Wheel/Rail Profile: Asymmetric Hollow Worn Wheels

Written by Ulrich Spangenberg, Principal Investigator; Scott Cummings, AVP – Research & Innovation; and Kenny Morrison, Senior Engineer, MxV Rail
Figure 1: Hollow wear difference as a function of initial diameter difference for (a) 28-inch, (b) 33-inch or (c) 36-inch wheels. (MxV Rail)

Figure 1: Hollow wear difference as a function of initial diameter difference for (a) 28-inch, (b) 33-inch or (c) 36-inch wheels. (MxV Rail)

MxV RAIL R&D, RAILWAY AGE MAY 2023 ISSUE: Hollow wear is a typical form of wheel wear often measured and reported by wayside wheel profile detector (WPD) systems that allow the monitoring of hollow wear throughout the service life of the wheels. According to Rule 41.A.1.ab and Rule 41.A.2.b in the Association of American Railroads (AAR) Field Manual of Interchange Rules, excessive hollow wear is a cause for removal. Hollow worn wheels have been known to cause rail damage in the transition zones of special trackwork and contribute to rolling contact fatigue on rails.

As part of the AAR’s Wheel/Rail Profile Design and Maintenance Strategic Research Initiatives project, MxV Rail investigated the formation of hollow worn wheels to discover how to reduce asymmetric hollow wear (i.e., a substantial difference in hollowing between the two wheels of a single wheelset). To better understand the potential causes of asymmetric hollow wear, the MxV Rail team studied the prevalence of hollow wear on given railcar and wheelset assembly practices.


MxV Rail researchers analyzed a dataset containing more than 4 million records of wheel wear measurements from wayside WPD systems across North America. 

The mate wheel wear measurements were also processed to determine the difference in hollow wear by subtracting the measurement on the railcar left side from the measurement on the railcar right side. Railinc’s Umler® System was used to determine the railcar type associated with each measurement. A dataset of approximately 41,700 manufacturing records listing the as-manufactured wheel diameters was obtained, and the wheel measurement dataset was used to determine the hollow wear of these wheels over the service life of each respective wheel.

Results and Discussion

The hollow wear difference associated with each railcar type was studied to determine if asymmetric hollow wear is more prevalent on a given railcar type. This study found that, when compared to other railcar types, intermodal railcars and the vehicular flat cars exhibited more asymmetry in hollow wear. 

The wheel sizes used on these cars are sometimes smaller than the more common 36-inch wheels. MxV Rail researchers theorized that wheel size and matching the diameters of mate wheels within the tape size requirements of AAR Standard S-659 could drive some of the asymmetry.

The measured as-manufactured diameters of mate wheels were determined from the manufacturing dataset. The hollow wear was measured on these wheels while in service. The hollow wear differences were calculated and compared to the initial mate wheel diameter differences. 

Figure 1 shows the results of the comparison for wheels with a nominal diameter of 28, 33 and 36 inches. The results show that severe hollow wear difference can occur on mate wheels with small initial diameter differences. Conversely, wheelsets with a maximum initial diameter difference do not show significant differential hollow wear. The diameter difference initially present on mate wheels is not driving the severe difference in hollow wear.


Asymmetric hollow wear was found on wheels in service in North America from WPD data and was found to develop more frequently on intermodal and vehicular flat car types based on a cross-reference with railcar types from Umler®. Initial diameter difference of mate wheels within the allowable variation of the same tape size did not cause asymmetric hollow wear. The tape size matching required by AAR Standard S-659 is deemed adequate since the requirement did not cause unacceptable asymmetric hollow wear performance.


1. AAR Field Manual of the Interchange Rules, 2022.

2. AAR Manual of Standards and Recommended Practices, Section G, Part II, Wheel and Axle Manual, Standard S-659, 2014.  

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