The Federal Highway Research Institute (BASt) has carried out a comprehensive study to improve the side‑force measurement method (SKM) that has been used in Germany for more than 25 years to assess road grip. The main goal of the project was to enable reliable evaluation of much shorter road sections than the traditional 100 m interval, thereby allowing a more detailed characterization of grip after localized repairs or in high‑wear zones such as junctions and acceleration lanes. To achieve this, the project focused on a uniform acquisition of raw measurement data, the development of a robust vertical‑force correction, and the creation of a standardized raw‑data format that can be adopted by existing SKM systems.
The technical work began with a theoretical description of the measurement mechanics and a laboratory investigation of the vibration characteristics of both SKM and SCRIM vehicles. The experiments revealed that the dominant dynamic influence on the side‑force signal originates from the eigenfrequency of the measurement mechanism itself, while the influence of wheel‑axis oscillations and manual “wank” movements was negligible. Consequently, any correction algorithm must target only the frequency band associated with the measurement mechanism’s eigenfrequency. A low‑pass filter was therefore designed to suppress high‑frequency fluctuations caused by load changes, and a phase‑correction routine was introduced to align the vertical‑force data with the side‑force signal.
Several calculation models were evaluated. The conventional approach, which divides the mean side force by the mean vertical force over the chosen section, was found to produce inconsistent results when applied to short 1 m or 0.1 m intervals because the mean values of the two signals are not directly comparable. To overcome this, a quotient method was implemented that operates on the raw data before averaging, thereby preserving the correct relationship between the two forces. A new, simplified correction approach was also proposed, which reduces the complexity of vertical‑force calibration without sacrificing accuracy. The models were validated against laboratory data and then implemented on an existing vehicle equipped with the new acquisition system. Extensive field tests on four different SKM vehicles confirmed the validity of the correction methods and demonstrated that realistic side‑force coefficients can be obtained for short sections while maintaining the established 100 m performance level.
Based on the field results, the project team developed a proposal for a future regulation that would incorporate the new raw‑data format and the validated correction algorithms. The proposal also outlines a pathway for further testing on additional vehicles to ensure robustness across different platforms. The entire effort was documented in a report that is available electronically through the BASt archive at https://bast.opus.hbz-nrw.de.
The project was led by BASt and carried out by a multidisciplinary team that included researchers Jürgen Schmidt, Julius Schmidt, Johannes Schmidt, and Müller. The work was funded by the German federal government as part of BASt’s road‑surface research program. The collaboration ensured that the developed methods are compatible with existing SKM equipment, thereby facilitating a smooth transition to the new measurement standard.