###### Abstract  
To decrease track derailment of tracked fire trucks in forested areas, a fish-bellied swing arm torsion bar suspension system is proposed in this research. Derived from a tracked forest fire engine, this study converts the shaft tube swing arm of the original vehicle to a fish-bellied swing arm, improving the semi-rigid shaft tube suspension to a torsion bar suspension. Static and kinematic simulation analysis of the improved virtual sample vehicle is carried out, and the stress and dynamic characteristics before and after the improvement are analyzed. The simulation force cloud diagram of the improved swing arm and the motion simulation curve of the supporting wheel is obtained. The results show that the design of the fish-bellied swing arm can effectively reduce the bending moment caused by force acting on the swing arm, and that the design of the torsion bar spring suspension can reduce vertical displacement of the supporting wheel by 58.53%, and reduces horizontal displacement by 46.58% under the same impact force. According to the design of the virtual sample to build a prototype vehicle, a comparative test is carried out to determine an optimized virtual sample vehicle. The results show that the trend of the test curve is essentially consistent with that of the simulation curve. torsion bar suspension chassis fish-bellied swing arm; kinematic model; virtual simulation; tracked fire engine +
Footnote †: journal: Applied Sciences  
track shoes on soft ground for a splayed grouser; the horizontal soil thrust of the track plate was calculated, which then provided a theoretical foundation for the design of a high strength track plate. [PERSON] et al. [12] put forward a method of controlling track tension by a constant hydraulic drive guide wheel, which improves track stability and reduces the risk of track derailment; [PERSON] et al. [13] designed a torsion bar suspension structure, meeting the requirement of dynamic and cushioning performance in all-terrain articulated tracked transport vehicles, and improved the stability of the vehicle and reduced the probability of track derailment. When a forest fire breaks out, the burning temperatures near the fire site are high, as well as the fire radiation intensity, so tracked vehicles with rubber wheels cannot come into close contact with the fire site. Additionally, the fire situation rapidly changes. If the forest-tracked fire engine falls off its track while fighting a fire, this can cause fatal results. Therefore, forest-tracked fire engines urgently need a rigid chassis structure and a suspension system that is more resistant to falling off the track [14]. This study is based on an LF1352 JP forest fire truck, produced in the northern forest region of the Daxing'an Mountains in China. The truck's overall firefighting impact is good. However, when the tracked vehicle is climbing over a gully, track derailment sometimes occurs. The on-site observations found that when track derailment occurs, the swing arm of the supporting wheel is damaged and seriously deformed; as shown in Figures S1 and S2. It can be inferred that the inefficient structure of the vehicle body swing arm suspension is the main reason for the track derailment of the LF1352 JP tracked forest fire engine [10]. The LF1352 JP tracked fire engine adopts a shaft tube swing arm and semi-rigid suspension structure. The shaft tube swing arm structure is a general swing arm structure of tracked vehicles, which is simple in form but not strong in terms of bending and torsion resistance. There has been little research on semi-rigid suspension structures, and scholars from various countries mostly use torsion bar suspension structures in the design of tracked vehicles, and those studies are more in-depth. [PERSON] et al. [15] improved the off-road performance of tracked vehicles using a spatial motion analysis model of independent torsion bar suspension tracked vehicles; the motion of the tracked vehicles, including wheels, was numerically simulated.