Yeah, but that one’s FWD

The last time my Wife was shopping for a car she was looking at a lot of different brands and models trying to size up exactly what she wanted.   She was thinking one possibility was a red convertible with leather interior, and so we went and test drove a variety of models that offered this combination, or as close to it as we could find.

As we waiting for the Sales Person to grab the keys for one test drive I explained that the vehicle we were about to drive was Front Wheel Drive (FWD).  My Wife asked, “Why are you telling me that; Do I care?”

This gave me pause.

To me, the fact that this car was FWD meant that it would not corner or accelerate as well as a similar Rear-Wheel Drive (RWD) car.  Further, it also meant that the car was not intended for a ‘serious’ car enthusiast / driver, but was being marketed or targeted for the features offered and not as a driver’s car.  On the other hand, a FWD vehicle can be very good in the snow or weather.  So one might choose a FWD performance car if one lived in parts of the country that get a lot of snow or harsh weather.

However, my Wife never plans to drive the car on the track.  If the wheels are screeching around a corner she will likely just slow down.  She probably prefers understeer to oversteer, for the same reason many manufacturers tune their cars to understeer — it gives the driver time to react and slow down, even if at the same time it tends to give up a bit of speed through the corner.  The fact that the car does 0-60 in 0.5 seconds less or more is immaterial in her weighing of the virtues of a car.  A very detailed argument on weight transfer under heavy acceleration being advantageous to a RWD car and disadvantageous to a FWD car would bore her.  And I paused in my response because it occurred to me, weighing these fact, that perhaps she did not care.

I think that BMW and I were very surprised recently to find that 80% of buyers of the rear-wheel drive 1-Series BMW think that the 1-Series incorrectly believe the 1-Series is FWD.  This reminded me of my Wife’s question.  Many buyers may not care that a car is FWD or RWD, only that it has the nameplate and features for the price range they are shopping.  I would have guessed that all BMW drivers were technically oriented, and were buying their BMW because of its driving and handling characteristics, and because it was a RWD performance sedan.  Not so apparently.

I am very glad that Cadillac went to RWD cars for the CTS family.  There is something re-assuring to me about the dynamics of a RWD car when cornering, and about the packaging of the engine longitudinally under the hood, with better associated engine access.  I am interested in how the new XTS does, and I am willing to believe that AWD can be a viable approach given tuning that allows a high percentage of torque to be vectored to the rear wheels when needed.  But I continue to believe that the best performance in acceleration and cornering will be available from a RWD configuration, and I am glad that Cadillac continues to offer RWD luxury performance sedans.

Cadillac AWD Technology, the videos

Driver interview while driving an All-Wheel Drive 2010 SRX  helps explain in lay-person’s terms why an All-wheel drive vehicle is superior as an all-weather vehicle:

For 2010 we have an all new, All wheel drive system.  The new feature is an electronic limited slip differential that is able to deliver torque across the rear axle of the vehicle to put power to the tire that has traction available to it.   This is really helpful in terms of your safety and security, making the car go where you want it to, getting started in tough conditions on hills and so on.  Just a great feature, and makes the car really enjoyable to drive when the going gets tough like this.

One of the great things about our Cadillac systems is the refinement that we offer, the smoothness of application and the sense of security that  the systems will give to you as you drive under tough conditions.  The vehicle  feels very sure footed, and makes you confident that you are going to get where you want to go, and get there safely.

This animation shows the system with shift in torque front to back or left to right:

You can read more about the AWD Haldex System here.  The new Cadillac XTS Platinum Concept car uses a similar AWD system as standard equipment.

To date all-wheel drive systems do not give advantages in terms of absolute cornering on a dry surface.  They can be an advantage in acceleration on a slick or dry surface, but because traditional AWD systems also brought a significant weight penalty the net advantage of the additional traction is more than offset by the added weight and thus reduced acceleration.  The new system is somwhat lighter, but also responds more quickly than traditional systems.

Cadillac All Wheel Drive – using the Haldex System

The new 2010 Cadillac SRX is available in front-wheel drive or all-wheel drive.  The all-wheel drive is the high tech choice, and makes the SRX a true all-weather, year-around vehicle.  The All-wheel drive system used is a 4th Generation version that can not only distribute power front to rear, but also left to right within the front/rear drive.

This system is likely to be a key feature / offering for the upcoming Cadillac XTS.  The new AWD system is different from that used on the current Cadillac CTS AWD.  The CTS AWD uses the Borg Warner InterActive Torque Management Transfer Case (ITM(R) tc) system.  The new Buick Lacrosse uses a similar AWD system to the Cadillac SRX.

Use of this new, light and efficient AWD system in the upcoming Cadillac XTS should enable the type of RWD sports luxury performance one would expect from a Cadillac while maintaining great all-weather performance.  In the Buick Lacrosse, the AWD system can send up to 85% of available torque to the rear wheels.  The Lacrosse specs show a weight impact of AWD vs RWD of 181 lbs.  Because the XTS will be on a similar chassis to the Lacrosse comparisons are apt, although the XTS is likely to have a 115″+ wheelbase vs the Lacrosse’s 112″ wheelbase.

Let’s examine this advanced AWD system used in the Cadillac SRX and Buick Lacrosse, and likely to be used in the upcoming Cadillac XTS.  From an article in the Haldex newsletter:

Mechanically, the SRX’s most notable new feature is an active and advanced electronically controlled, all-wheeldrive (AWD) system by Haldex that continuously distributes and transfers torque between the front and rear axles and between the rear wheels. Honed through more than 600,000 miles of tests on roads and test tracks across Europe – including Germany’s famed Nürburgring circuit, where Cadillac engineers have developed and validated new vehicle features for years – the system provides best-inclass levels of driving stability.

The SRX AWD hardware consists of a power take-off unit (PTU) in the front final-drive that transmits engine torque through a prop-shaft to the rear drive module (RDM) that includes a torque transfer device (TTD) and the optional eLSD. Both are wet, multi-plate clutch units from Haldex. Operating seamlessly and programmed to keep the car stable even when cornering (by splitting drive torque to counteract over- or under-steer situations), the AWD system also incorporates an innovative pre-emptive engagement of the rear wheels that eliminates the need to detect front-wheel slip before rear-drive activation, helping to both enhance and exploit the 265 horsepower generated by the SRX’s new standard 3.0-liter DOHC direct-injected V6 engine or the 300 hp delivered by the optional 2.8-litre turbocharged V6, both of which are paired with six-speed automatics. “Performance-wise, you get more horsepower and better economy [18 mpg in the city, 25 on the highway] with a V6,” Reuter says when asked about the change from the V8 in the 2009 model. “Fuel mileage is very important, and the price tag is also a little lower with a V6.”

Haldex Image
Haldex internal view

Basic Function

The unique design of the Haldex Gen IV compromises four functional parts:

* Hydraulic power pack with electrical motor and accumulator
* Wet multi-plate clutch
* Controllable pressure reducing valve.
* Electrical control unit (ECU).

The front and the rear axle of the car is connected via the wet multi-plate clutch which makes it possible to vary the torque distribution between the two axles. As the function of the Haldex Gen IV is independent of the differential speed between the front and the rear axle full lockingtorque, if needed, is available at any given time and speed.

When starting the vehicle the electrical pump in the Haldex Gen IV is started, swiftly providing the system with pressurized oil and thereby making the system ready for operation. The control valve sets the pressure to the piston which in turn compresses the disc package. The level of pressure set depends on the torque level needed which in turn depends on the driving situation. In traction/high slip conditions, a high pressure is delivered: in tight curves (i.e. parking), or at high speeds – a much lower pressure is provided.

Coupling Control Concept

Haldex Gen IV connects to the vehicle’s electrical system and to the data communication bus (i.e. the CAN-system), transmitting information on the driver’s actions, the engine, the transmission, the brakes and other on board systems.

Haldex Gen IV uses this information entirely for its control without the need for additional sensors. The available signals are received by the coupling’s processor and interpreted by the software. Continuous analysis of these data adjusts the characteristics of the coupling according to actual demand, without any active intervention by the driver.

The function of the coupling is automatically adjusted to prevailing conditions. When starting in sand for example, the shafts are coupled together as firmly as possible to obtain the best traction. When close cornering, i.e. parking, the shafts are uncoupled to allow easy maneuvering.

Haldex Drivetrain Model

The controllability of the Haldex Gen IV also makes it possible to use differently worn tires, to tow the vehicle with one axle raised, and to maintain function when using Run-flat-tyres or Mini-Spare.

Haldex Gen IV can use any suitable signal available on CAN. Signals from the ABS and ESP systems and the engine control are vital for performance. For enhanced performance, signals from a steering wheel sensor, yaw sensor, lateral acceleration sensor can be used if available.