A Bond Graph Model of a Full-Suspension Mountain Bicycle Rear Shock

Abstract

As the sport of mountain biking matures, equipment continually evolves to afford better biking performance, enjoyment, and safety. In the arena of suspension systems, mountain bikes have moved from rigid suspensions with large, knobby tires to front fork suspensions, and finally full suspensions. Suspensions have gone from elastomeric compliance to air and coil springs with adjustable travel. Damping has progressed from fixed to adjustable rebound, compression, and lockout. The current trend is to add force or frequency dependent damping to minimize response of a suspension from pedal input. A bond graph model of a mountain bike rear shock is developed incorporating adjustable rebound/low-speed compression, high-speed compression, and adjustable, compression damping initiation. An air shock with a nitrogen charge is modeled with velocity across the shock as input. The dynamic equations that come from a bond graph model are simulated to predict key responses. Experimental response of the modeled shock is acquired subject to periodic velocity inputs. The experimental response is used to tune the design parameters of the model and for validation. Future use of the model is to better understand the physics and performance of the mountain bike shock and to relate performance to the requirements of expert mountain bikers.