[randomx]

Hi all,

I'm keen to find out how this new xDrive system distributes its torque. Some sources say the rear will receive a max of 50%, others say its fully variable.

Can anyone confirm?

Thanks,

Josh

[ttimbo]

My understanding is that it uses the Haldex 5 system, which is fully variable and can transfer up to 100% of the torque to the rear in other applications (i.e., VW/Audi) but I've not yet read if BMW implements the system the same way.

[ShinySteelRobot]

Originally Posted by randomx

Hi all,

I'm keen to find out how this new xDrive system distributes its torque. Some sources say the rear will receive a max of 50%, others say its fully variable.

Can anyone confirm?

Thanks,

Josh

According to the X1 press release, the system can transfer up to 100% of the torque to the rear wheels in a fraction of a second.

[r999]

Originally Posted by ttimbo

My understanding is that it uses the Haldex 5 system, which is fully variable and can transfer up to 100% of the torque to the rear in other applications (i.e., VW/Audi) but I've not yet read if BMW implements the system the same way.

BMW's system is based on a Haldex clutch, but modified by BMW. There is a non-technical description here: http://www.autocar.co.uk/car-review/...ourer-review-0

The formal description from BMW is to be found in the Technical Training document on the F48's powertrain, available here: http://f48.bimmerpost.com/forums/sho....php?t=1177240 Notably, it repeatedly avoids saying that all of the power can be sent to the rear wheels. For example on p. 18: "With xDrive some of this driving power is transmitted to the rear output shafts". And on p. 28: "Depending on the driving situation, the longitudinal torque distribution can direct some of the driving power (up to maximum torque of 1300 Nm) to the rear axle, sometimes before the wheels even spin." That is a slightly strange thing to say since the max torque on any F48 model is only a fraction of 1300Nm. But the repeated use of "some" makes it fairly clear they are avoiding saying that all of the torque can be sent to the rear.

Why are these Haldex-based systems popular? Well, a large part of their reason for existing is to avoid sending power to the rear axle until it becomes necessary, since that is what improves fuel economy. So a Haldex-equipped car is essentially a FWD car that can become a 4WD car or even a nearly-RWD in less than half a rotation of a road wheel. It will revert to FWD just as fast.

How good are these systems? This article http://www.pistonheads.com/news/feat...he-truth/31854 has quite a lot to say about it. And I have been driving a good Haldex for 4 years, the 4WD Yeti, and can say it handles quite like the Golf GTi on which it is based but with the added benefit of 4WD in a fraction of a second whenever required. It's a clever and very effective system, though all the tests show that in really serious off-road stuff it falls well short of Land Rover or Subaru abilities. And on smooth straight level tarmac, you have to accept you are driving a FWD car.

[ShinySteelRobot]

Originally Posted by r999

Notably, it repeatedly avoids saying that all of the power can be sent to the rear wheels.

Great catch. Looks like I had incorrectly interpreted what BMW says in their press release, "The weight-saving, compact and efficient system uses an electro-hydraulically controlled multi-plate clutch to distribute drive between the front and rear axle just as required for the situation at hand."

I had interpreted that as "they transfer everything", but in retrospect they don't say "distribute all the drive". Hmm. It's actually a little bit misleading, IMHO.

Now I'm curious exactly what percentage can actually be transferred to the rear wheels.

[brissim]

Volvo have been using Haldex for more than 15 years on their AWD models - probably ever since they switched from RWD to FWD in the 90s. All their current AWD models use the "Gen 5" system. As r999 says. the real advantage is that for the vast majority of the time most work is being done by the front wheels alone and so you gain the fuel economy advantage that this gives, while still having AWD traction when needed. From Volvo forums over the years I've gathered the AWD system is considered very useful in snow and ice (something we don't have a lot of in Australia). From about Generation 3 onwards, the Haldex system has been very reliable for Volvo.

Before I switched to BMW with my current X3 we came from a Volvo background with 2 of our last 3 Volvos being AWD.

Tony

[ttimbo]

Thanks for that reference. Interesting reading.
I think the use of "some" probably reflects criticism of other Haldex 5 equipped vehicles, which repeatedly marketed themselves as transferring "up to" 100% of the torque to the rear axle. This is, at best, probably theoretically possible. Realistically, you can expect 50% or greater, depending on the situation. VW/Audi forums used to have quite a few demonstrations, showing the rear wheels spinning on loose gravel roads.

I had this system on my Audi S3. At the same time we also had a Golf GTD. While the Audi is FWD most of the time. A hard shove on the throttle quickly alerts you about the additional traction, and made the car feel very secure on slippery roads relative to the FWD Golf.

[randomx]

thanks for all the info : )