Wheels and Tires: Service and Repair
Balancing/optimizing wheels
Balancing/optimizing wheels
General
Excessive wheel imbalance not only produces vibrations and causes the steering wheel to shake, but also results in increased wear on various parts of the running gear.
Even if the wheels are correctly balanced, excessive geometry errors (radial and axial runout) and misshapen tires, e.g. accumulation of material at one point in the tire structure (spring action of tire changes as the tire rolls), can cause rolling and steering imbalance.
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 tire.
Use a tire 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 optimize smoothness of rolling, it is a good idea - and even necessary in individual cases - to match the tire (i.e. to fit it in a favorable position with respect to the wheel). Follow tips and fitting instructions as well as information on tire mounting paste for tires.
If the rims (wheels without tires) 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 tire.
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.
Correct tire 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/tire assembly.
Flat spots
Flat spots can occur on the tires if the vehicle is idle for extended periods.
Radial force variations
The radial force is a force that compresses a tire. Based on its size, a tire has softer (-item A-) and more rigid areas (-item B-) due to design and production-related factors. Given this, the tire compresses to various extents at a constant wheel load (-arrows 1-) and a constant tire pressure (-arrows 2-). This varying spring (action -X-) in the tire 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 tire on a wheel balancer that measures radial force variation while applying a constant load to the tire. This simulates the rolling of the tire on a driving surface. Variations in radial force occur independently of the direction of rotation of the tire and are measured as the difference between the maximum and minimum radial force ("peak to peak" value).
1st 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 tire 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 tire on the wheel by 90 degrees or 180 degrees 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.
Tire seating pressure
This is the filling pressure after the tire has been fitted on the rim at which the tire bead, coming out of the deep bed, must pop over the hump of the rim bead.
Balancing/optimizing wheels
The entire process is divided into three operations.
1. Fitting the tire on the rim.
2. A - Measure radial force variations and optimize 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 optimization of rolling smoothness.
3. Fitting the wheel on the vehicle.
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 tire air 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 tire must be matched on the rim.
If these values are not reached, the wheel must be balanced.
Information
A new tire must be fitted if the radial force variation cannot be reduced to the values listed in the table.
Further to 2B: Stationary balancing with optimization of rolling smoothness
- Clamp wheel on stationary balancing machine.
- Observe the operating instructions for the balancing machine. Adjust the machine according to the manufacturer's instructions (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. 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. 25/35 g) means that the tire was fitted correctly and the quality of the tire and rim is good.
A value of 80g/80g (90g/90g using the tire pressure monitoring system) should not be exceeded with interior balancing behind wheel centre. A considerably different distribution, e.g. 35/90 g is very unfavorable; this usually indicates incorrect fitting. Problem wheels of this kind often have pronounced radial and axial 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 optimization program (controlled matching).
Tire pressure during balancing at required pressure.
Permissible residual imbalance of less than 3 g, or max. 3 g per plane.
Further to 3: Fitting the wheel on the vehicle
Information
Never use an impact bolter.
Observe information on the two-piece wheel bolt.
- 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 130 Nm (96 ftlb.).
- Tire pressure according to the specifications.
Balancing weights
Balancing weights: Balance stick-on weights (current version is a fine zinc alloy).
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.