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Discussion Starter · #1 ·
I have a very simple question! what kind of awd systym does my 05 srx have, is it full time 50/50 split, or does it kick in when i start slipping, and am i always in awd. also..if i hav an 05 do i have the most recent nav dvd>?.

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2007 N* Thunder Gray/Ebony AWD/NAV/MRC
877 Posts
How does the SRX AWD work? Probably the following is more than you wanted to know.

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.

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