All-Wheel Drive Like You've Never Seen It!
The technology of Holden's high performance four-wheel drive breakthrough.
Extracted article by Julian Edgar, October 1 2002, www.autospeed.com

If the idea of a V8-powered, all-wheel drive curvaceous and beautiful Monaro or Pontiac GTO coupe rings your bell, here's the final ingredient to really get the excitement rising. Because we can now guess with near-certainty the type of four-wheel drive system that will be used in the cars - and it's breakthrough engineering that has huge implications for how well they'll drive.

Stand back - we're talking about the potential for performance-orientated all-wheel drive not seen since the release of the Skyline GT-R!

Holden Plans

It's no secret that Holden is developing a range of all-wheel drive vehicles. How extensive that range is remains to be seen, however the pictured Cross8 concept car - released in February of this year - shows that at least a jacked-up all-wheel drive wagon will be available. But it'd be economic madness to engineer all-wheel-drive for just that one car - if the system fits under the Cross8 (based on the long wheelbase Statesman platform) then it will also be happy under the Commodore and Monaro (the latter to be sold as the Pontiac GTO in the US).

So? Well, all-wheel drive is becoming increasingly popular, and while the idea of an all-wheel drive performance V8 is pretty damn attractive, how will it drive? Will it use a viscous coupling to give a Subaru WRX-style torque split? This gives handling that is safe and secure but prone to understeer. Or will it use a torque split favouring the rear wheels, giving a traditional rear-wheel drive power oversteer bias? Or what about an Audi-style Torsen centre diff, which feels a lot like a 50/50 viscous coupling but gives better throttle control mid-corner?

Well, now we can tell you the almost certain answer: the Holdens will use one of the most technically fascinating all-wheel drive systems in production. Electronics will control not only the front/rear torque split, but also look after the side-to-side traction. Not only will the handling outcome be easily able to be software-tailored from model to model, but the aftermarket should be able to tweak the torque split to suit individual preferences!

Add to that the likelihood of a 6.2-litre Gen III engine punching out 350kW and 600Nm, and you're talking about a car that has the potential to be an absolute performance king....

All-Wheel Drive Systems

All constant all-wheel drive cars use a centre diff. In the same way that a normal differential allows the inner and outer wheels to turn at different speeds while the car is cornering, a centre diff is needed in all-wheel drive cars to cope with the variation in front and rear axle speeds. (In older cars where four-wheel drive could be manually selected and no centre diff was used, driving in tight circles on bitumen resulted in the whole driveline groaning as it attempted to bind itself up....)

The centre diff has always needed to be of a limited slip design, otherwise the wheels at one end of the car could spin under power, while the wheels at the other received no drive at all. A viscous coupling (a device where plates in close proximity to each other are immersed in a silicone fluid that gets more viscous as the speed differential between the plates increases) is the most common approach - it's cheap, easy to 'tune' in its behaviour during development, and has no parts in contact with each other to wear out. The downside is that it reacts only to the speed differential between the front and rear axles. All manual gearbox four-wheel drive Subarus use a centre viscous coupling.

The Torsen ("torque-sensing") diff alters in its behaviour on the basis of the amount of torque going to each axle, and in practice works much better than a viscous coupling - Audi has long been champions of Torsen centre diffs in their all-wheel drives.

The final common all-wheel drive approach is to power the wheels at one end of the car most of the time, feeding torque as necessary to the other pair by means of a wet multiplate clutch. This clutch can be engaged electronically, or be under the control of a pump that operates when the front and rear wheels rotate at different speeds. The electronic control can be based on inputs from wheel speed, longitudinal and lateral acceleration, throttle opening - and so on. The famous Skyline GT-R uses the electronically-controlled multiplate clutch approach to feed power to the front wheels as required.

In addition to a centre LSD, the majority of high performance all-wheel drive cars run a rear LSD. Without this, the situation could develop where one front wheel and one rear wheel were spinning - with the car going nowhere. The older model 5-cylinder turbo Audi S4 could get front or rear wheelspin when really thrown around to the extent that individual wheels went light. Finally, some cars - eg the current Subaru WRX STi - use three LSDs... every differential in the car is limited in slippage.

OK, so what would you say if we told you that the Holden all-wheel drive system uses three open diffs - not one of 'em is an LSD design!?

To make sense of how the Holden system will work, think for a moment not of four-wheel drive cars, but of those with two-wheel drive. Some two-wheel drive cars have a traction control system that works by braking the spinning wheel. The "electronic diff lock" Golfs use this approach, as do a number of other cars. They are fitted with an open (ie non-LSD) diff.

Imagine that one of these cars is placed so that one wheel is on a slippery surface. The driver dumps the clutch and that wheel starts to spin. However, as soon as it's turning faster than the other wheel, it is automatically braked. This immediately sends lots of torque across to the other wheel, which is therefore getting most of the power. The car drives off. So on this car the torque split from side to side is being controlled by the braking of individual wheels.

Now, take that idea and apply it to not only the side-to-side torque split, but also the front-to-rear....

The Cadillac SRX

General Motors in the US has just released information on a 2004-model SUV which uses this 'braking individual wheels' approach to controlling the four-wheel drive torque split. Due for release mid-2003 (not coincidentally, the all-wheel drive Holden is also due out next year), it's our bet that the Holdens will use exactly the same technology - it's the only four wheel drive system that uses three open diffs...

To quote GM on the four-wheel drive system of their new 4.6-litre V8 Cadillac SRX:

"The uniquely designed and integrated full-time all-wheel-drive system provides the Cadillac SRX with superior handling performance, acceleration, traction, and control for dry, wet, icy, and snowy road conditions. Its three open differential layout, coupled with four-wheel traction control, balances driving torque distribution to each wheel and minimizes wheelspin to optimize on-road performance.

"The transfer case includes an open centre differential, which has a torque split of 50% to front and 50% to rear, providing a distribution of torque consistent with the vehicle's weight distribution, allowing for better handling and traction. The housing is a lightweight aluminium casting. The transfer case directs power to the rear coaxially along the principle powertrain centreline while providing power to the front via a quiet chain drive to offset front output on the passenger side of the vehicle.

"The open front differential is very compact, allowing for flexible packaging and providing a 3.23:1 final drive reduction for the V8 version of the SRX. It rigidly attaches to the engine oil pan, with its intermediate shaft passing through the pan and supported by a bearing housing on the opposite side. The design also includes a lightweight aluminium die-cast housing.

"The Traction Control system uses four-channel brake controls to limit excessive wheel slip and preserve the torque distribution provided by the three open differentials. During traction control events, the system individually applies the brake(s) to an excessively spinning wheel(s) to slow the wheel down and re-establish torque balance in the driveline. If necessary, the advanced control algorithms also manage engine torque [ie vary throttle opening]. This integrated control approach allows the wheel(s) with the greatest tractive potential to apply torque to the road and deliver the performance and handling the driver is requesting - on dry pavement as well as surfaces degraded by rain, snow or ice.

"The system-controlled individual wheel brakes take over the function of any of the torque-biasing driveline devices that may be used by other systems to overcome a free-spinning wheel, such as a limited slip, locking rear differential, viscous coupling and clutch plates.

"Unlike the intrusive systems of some European sports sedans, the Stability and Traction Control systems in the SRX system are calibrated to reduce wheel slip and yaw in an imperceptible manner that allows drivers to continue driving their vehicle as they want. It also provides a driver-selectable button on the console, which allows increased wheel spin/speed for high performance driving on gravel, rainy or dry surfaces, while maintaining basic system benefits and intervening to protect driveline components and driver safety.

"Unlike many systems, such as those with mechanically locking devices, the SRX's system is completely compatible with ABS and Stability Control.

"Traction control can be disabled independently of the StabiliTrak system. By momentarily pressing a button on the console, the driver can shut off the traction control to get increased wheel slip for certain surface conditions, such as in sand, and still maintain the directional benefits of StabiliTrak. Pushing and holding the button for five seconds enables the driver to shut the whole system down. Competitive systems typically don't provide such options. They only allow the driver to shut the whole system down."

StabiliTrak is GM's stability control system - and it's worth exploring the all-wheel drive implications for stability control as well. Stability control is used to yaw a car around its vertical axis, to keep it on the road when otherwise it'd be sliding off. When a car is understeering, stability control brakes the inside rear wheel to pivot the car back on line. When it is oversteering, the outside front wheel is braked.

The Implications

Now, put all of this together and what do you have?

First-up, in 'normal' conditions the torque split front/rear and side/side is 50 per cent each - in other words, each wheel is apportioned 25 per cent of the available torque. But what about in a straightline drag? If the loud pedal is tromped, there will be a weight shift rearwards and the front wheels will go light. They'll just start to spin before they are braked to reduce their speed to being the same as the rear wheels. This braking of the fronts will send more torque rearwards - the result will be strong acceleration.

But let's get a bit more complex. What happens if the car is being cornered at high speed? With a 50/50 front/rear torque split, the car will start to power understeer - the front tyres being overloaded with their combined torquing/turning duties. Now with this system there're a whole heap of possible corrections for that. The front wheels could be both braked a little to send more torque to the rear wheels. Or, the inside rear wheel could be braked, as would normally occur with stability control. But in this case, that would send even more torque to the front wheels - and the outside rear wheel. So perhaps all the wheels but the outside rear wheel could be subtlety braked?

Start thinking it through and this is a phenomenal system - it makes the distribution of torque by wet multiplate clutches and viscous couplings look old-hat. Like, the torque can be separately directed to any individual wheel! And not only for the purposes of gaining traction, but also for the achievement of stability control outcomes. Yes, rather than just slowing a wheel to pivot the car, slowing three wheels - and so increasing the torque going to the fourth - could be used to literally power the yaw movement of the car. In an understeer situation, the outside-rear wheel could be fed 75 per cent of the torque!

Hell! Talk about the ability to actively control the car in performance driving!

And there's nothing to stop the software being set up to allow power oversteer - or even power understeer going to neutral going to power oversteer. Simply by changing the software calibrations it will be possible to have anything from a lair-arse power oversteering car to one which remains stable and competent with even the most outlandish driver inputs. And variations in the torque split and stability control strategies are likely to be driver-selectable - after all, even on the Cadillac SRX there are different levels available.

If Holden really exploit the capabilities of this system in a sporting V8 package, there'll be nothing coming close....