All-wheel Drive systems all have their advantages and disadvantages. This BMW bunch in the video (more than likely just a dealer doing their own X3 launch to prospective customers) know how to exploit a potential weakness in Torsen centre differential-based Quattro. A Torsen differential is an exquisite (for nerds like me at least) piece of
mechanical engineering. It’s purely mechanical unit – not making use of viscous fluids or electronically actuated clutches – that comprises of helical gears located in a planetary gear arrangement. Purely through the laws of physics this Torque-Sensing (Torsen – get it?) differential apportions torque front to rear – when employed as a centre diff – in infinitely variable amounts depending on which
side of the diff has the most frictional resistance. So, depending on which
axle has the most grip Torsen will send as much of the torque as possible to that axle. It’s a superb device for road cars, infinitely apportioning torque back-and-forth to the grippier axle.
Disadvantages: a Torsen diff is a torque multiplying device. This means that if a complete loss of frictional resistance is experienced at one end, then no torque can be delivered to the other side.
Quite simply: Torque x 0 = 0. Another, lesser disadvantage is the mechanical limitation of a Torsen diff to apportion more than a certain percentage of torque. If I recall correctly, only about 80% of the drive can be apportioned to either side. Now here’s the cool thing: as soon as any resistance is available at an axle, just like a tyre gripping the road, Torsen works – beautifully. Modern traction control systems do exactly this; by braking the disc of a spinning wheel (or wheels) resistance is restored and the drive can be sent to the axle with the grip.
I am 95% certain that unscrupulous demonstration techniques are at work here: the Q5 has had its traction control system disabled (more than likely the fuse has been pulled) because the demonstration is purposely and childishly designed to exploit the mathematical limitation in Torsen and not the complete Quattro solution as a whole. In exactly the same way that a road provides resistance to the wheels, the traction control system will brake the spinning wheels restoring resistance for Torsen to work.
BMW’s xDrive uses clutches electronically to engage either side of the differential and as a result xDrive can apportion the full 100% of drive to the front or rear axle. Hence the format of the demonstration, which I might add is very unrepresentative of a real world condition that you may encounter. Both wheels of an axle losing traction at once? What were you trying to do? Reverse off a ledge? Drive on to a frozen lake? Ahh, of course, you were doing a BMW technology demonstration.
Every permanent all-wheel drive system needs some form of centre differential to allow the front and rear wheels to rotate at different speeds (because they do when cornering as the front axle tracks a different path to the rear) and some of these devices work better than others in certain conditions and vice versa. This is purely as a result of the compromises arising from the elected mechanical solutions than some my-brand-of-4x4 –is-better-than-yours nonsense.
A much more telling measure of an AWD system’s ability to cope with a greater variation of low-grip scenarios is to check out what kind of limited slip differentials are located at one or more axle. The reason I say this is that the marketers use the term “transferring torque to the wheels with the most grip”. Notice the use of “wheels” plural. If they said “...
wheel with most grip” and all they have is an ordinary open diff front and rear then I’d say that their claim is contentious and not technically correct. Both Audi’s Quattro and BMW’s xDrive centre differentials only apportion torque to the
axle with the most grip. It’s the ABS/Traction Control system (completely removed from a mechanical perspective) that brakes a spinning wheel in order to restore resistance and thus apportion drive to the wheel that does have grip.
A far more realistic scenario for an AWD system to lose drive altogether is the dreaded cross-axle loss of grip often found when traversing uneven off-road terrain. In this case a front wheel at one corner loses grip and the opposite rear wheel loses grip. Now, if you’ve just got an ordinary open differential and no traction control system to brake the spinning wheels, you’re going nowhere. Interestingly, BMW’s xDrive sans DPC rear-diff has two regular open differentials at either end and will rely on wheel speed sensors in the ABS system to signal that a wheel needs to be braked. So nothing all that special to write home about.
A really cool AWD system – like that on a WRX STI - will use three limited slip diffs, front, centre and rear to apportion as much drive to the grippiest wheel (not just axle) before having to rely on braking a wheel. Combine this with a sophisticated 3 level traction control system and now that’s what I call an AWD system.
BMW DPC or Quattro sport differentials are nice examples of great OEM supplier innovation finding its way into production vehicles but should not be construed as one brand of AWD system being better than another.
I found this post of mine [bad etiquette, I know] from October 2005 – 5 years ago
just for a bit a laugh. The fundamentals are the same but my views have changed slightly.
martinbo said:
There are 3 main types of AWD on any given vehicle and their fundamental behavior is governed by the single most important component of any AWD system: the centre differential.
1. Semi-permanent, reactive AWD
You'll notice the list of cars in that Haldex page, most of them are FWD cars and the Haldex LSC acts as a limited slip centre differential that reacts to front wheel slippage (very quickly mind you) to send torque aft to the rear axle. Once the slipping stops, so does the all-wheel drive. So, the ultimate benefits of AWD *2 aren't attained and the car behaves predominantly in much the same way as its FWD counterpart might do i.e. the vehicle has a natural tendency to understeer quite dramatically when pushed close to the limits of adhesion. Examples are the Audi S3, Golf R32 and Volvo XC70 Cross Country.
2. Permanent, reactive AWD
This is a much better system - especially if the torque split is 50:50 or close to that - as the benefits of AWD are already in place and working for the driver. Because the torque split is so even, the wheel that has the most grip is doing most of the work and so, even in the event of wheel spin the limited slip device such as a viscous coupling has much less work to do in terms of apportioning a percentage of torque to the axle with most grip. It's quite simple: take a powerful car like an R32 with 320 Nm and divide that between the two front wheels = 160 Nm per tyre. Take a US spec STI with 400 Nm divide by all four wheels = 100 Nm per tyre. So the limited slip diff has a more even spread of torque and thus has less to do when moving it from front to rear. The LSD reacts to losses in grip at a given axle. Subaru is a prime example of this system as is Porsche and to a lesser extent, BMW's x-Drive which uses electronic sensors to signal the transfer of torque. Such a vehicle has a natural tendency to understeer when pushed beyond the limits of adhesion.
3. Permanent, proactive AWD
The ultimate AWD systems, the most notable example being Torsen as found in the Audi S4, RS4 and RS6 for example. It takes the above concept of AWD to a different level. The Torsen centre differential is a worm / planetary gear arrangement that is a mechanical work of art. It's name is derived from TORque & SENsing because really that's what it does: senses which axle has the most torque based on the frictional co-efficient or the amount of grip that that axle has. It reacts early on to subtle changes in torque before grip is lost. Hence, my term proactive. This is the ultimate in LSD technology in my opinion.
