Chassis Dynamics - Overview BMW X3 E83

E83 Chassis Dynamics

As with all BMW vehicles, the X3 chassis and suspension was designed with particular emphasis on driving pleasure, dynamics and favorable handling.

The main aim in the full development of the X3 was to achieve

1. Agility on the road
2. Driving dynamics
3. Stability
4. Traction

The front axle is a double joint spring strut axle with tension arms and a rack and pinion steering system with hydraulic power assist. Conventional rack and pinion power steering is installed to the E83 as standard. Servotronic is available as an option. Spring struts with coil springs and twin tube gas-pressure dampers are used on the front axle. It is similar the front axle of the E53.

The rear axle design is based on the E46/16 with barrel springs and separate twin tube gas-pressure dampers.

The brake system is a hydraulic dual-circuit brake system with "front/rear split" vacuum boost power assist and EBV. The parking brake actuating unit is located in the center console and is equipped with an automatic cable adjuster (ASZE) and a compensating element.

Several different wheel and tire combinations are available for the E83. The spare wheel (steel) is located in a special bracket under the luggage compartment floor in the rear. The tire deflation warning (RDW) is a standard feature on the E83.

Front Axle

Specific components in the E83 (subject to additional load) are designed as forged steel parts.

1. Control arms: forged steel with rubber mount
2. Tension arms: forged steel with hydraulic bushing
3. Swivel bearing: forged steel, track arm forged on

A reinforcement plate (thrust panel) is bolted to the front axle carrier. This makes a contribution to the rigidity of the front structure and suspension. There are two recesses in this thrust zone for accessibility to the engine oil drain plug (rectangular recess on 6 cylinder engines).

Scheme 47

Technical Data

FRONT AXLE TECHNICAL DATA

Alignment And Camber Adjustment

Toe-in adjustment is performed at the tie rods. If necessary, the camber is corrected by driving out the pin on the strut tower (spring cup).

Strut Replacement

When replacing the strut, the mounting on the swivel bearing must be expanded with a special tool.

Spring Replacement

There are new spring tensioners to be used during spring replacement.

Reinforcement Plate

The vehicle must not be driven without the reinforcement plate in place. The reinforcement plate ensures the transversal rigidity and contributes to the strength of the front axle.

Rear Axle

The E83 is fitted with a central rear axle and subframe. The design of the rear axle is similar to the E46/16. Two tension arms have been added to the rear axle of the E83 and are secured to the bodyshell.

Mechanical Components

Scheme 48

Modifications to the E46/16 rear axle to adapt it to the E83

1. Anti-roll bar secured to the rear axle carrier by clamps.
2. Front of rear axle carrier suspension converted to special bolts with additional thrust washer.
3. Thrust brace and tension arms.
4. Surface of control arms are galvanized steel plates.
5. Anti-roll bar link with ball joint attached directly to the control arm.
6. Dampers with three point flange (bolt) plate.

REAR AXLE TECHNICAL DATA

Adjustment Of Rear Wheel Alignment

Rear wheel alignment is adjusted by a special tool on the lower trailing arm. The camber is adjusted by an eccentric element on the lower control arm at the axle carrier connection.

Lowering (Removing) The Rear Axle

The handbrake cables (routed through the rear axle carrier and the body console) must be disengaged before the rear axle is lowered. This is to prevent shearing of the handbrake cables.

Brakes

The brake system is a hydraulic dual-circuit brake system with "front/rear split", vacuum boost power assist and EBV. One brake circuit each for the front and rear axles. The parking brake actuating unit is located in the center console and is equipped with an automatic cable adjuster (ASZE) and a compensating element.

The advantages of the parking brake with automatic cable adjuster are

1. The 2 cables no longer have to be adjusted during installation.
2. Cable extensions (cable conduit) contractions over the operating time are automatically compensated so that the parking brake lever travel is consistent.
3. Prestretching of the cables is no longer necessary.
4. The parking brake does not have to be adjusted at the end of the assembly line and in the service department.

Mechanical Components

The brake system consists of the following components

Scheme 49

Four Wheel Hydraulic Disc Brakes

The front brakes are constructed of ventilated brake discs 325mm diameter x 25mm thick (Geomet coating) with FN57/25 single piston floating calipers.

The rear brake brakes are constructed of ventilated brake discs 320mm diameter x 22mm thick (Geomet coating) with FN42/22 single piston floating calipers.

Hydraulic Unit With Vacuum Brake Booster And DSC Hydraulic Module

The hydraulic unit is located at the front on the left hand side under the main brake cylinder. The electric precharging pump is no longer required. All vehicles are equipped with an 8"/9" tandem brake booster and have a tandem brake master cylinder. The DSC hydraulic module is a DSC8 system.

Foot Controls With Brake And Clutch Pedals

The essential components of the foot controls are

1. Pedal mounting block
2. Brake pedal
3. Clutch pedal
4. Clutch master cylinder

The pedal mounting block is made of aluminum and is bolted to the bulkhead. The vehicle has a wide steel brake pedal. The clutch pedal is made of plastic.

Parking Handbrake Lever, ASZE, Cables And Duo-Servo Brakes.

The duo-servo brakes are similar to the duo-servo brakes on the E65 and E53 (185mm diameter x 30mm wide).

The handbrake lever is bolted to the floor pan and is equipped with an automatic cable adjuster (ASZE) and a compensating element.

Changing Cables

If there is a broken cable, the automatic cable adjuster is in the untensioned position. To replace the cables, it is necessary to remove the center console and the rear compartment ventilation ducts. For the cables to be removed or parking break shoe replacement, the parking-brake lever must be in the released position and the ASZE unit must be deactivated.

Steering

Conventional rack and pinion power steering is used in the E83 and Servotronic is available as an option. The design and functioning principle of the Servotronic option are nothing new but have been modified as described in this chapter.

The servotronic steering system controls the amount of power assistance based on the current road speed. The hydraulic pressure is electronically adapted to the current road speed, with greater power assistance available at lower road speeds and less power assistance available at greater road speeds.

Mechanical/Hydraulic Components

Scheme 50

The E83 is equipped with hydraulic assisted rack and pinion steering. The total ratio of the steering is: 18.9 : 1. The total rack stroke is 81.0 mm (1.7 turns of the steering wheel). The hydraulic ports on the steering gear have quick release couplings. The hydraulic pump maximum pressure is limited to 127 bar.

E83 vehicles are equipped with "W" shaped cooling loops (hoses) to assist in cooling.

Wheels And Tires

Several different wheel stylings are available. Different 17" wheel stylings are standard equipment. Other 17" and 18" wheels are available as options, depending on the type of engine.

A compact wheel (spare/emergency wheel) is standard, regardless of the vehicle version.

The tire deflation warning (RDW) is a standard feature. The RDW function is integrated in the DSC control unit.

The following wheel/tire combinations are available for US models

2.5 Liter Engine (Optional For 3.0 Liter)

Note. IS = insert size LI = load index A = all season tires S = summer tires

Scheme 51

1. 8.0Jx17 EH2 IS46
2. 235/55 R17 LI 99 tires
3. A/S
4. H-rated tires, M+S

3.0 Liter Engine (Optional For 2.5 Liter)

Note. IS = insert size LI = load index A = all season tires S = summer tires

Scheme 52

1. 8.0Jx17 EH2 IS46 (shown below)
2. 235/55 R17 LI 99 tires
3. A/S
4. H-rated tires, M+S

Optional (for 2.5 And 3.0)

Note. IS = insert size LI = load index A = all season tires S = summer tires

Scheme 53

1. 8.0Jx18 EH2 IS46 (shown below)
2. 235/50 R18 LI 97 tires
3. A/S
4. H-rated tires, M+S

Note. IS = insert size LI = load index A = all season tires S = summer tires

Scheme 54

1. 8.0Jx18 EH2 IS46 (shown below)
2. 235/50 R18 LI 97 tires
3. A/S
4. H-rated tires, M+S

The spare/emergency wheel (compact wheel) is standard equipment. The E83 has a special bracket for the compact wheel which is located in the vehicle floor under the luggage compartment and is operated from inside the vehicle.

The release knob is located under the luggage compartment floor covering next to the tool kit.

The compact wheel is a 4Bx17 IS18 steel wheel with T135/90 R17 104 M tire.

The compact wheel bracket has a special service access cap (1) to check the tire pressure without having to lower the bracket/wheel.

Scheme 55

Tire Deflation Warning (RDW)

The RDW function is integrated (and diagnosed) in the DSC control unit. The system uses the rotation speed of the diagonally opposite wheels to compare the dynamic circumferences of all four wheels.

The RDW system does not monitor uniform air pressure loss over all 4 tires. If the pressure loss is the same in all four tires, the wheel speeds change equally and pressure loss cannot be detected. The customer must continue to check inflation pressures on a regular basis.

The system must be reinitialized if tire inflation pressures are modified or if the tires/wheels are replaced. Initializing RDW

1. Start the engine but do not drive away.
2. Press and hold the RDW button in the center console until the indicator light in the instrument cluster illuminates in yellow for several seconds.
3. Drive away.

After a certain distance, the system stores the new wheel speeds as reference values and is then able to display a detected deviation. RDW has an indicator light in the instrument cluster that may illuminate in yellow or red.

Indicator illuminates in red

1. Indicates a loss of more than 30% of inflation pressure in one of the tires
2. Accompanied by a "gong" sound

Indicator illuminates in yellow

Scheme 56

1. Possible faults in the system
2. Initialization