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martinbo · 09-01-2006, 06:22 AM

Oh dear... here I go again. Sorry if I bore the pants off you but here it is:

Well then, it seems that there is a fair deal of consternation, misperception and conjecture being bandied about in this debate on which AWD system is superior. The sad fact is however, in plain and simple terms: there is no such thing as the perfect all-wheel drive in existence today. Some are better than others in certain aspects but no outright superior AWD system exists because of one simple concept. All wheel drive systems cannot be judged as merely drivetrains in isolation but need to be assessed together with the engine (including its placement and integration with the drivetrain) as a cohesive mechanical solution.

All wheel drive systems are in each example an object lesson in engineering compromise. Take Audi’s Quattro system for longitudinally mounted engines for example. It’s a magnificently elegant, symmetrical (more on the importance of that later) and compact design. However, it necessitates the placement of the engine ahead of the front drive shafts that emerge from the transmission casing, causing the wheels to be mounted further back relative to the engine thus giving the car an undesirable nose heavy front-to-rear weight bias. So, by hanging a big V8 or V10 ahead of the gearbox, Audi engineers then have to grapple with the extremely difficult task of tuning the front suspension to cope with all of that mass out front whilst still ensuring that the ride, handling and turn-in characteristics remain competitive. Ever noticed the A6 quattro’s jarring ride quality over sharp extrusions? Compromise. And certainly not a sensation that you’d experience in a 5 series.

Subaru’s symmetrical all-wheel drive suffers the same limitation, hence their adoption early on of the lightweight alloy boxer engine in an attempt to alleviate the situation somewhat. As a cohesive, integrated mechanical layout, Subaru’s is one of the more elegant and well-rounded ones out there. But it’s far from perfect. The benefit of Audi and Subaru employing this centre-aligned layout is symmetry, particularly important at the front axle where (because the front wheels are now given the duty of both propulsion and steering) equal length driveshafts negate the torque steer that results from uneven torque distribution caused by unequal length driveshafts. So, as far as all-wheel drive systems go, this layout interferes less with the steering loading and provides the best steering “feel”. Note that these types of AWD systems where designed from the outset to be all-wheel drive and never started out life as front-wheel or rear-wheel drive systems. Another important benefit is the balanced left to right weight distribution that such a layout provides.

A typical example of Audi’s longitudinally configured AWD system:

And here’s Subaru’s elegant and compact layout:

So, what of xDrive? As you’re all aware, BMW choose to mount their engines further back to achieve that hallowed 50:50 front to rear weight distribution. Hence, xDrive was engineered as a mechanical add-on or enhancement to this pre-existing layout. In that it necessitates rather more complex, bulky and heavy re-engineering in order to arrive at an all-wheel drive solution, BMW’s xDrive too is compromised in a number of respects. This is with particular regard to weight and maintaining a left to right balanced symmetrical drivetrain layout. The added complexity further compounds the inherent inefficiencies that characterise all-wheel drive systems: the loss of engine power reaching the wheels through driveline drag or frictional losses. The heavier and more complex your AWD system, the less efficient it is. This affects performance and fuel economy markedly. Note the large and heavy componentry and multi-clutch pack of the xDrive transfer case in the image below. Also note the external propshaft leading to the front axle and how this results in an off-centre front differential which isn’t desirable as this then necessitates the fitment of additional components to achieve equal length driveshafts. Quite a long way off the traditional BMW ethos wouldn’t you agree?

End of post 1 – continues…

09-01-2006, 06:22 AM	#7
martinbo Fanatic Moderator Emeritus Join Date: Sep 2005 Location: Johannesburg Posts: 1,701 Thanks: 324 Thanked 531 Times in 269 Posts	Re: AWD comparisons Oh dear... here I go again. Sorry if I bore the pants off you but here it is: Well then, it seems that there is a fair deal of consternation, misperception and conjecture being bandied about in this debate on which AWD system is superior. The sad fact is however, in plain and simple terms: there is no such thing as the perfect all-wheel drive in existence today. Some are better than others in certain aspects but no outright superior AWD system exists because of one simple concept. All wheel drive systems cannot be judged as merely drivetrains in isolation but need to be assessed together with the engine (including its placement and integration with the drivetrain) as a cohesive mechanical solution. All wheel drive systems are in each example an object lesson in engineering compromise. Take Audi’s Quattro system for longitudinally mounted engines for example. It’s a magnificently elegant, symmetrical (more on the importance of that later) and compact design. However, it necessitates the placement of the engine ahead of the front drive shafts that emerge from the transmission casing, causing the wheels to be mounted further back relative to the engine thus giving the car an undesirable nose heavy front-to-rear weight bias. So, by hanging a big V8 or V10 ahead of the gearbox, Audi engineers then have to grapple with the extremely difficult task of tuning the front suspension to cope with all of that mass out front whilst still ensuring that the ride, handling and turn-in characteristics remain competitive. Ever noticed the A6 quattro’s jarring ride quality over sharp extrusions? Compromise. And certainly not a sensation that you’d experience in a 5 series. Subaru’s symmetrical all-wheel drive suffers the same limitation, hence their adoption early on of the lightweight alloy boxer engine in an attempt to alleviate the situation somewhat. As a cohesive, integrated mechanical layout, Subaru’s is one of the more elegant and well-rounded ones out there. But it’s far from perfect. The benefit of Audi and Subaru employing this centre-aligned layout is symmetry, particularly important at the front axle where (because the front wheels are now given the duty of both propulsion and steering) equal length driveshafts negate the torque steer that results from uneven torque distribution caused by unequal length driveshafts. So, as far as all-wheel drive systems go, this layout interferes less with the steering loading and provides the best steering “feel”. Note that these types of AWD systems where designed from the outset to be all-wheel drive and never started out life as front-wheel or rear-wheel drive systems. Another important benefit is the balanced left to right weight distribution that such a layout provides. A typical example of Audi’s longitudinally configured AWD system: And here’s Subaru’s elegant and compact layout: So, what of xDrive? As you’re all aware, BMW choose to mount their engines further back to achieve that hallowed 50:50 front to rear weight distribution. Hence, xDrive was engineered as a mechanical add-on or enhancement to this pre-existing layout. In that it necessitates rather more complex, bulky and heavy re-engineering in order to arrive at an all-wheel drive solution, BMW’s xDrive too is compromised in a number of respects. This is with particular regard to weight and maintaining a left to right balanced symmetrical drivetrain layout. The added complexity further compounds the inherent inefficiencies that characterise all-wheel drive systems: the loss of engine power reaching the wheels through driveline drag or frictional losses. The heavier and more complex your AWD system, the less efficient it is. This affects performance and fuel economy markedly. Note the large and heavy componentry and multi-clutch pack of the xDrive transfer case in the image below. Also note the external propshaft leading to the front axle and how this results in an off-centre front differential which isn’t desirable as this then necessitates the fitment of additional components to achieve equal length driveshafts. Quite a long way off the traditional BMW ethos wouldn’t you agree? End of post 1 – continues… Last edited by martinbo; 09-01-2006 at 09:13 AM..