Wheel alignment using the BMW E36 as an example

The E36 model (the 3 Series BMW) was chosen because it uses a developed form of the multi-link rear suspension first adopted on the Z1. The E36 front suspension has a lateral control arm (a lower wishbone) and a suspension strut. A stabilizer (anti-roll bar) limits body roll when cornering. The resilient wheel locating pivot is mounted on the outer end of the lateral control arm. The E36 rear suspension consists of a rigid trailing arm linked to the wheel carrier and two lateral control arms, one above the other. To improve insulation from driveline and rolling noise, the trailing arm has a resilient mounting on the body. A special feature of this design is the very small variation in toe-in when the suspension moves in the vertical, longitudinal or lateral axis.

Preparatory work and condition of car

The specific preparatory work needed on the E36 is:

• Position the turntables and sliding plates and set the width of the vehicle hoist to suit the car's track and wheelbase.
• Drive the car on to the turntables and plates. The tyres must be located centrally on them.
• Remove the keeper pins from the turntables and plates, to prevent stresses from being trapped in the vehicle's suspension.
• Apply the brakes and hold them on with the brake lock.
• Take measurements with the fuel tank full and at the normal load specified by BMW. If the tank was not filled, compensating weights in the form of ballast sacks must be placed above the fuel tank. An empty 70-litre fuel tank, for example, reduces the weight by approx. 50 kg.
• Apply the normal load to the car. For the E36, this is 68 kg on the front seat and 68 kg on the rear seat, with 21 kg in the luggage compartment. Sandbags weighing between 3 kg and 15 kg are available for ballasting purposes.
• Attach quick clamping devices to all wheels and insert the measured-value sensors, using the integral spirit levels to keep the equipment correctly aligned.
• Input the figures 1 to 4 to identify the positions of the measured-value sensors.
• Switch on the wheel alignment computer or, if it was already on, start a new wheel alignment session. This erases all previous values, after which input of the four new measured-value sensor positions takes place.
• Use menu guidance to call up the car's desired values from the data memory.
• Check that the height setting is within the tolerance of + 10 mm. Before this check is made, compress the car's suspension firmly to ensure that it settles to a consistent center setting.

If the car is within the tolerance of +/- 10 mm, vary or increase the load to correct its height to within +/m 1 mm of the desired value specified by BMW.

Measurement before adjustment

Like the subsequent adjusting work followed by the measurement after adjustment, this work can take place under program control or as a free-choice procedure.

Program-controlled measurement

The software determines the order of vehicle measuring points called up in the program-controlled measurement mode, and controls the procedures. For each measuring point the appropriate screen graphics are displayed with the target and actual measured data for comparison. The entire measurement procedure is controlled by the "Program step forwards" and "Program step back" keys.

Free-choice measurement

In this case the operator can choose the measuring points in any desired order. The desired measuring point can be selected by applying the sensor pen to the digital keypad on the screen.

When free-choice measurement has been selected, certain measurements have to be performed in the correct order in order not to falsify the results.

For this reason, wheel alignment in the free-choice mode should only be attempted by workshop personnel with suitable prior knowledge and ample experience.

The individual stages in before-measurement adjustment are:

• Align the wheels pointing straight ahead so that toe and camber can be recorded correctly at the rear axle.
• Perform the steering-lock routine (turn the steering to an angle of 20° in each direction in order to determine caster, kingpin inclination and toe-out on turns can be determined).
• Record front-axle toe and camber with on-screen references; first determine the center point of the steering.
• Perform the full steering-lock routine in order to measure maximum steering angle at left and right.
• Check the summary of measured values with target-actual comparison for all values. If all measured values are within the permitted tolerances, a wheel alignment report can be printed out immediately and wheel alignment work on this car terminated.

If out-of-tolerance values are found, renew damaged components and/or perform the necessary adjustment work. All values that are adjustable on the E36 are marked with a tool symbol in the "Target values" column of the measured value summary. The appropriate pictures and written instructions for adjustment work can be called up on the screen by pressing a key.

Adjustment work

The individual steps in adjustment work are:

• Set the steering to the straight-ahead driving position according to the screen guidance provided.
• Adjust the rear axle; on the E36, both camber and toe can be adjusted separately. By pressing a key the relevant adjustment diagrams and written instructions can be shown, before the actual measured values are corrected by a permanent target-actual comparison showing all values for left/right camber, left/right individual toe and total toe at the rear axle.

  Caster adjustment at the left and right front wheel. On the E36, caster is a design value and therefore cannot be adjusted. If the need does arise, a 3-way display of caster, camber and individual toe can be called up for the front wheel concerned, with the continual value changes displayed as a target-actual comparison during adjustment. The reason for this is that the compact construction of modern multi-link suspensions prevents the eccentric adjusters for caster and camber from being placed at a right-angle to one another, so that when one is adjusted, the other is also affected.

  In view of these mutual influence values, it has been found best to adopt the adjusting order "Caster, camber, individual toe".

• Front axle adjustment: on the E36, camber and individual toe are adjustable. Adjusting diagrams and instructions, with a 5-fold target-actual measured value display also appear on the screen.

  During the concluding adjustment of individual toe angles, make sure that the steering center point is adjusted first and the steering wheel lock is tightened into position before completing the adjustment procedure by altering the length of the two track rods.

• Adjusting the maximum steering angle. On series-production E36 cars there is no additional means of adjusting the maximum steering angle at both sides apart from the stops in the steering system. However, extremely wide (low aspect-ratio) tyres could make it necessary to restrict the maximum steering angle. This can take place by resetting the steering stop screws or by inserting suitable shims. In either case, the full steering-lock routine can then be repeated and a check made on the screen that the desired value has been obtained by the correction method which was adopted.

Measurement after adjustment

The individual after-adjustment measuring steps are:

• Measure front-axle toe and camber, with on-screen reference, but only after having adjusted the steering center point. The toe and camber measurements can be omitted if these values were adjusted at the end of the before-measurement work, and valid figures were memorized.
• Adjust straight-ahead running in order to permit the rear-axle toe and camber values to be measured correctly.
• Perform the steering-lock routine: turn the steering to an angle of 20° on either side in order to determine caster, kingpin inclination and toe-out on turns.
• Perform the full steering-lock routine in order to measure the maximum left/right steering angles.
• Check the measured value summary, with target-actual comparison of all measured values. If all the after-adjustment measured values are within the specified tolerances, the final wheel alignment report can be printed out and work terminated on this car.

Data print-out

The report printed out by the integral A4-format printer is divided into three zones:

• The header section, with customer and vehicle identification data. This is where the data input before measurement starts, namely customer data (name, customer number) and the vehicle data (license plant number, chassis number, distance reading), are shown.
• Center zone with car data. This contains the make, type, model and year of manufacture of the vehicle defined when the target data set was selected. It also contains the previously measured values for ride height, tyre pressures and tyre tread depth.
• The lower half of the sheet, with all rear- and front-axle wheel alignment data, listed in three columns headed "Before adjustment", Target values" and "After adjustment".

The data in the "Before adjustment" column show the condition in which the vehicle was received. Measured values beyond the permitted tolerances, which therefore need adjustment, are marked here by an asterisk (*).

In the "Target data" column, all available specified values with their tolerances are listed, and in addition the maximum permissible tolerance between the left and right wheels is stated.

If no target values are specified for a particular measured value,. a "#" sign appears in this line of the print-out.

In the "After measurement" column, a note appears of all measured values which were determined in a second complete measuring routine after such adjustment work as was necessary.

If adjustment work was correct and all measured values are then within the permitted tolerances, no further "*" symbols should be seen in this column.

The complete wheel alignment report sheet, with the identification data in the header, is saved on to the wheel alignment computer's hard disk after printing out, and can be called up again with the aid of the relevant search criteria if queries arise, displayed on the screen and printed out in A4 format.