Rear Leaf Spring Fractures

On June 18, 2019, the Office of Defects Investigation (ODI) opened PE19-007 to investigate instances of leaf-style suspension springs fracturing in various model 2008-2018 Thomas Built Buses (TBB) school buses, which could lead to a road hazard if a piece of the leaf spring detaches from the vehicle while driving. This investigation was prompted by ODI’s review of field reports in Daimler Truck North America's (DTNA) Early Warning Reporting (EWR) data, describing cracked rear leaf-style suspension springs in Thomas Built school buses. ODI contacted three municipal school bus fleets with subject vehicles. These fleets described similar failures where a fractured piece of leaf spring separated from the vehicle while driving. In addition, ODI contacted a nationwide school bus transportation service provider who identified several failures of this component. DTNA’s response to NHTSA’s Information Request (IR) in this investigation acknowledged that TBB school bus leaf springs may corrode and fracture. DTNA data provided in response to the IR did not show a high frequency of failures that result in portions of a cracked leaf spring detaching from the vehicle. In the response, the majority of the information concerned C2 springs. While the C2 represents approximately 71% of the sales, 136 of 141 warranty claims (97%) were for C2 buses, and 94% of all of the replacement parts sales were for C2 springs. In 2021, DTNA retained Exponent to assess reports of rear leaf spring fractures on TBB C2 school buses. As part of its assessment, Exponent inspected in-service buses and leaf springs, conducted material and metallurgical analyses of exemplar leaf springs, and analyzed manufacturer- and fleet-generated claims data. Exponent concluded based on its assessment that DTNA’s leaf springs are typical for the industry and within DTNA’s specification, and it found a low frequency of a cracked leaf springs detaching from vehicles. Exponent also asserted that it could not confirm any i