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Disc Wheel Radial Runout Low Point Marking: Optimizing Commercial Vehicle Tire-Wheel Assembly
The SAE J2133 recommended practice provides standardized specifications for marking the low point of average radial runout first harmonic on steel disc wheels for Class 5–8 commercial vehicles. This marking enables precise match-mounting of tires to wheels, minimizing assembly radial force variation and improving vehicle ride quality and tire wear.
Understanding Radial Runout and First Harmonic
Radial runout is defined as the total indicator reading in the radial direction at the rim bead seat for one revolution, with the wheel located on a specified datum. The average radial runout is obtained by simultaneously averaging signals from both bead seats. The first harmonic of this average runout represents the magnitude of the dominant sinusoidal component at one cycle per revolution. The low point is the angular position where this first harmonic reaches its minimum value.
By marking the low point, tire fitters can align the tire’s high radial force point with the wheel’s runout low point during assembly, significantly reducing radial force variation. This match-mounting process is critical for optimizing ride comfort and minimizing vibration in heavy-duty trucks.
Measurement Procedure and Datum Requirements
Accurate measurement of radial runout requires a proper datum. According to SAE J2133, the wheel must be located using its disc mounting face and center-hole periphery. The measuring system must collect average radial runout data at rotational intervals of no more than 3° and compute the first harmonic and its low point location.
The maximum sampling interval of 3° ensures sufficient resolution to capture the first harmonic waveform correctly. The system typically uses Fourier analysis to extract the one-cycle component and determine the angle of its minimum. Repeatable measurement is essential for reliable marking.
Marking Specifications, Tolerances, and Practical Insights
The low point mark is a permanent circular indentation with a minimum diameter of 3.0 mm and a minimum depth of 0.13 mm. It must be placed on the rim weather side in the bead seat area, on the side used for tire mounting. A duplicate mark may be added optionally on the opposite side for flexibility during assembly.
The required location accuracy depends on the magnitude of the first harmonic of average radial runout, as shown in the table below.
| First Harmonic of Average Radial Runout | Marking Location Tolerance |
|---|---|
| 0.13 mm (0.005 in) or less | ±180 degrees |
| Greater than 0.13 mm (0.005 in) but less than 0.66 mm (0.025 in) | ±45 degrees |
| Greater than 0.66 mm (0.025 in) | ±20 degrees |
🛠️ Engineering Design Insight
The three-tiered tolerance system balances manufacturing practicality with effective reduction of radial force variation. For very low runout (≤0.13 mm), a ±180° tolerance is acceptable because the first harmonic magnitude is so small that any misalignment has negligible effect on assembly force variation. For higher runout amplitudes, tighter tolerances ensure that the mark provides a meaningful reference for match-mounting.
The placement on the rim weather side protects the mark from wear and allows easy access during tire mounting. The minimum mark depth of 0.13 mm ensures visibility and permanence without compromising wheel integrity.
⚠️ Common Mistakes to Avoid
- Using an improper datum (e.g., not including the center-hole) leads to inaccurate runout measurement.
- Collecting data at intervals larger than 3° compromises the first harmonic calculation.
- Applying the low point mark based on instantaneous maximum runout instead of the first harmonic low point.
- Misreading the first harmonic amplitude and applying the wrong tolerance category.
- Placing the mark only on the non-tire-mounting side without the optional duplicate.
Frequently Asked Questions
How is the first harmonic of average radial runout computed?
The first harmonic is calculated by performing a Fourier analysis on the average radial runout data collected at ≤3° intervals. The magnitude and phase of the one-cycle-per-revolution component are determined, and the low point is the angular position where this component reaches its minimum.
Why is the low point marked on the rim weather side?
Marking on the rim weather side (the side facing outward when mounted) protects the mark from harsh environmental conditions and keeps it accessible during tire mounting. The bead seat area provides a consistent reference surface that is directly related to the tire mounting interface.
What are the consequences of using a sampling interval larger than 3°?
Using a larger sampling interval can cause aliasing and inaccuracies in the computed first harmonic. It may not capture the true sine wave shape, leading to an incorrect low point location and reduced effectiveness of match-mounting.
Does this standard apply to all commercial vehicle wheels?
No, SAE J2133 specifically applies to tubeless 15° drop center steel disc wheels for Class 5 through 8 commercial vehicles. Other wheel types, such as aluminum wheels or different rim profiles, may require different standards.
