Thermal modeling in mountain bike air shocks

Abstract

The demands on mountain bike (MTB) components continue to grow as the sport matures. Components are driven toward increased functionality, reliability, and weight savings. In the arena of mountain bike rear shocks, air shocks lead the way in both weight savings and user adjustment over their coil spring counterparts. Not only do air shocks allow multiple controls over both compression and rebound damping but they also allow easy control for rider sag, adjusting for varied sized riders and rider suspension travel preferences. They do so with significant weight saving over coil spring competitors. A downside to air shocks over coil are the thermal issues surrounding housing an air spring concentric with the shock damping mechanism as well as the seal-friction issues associated with containing the air. The continual compressing of the air and the air seal friction are thermal dynamics that coil shocks do not experience. Also, the insulation of the damper mechanism by the air spring constrains more heat in the air shock. The net result of these thermal complexities is that air shocks typically get much hotter than coil shocks and as such cannot easily be used in continual high load riding scenarios such as downhill racing and long distance downhill riding. This paper examines some of these issues by developing a mountain bike rear air shock model incorporating air spring and frictional thermal effects. Heat is generated by compression and friction, stored in material and air thermal capacitances, and transferred between system elements and eventually to atmosphere. Damper energy generation effects are ignored in this paper and saved for a future study. The effects of shock design on thermal time constants and maximum temperatures are evaluated. Certain model predictions are compared to laboratory data.