Influence of Available Rear Wheel Travel and Target Sag on Suspension Performance of a Cruiser Motorcycle

The design and optimization of two-wheel vehicle suspension provides an exciting design challenge due to the multitude of potential layouts and interrelated variables to consider. Balancing these design factors to achieve the desired comfort and road holding performance while also ensuring the vehicle achieves the desired trim state under the various operating conditions, termed chassis control for the purposes of this paper, requires a deep level of technical understanding to execute successfully. Consequently, a specific area of two-wheel vehicle suspension development that has received little attention is defining the nominal vehicle trim state in terms of target sag and the associated proportion of vertical wheel travel to be used in compression versus that available for extension. For closed course racing vehicles, both on-road and off-road, the suspension travel and target sag are determined experimentally based on simulation or testing to obtain the primary objective of minimum lap time. However, for commercial on-road vehicles, suspension travel and target sag are often constrained by numerous vehicle design requirements such as aesthetics, seat height, and packaging limitations. These design constraints require production-intent suspension travel and target sag to be selected early in the product development cycle. Until now, limited literature has been published regarding nominal target sag and how best to proportion suspension travel between compression and extension, though a general guideline proposes ~33% target sag as the starting point. The intention of this paper is to provide a deeper technical understanding of suspension performance trade-offs between available suspension travel and target sag using physical vehicle testing and multibody simulations.