44 Balancing/Optimising Wheels
WM 440093 Balancing/optimising wheels
Tools
Balancing/optimising wheels
General
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 is turn leads to increased wear of various parts of the chassis.
Even if wheels are correctly balanced, rolling and steering imbalance can still occur, for example in the case of
- 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/requirements
Wheel suspension elements must be in good technical order.
The wheels must be clean with no foreign objects in the tyre.
Use a tyre measuring gauge, e.g. measuring gauge NR.136 to check radial and lateral runout values on wheels without flat spots. Values of less than 1.0 mm are desirable - around 0.5 mm would be better.
To limit the number of radial runout errors and to optimise smoothness of rolling, it is a good idea - and even necessary in individual cases - to match the tyre (i.e. to fit it in a favourable position with respect to the wheel). 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 axial and radial 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 in the case of flat spots. In the event of flat spots, wheels must be warmed up by driving 25 - 30 km 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.
Explanation of terms
Geometry 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
Figure 1
The radial force is a force that compresses a tyre. Based on its size, a tyre has softer ( -item A- ) and more rigid areas ( -item B- ) due to design 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. Other harmonic variations (2nd - 4th Harmonic) can also be seen in the changes in radial force. 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 may 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. A - Measure radial force variations and optimise the wheels if necessary. Afterwards, balance the wheels.
B - 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 optimisation of rolling smoothness.
3. Fitting the wheel on the vehicle.
Further to 1: Fitting the tyre on the rim
- Remove rubber residues and dried-on tyre paste from the rim, especially from the rim bead and the hump.
- Examine rim for damage; remove old balancing weights.
- Use the prescribed tyre mounting paste (only TIP TOP Universal, order no. 593 0601, 3.5 kg bucket, or Contifix) so that the tyre does not turn on the rim the first time the car is driven, thereby counteracting all your careful work. If the tyre turns by as little as 20 mm with respect to the rim, this can worsen an optimum balancing result.
- Fit tyre on the rim. When doing so, it is a good idea - and even necessary in individual cases - to fit the tyre in a favourable position with respect to the wheel (matching). Compare Controlled and Uncontrolled matching below.
WARNING
Danger of accident due to high tyre seating pressure.
- Personal injury may occur in the event of tyre or wheel damage (due to the coincidence of several unfavourable factors).
-> Maximum tyre seating pressure 4 bar. This value must not be exceeded.
-> Tyres must be filled in the filling cage.
Tyre filling cage
- Fill tyres to a maximum pressure of 4.0 bar. By the time the pressure reaches 4.0 bar, at the latest, the tyre beads must pop out of the deep bed and over the hump of the rim bead in order to avoid fractures of the bead core. If necessary, interrupt the process and coat all necessary surfaces generously with sliding compound (tyre mounting paste). Then repeat the process.
- 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.
- Check the fit of the tyre on the rim using the bead centering line.
- Set the prescribed tyre pressure.
Further to 2A: Measuring radial force variations
- Tension wheel on balancing machine with radial force variation measurement (vibration monitoring system).
- Observe the operating instructions of the balancing machine (adjust the machine from time to time).
- The contact surfaces of the wheel on the balancing flange (levelling surface) and the centering surface must be clean.
- The display disabling function must be switched off.
- Adjust tyre pressure to 2.6 bar (best test pressure) when balancing and measuring radial force variations.
- Particular attention must be paid to the wheel mounting when measuring radial force variations and when balancing. Using the special pressure pieces provided by the manufacturer of the balancing machine will ensure that wheels are balanced accurately and there is the highest possible repeat accuracy. Observe the operating instructions.
Maximum deviations permitted in radial force variation results:
If these values are exceeded, the tyre must be matched on the rim.
If these values are not reached, the wheel must be balanced.
Information
A new tyre must be fitted if the radial force variation cannot be reduced to the values listed in the table.
Further to 2B: Stationary balancing with optimisation of rolling smoothness
- Clamp wheel on stationary balancing machine.
- Observe the operating instructions of the balancing machine; adjust the machine from time to time, if necessary.
- The contact surfaces of the wheel on the balancing flange (levelling surface) and the centering surface must be clean.
- Centre (clamp) the wheel from the inside with centering clamping device - see Tools and Equipment, Section 3. This centering clamping device fits all wheel-balancing machines approved by Porsche.
- Set required tyre pressure (tyre-pressure plate).
- Centre (clamp) the wheel from the inside with centering clamping device - see Tools and Equipment, Section 3. This centering clamping device fits all wheel-balancing machines approved by Porsche.
Check radial and lateral runout during the first measuring run. Values of less than 1.0 mm are desirable - around 0.5 mm would be better.
Consider the size of the measured balancing weights and their distribution on the inner and outer planes (inner and outer rim flanges) critically.
A uniform distribution with low values (e.g. 35/40g) means that the tyre was fitted correctly and the quality of the tyre and rim is good.
A value of 110 g/110 g (120 g/120 g with Tire Pressure Monitoring) should not be exceeded with interior balancing behind wheel centre. A considerably different distribution, e.g. 40/90g is not good; this usually indicates incorrect fitting. Problem wheels of this kind often have pronounced radial and lateral runout values.
This can be remedied by matching (use correct mounting paste). This can be improved with uncontrolled matching (manual); significant improvement is possible in almost all cases using the rolling smoothness optimisation program (controlled matching).
Tyre pressure during balancing at required pressure.
Permissible residual imbalance of less than 3g, or max. 3g per plane.
Further to 3: Fitting the wheel on the vehicle
Information
- Never use an impact bolter.
- Observe the description on wheel removal and installation (fitting wheel on vehicle) -> 440519 Removing and installing wheel Removal and Replacement.
- Fit the wheels with the lowest residual imbalance values on the front axle. Observe the rolling direction of the wheels.
- Fit wheel using a suitable commercially available tool.
- Screw in wheel bolts by hand and then tighten evenly. Start at the top when tightening. Always observe the specified tightening torque of 160 Nm (118 ftlb.)
- Set required tyre pressure (tyre-pressure plate).
Balancing weights
Balancing weights: For adhesive weights, refer to the spare parts catalogue.
The adhesive weights used up until now may still be installed until 30 June 2005.
As of 01 July 2005, only zinc weights may be used. These may not be broken or cut.
Attach both weights to the inner side.
Information
- Observe program selection and operating instructions of the balancing machine.
Attaching the balance stick-on weights
Information
- Remove the protective film just before sticking on the weight, as prolonged exposure to the air will reduce the bonding strength.
- Pay attention to cleanliness.
- Determine exact position of the balancing weights (if necessary, temporarily stick on the balancing weights with a strip of adhesive tape until the correct position is determined).
- Prepare bonding surface on the rim. The bonding surface must be perfectly clean and free of grease.
- Remove protective paper from the bonding surface of the weight and press the weight firmly into place.
- The balance stick-on weight must be in uniform contact over the entire surface.
- Check the balancing weight is seated firmly. A weight freshly affixed weight must not detach itself from the rim under application of a shear load at right angles to its longitudinal axis.