[GWA]

I am mystified about how 4WD is X Drive. The broad message is that it operates maybe 40/60% power to front/rear. The implication is that it is sort of permanent four wheel drive. Operating all the time. The percentage varies in special circumstances.

But maybe the front wheel power is only triggered when absolutely necessary for stability correction or to gain traction. The reason I suppose this is that there is no centre differential but rather a multiplate clutch to power the front axle. If this is the case it cannot presumably operate continually or the slippage sending under 100% would surely burn it out?

Put more simply, if you were driving through winding mountain roads would the 4wd operate for greater nimbleness all the time or does it only switch on briefly in semi- emergencies?

If someone reckons front and rear are pretty much always powered - how does the multiplate clutch not burn out?

Thanks in advance for the expert response.

[richard in NC]

Per the tech guides (see the training manuals sticky above).
Sorry about the formatting....

4.3.2.￿Power￿flow
The￿Dynamic￿Stability￿Control￿(DSC)￿activates￿the￿ fully-variable￿torque￿distribution￿between￿the￿front
axle￿differential￿and￿rear￿axle￿final￿drive.￿The￿r equired￿nominal￿torque￿at￿the￿multidisc￿clutch￿of￿ the
transfer￿case￿is￿adjusted￿via￿the￿integrated￿contr ol￿unit￿of￿the￿transfer￿case￿(VTG).￿This￿process￿i s￿subject
￿to￿correction￿functions￿in￿relation￿to￿the￿wear￿a nd￿run-in￿behavior￿in￿order￿to￿ensure￿optimum￿positio ning
￿accuracy￿throughout￿the￿entire￿service￿life.￿Cont inuous￿ongoing￿calculation￿of￿thermal￿loading
models￿in￿the￿control￿unit￿of￿the￿transfer￿case￿(V TG)￿protect￿the￿transfer￿case￿from￿destruction￿as￿ a￿result
￿of￿overheating.
The￿variable￿distribution￿of￿torque￿to￿the￿front￿a xle￿is￿superimposed￿on￿the￿rigid￿through￿drive￿to￿ the
rear￿axle.
When￿the￿multidisc￿clutch￿is￿open,￿the￿entire￿driv e￿torque￿is￿transmitted￿to￿the￿rear￿axle￿final￿dri ve
(0/100%).￿If￿the￿multidisc￿clutch￿in￿the￿transfer￿cas e￿is￿activated,￿the￿drive￿torque￿will￿normally￿be￿ distributed
￿between￿the￿front￿and￿rear￿axle￿according￿to￿the￿ BMW-typical￿characteristic￿(40/60%).
Due￿to￿the￿xDrive￿functionality￿in￿the￿DSC,￿the￿to rque￿distribution￿between￿axles￿can￿also￿be￿varied ￿arbitrarily
￿depending￿on￿the￿driving￿conditions￿(in￿response￿ to￿different￿road￿surface￿coefficients￿of￿friction ).
￿For￿more￿information￿on￿the￿xDrive￿functionality, ￿refer￿to￿the￿"F25￿chassis￿and￿suspension￿tr aining
￿material.

The later basically describes the torque front to rear is varied based on understeer/oversteer and traction (DSC & ABS sensors).

[The X Men]

It is basically a haldex awd system. Some Audi use haldex awd as well and it is permanent power to all wheels.

[bagekko]

X Drive is not 4WD, as it does not have a lockable transfer case. X Drive is a variable system that normally operates at 40/60 but can send 100% to either axle. True 4WD where the transfer case is locked is not nimble at all, difficult to turn and horrible turning radius. 4WD in high speed mode where the trasnfer case is not locked is still not as nimble as a AWD system IMHO. The multi clutch in the XDrive is lubricated that prevents wear and prolongs life, but eventually someday it will need to be replaced. The multiple plates in the clutch prevent slippage.

[xDrive35i]

Originally Posted by The X Men

It is basically a haldex awd system. Some Audi use haldex awd as well and it is permanent power to all wheels.

Audi uses a Torsen system in all cars but the A3, where the Haldex system is used. Since the B7 RS4, all of their Torsen systems operate with a variable but full time 40/60 distribution. Haldex usually provides a 50/50, 90/10, or 100/0 split in normal driving, depending on the system. There are no rear biased Haldex systems to date, but again, depending on the system, torque can be sent to the rear wheels in varying percentages. Some 50/50 Haldex systems can send 100% to the rear, some 90/10 and 100/0 Haldex systems can send 50%, 70%, or 100% to the rear. But universally, the Haldex system operates on a front drive basis. BMW use their own system with a full time 40/60 torque split, but 100% can be sent to either axle. Their old system (before they started calling it xDrive) was much like Mercedes' old 4Matic, which only sent power to the front wheels when needed. Mercedes' current 4Matic uses a full time variable 45/55 split (though some of their performance cars operate with 20/80 or 30/70), and is designed entirely in house. The CLA45 AMG 4Matic, however, uses a Haldex system because it is front biased.

[GWA]

I remain muddled on this.
A popular interpretation is that the system allocates power from 0/100 front rear to 100/0. The inference many make is that the system is apportioning power and that the base position is something like 40/60 front to rear.
However, a BMW workshop manual indicates that the rear axle is always fully connected. So it is always at 100%. Subject to the implication of braking mentioned below.
Consequently the electronics are essentially only able to adjust power to the front when necessary and then the tightness of the multi-plate clutch determines how much power the front axle gets.
So in a sense the rear axle is always at 100% and the front runs at 0 when disengaged to 100% when fully engaged.
The only time there is a sort of limit to what goes to the rear axle is when the stability control finds rear wheels are spinning and jams on the rear brakes. Only in that sense does power shift from rear to front.
What remains a mystery is how much power goes to the front when the electronic systems are dormant because no traction or handling problems are detected. So if you drive for 10,000 miles and you never hit ice, slippery roads, gravel, over ambitious cornering etc etc is the front axle running at 0% power because the multi plate clutch is not needed to be engaged? Or is its neutral position something between 0 and 100%?

[The X Men]

The X3 has a 40/60 split F/R. When its detect a wheel slip it can adjust torque to the front or rear. Front what I understand, when at very low speed or very high speed, 100% of the torque goes to the rear.

[clivem2]

With Performance Control the split is 20/80.

[lovec1990]

Originally Posted by clivem2

With Performance Control the split is 20/80.

Isnt performance control just sending power to and braking invidual rear tyre?

[HighlandPete]

Clearly with the transfer box clutch pack open, 100% of drive torque goes to the rear wheels. The clutch pack has to operate to get drive to the front wheels.

In data for the E83 xDrive system BMW refer to "pre control" and also state...

In normal driving, the clutch is operated with minimum slip so that a permanent four-wheel drive with a driving-torque distribution of 40% on the front axle and 60% on the rear axle is available.

Note "normal" driving = 40/60% split. Then we have all the specific conditions when torque transfer is governed by adjustment to the clutch pack and/or brake intervention.

I've not read any data that contradictes those comments for the F25 models.

HighlandPete

[clivem2]

Originally Posted by lovec1990

Isnt performance control just sending power to and braking invidual rear tyre?

I've read in a few place that the split is 20/80 though not on offical BMW sites, a couple of examples here:

http://www.thecarconnection.com/revi..._performance_3

http://www.rubensimports.com/reviews...troduction.htm

[Ranger 6]

Originally Posted by bagekko

X Drive is not 4WD, as it does not have a lockable transfer case...

X Drive is 4wd as it does use a transfer gearbox and the drive is always sent to all 4 wheels. 4wd is not defined by what's in the transfer box it is very simply defined by whether all 4 wheels are driven. So even the Haldex system is 4wd - after all even Land Rover use it....

Originally Posted by xDrive35i

Audi uses a Torsen system in all cars but the A3, where the Haldex system is used.

Not just the A3 - the TT, A1, Q3 and new A4 all use the Haldex system. Don't forget the Torsen centre diff was not used on the early Quattros either.

[The X Men]

Audi also recently started using a crown gear center differential in place of the Torsen.

[philtrick123]

Originally Posted by The X Men

Audi also recently started using a crown gear center differential in place of the Torsen.

http://www.audiworld.com/articles/au...xt-generation/

[GWA]

Thanks for the explanations. The significant point is how the percentage allocation front and rear is to be interpreted. If both front and rear are fully connected that is not expressed as 100% front and 100% rear. But rather 50/50 as the percentages are a measure of the share of gearbox output.
A BMW technical description states that the rear is "always" directly connected. Consequently the rear gets what is left after a little or a lot goes to the front. So if the multiplate clutch is fully engaged then the front ends up with 50% and the rear gets the 50% left over. So the rear can never ever go below 50%. The standard sort of benchmark allocation is 40/60.

[n55x3]

Originally Posted by Ranger 6

X Drive is 4wd as it does use a transfer gearbox and the drive is always sent to all 4 wheels. 4wd is not defined by what's in the transfer box it is very simply defined by whether all 4 wheels are driven. So even the Haldex system is 4wd - after all even Land Rover use it....


Not just the A3 - the TT, A1, Q3 and new A4 all use the Haldex system. Don't forget the Torsen centre diff was not used on the early Quattros either.

This is not correct. XDrive is not a 4WD system. A 4WD system has the ability to permanently run in 2WD or 4WD, via a transfer case. Further more, when the T-case is engaged you should avoid driving on dry pavement, especially when turning since the outside wheel will travel farther than the inside wheel and in turn cause binding within the front diff. If 4WD is engaged on dry pavement and you try to turn you will instantly feel the binding from the front diff.

AWD allows for all 4 wheels to be powered while still being able to turn on dry pavement by allowing slippage to occur.

Generically speaking you can easily differntiate between a conventional 4WD system and an AWD system just by knowing if the truck can be manually locked in 2WD only (Tcase not engaged) or 4WD.

[Booty]

Lots of differing opinions.... X drive helps you get out of the sticky stuff easier! Simples.

[xDrive35i]

Originally Posted by Ranger 6

X Drive is 4wd as it does use a transfer gearbox and the drive is always sent to all 4 wheels. 4wd is not defined by what's in the transfer box it is very simply defined by whether all 4 wheels are driven. So even the Haldex system is 4wd - after all even Land Rover use it....

Not just the A3 - the TT, A1, Q3 and new A4 all use the Haldex system. Don't forget the Torsen centre diff was not used on the early Quattros either.

My A4 does not use a Haldex system, nor does any vehicle in the B8 lineup.

[Ranger 6]

Originally Posted by n55x3

...Some 4WD systems have the ability to permanently run in 2WD or 4WD, via a transfer case....

There, fixed that for you....