General information on balancing/optimising wheels

Excessive rough running of the wheels, caused by wheel imbalance, flat spots and securing the wheel on the vehicle, causes vibrations and shaking in the steering wheel. This in turn leads to increased wear on various parts of the chassis.

Even if wheels are correctly balanced, rolling and steering imbalance can still occur. E.g.:

• excessive geometric errors (radial and lateral runout),
• radial force variations (softer and more rigid areas in the tyre).

Information 

• Achieving a good result requires utmost care, observance of the instructions and procedure, and attempting to eliminate even the very last gram of imbalance  .

Recommendations/prerequisites:

Wheel suspension elements must be in good working order.

The wheels must be clean with no foreign objects in the tyre.

Use a tyre measuring gauge, e.g. Dial gauge WE 1630  to check radial and lateral runout values on wheels without flat spots. Values of less than 1 mm are desirable - around 0.5 mm would be better.

To limit the number of radial runout errors and to optimise rolling smoothness, it is a good idea - and even necessary in some cases - to match the tyre (i.e. to fit it at a favorable position with respect to the rim). Follow tips and fitting instructions as well as information on tyre mounting paste for tyres.

If the disc wheels (wheels without tyres) are perfect but the complete wheels have large radial and lateral runout values (due to ply breakage or other damage), it may be necessary to replace the tyre.

Balance used wheels only after they have been warmed up by driving, otherwise flat spots will simulate imbalance. Never perform matching on wheels with flat spots. In the event of flat spots, wheels must be warmed up by driving 25 - 30 km (16 - 20 miles) on a motorway. Lift up the vehicle immediately on return. This prevents new flat spots from forming.

Correct tyre pressure is an important prerequisite.

Place the balancing weights at the prescribed position.

On wheel balancing machines with vibration control system (Vibration Control System WE 1296  ), the radial force variation value must be set in the machine's basic settings as follows:

• Porsche Cayenne: 120 N
• All other models: 80 N

Explanation of terms 

Geometric error (radial and lateral runout) 

Radial and lateral runout of the rim and/or rim/tyre assembly.

Flat spots 

Flat spots can occur on the tyres if the vehicle is idle for extended periods.

Radial force variations 

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

The radial force is a force that compresses a tyre. Based on its size, a tyre has softer (-item A-  ) and more rigid zones (-item B-  ) due to constructive and production-related factors. Given this, the tyre compresses to various extents at a constant wheel load (-arrow 1-  ) and a constant tyre pressure (-arrows 2-  ). This varying spring action (-X-  ) in the tyre can cause vibrations in the chassis on even road surfaces and the driver may notice this as shaking in the steering wheel, for example. Measuring the radial force variation involves running the tyre on a wheel balancer that measures radial force variation while applying a constant load to the tyre. This simulates the rolling of the tyre on a driving surface. Variations in radial force occur independently of the direction of rotation of the tyre and are measured as the difference between the maximum and minimum radial force ("peak-to-peak" value).

1. Harmonic 

The changes in radial force variation can be broken down mathematically into individual variations. Porsche uses the value of the 1st Harmonic Variation (basic variation) to assess the tyre from the point of view of rigidity. The 1st Harmonic Variation relates to the changes in radial force that cause the strongest vibrations. Changes in radial force can also result in further harmonic variations (2nd-4th Harmonic). These relate to the changes in radial force that cause weaker vibrations.

Uncontrolled matching 

Turn the tyre on the wheel by 90° or 180° if necessary in order to achieve an acceptable value with regard to rolling smoothness (true running, imbalance and distribution of balance weights).

Controlled matching 

With a balancing machine with matching program. In most cases, this produces an even better result with regard to the rolling smoothness (true running, imbalance and distribution of the balance weights) than can be achieved with uncontrolled matching.

Tyre seating pressure 

This is the filling pressure after the tyre has been fitted on the rim at which the tyre bead, coming out of the deep bed, must pop over the hump of the rim bead. This seating pressure must not exceed 4 bar.

Balancing/optimising wheels 

In certain cases, it may be necessary to measure radial force variations. This should be done before normal balancing is performed.

The entire process is subdivided into several operations:

1. Fitting the tyre on the rim.
2. 1. Measuring radial force variations and optimising the wheels if necessary. Then balancing the wheels.
   2. As an alternative to item 2 A, if there is no device available to measure radial force variations: Stationary balancing of the wheel (eliminating static and dynamic imbalance) with optimization of rolling smoothness.
3. Fitting the wheel on the vehicle.