Yesterday we reported that MINI is planning to release a all wheel drive Clubman using a heavily revised version of ALL4. Today we give you all the technical glory of MINI’s new ALL4 system.
Thanks to our inside knowledge of MINI’s plans and BMW’s xDrive system just released for the UKL based 2 Series Active Tourer, we have the full technical briefing on MINI’s new Clubman ALL4. We expect the system to be available on Cooper, Cooper S and (outside the US) high output diesel models.
It’s worth noting that this is the system that will not only appear in the Clubman ALL4 but also the 2017 Countryman ALL4.
In case you missed our previous article, the 2016 Clubman ALL4 will begin production in March of 2016 and will likely retail for approximately $1,500 more than its front wheel drive equivalent. Then later in the year MINI will release the JCW Clubman with a slightly revised version of the same ALL4 system. Production of that car should begin in November of 2016.
Upsides with Limited Downsides
All wheel drive is all about priories. If traction and safety is a priority over outright performance and efficiency, ALL4 should be paramount. But as we’ll find out, MINI has further reduced those traditional downsides that all wheel drive has brought. In fact performance has a good chance of being on par with the two wheel drive model and the MPG figure will likely only be marginally worse.
This is partially due to the fact that the traditional ALL4 weight penalty has gone down to only 134 lbs with this new revised version. Furthermore, measures for reducing system-related losses and an energy-efficient operating strategy have made MPG loss much less of an issue. But enough of the high level. Let’s talk details.
The system is major revision to the one that debuted on the R60 Countryman. Power from the front drive to the rear axle is transferred by means of an angular gear (Power Take-Off) on the front differential and a two-part cardan shaft. The central component of the four-wheel drive system is an electro-hydraulically controlled multiple-disk clutch (Hang-On) inside the rear axle drive, which facilitates infinitely variable distribution of torque to the front and rear wheels. The corresponding commands are provided by an electronic control unit which, like the hydraulic pump, is located on the rear axle. The idea is completely invisible engagement resulting in constant traction in any condition.
The angular gear is mounted behind the engine on the automatic transmission and crankcase. The input shaft is a hollow shaft construction and directly connected to the front axle differential. In this way, part of the drive force is transferred from the differential basket to the cardan shaft via the hollow shaft, the crown wheel and the pinion shaft. The angular gear operates at a fixed gear ratio (1:1.74) and is permanently engaged, meaning that the cardan shaft always rotates when the vehicle is driven. Reversal of transmission takes place in the rear axle drive so that the front and rear axle drive shafts both rotate at exactly the same speed.
The multiple-disk clutch located in the rear axle drive (Hang-On) directs a proportion of torque to the rear wheels according to each driving situation, ensuring optimal power distribution between the front and rear. In extreme cases (e.g. the front wheels are standing on ice), the ratio can be 0:100. The required operating pressure (0 to 40 bar) is delivered by an electro-hydraulic pump, the speed of which is defined by a pulse-width-modulated signal from the electronic control unit. Pressure is not measured by a sensor, but extremely accurately by means of voltage and power alignment. In order to ensure maximum positioning accuracy, run-in behaviour and temperature influences are independently compensated, the system constantly adapting to ever changing operating conditions. We told you this is an improvement.
As before, when extra traction isn’t required the ALL4 system reverts to front wheel drive improving efficiency. That means the pump is deactivated, rendering the system unpressurised. In order to make use of additional saving potentials, the system has a multiple-disk clutch with a spring-loaded Efficient valve, which lowers the oil level in the clutch and significantly reduces friction losses (oil splash losses). When required, the system takes only fractions of a second to build up maximum operating pressure in the Efficient mode and thus deliver maximum torque to the rear wheels. As you’d expect this all happens without loss in traction or any discernible change in the drivetrain.
Like all MINI and BMW all wheel drive systems, everything is managed from the control unit of the Dynamic Stability Control feature (DSC). DSC analyses a large amount of data providing information in an effort to ensure optimum distribution of drive torque. This information includes vehicle speed, lateral and longitudinal acceleration, steering angle, wheel speed, longitudinal inclination, accelerator position and the setup via Driving Experience Control. Any adjustments are made within fractions of a second in an effort to make power distribution between the two axles virtually unnoticed by the driver. In addition to this, torque distribution is precisely regulated as to avoid any loss of power due to wheel spin.
Due to the DSC controlling everything, wheel slip can be detected at an early stage. In an example where a Clubman threatens to drift outwards over the front wheels (understeer), increased tractive force is supplied to the rear axle, allowing the vehicle to turn in more accurately. On the other hand, ALL4 directs excess force to the front wheels, should the rear of the vehicle threaten to swerve outwards. As a result, maximum four-wheel performance is available even before slippage occurs. Put simply, the updated system not only ensures best possible traction and safety in wet or snowy road conditions, but also enhances vehicle stability, cornering dynamics and ride comfort. Further, in driving situations where the interconnection of all four wheels is disadvantageous – i.e. in an emergency stop – the system opens the multiple-disk clutch completely within milliseconds.
Only if optimum power distribution to the front and rear axle is not sufficient to keep the Active Tourer on the desired course, DSC intervenes by reducing engine output and/or by decelerating individual wheels. Moreover, DSC assumes the function of a transverse differential lock: If a wheel spins without transferring power, it is automatically slowed down, whereby the axle differential directs more power to the wheel opposite.
Look for the ALL4 Clubman to officially debut this winter with production beginning in March at the Oxford Plant. The highly anticipated JCW Clubman ALL4 will begin production in November. Look for more about that in the coming days in MotoringFile.