[ajsalida]

Originally Posted by issabmw

He is correct. The x-drive can shift completely to the front axles, in cases where the rears are stripped of traction.

Wrong. He is quoting marketing. There is no center diff. There is only a clutch pack hanging off an uninterrupted rear drive shaft. So all that happens is you variably engage or disengage the front. The rear cannot be disengaged.

This has been discussed at length here, do a search. Look at a cutaway diagram of the transfer case, it is clear.

On ice where both the rears have absolutely zero traction, and the front have traction, technically in a limited physics sense no torque can go to the rears for drive purposes. The rears will still spin (torque is required for that) as they cannot be disengaged from the tranny. The xDrive cannot dynamically shift any more than 50% to the fronts in any kind of normal real world sense, all that happens is the front and rear shaft lock up and turn at the same speed.

This is BMW marketing trying to imply there is a center diff, as in better AWD systems, when there isn't one.

edit: Another way to look at this is, jack the rear wheels off the ground, start the car and voila! the fronts get all the power. The rears will turn at the same rate. That is not the same as having a true center diff capable of transmitting 50-100% to the front dynamically depending on traction. But BMW marketing wants you to believe it is.

[Slops]

Really? Your telling me that they are straight up lying to us?

[Lenaxia]

Originally Posted by Sloppy

Really? Your telling me that they are straight up lying to us?

He's saying it's marketing spin and they couldn't care less how the average lay person interprets it as long as it makes their car look better.

If you believe everything that marketing tells you, you have some serious self reflection to do.

For those of us who care how our car actually works, it is only 50% because that is all that is mechanically possible given the way the car is designed. For everyone else who just likes to look at numbers and quote marketing hype, its 100% because that's what the car sales man told them.

[ajsalida]

Originally Posted by Sloppy

Really? Your telling me that they are straight up lying to us?

Not exactly lying, just being very weaselly & misleading. Look at the engineering. It is a very good AWD system but it is not physically possible to send 100% of TQ to the front axle if the rear cannot be disengaged.

It is like someone with a RWD only car + welded diff saying he can send 100% of TQ to either side*, (*if one wheel is up in the air or on ice). Technically that is "true" but profoundly misleading and stupid.

[ajsalida]

Originally Posted by Lenaxia

He's saying it's marketing spin and they couldn't care less how the average lay person interprets it as long as it makes their car look better.

If you believe everything that marketing tells you, you have some serious self reflection to do.

For those of us who care how our car actually works, it is only 50% because that is all that is mechanically possible given the way the car is designed. For everyone else who just likes to look at numbers and quote marketing hype, its 100% because that's what the car sales man told them.

This.

[issabmw]

Originally Posted by ajsalida

Wrong. He is quoting marketing. There is no center diff. There is only a clutch pack hanging off an uninterrupted rear drive shaft. So all that happens is you variably engage or disengage the front. The rear cannot be disengaged.

This has been discussed at length here, do a search. Look at a cutaway diagram of the transfer case, it is clear.

On ice where both the rears have absolutely zero traction, and the front have traction, technically in a limited physics sense no torque can go to the rears for drive purposes. The rears will still spin (torque is required for that) as they cannot be disengaged from the tranny. The xDrive cannot dynamically shift any more than 50% to the fronts in any kind of normal real world sense, all that happens is the front and rear shaft lock up and turn at the same speed.

This is BMW marketing trying to imply there is a center diff, as in better AWD systems, when there isn't one.

edit: Another way to look at this is, jack the rear wheels off the ground, start the car and voila! the fronts get all the power. The rears will turn at the same rate. That is not the same as having a true center diff capable of transmitting 50-100% to the front dynamically depending on traction. But BMW marketing wants you to believe it is.

Very enlightening. I haven't dissected the system (I will soon), and I guess I was fooled into thinking this as well.

In this video, the rear wheels begin to hover, not enduring any speed whilst the front brings the car off the rollers. How is this so?

[Slops]

very interesting... thanks for the education. I had no reason not to believe them as i'm not going to disect my car to make sure everything they say is true... as most people wouldn't.

[issabmw]

Originally Posted by Sloppy

very interesting... thanks for the education. I had no reason not to believe them as i'm not going to disect my car to make sure everything they say is true... as most people wouldn't.

This. It's not common to come across false advertisement, but recently BMW has been embellishing some statements.

[Slops]

I think 50% is plenty though! I primarily bought it just to help me out in the snow and rainy conditions and with 50% going up front it does just that. Launchin it at the strip also helps out!

[ajsalida]

Originally Posted by issabmw

Very enlightening. I haven't dissected the system (I will soon), and I guess I was fooled into thinking this as well.

In this video, the rear wheels begin to hover, not enduring any speed whilst the front brings the car off the rollers. How is this so?

Not sure what you mean here. Rears have no traction, transfer case clutch packs lock up front & rear drive shafts, front pulls car off rollers. That is not inconsistent with my statements. Keep in mind both front and rear diff are open, so it is possible for example for one rear wheel to not rotate at all during this. Clearly intent of this video is also a bit misleading.

It is a shame what BMW is doing with this marketing approach, xDrive is a good system it is just not based around a true center diff (or even true LSD diffs for that matter), like some others are.

[Tortfeasor 335xi]

With all the complaints about not having a front or rear limited slip differential, I figured it was important to point out that xDrive is connected to the anti lock brakes and stability control systems. In the case that wheelspin or directional instability still occurs while xDrive is or has been modulating the front to rear torque split, DSC will brake each wheel independently to regain traction and improve directional stability. This is how the car controls wheel spin on individual wheels without having a limited slip differential.

Worth pointing out is that most limited slip differentials use friction to limit the spin of the wheel with less traction. So essentially, the x-drive system uses the brake pads and rotors as the friction elements instead of clutch plates or worm and scroll gears in a limited slip differential. The benefit is that its easy to service warn friction elements by replacing brake pads (compared to opening up the diff, replacing warn clutch plates, and reinstalling the diff). The bad part is that it means that the brakes are not used solely to slow the car. This could conceivably cause cooked brakes in motorsports applications and premature brake wear.

[Lenaxia]

There is discussion on this elsewhere in the forum and I'm not entirely versed in the full mechanics behind it but the dsc simulated LSD setup they have in the xdrive is pretty far from a real LSD.

I am fairly certain that this is also marketing spin and as a consumer I do not buy it one bit as if it was as effective as claimed every consumer level car would claim this technology and call themselves a sport car (sport van anyone?).

There is no good reason BMW did not come with a factory LSD.

[ajsalida]

I think you are both right and make some good points. Where general road traction conditions are low, like snow ice gravel etc. the e-brake LSD feature only needs to apply relatively small forces to individual brakes to make an open diff simulate a true LSD. This causes very little additional brake wear and is a very good street AWD system.

However when you are doing burnouts or track days on clean pavement that is not the case, and the brakes (sometimes only 1 wheel!) are dealing with modulating the engine's entire peak TQ output. Goodbye brakes.

The wear inside a true LSD diff is minimal compared to the latter activity. Again with the marketing, if they want you to think xDrive can run with Mitsubishi Evo's and Nissan GTR's, certain Audis, no way.

I personally did not buy this car to turn it into a rally car or a track car, and for my purposes it is fine as a snow-capable german luxo-sport mountain tank. Nevertheless it burns somewhat to have now seen through the marketing hype, which I did not understand fully when I bought it. And BMW makes it very hard to see through it.

Some newer xDrive systems on say the X6 (and M3?) have true torque sensing diffs and some other cool stuff. But of course this is not available on any e9x AWD cars and marketing again is carefully not very clear about what is available on what.

[4corners]

It is 100%. The front end is 100% engaged with the transfer case, not 50% engaged. Semantics.

[ajsalida]

Originally Posted by 4corners

It is 100%. The front end is 100% engaged with the transfer case, not 50% engaged. Semantics.

The 100% or not is used in the context of what the possible TQ split is out of the transfer case. Usually it is 40/60, F/R, but it can go 0/100 F/R in certain cases (low speed parking lots and at high speed over 70 MPH I have read).

This thread is about whether it can ever truly go 100/0, as some BMW marketing literature claims, and it cannot. It cannot go higher than 50/50, or in plain english, no more than 50% of available TQ can go to the front axles, when the TC is locked up.

If we adopt your convention then 200% of ? can go to the wheels? Well it is not TQ or power as defined by engineers, maybe it is ///Marketing energy.

[4corners]

Let's call it 100% engaged to both axles then.

[Lenaxia]

Originally Posted by 4corners

Let's call it 100% engaged to both axles then.

Like I said before, as a lay person you are welcome to quote any number you want between 0 and ten quadquintseptrillion. The marketing folks couldn't care less because as we all know, the bigger the number the better.

However for those of us who actually care, like ajsalida, the others and I; and aren't trying to be difficult for the sake of being difficult, like you, the number is 50%.

You can call it whatever you want that makes you happy but take it elsewhere.

[4corners]

My point is if the transfer case is engaged completely there is equal amounts of power, torque or whatever going to both back and front. If it is set up to be some ratio, fine.

And SVP don't tell me I don't care, or I'm being difficult, or take it elsewhere if you don't mind. You should be more tolerant of people who don't already know everything.

[Lenaxia]

Originally Posted by 4corners

My point is if the transfer case is engaged completely there is equal amounts of power, torque or whatever going to both back and front. If it is set up to be some ratio, fine.

And SVP don't tell me I don't care, or I'm being difficult, or take it elsewhere if you don't mind. You should be more tolerant of people who don't already know everything.

In light of the discussion that we've had for the past 2 pages, there is no other way to interpret what you're saying other than being difficult. You could've just as well waltzed in here and insisted that BMW's were made of smurfs and it would've added the exact same amount of value to the conversation.

[Keenan35i]

hey so i am thinking about buying a wavetrac lsd thats been on my friends rwd car he wants to just exchange pumpkins, my question is does xdrive have the same rear pumpkin as rwd they are both automatic, also ik that if the tire sizes are different front and rear it messes with the xdrive, does the lsd change the ratio at which the wheels spin to where it would create different front and rear wheel rotational speeds?

[n55jb4]

I'm approaching this AWD=LSD from a little different perspective. From purely functional/consumer one. On track - xi models have faster 60' times - this is a fact. On regular roads in winter - from my experience going uphill was a lot easier in xi than in i. To the point that even with good tires 328i could not go up frozen hill, while my xi did it. So it's a win-win. I don't see much sense in discussing little details and nuances of why x-drive is not a real AWD and such. It's more than good enough for 99% of applications - unless you autocross lol

[Rocklicker]

Don't know why you folks are saying it's "not a real AWD". From both a semantic and practical standpoint a transaxle transferring power through a viscous coupling or torsen differential and a transmission transferring power through a variable slip transfer case are both AWD systems that achieve the same results in fundamentally different ways, primarily for different architectures.

Sure some systems are better. I'd take the two center diffs in an STI over a Honda Rav4 center diff any day but they are both still AWD.

Oh and reading through this was funny. I knew BMW went off the deep end of consumer relations end when I got this car and saw my mpg needle swing from 12 to 100 when I came to my first stop. We apparently need to be told we are special snowflakes that can't possibly get 0 mpg, even if it's a complete lie.