[Ali Shiralian]

Originally Posted by Jbrock22

But it's still a Subaru.

It sure is

Watch the video below

[Beek]

Yeah but can it Ghostwalk?

[vanbmw]

My WRX with snow tires was great in snow but no better then my A4 or S4. All of them had manual transmissions and snow tires in winter. All of them had great grip. Handled snow really well. None stood out as being much better. Even the G35 was really good but it felt heavier.

[Jbrock22]

Originally Posted by Ali Shiralian

It sure is

Watch the video below

Very cool.

[hollandog]

I just put my Blizzak WS80 on last week (RWD) but no real snow accumulation to try it yet.

[qgshadow]

Originally Posted by Ali Shiralian

Hmmm, they are. They all use symmetrical all wheel drive, meaning the car is always AWD drive all the time even when not needed. The only exception is BRZ.

It distributes equal power to all wheels as opposed to Xdrive where it sends powe to front wheels only when needed.

What??? Xdrive always sends power the all wheels 60/40. Only if there is slippage does it change that ratio.

[T4NZ4NIT3 335i]

Originally Posted by qgshadow

What??? Xdrive always sends power the all wheels 60/40. Only if there is slippage does it change that ratio.

I know right..so much misinformation floating around.

[Araemo]

Originally Posted by qgshadow

What??? Xdrive always sends power the all wheels 60/40. Only if there is slippage does it change that ratio.

Originally Posted by T4NZ4NIT3 335i

I know right..so much misinformation floating around.

Incorrect. xDrive does not 'always' send power to all wheels at 60/40(or 40/60). It's not a full-time-connected differential, it's a part-time connected clutchpack. It disconnects at low speeds until there is rear wheel slip. At other speeds, I have not seen any technical documentation from BMW on how pre-emptively it locks up the clutchpack to allow the fronts to take power.. but based on the current governmental focus on fuel economy, I'd assume that at steady state (constant speed, no turning, etc), the front wheels are disconnected. (And after re-reading the link below, I'm right..) If they do actually partially apply the clutchpack in any dynamic driving (and I don't mean 'sporty', I mean changing loads on the drivetrain and suspension), you'll get something akin to a 40/60 torque split, but it has more loss and less control than a torque biasing differential could give you.

The other thing to note about the 40/60 split.. since it's based on a slipping clutch, if you suddenly have a traction imbalance between the front and rear wheels while under power, the clutch will not be able to maintain 40/60 without changing the pressure, so you'll get more torque to the rear momentarily (This paragraph is supposition on my part... the rest of this post is not supposition, and is the result of lots of research). So, depending on how quickly the DSC can detect the wheel speed difference and any yaw changes, that's how quickly the clutch pressure can START changing, and of course that takes time (probably measured in milliseconds). Technically, a torsen-style pure gear differential could respond 'instantly' to the wheel speed difference, but those are extremely rare in AWD systems anymore (Since you can't disconnect them, low speed maneuvering is jerky and uncomfortable, and highway gas mileage suffers, so almost everyone is moving away from true full time AWD systems for general consumer vehicles.. the exception being some subarus, but they are based on viscous coupling, so they are more tolerant of low speed differences, and are less jerky when they do start transferring torque).

Also, note that the most front-biased torque split possible is '50/50', which is a side-effect of there being no center diff. Either the front and rear axle are moving at the same speed, or the rear axle is moving faster. And since it isn't a differential, the output speeds of the front+rear axles don't have to 'add up' (when accounting for gearing) to the input speed from the transmission.

You'll occasionally see comments from marketing people that it can send 'up to 100% of the torque' to the front wheels.. there's a big invisible asterisk on that: In order for 100% of the torque to be sent to the front wheels, the xDrive transfer case must be 100% locked (50/50 nominal torque split), and the rear wheels must be in the air/on completely 0 friction ice. In that case, since the front and rear wheels must turn at the same speed, all the torque is applied to the ground through the front wheels. This is the same way Audi claims that the haldex-based quattro systems can send 'up to 100%' of the torque to the rear (since haldex is functionally identical to xdrive but the front is permanently-connected instead of the rear).

Ref: http://www.awdwiki.com/en/xdrive/

[Ali Shiralian]

Originally Posted by Araemo

Incorrect. xDrive does not 'always' send power to all wheels at 60/40(or 40/60). It's not a full-time-connected differential, it's a part-time connected clutchpack. It disconnects at low speeds until there is rear wheel slip. At other speeds, I have not seen any technical documentation from BMW on how pre-emptively it locks up the clutchpack to allow the fronts to take power.. but based on the current governmental focus on fuel economy, I'd assume that at steady state (constant speed, no turning, etc), the front wheels are disconnected. (And after re-reading the link below, I'm right..) If they do actually partially apply the clutchpack in any dynamic driving (and I don't mean 'sporty', I mean changing loads on the drivetrain and suspension), you'll get something akin to a 40/60 torque split, but it has more loss and less control than a torque biasing differential could give you.

The other thing to note about the 40/60 split.. since it's based on a slipping clutch, if you suddenly have a traction imbalance between the front and rear wheels while under power, the clutch will not be able to maintain 40/60 without changing the pressure, so you'll get more torque to the rear momentarily (This paragraph is supposition on my part... the rest of this post is not supposition, and is the result of lots of research). So, depending on how quickly the DSC can detect the wheel speed difference and any yaw changes, that's how quickly the clutch pressure can START changing, and of course that takes time (probably measured in milliseconds). Technically, a torsen-style pure gear differential could respond 'instantly' to the wheel speed difference, but those are extremely rare in AWD systems anymore (Since you can't disconnect them, low speed maneuvering is jerky and uncomfortable, and highway gas mileage suffers, so almost everyone is moving away from true full time AWD systems for general consumer vehicles.. the exception being some subarus, but they are based on viscous coupling, so they are more tolerant of low speed differences, and are less jerky when they do start transferring torque).

Also, note that the most front-biased torque split possible is '50/50', which is a side-effect of there being no center diff. Either the front and rear axle are moving at the same speed, or the rear axle is moving faster. And since it isn't a differential, the output speeds of the front+rear axles don't have to 'add up' (when accounting for gearing) to the input speed from the transmission.

You'll occasionally see comments from marketing people that it can send 'up to 100% of the torque' to the front wheels.. there's a big invisible asterisk on that: In order for 100% of the torque to be sent to the front wheels, the xDrive transfer case must be 100% locked (50/50 nominal torque split), and the rear wheels must be in the air/on completely 0 friction ice. In that case, since the front and rear wheels must turn at the same speed, all the torque is applied to the ground through the front wheels. This is the same way Audi claims that the haldex-based quattro systems can send 'up to 100%' of the torque to the rear (since haldex is functionally identical to xdrive but the front is permanently-connected instead of the rear).

Ref: http://www.awdwiki.com/en/xdrive/

Best explanation of differences between various AWD systems. Very informative, thank you

[Ali Shiralian]

Originally Posted by T4NZ4NIT3 335i

I know right..so much misinformation floating around.

Not misinformation, read below

[T4NZ4NIT3 335i]

Originally Posted by Araemo

Incorrect. xDrive does not 'always' send power to all wheels at 60/40(or 40/60). It's not a full-time-connected differential, it's a part-time connected clutchpack. It disconnects at low speeds until there is rear wheel slip. At other speeds, I have not seen any technical documentation from BMW on how pre-emptively it locks up the clutchpack to allow the fronts to take power.. but based on the current governmental focus on fuel economy, I'd assume that at steady state (constant speed, no turning, etc), the front wheels are disconnected. (And after re-reading the link below, I'm right..) If they do actually partially apply the clutchpack in any dynamic driving (and I don't mean 'sporty', I mean changing loads on the drivetrain and suspension), you'll get something akin to a 40/60 torque split, but it has more loss and less control than a torque biasing differential could give you.

The other thing to note about the 40/60 split.. since it's based on a slipping clutch, if you suddenly have a traction imbalance between the front and rear wheels while under power, the clutch will not be able to maintain 40/60 without changing the pressure, so you'll get more torque to the rear momentarily (This paragraph is supposition on my part... the rest of this post is not supposition, and is the result of lots of research). So, depending on how quickly the DSC can detect the wheel speed difference and any yaw changes, that's how quickly the clutch pressure can START changing, and of course that takes time (probably measured in milliseconds). Technically, a torsen-style pure gear differential could respond 'instantly' to the wheel speed difference, but those are extremely rare in AWD systems anymore (Since you can't disconnect them, low speed maneuvering is jerky and uncomfortable, and highway gas mileage suffers, so almost everyone is moving away from true full time AWD systems for general consumer vehicles.. the exception being some subarus, but they are based on viscous coupling, so they are more tolerant of low speed differences, and are less jerky when they do start transferring torque).

Also, note that the most front-biased torque split possible is '50/50', which is a side-effect of there being no center diff. Either the front and rear axle are moving at the same speed, or the rear axle is moving faster. And since it isn't a differential, the output speeds of the front+rear axles don't have to 'add up' (when accounting for gearing) to the input speed from the transmission.

You'll occasionally see comments from marketing people that it can send 'up to 100% of the torque' to the front wheels.. there's a big invisible asterisk on that: In order for 100% of the torque to be sent to the front wheels, the xDrive transfer case must be 100% locked (50/50 nominal torque split), and the rear wheels must be in the air/on completely 0 friction ice. In that case, since the front and rear wheels must turn at the same speed, all the torque is applied to the ground through the front wheels. This is the same way Audi claims that the haldex-based quattro systems can send 'up to 100%' of the torque to the rear (since haldex is functionally identical to xdrive but the front is permanently-connected instead of the rear).

Ref: http://www.awdwiki.com/en/xdrive/

very informative. Thanks for taking the time out to explain that. I am suddenly liking the xdrive more now as opposed to quattro due to its rear bias.

I stand corrected Ali

[Araemo]

Quattro in the S4 has a rear bias created by a center differential, very different from the haldex based Quattro in the S3 and TT-S. Debatably better than xdrive, but it's matched to an engine that is too far forward and creates a ton of understeer. Also, the S4 has an optional rear limited slip differential that can overdrive the outside rear wheel to do torque biasing in turns without the use of brakes. Combine that tech with bmw balance and it's be awesome. As it is, every car is a compromise on some aspect.

[Polo08816]

Originally Posted by Araemo

Quattro in the S4 has a rear bias created by a center differential, very different from the haldex based Quattro in the S3 and TT-S. Debatably better than xdrive, but it's matched to an engine that is too far forward and creates a ton of understeer. Also, the S4 has an optional rear limited slip differential that can overdrive the outside rear wheel to do torque biasing in turns without the use of brakes. Combine that tech with bmw balance and it's be awesome. As it is, every car is a compromise on some aspect.

+1.

The S4's AWD system is better than the xDrive.

[Araemo]

Originally Posted by Polo08816

+1.

The S4's AWD system is better than the xDrive.

In isolation, if you ignore the car it's under. With the dynamic deficiencies of the S4 (too much weight forward of the front axle, ruining the polar moment of inertia, and taking too much weight off the rear axle), they're probably closer in real world performance than the center diff would suggest. Realistically, the S4 is the competitor to the 335/340, and I guarantee they offer the sport diff option to overcome the strong understeer tendencies, and bring it into closer competition with the BMWs, from a 'fun' (IE, RWD-feeling) standpoint.

I honestly like both cars, and I think they are both fun to drive, and until very recently, expected to find myself in an S4 some day.

Also, I do wonder if the B9 S4 still has the crown gear center diff, or if is switching to the 'ultra' quattro like the A4... 'ultra' quattro is lacking a center diff, and replaces it with a clutchpack that can disconnect the rear wheels, very similar to how xDrive and haldex work (though technically ultra quattro uses two clutches, on at each end of the driveshaft, to further reduce drag/drivetrain losses while driving in steady state).

[RM7]

My WRX was pretty terrible in the snow, but that was simply because of the performance tires. The car didn't handle well at the limit of course, due to the engine so far forward, but the power delivery while turning was very good.

My xdrive is needed because I live in a climate where the roads will be iced/snowed over for a majority of the winter. Not just that, but lots of stopping and starting due to the city and intersections. Trying to do this with a RWD car is usually an absolute joke, I know, I've done that here too. If you have enough weight on the wheels, it's better, but there are some places where AWD makes a lot of sense, where you can actually clear the intersections safely, rather than constantly trying to not spin the wheels and have the rear end slide out again. The studded tires help immensely too, but above all I want to be in control, so I can get out of the way of people that are out of control, which has already happened a few times this winter.

[Kingpin4]

AUDI

ASSHOLE
U
DESERVE
IT

[Twodawgs]

Originally Posted by Ali Shiralian

Bridgestone Blizzak WS80 on 18 inch rims for both cars...

Are these tires the same size? I ask because a narrower tire tends to do better in the snow.

[floydarogers]

Originally Posted by Araemo

Incorrect. xDrive does not 'always' send power to all wheels at 60/40(or 40/60). It's not a full-time-connected differential, it's a part-time connected clutchpack. It disconnects at low speeds until there is rear wheel slip. At other speeds, I have not seen any technical documentation from BMW on how pre-emptively it locks up the clutchpack to allow the fronts to take power.. but based on the current governmental focus on fuel economy, I'd assume that at steady state (constant speed, no turning, etc), the front wheels are disconnected. (And after re-reading the link below, I'm right..)

I've been concerned about this since you and Ali posted about this "mostly disconnected" thing. While the text in the link you provide says " For example, when driving at high speed, or when parking, the clutch is disengaged and all power goes to the rear wheels"

I have not been able to find confirmation of that statement from any other source, especially from any "official" or semi-official BMW site. In fact, the video in the last comment on your link provides (at least some) evidence that the fronts have power from zero mph. The following video also provides similar evidence:

Do you have better (more authoritative) references?

[Ali Shiralian]

Originally Posted by floydarogers

I've been concerned about this since you and Ali posted about this "mostly disconnected" thing. While the text in the link you provide says " For example, when driving at high speed, or when parking, the clutch is disengaged and all power goes to the rear wheels"

I have not been able to find confirmation of that statement from any other source, especially from any "official" or semi-official BMW site. In fact, the video in the last comment on your link provides (at least some) evidence that the fronts have power from zero mph. The following video also provides similar evidence:

Do you have better (more authoritative) references?

I don't know if I'd put too much value in these type of videos but here's the Audi answer to the video you posted. Seems like the Audi was equipped with winter tires but the bimmer was not

[Billfitz]

Originally Posted by floydarogers

the video in the last comment on your link provides (at least some) evidence that the fronts have power from zero mph.
Do you have better (more authoritative) references?

I have a steep driveway. With all of my previous RWD and FWD vehicles, going back 30 years, if I parked midway up my driveway and there was snow the only way I could get it all the way up my driveway was to first back down, taking it across the street, into the flat driveway of my neighbor across the street. Then I could get enough of a head start to make it up the driveway, though usually still with some fishtailing. This was with winter tires. With my xDrive, and AS tires, I can park it midway up the driveway, and then with even four inches of snow continue up the driveway from a standing start with no more difficulty than when the driveway is bare. It will do this going forward, it will do this going backward. Will an Audi or Subaru doe the same? I don't know, I don't care, my BMW does and that's all that matters to me.

[Araemo]

Originally Posted by floydarogers

I've been concerned about this since you and Ali posted about this "mostly disconnected" thing. While the text in the link you provide says " For example, when driving at high speed, or when parking, the clutch is disengaged and all power goes to the rear wheels"

I have not been able to find confirmation of that statement from any other source, especially from any "official" or semi-official BMW site. In fact, the video in the last comment on your link provides (at least some) evidence that the fronts have power from zero mph. The following video also provides similar evidence: <video>

Do you have better (more authoritative) references?

Well, I (and my references) never said that it has a minimum speed for the transfer case to lock up. Just that is not locked up by default. Any wheel speed difference between front and rear is enough to tell the computer that some slipping is going on. And there's also the possibility of preload based on outside temp, and other factors (IE, if the outside air temp is under 0C, default to some lock-up in forward gears, or all gears, for initial starts). Those kinds of details are locked away in the control modules, and getting someone capable of answering them knowledgeably to do so is really difficult (You'd have to talk to engineers or product managers on the development side, and hope they'll actually answer).

Realistically though, and this is why I think the argument is pretty silly when it comes to street cars: the control units can react to the slipping wheels before you realize they're slipping, and start engaging the front wheels in less than your reaction time. Does that delay matter if you're ice racing or rallying on gravel? Maybe. Does that delay matter if you're just trying to get up your driveway? Not really.

If you do enough searching, you can find versions of those videos from Audi, BMW, Mercedes, Volvo, and Subaru.. where only their brand is successful. This suggests to me that there's more to meets the eye on all of them. For example, with old school audi quattro (3 torsen diffs), if one wheel on each axle is spinning, it will go nowhere (unless you apply the brakes). Traction control will do this for you, and only to the spinning wheels, but even with TCS off, you can apply the brakes lightly and move off the rollers. They may say 'it was fair, we hit the TCS button on all cars so 'tcs was off', but on some brands, that enables full TCS off, while on other brands, it first activates snow/gravel mode, etc., but it may explicitly be the wrong thing to do for a specific vehicle in that situation.

Basically, marketing can come up with a test their car will be the only one to pass, even if it's 'fair'.. and in some of those videos (not all), I strongly suspect that TCS is turned off for the losers, and left on for the winners. Or, they just never apply enough throttle for the systems to work as intended, but having wheels on rollers makes it look dramatic. (There probably are some cases of large SUVs where the front (or rear) wheels alone can't receive enough torque to accelerate all 3 tons of vehicle up the grade they're resting on, but that's equally BS in my mind, since if you keep your tires in good enough shape (either use snows in winter, or keep your all seasons above 4/32" of depth all winter), you'll basically never have that little traction to 3 of the wheels, but full traction to 1.)

Realistically: Find reviews from people using the cars as intended in real conditions. I can say pretty confidently that if you know how to drive it, F30 xDrive is awesome. I will say I think you'd probably need to use DTC / 'traction' mode sometimes for the best behavior, but read the manual, learn when it's appropriate, and use it when needed. I had no problem with snow that was halfway up my bumper, and I could tell the front wheels were involved.

Edit: Those two videos specifically - can't even be compared. First video is longitudinal/rear-biased vs longitudinal/rear-biased, second video is tranverse/front-biased vs transverse/front-biased. BMW X1 and Audi Q3 are both transverse platforms not using their 'signature' awd systems, while X3 and Q5 are both longitudinal platforms using the primary implementation of both their respective AWD systems (as of the year of introduction anyways, that Q5 is not using 3 torsen diffs. It's probably using an open front and rear and a 'crown gear' torque-biasing limited-slip center diff, as I don't think the 'sport diff' torque biasing rear LSD was available on the Q5). While the Q3 is using 'haldex', which works a lot like standard xDrive, but with only the fronts always connected. I have no direct info how the X1 is hooked up, mechanically, but I wouldn't be surprised if it is literally haldex, or one of their competitors. (Note that even the same hardware (haldex gen3/gen4/gen5) can be programmed differently in different implementations, such as Honda vs Volvo vs VW/Audi, which all use haldex for many of their smaller AWD vehicles, and as I hint at above, it matters more how the entire system is integrated - how does the AWD controller talk to the TCS controller (which is usually the ABS controller) and the ESC controller (which may or may not be the ABS controller), etc. Or even if they don't literally 'talk', how tightly integrated are their calibrations.).

Edit2: After looking up the "All4" system from the Mini Countryman (which the X1 shares many mechanical parts with), it sounds mechanically very similar to haldex, even if it is a different specific design, and it sounds like at least in the JCW mini, it defaults to lock-up mode, not FWD-only mode.. but the point is pretty moot.

[floydarogers]

Originally Posted by Araemo

...
If you do enough searching, you can find versions of those videos from Audi, BMW, Mercedes, Volvo, and Subaru.. where only their brand is successful. This suggests to me that there's more to meets the eye on all of them...

I think we can all agree that the marketing departments put out specious stuff all the time. I've seen at least the Audi counters... maybe another from MB. I'm just concerned that your postings were "mis-information". Having no torque to the front at slow speeds makes no sense to me - that's when you need all the traction available, to get going. After all, starting in 2nd is a long-time trick to reduce wheelspin due to excess torque in low gears.

I've not gone out in my wife's 328xd looking for "problems". I had 9 years/145K miles experience in my pre-xDrive '01 330xi; only got high-centered once and needed a tow. With Pilot Alpine or xIce the thing was a tank.

Frankly, snow tires make my rwd 335d pretty adequate in most conditions. It has out-performed poorly-driven Audi A6 Quattro and Jeep Grand Cherokee cars in 10-12 inches of snow...