Suspension Systems - Overview: Other BMW Z3 E36 рестайлинг

Purpose of the System

The safety, stability, handling and performance of a vehicle depends on many factors. One of the most important aspects of these characteristics is the design of the suspension and steering systems. BMW vehicles are known for their superior handling and road performance. The design of the BMW suspension systems is a key factor in achieving these goals.

Suspension geometry is defined as: "The angular relationship between the suspension, the steering linkage and the wheels - relative to the road surface."

There are several alignment geometry angles which relate to the suspension components and steering linkages including

1. Caster
2. Camber
3. Toe-In/Toe-Out
4. Steering Roll Radius (steering offset)
5. Steering Axis Inclination (SAI) and Included Angle (IA)
6. Toe Out on Turns
7. Thrust Line and Thrust Angle

All of these angles influence

1. The ease with which the vehicle can be steered
2. The steering stability (handling, tracking and safety)
3. Tire wear

The suspension geometry for any given vehicle is a result of the design engineers development of the vehicle and the design criteria for that particular vehicle.

Caster

Caster is the forward or rearward tilt of the steering axis centerline, as viewed from the side of the vehicle, and is measured in degrees from a vertical. It is a directional control geometry angle that helps keep the vehicle moving straight ahead.

If the steering axis is tilted rearward, it is called "Positive Caster". A forward tilt is called "Negative Caster".

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On BMW vehicles, Caster is always positive. Positive Caster increases stability at high speeds. Positive Caster also causes increased steering effort at low speeds. Systems such as Servotronic help minimize this negative effect. In addition to increased high speed stability, cornering is enhanced and steering wheel returnability is improved by positive Caster.

Caster is measured in degrees. On BMW vehicles, Caster is a non-adjustable angle. But Caster is influenced by damaged suspension components. When performing a wheel alignment, always check Caster to insure there is no hidden damage.

Cross Caster

Cross Caster is the difference in the Caster measurement from left to right. Excessive Cross Caster can cause the vehicle to pull to the side with the least positive caster.

Camber

Camber is the inward or outward tilt of the wheels when viewed from the front of the vehicle. The amount of tilt is measured in degrees from the vertical and is called the camber angle.

If the wheel tilts out at the top, the camber angle is positive and if the wheel tilts in at the top it is negative.

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Excessive Positive Camber will cause abnormal wear on the outer edge of the tire.

Excessive Negative Camber will cause abnormal wear on the inner edge of the tire.

Camber is measured in Degrees and Minutes and is measured at the front and rear of the vehicle. Camber is adjustable on some BMW models.

Cross Camber

Cross Camber is the difference between camber angles from left to right. If Cross Camber is excessive, the vehicle will pull to the side with the most positive Camber.

Toe In/Toe Out

Toe is the difference in length by which the wheels of each axle differ from each other, front to rear, in the straight ahead position. The rear wheels are also subject to toe measurement.

Toe is measured at the center of the wheels from one wheel rim to the other. When the distance is greater at the rear of the wheels, it is called toe-in.

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When the distance is greater at the front of the wheels, it is called toe-out.

Rear wheel drive vehicles generally will have a small amount of toe-in at the front wheels. This will allow the wheels to toe out when rolling to achieve a zero running toe.

Toe is measured in Degrees when using BMW specifications. Front toe is adjustable on all BMW vehicles. Rear toe is only adjustable on some models.

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Steering Roll Radius (Steering Offset)

The steering offset is the distance between the point of contact of the projected line drawn through the steering axis to the road surface and the center point of the tire contact area (foot print). The roll radius is the distance between these two lines.

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1. A positive roll radius exists when the steering axis line is inside the center line of the tire.
2. A negative roll radius exists when the steering axis line is outside of the tire center line.

When the Steering Roll Radius is excessively positive, stability during braking is reduced. When the Steering Roll Radius is excessively negative, the directional stability is reduced and there is reduced feedback to the driver through the steering wheel. BMW vehicles are designed with a slightly positive Steering Roll Radius. This gives the driver a better "road feel" without compromising braking stability.

Steering Roll Radius is not adjustable, but can be influenced by improper tire and wheel combinations. Wheels with incorrect offsets can compromise handling characteristics. Scrub Radius is another term used to describe Steering Roll Radius.

Steering Axis Inclination (SAI)

Steering Axis Inclination is the inward tilt (angle) of the strut assembly with respect to a vertical line to the road surface.

SAI results in self-correcting forces that cause the front wheels and steering wheel to return to a straight ahead position after cornering.

SAI is not adjustable, but is affected by damaged suspension components. Most current alignment equipment can measure SAI and can aid in the diagnosis of damaged parts. Bent strut or spindle assemblies are common causes of incorrect SAI readings.

Included Angle (IA)

Included angle is the Camber angle and SAI combined. IA is also helpful when trying to diagnose bent suspension components. Knowing the IA and SAI is helpful when adjusting Camber. If the desired Camber angle cannot be achieved, then looking at SAI and IA could help determine the cause.

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Toe Out On Turns

Also referred to as "Turning Angle", Toe out on turns results from the different angles (arcs) taken by the front wheels when driving through a corner. When turning a corner, the outside wheel must travel a greater distance than the inside wheel. The additional toe angle is determined by the steering arm design. Deviations from the specified value could indicate possible bent steering linkage. A typical complaint that would be associated with this condition would be excessive tire squeal or "scrubbing" on turns.

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Geometric Axis

The Geometric Axis (Centerline) is an imaginary line that is drawn between the midpoints of both front and rear wheels. The Axis is perpendicular to front and rear axles at 90 degrees. This is an imaginary angle that is not adjustable.

Thrust Line/Thrust Angle

The Thrust Line is represented by an imaginary line that bisects the rear toe angle. This angle represents the overall "direction" in which the rear wheels are pointing. The Thrust Angle is the difference between the Geometric Axis and the Thrust Line. The optimum Thrust Angle is Zero Degrees, any deviation from this will affect the position of the steering wheel.

Positive Thrust Angle - is formed when the Thrust Line is to the right of the Geometric Axis (Centerline). When this situation occurs, the steering wheel position will be off to the right as well. The rear of the vehicle will tend to move to the right which will cause the front of the vehicle to steer left, the driver will move the steering wheel to the right to compensate.

Negative Thrust Angle - is formed when the Thrust Line is to the left of the Geometric Axis (Centerline). When this situation occurs, the steering wheel position will be off to the left as well. The rear of the vehicle will tend to move to the left which will cause the front of the vehicle to steer right, the driver will move the steering wheel to the left to compensate.

Front Axles

Two basic design front axles are used on BMW vehicles including

1. Single-Joint Spring Strut Axle
2. Double Pivot Spring Strut Axle

Single Joint Spring Strut Axle

The single-joint spring strut axle is used on E30, E36 (all incl Z3 and 318ti) and E85 models. The axle beam is bolted to the body through four locating bushings. It supports the engine mounts, steering rack, anti-roll bar and lower control arm mount.

The "C" shape lower control arm is additionally mounted through a rubber bushing to the body and serves as the lower pivot for the steering system.

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E46 Front Suspension

The E46 front suspension is based on the E36 design. There have been some design changes to improve ride quality and handling characteristics. The following changes were incorporated into the E46 front suspension system

Scheme 78

1. A new forged aluminum lower control arm is used (Except all-wheel drive). If offers the advantages of weight reduction and lower unsprung mass.
2. Hydraulic bushings are used for the rear lower control arm support.
3. Hollow strut piston rods are used to reduce weight.
4. The steering knuckles are press fit into the strut tubes which reduce the tolerances of the front suspension geometry.
5. The Caster has been increased to improve straight line stability.
6. The track has been widened for improved cornering
7. Aluminum brake dust shields are used for weight reduction. Altogether there is 5.72 lbs. less unsprung weight in the front suspension which enhances ride comfort and handling.

Double Pivot Spring Strut Axle

The "Double Pivot Spring Strut Axle" is used on the following vehicles

Scheme 79

1. All 5 Series including E34 and E39
2. E53, X5 All Models
3. All 7 Series including E32, E38 and E65/E66.
4. E31 8 Series The axle beam of the double pivot is bolted to the sub-frame through three triangulated mounting bolts. The axle beam supports the steering box, engine mounting and lower control arms. The tubular cross member and compression struts complete the double pivot suspension frame work.

The double pivot refers to the lower mounting of the strut and spindle assembly around which the wheel turns.

The lower control arm and the compression strut form the two lower pivot points.

The lower pivot point is actually an imaginary point formed by the extension of the two lower mounting arms.

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Rear Suspension Systems

BMW utilizes several different configurations for the rear suspension systems depending on the model and specific needs of the vehicle in terms of handling, ride and performance. Alignment angles are not adjustable on the early rear suspension systems. The Central "C" Link rear suspension and the Integral Link rear suspension have adjustable Camber and Toe. The different rear suspension systems used include

Scheme 82

1. Semi-Tailing Arm - E30, E36/5 318ti and E36/7 Coupe and Roadsters
2. Semi-Trailing Arm with Track Link - E32, E34, E28, E24
3. Central "C" Link - E36, E46 and E85
4. Integral Multi-Link - E38, E39, E31, E53, E65, E66 and E52.

Semi-Trailing Arm

The Semi-trailing arm rear suspension is used on the following vehicles

1. E30 All models including convertible, M3 and iX
2. E36/7 Z3 Roadster and Coupe including M Roadster and Coupe
3. E36/5 318ti (Compact)

The semi-trailing arm is an independent suspension system. The axle geometry is such that all points on the hub carrier move in a circular path, on a single plane during bounce and rebound. As a result of this, the rear camber and track will vary with the up and down movement of the wheel. The are no adjustable angles on this suspension system. Rear Camber is influenced by ride height. In the event of an incorrect rear toe situation, offset bushings can be installed on the trailing arms to make slight corrections.

Scheme 83

Semi-Trailing Arm With "Track Link"

The "Track Link" rear suspension is used on the following vehicles

1. E24 All models including M6 and L6
2. E28 All models including M5
3. E32 All models
4. E34 All models

The Track Link is a modified semi-trailing arm rear suspension. It incorporates an additional link that is mounted between the trailing arm and the rear axle carrier beam. The track link is mounted through rubber bushings that allow it to flex during bounce and rebound. The track link reduces the pivot axis from 20 degrees to 13 degrees which reduces the amount of camber and track change during bounce and rebound.

It provides better handling, reduces "squat" and "dive" tendencies and allows less camber change due to heavy loading.

There are no rear suspension adjustments, small corrections can be made with offset bushings.

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Central "C" Link Rear Suspension

The Central "C" Link rear suspension is used on the following

1. E36 All Models except E36/7 and E36/5.
2. E46 All Models
3. E85 Z4 Roadster

The Central Link rear suspension consists of a cast iron longitudinal are that is bolted to the body through a rubber bushing. The wheels are additionally supported by two transverse arms that are connected to the rear axle sub-frame.

The design of this suspension allows for individual tuning of longitudinal and lateral forces acting on the vehicle. Longitudinal forces, such as starting and stopping are absorbed by the central arm. Lateral forces, such as cornering, are absorbed by the transverse links. The complete suspension exhibits little or no change in toe-in while driving.

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E46 Rear Suspension

The Central "C" Link rear suspension on the E46 is almost identical to the E36 with some minor modifications

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1. Upper Transverse Control Arms (Lateral Links) are made from aluminum.
2. The Subframe (rear axle carrier) is of tubular steel construction.
3. The differential is mounted to the subframe with a hydraulic mount.

E85 Rear Suspension

The E85 rear suspension is taken from the E46 design with some minor changes for the roadster. The track width has been increased by 30 mm. A reinforcement plate has been added to increase rigidity. Due to underbody aerodynamics, a duct has been added to direct airflow to the differential. The differential cover has cooling fins which will help keep the differential within it's operating temperature range.

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Rear Toe

Toe is adjusted by moving the forward central arm bushing mount with a special tool. The tool part number varies between models.

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Tool # 90 88 6 323 080 for E36

Not Used for Z3 and 318ti

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Tool # 90 88 6 323 030 for E46 and E85

Rear Camber

Rear Camber is adjusted by rotating an eccentric bolt located at the outer end of the rear lower lateral link. No Special tools are required to make the Camber adjustment.

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Multi-Link Rear Suspensions

Multi-Link Rear suspension systems are used on the following vehicles

1. E31 8 Series including 850i, 850Ci, 850CSi and 840Ci
2. E38 7 Series - All models
3. E39 5 Series - All models including M5 (Touring version is slightly modified)
4. E53 X5 3.0i, 4.4i and X5 4.6is
5. E52 Z8
6. E65 and E66

The "integral" multi-link suspension was introduced on the E31 and variations of it are used on the 5 and 7 series vehicles. Slightly modified versions are also used in the X5 and E39 Sportwagon with EHC. The multi-link suspensions incorporate what is called "elastokinematics" that allow each wheel to move and flex individually without transmitting loads and forces through the sub-frame to the other wheel.

These suspension systems incorporate anti-dive, anti-squat geometry which serves to keep the vehicle level during hard acceleration and braking.

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Elasto-Kinematics

Elasto-kinematics relates to the suspension system design type. The term "elasto" implies stretching, which in fact the system does. Under extreme load (acceleration, turning, braking) the suspension changes its geometry to counteract inherent changes induced by the increased loads.

Scheme 93

The system changes are pre-determined and built into the system. The geometry changes provided by this system correct for unwanted changes that occur under load in non-elastokinematic systems.

E38/E39 Multi-Link Suspension

The E38 and E39s use a modified version of the E31 multi-link rear suspension system. The system was modified to be more compact yet provide the same handling and ride characteristics as the E31 system.

The trailing arm was eliminated and all suspension mounting is to the sub-frame of the vehicle. The E38/E39 Multi-Link Rear Suspension is made up of the following

Scheme 94

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E39 Sportwagon And X5 Rear Suspension

With the introduction of the E39 Sportwagon and the E53 X5, the rear suspension system has been modified to accommodate the increased cargo capacity. The rear suspension is also modified to allow for the optional EHC system. The X5 and Sportwagon rear suspensions differ from the standard E38 and E39. The springs are now located on the perches that are part of the wheel carrier assembly. The E39 and E53 rear suspensions are also slightly different from one another, the E39 upper shock mounts are part of the subframe assembly. The E53 upper shock mounts are still located in the body.

Both the E39 and E53 are available with optional EHC, this means that the rear springs will either be coils springs or "air springs".

The E39 rear suspension also uses special "hydro mounts". This modification is necessary due to the fact that the upper shock mounts are on the subframe. The "hydro mounts" are used to isolate the subframe to suppress road noise and vibrations from being transmitted to the body.

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E65/E66 Rear Suspension

The rear axle on the E65/E66 is also the integral type. It is very similar to the E38/E39 rear suspension. The rear axle carrier (subframe) is made from tubular aluminum. The E65/E66 is also available with optional systems such as

Scheme 97

1. EDC-K Electronic Damping Control (Continuous)
2. ARS Active Roll Stabilization
3. Air Suspension System

The Rear Toe and Rear Camber are adjusted by means of eccentric bolts. No special tools are needed to make these adjustments.

Ride Height Measurement

When performing alignments on BMW vehicle, the ride height must be set and checked before proceeding with any measurement or adjustments. Ride height is measured from the lower edge of the wheel rim to the lower edge of the wheel housing. Ride height specifications are in millimeters. In order to obtain the correct specification, the vehicle and suspension type must be identified. Ride height specifications depend upon rim size (15", 16", 17" etc.), engine size and the type of suspension. There are 4 different types of suspension packages

1. Series - Standard production vehicle
2. Low Slung Sports Suspension - This is used on vehicle with Sports Package Option.
3. Rough Road Package - For Eastern European Rough roads. Not used in US.
4. M Sports Package - Used on Motorsport vehicles or those with M Package Option.

The example shown below is the ride height specifications for an E39 540i (Front Axle).

HEIGHT SPECIFICATION

Ride Height

When measuring ride height, measure from the lower edge of the wheel rim to the lower edge of the wheel opening. Use a metric tape measure with divisions of 1 mm.

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Z3 Ride Height Tool

When measuring ride height on the E36/7 (Z3). Use special tool # 313 010. Due to the design of the hood on the Z3, this tool is used to simulate the lower edge of the wheel opening.

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Vehicle Loading

In order to obtain the correct ride height, the vehicle must be loaded with the specified weight. The normal specified weight for most vehicles is as follows

1. 68 kg. in each front passenger seat
2. 68 kg in the rear seat
3. 21 kg in the trunk
4. Fuel tank full.

Not all vehicle need to be weighed down. Usually E39 M5 and E36/E46 M3 are measured without any weight. The proper test conditions can be found under "Normal Position/Inspection Conditions".

If the vehicle fuel tank is not full, compensate by adding weight in the trunk. Fuel is approximately 7 pounds (about 3.2kg) per gallon. For example: If the vehicle has an 18 gallon tank and it is 1/2 full, add about 63 lbs. (about 28kg.)