% of the rear power to a single wheel already.

Braking Torque Management is basically what VSA does in FWD cars, As you mention it will brake the spinning wheel in order to act as an ELSD to force the differential to send more power to the wheel with traction. But this system is detrimental to SH-AWD... As SH-AWD needs the front axle to spin in order for the rear to move, So if you would brake one of the front wheels you will actually reduce torque in the rear, SH-AWD counters this by allowing the front tire to spin but giving more torque to the rear wheels via the SH-AWD diff under-drive ratio.

In resume:
1.- SH-AWD has NO REAR COMMON DIFFERENTIAL as the Front Trans-axle Setup, It uses Active Torque Vectoring instead.
2.- SH-AWD has no brake torque management (It wont brake the front spinning wheels to regain traction as this will defeat SH-AWD).
3.- SH-AWD does not replace Snow Tires, Its not a Weather AWD System! It is designed to turn fast.
 

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Discussion Starter #16
Good info - thanks skirmich. This is my first AWD/4WD car; I did do some research before buying but the many different configurations of AWD are mindboggling.

BTW I've never had a problem getting around in snow with RWD or FWD, and I didn't here either. It was just an observation when backing into deep snow (my brother doesn't believe in shoveling his driveway, or any other "unnecessary" work). I wanted to understand what was happening, and now I am better informed thanks to the posts. IMHO skill/experience at driving in snow and ice is far more valuable than technology, although I will certainly avail myself of the latest given the chance. I used to take my 240Z up Mt. Hood to ski, and it was my only car in Detroit when I lived there. The only problem was having to dig out the snow under the car if I got in too deep and the wheels were hanging - always carry a small shovel or better a surplus entrenching tool. The MDX has almost double the ground clearance so probably not going to be a problem (but I still have a small shovel aboard).
 

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3.- SH-AWD does not replace Snow Tires, Its not a Weather AWD System! It is designed to turn fast.
But, SH-AWD with snow tires creates an AWD system that beats any other winter traction-focused system that I've ever driven. Just sayin'..... :)
 

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Most AWD cars are not made for serious snow, the car that I know that can handle serious snow is the Subaru which has the best AWD system on the market by a large margin, however Acura AWD will handle better in performance situations than the Subaru will, so what's the difference you scream? Subaru uses symmetrical AWD and is the best system on the market for deep snow, mud, even offroad if you have the ground clearance, it provides tenacious grip, it's the same system that you would find in a Jeep...old school 4 wheel drive with no torque bias toward the front or rear, it's a safety oriented system but not a performance oriented system; Acura uses asymmetrical AWD, it's the best system for high performance handling, the Acura system, unlike other asymmetrical systems in most other cars, sets the computer up to be torque bias on the rear wheels, the other car makers set theirs up to be torque bias to the front wheels...a BIG difference by the way in terms of handling, Acura's bias is the same as one would find in the newer AWD Ferrari and other such high performance cars. Acura takes the performance to another level over other cars that instead of just shifting torque from front to rear, but Acura can also send it side to side, which I think Acura at this time is the only car company to do this stunt. But this is why your Acura did not fail well in snow, and the only real improvement you can make on the car is to get winter tires like the Bridgestone Blizzak and put them on about the first of November and take them off about the first of April, otherwise as things heat up the Blizzak tires will wear out faster and faster.

The odd duck in all of this is the Subaru WRX STI, this one the driver can change the degree of asymmetry based on the driver's preference which could be for snow, high performance dry pavement handling, baja type of driving, whatever suits the driver.

Subaru offers 4 different types of AWD, if you want more detail then my half lame explanation then read this: https://www.qualitysubaru.com/symmetrical-all-wheel-drive-explained.htm

The only thing I don't like about all the Subaru models is the CVT transmission, I don't like these transmissions at all, they don't last long and can be very expensive to rebuild from $6,000 to $8,000 dollars! not sure why Subaru would put their cars history of strong reliability to shame by putting CVT's in. On average a CVT will last about 100,000 miles, sure you read about some guy who got 200,000 out of one but those are not common by any means, and I did read where the few that got 200,000 miles changed the tranny fluid every 30,000 miles where the owners manual says you don't have to change it ever! Right, and that's why people who followed the owner's manual of never having to change the fluid ever ran into tranny problems 60,000 to 75,000 later.
 
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