Linkage suspension sag estimation using the static equilibrium method

Abstract

Small amateur teams often struggle to estimate suspension performance in off-road motorcycle competitions due to limited access to advanced assessment equipment. Vehicle testing and validation are commonly performed through direct test rides, where improper setups can lead to loss of control, accidents, and costly repairs. This article aims to provide an economical and less sophisticated technique for engineering technicians working with linkage suspensions to estimate total sag and corresponding wheel travel. Initially, the wheel normal force and suspension geometry are measured and verified. The leverage ratio (LR) is then examined, and the forces in the relay arm ($F_{2r}$) and shock absorber ($F_{2a}$) are identified. By combining the graphs of $F_{2r}$ and $F_{2a}$, sag and wheel travel can be determined based on the swingarm angular displacement. Three adjustment approaches are demonstrated: tuning the initial position of the swingarm, changing the spring stiffness and/or preload, and modifying the leverage ratio. Using the described technique, technicians can assess and reuse the noted LR, $F_{2r}$ and $F_{2a}$ curves to approximate the optimal suspension setting for specific sections or terrains during competitions.