BMW all-wheel-drive vs Subaru AWD vs MB 4-matic

[etam]

Can someone educate me about the different of those system?

Regards,

etam

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[STiShawn]

The MB 4-Matic is an automatic 4 speed tranny....not all wheel drive. The VW 4-Motion is similar to Audis Quattro, but I think the VW uses a viscous center diff like the suby's, unlike Audi's torsen diff.....the BMW unit uses a transfer case like a 4x4 truck, albeit much smaller, lighter and compact. If I am wrong please correct me....

[STiShawn]

Check this other thread out too... http://www.i-club.com/ubb-files/Foru...ML/000635.html

hehe post#666

[This message has been edited by STiShawn (edited December 13, 2000).]

[RyanC]

NEgative, ghostrider, 4Matic is an AWD system. I have no idea on the specs for it though, try the Merc web site perhaps?

[red]

Here you go: http://www.fortunecity.com/silversto...ion_4wd_21.htm
"Active differential 4WD is the most sophisticated one available today. Basically, active LSD is actually a multi-plate clutch enabling variable torque split between front and rear axle. The torque spit is controlled by computer which gather information about tyre slip and others from many sensors. Depends on design and software, some systems allow more precise control of traction during hard cornering, some achieve desirable understeer / oversteer, some can even make the best use of traction for acceleration and braking during normal conditions. Since active differential was pioneered by Porsche 959, we take the Porsche system as an example.
Porsche 959's PSK system - the most sophisticated
Until today, Porsche 959's PSK (Porsche-Steuer Kupplung) system is still the only one which make use of variable torque split for maximum traction under normal conditions. In most of the time, torque split between front and rear is 40:60, that is, the same as the car's weight distribution. In hard acceleration, weight transfer to the rear axle increases traction in the rear tyres while reduces traction in the front. Then PSK will transfer up to 80% torque to the rear axle in order to make better use of traction. On slippery road (even tyre slip is yet to occur), 50:50 torque slit is used. In any time, computer determines the torque split ratio by analysing parameters such as throttle position, steering angle, g-force and even turbo boost. Therefore PSK system provides optimum traction under all conditions, unlike other 4WD systems which can only varies torque split whenever tyre slip occurs.
Mechanism

Porsche PSK uses a multi-plate clutch instead of center differential. You may call it a "differential clutch" as well. The multi-plate clutch has 6 pairs of frictional plate, each pair is independently controlled by computer and actuated by hydraulic pressure. This simply equals to 6 independent clutches.

To make this system work, the front and rear driveshafts must run at different speed in normal condition. (so 959 adopted a pair of front tyres with 1% larger diameter than the rear's) Because of the speed difference between front and rear driveshafts, the 2 frictional plates of each independent clutch are rotating relatively to each other. When apply hydraulic pressure to the first clutch, a small amount of torque will transfer to the front axle. But note that the two driveshafts cannot be fully locked up unless all 6 clutches are locked simultaneously.

Now, you may see how it works: lock up 2 clutches, 3 clutches ... and the torque to front wheels will be increased, subsequently, torque split could be 50:50 if all the clutches are fully engaged. Of course, all these action is controlled by computer.

This is only for "normal" condition. Like other 4WD systems, when tyre slip occurs, most of the torque could be
sent to either axle.

What about energy loss and wear due to the slipping clutches? As the speed difference is very small, Porsche claimed energy loss is no greater than 0.4% of the power developed by the engine. As for wear, the clutch is dimensioned that it was negligible and caused no problem during the whole life span.
Advantage: Variable torque split even in normal condition. Best use of traction.
Since computer monitors and controls everything, theoretically it can cope with any normal or abnormal condition without being limited by the design of mechanical parts. The result is fast response and adaptive.

Disadvantage: Heavy and expensive
Who use it ? Porsche 959 only


--------------------------------------------------------------------------------

Mercedes 4-Matic - use under emergency
Mercedes 4-Matic is very much like Porsche's PSK, but it works as a part-time 4WD. In normal condition, the clutches are disengaged so that the car runs just like any rear-wheel drive version. When it lose traction, the computer will signal the clutches to engage progressively so to transfer some torque to the front wheels.
This is rather irrational. It costs even more than Torsen system, but only offers part-time 4WD. However, unlike Porsche's PSK, it is not subjected to any energy loss or wear during normaly conditions, so the clutch can be made smaller and cheaper.
Advantage: No much
Disadvantage: Part-time 4WD only; not cheap.
Who use it ? Mercedes E-class 4-Matic "

[Necromancer]

Hey, that description of the Porsche 959 system sounds an AWFUL lot like the Subaru automatic tranny .... hmmmm...

[brainlock]

this is some of the info from vwvortex.com
that i gleaned when the above post was repeated
in their forums..
http://www.audiworld.com/tech/audi2.shtml

this one has been seen before http://www.eskimo.com/~eliot/awd.html

and here is the link to their discussion http://forums.vwvortex.com/vwbb/Forum1/HTML/012250.html

[STiShawn]

Thank you for correcting me, I was wrong.Sorry about that.