BMW TIS description

this section is some text from the BMW TIS describing some design elements of the 325ix
BMW TIS SBT permanent all wheel drive

SUBJECT: Permanent All-Wheel Drive
Model: 325iX
A high-powered, permanent All-Wheel Drive model, based on the rear-wheel drive 325i has been developed with only slight modifications to the standard body. In general, the new 325iX provides improved handling performance via superior transmission of engine power to the road. Directional stability and traction. even under extreme operating conditions, have been improved over those already appreciated in rear-wheel drive cars. The power output is now distributed to a doubled tire contact area.

A transfer case is attached to the rear of the transmission and transmits power to the front and rear axle final drives The standard 26015 transmission has a modified output cover with connections for the new single arm shift lever mount as well as attaching points for the transfer case. An opening between the two units provides access to the flexible joint coupling flange connections.

The ZF A 95 Transfer Case (with modifications for special BMW requirements) is used as a power divider. The torque of the planetary gear differential is divided with 37% of the power transmitted to the front wheels and 63% to the rear wheels. In this manner the greater portion of torque drive is transmitted by the rear wheels which promotes easy handling and maintains the typical BMW handling.

The transfer case transmits power laterally via an Internal-toothed drive chain to the front drive shaft The amount of torque transmitted will depend on the ratio of the planetary gear differential and the viscous clutch. The toothed drive chain system is known as a power transmission method having the lowest possible loss over a wide range of conditions and drive distances and also as the most compact system of this type.

Aside from periodic oil changes, the transfer case requires no adjustments and is maintenance-free. 1. Viscous Clutch 2. Planetary Gear Differential 3. Drive Chain

The drive line to the rear final drive is straight via the planetary gear differential and viscous clutch. Tension between the driven axles is prevented in the All-Wheel Drive System. Axle speed differences are compensated within the planetary gear differential and viscous clutch in the transfer case, so that any trapped stress or strain between axles is eliminated. The shift lever is mounted higher than on the standard E30. A cast aluminum strut is used for mounting. The transmission and transfer case are mounted on a large cushioned rubber mount.

Description of Operation

The power from the transmission is transmitted via the transfer case input shaft to the planetary gear carrier and the planetary gears. Torque division takes place in the planetary gear differential. 63% of available power is transmitted via the annulus gear to the rear final drive and 37% via the sun gear and drive chain to the front final drive. The viscous clutch is a sealed, self-contained unit and is connected in such a way as to provide a non-rigid link between the driven axles. The plates contained in the viscous unit are connected alternately with the hub(rear drive) or the case (front drive) and all plates are surrounded with a highly viscous fluid.

When the driven axles encouter differing frictional surfaces, the viscous clutch will lock for a redistribution of power and, therefore, increase the torque of the driven axle with the better frictional surface. In other words, when the speed between the case (front drive) and hub (rear drive) differ, a locking action accurs. The shearing force between the plates with holes and the plates with slots as well as the viscous fluid produces a locking torque relative to the difference in plate speeds and fluid temperature.

The viscous clutch operates without noise, shock or wear as the plates experience no metal to metal contact. As illustrated, torque redistribution if fully automatic requiring no manual input from the vehicle operator. The viscous clutch has a lifetime filling of a special oil. The tranfer case if filled with ATF Dexron II (capacity appx .5 liter). The ATF in the tranfer case has to be changed at the 1,200 mile inspection and at every inspection II.

Driveshafts

Power is transmitted to the new differential for the front axle using a short 25mm solid shaft, employing a flexible joint coupling at the final drive. Drive to the rear final drive is accomplished with a single piece driveshaft without intermediate mounts, similar to the standard version in design and strength. A flexible joint coupling is provided at the tranfer case output flange.

Rear Axle Final Drive

The rear axle final drive is the standard E28 type with a ratio of 3.91:1 and is fitted with a viscous clutch. While similar in function, the rear final dirve viscous unit differs in dimensions and applications from the larger unit employed in the tranfer case. The final drive unit is mounted 7mm closer to the underbody thus increasing the vehicle ground clearance.

The application of viscous clutch technology in the rear drive unit provides several advantages: 1. automatic and continuous rear wheel locking according to varying road surface conditions 2. quieter, softer engagement which rseults in less strain, wear and noise. 3. gentler lock/unlock action enhances front axle steering control.

Front Axle Final Drive

The 3.91:1 front axle final drive is of the conventional non-locking type and is rigidly mounted to the engine oil pan. Locking capability is deleted from the front drive unit to avoid any unusual feel or feedback in the steering. The forward drive unit is compact in design to conserve under-vehicle space and to save weight. The oil capacity is .7 liter. The differential oil has to be changed during the 1,200 Mile Inspection and at every Inspection II.

Engine Oil Pan

The oil pan has an opening to mount the front final drive unit and the right final drive output. While it is true that the final drive "passes thru" the engine oil pan, these two units remain completely independent of each other. Ther is never an intermix of oils, gasses orpressures between the engine and the front drive unit. The conficuration of the engine oilpan has changed substantially. While the relationship of the oilpump, pickup and sump location has changed to accomodate the All-Wheel Drive components, the total engine oil volume remains the same with the installation of the oil cooler.

The oil pan has a baffle with an integrated one-way valve to prevent the displacement of oil out of the lower oil sump during certain operating conditions. The BMW All-Wheel Drive uses a special supporting bracket for the intermediate output shaft to compensate for the off-center position of the front final drive unit. The bracket, which is attached to the oil pan, permits the use of front axle half shafts of equal length. This configuration assures that both sides are equal with minimal deflection angles, thus the "Torque Steer" effect is eliminated. Constant velocity joints on each enf of the half shafts allow for angular compensation.

Front Axle

The front axle, subframe and related components are of a completely new design. A cast aluminium subframe was developed to conserve space and conform to BMW requirements concerning strenght and performance. The subframe is located further forward necessatating a new stabilizer bar and mounting points. The new stabilizer bar and the related hardware are similar to the E28 type.

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