Influence of Rear Wheel Vertical Displacement and Target Sag on Suspension Performance of a Cruiser Motorcycle

Type of the Paper: Extended Abstract

Influence of Rear Wheel Vertical Displacement and Target Sag on Suspension Performance of a Cruiser Motorcycle

K. Peck^1,*, and J. Sadauckas²

¹ Vehicle Dynamics Group, Harley-Davidson Motor Company, Milwaukee, WI, 53222, USA; kasey.peck@harley-davidson.com; ORCID 0009-0004-8497-7691

² Trek Performance Research, Trek Bicycle Corporation, Waterloo, WI, 53594, USA; jim_sadauckas@trekbikes.com; ORCID 0000-0002-6055-9047

*corresponding author.

Name of Editor: Jason Moore

Submitted: 28/02/2023

Accepted: 26/04/2023

Published: 28/04/2023

Citation: Peck, K. & Sadauckas, J. (2023). Influence of Rear Wheel Vertical Displacement and Target Sag on Suspension Performance of a Cruiser Motorcycle. The Evolving Scholar - BMD 2023, 5th Edition.
This work is licensed under a Creative Commons Attribution License (CC-BY).

Abstract:

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 roadholding performance (Lot, 2021) while also ensuring the vehicle achieves the desired trim state under the various operating conditions (Cossalter, 2006), 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 that has received little attention is defining the nominal vehicle trim state in terms of target sag and the associated proportion of vertical wheel displacement to be used in compression versus that available for rebound. For closed course racing vehicles, both on-road and off-road, the suspension displacement and target sag are determined experimentally based on testing and iteration to obtain the sole objective of minimum lap time. Conversely, for commercial on-road vehicles, suspension displacement and target sag are often constrained by numerous vehicle design requirements such as seat height and packaging limitations. These design constraints require production-intent suspension displacement and target sag to be determined early in the product development cycle. Until now, limited literature has been published regarding nominal target sag and how best to proportion suspension displacement between compression and extension, though a general guideline proposes ~33% target sag as the starting point (Thede, 2010). The intention of this paper is to provide a deeper technical understanding of suspension performance trade-offs between available suspension displacement and target sag using physical vehicle testing and multibody simulations.

In this paper, two rear suspension layouts have been tested with different suspension displacements and target sags while maintaining the same seat height and vehicle ride height. For all configurations both subjective rider feedback and objective data were collected over discrete negative (downward) and positive events. Qualitative suspension performance was captured by a professional motorcycle test rider, trained to articulate any tangible differences between the configurations on closed-course suspension events. Quantitative suspension data was simultaneously collected on the vehicle including front fork and rear shock displacement and stroking velocity, acceleration at the steer head and mid-frame, as well as vehicle pitch rate and pitch angle to correlate to subjective ride feel. The analysis of the qualitative and quantitative data yielded suspension performance metrics that were used to correlate to a multibody model. Simulation was conducted over various discrete negative and positive events of increasing size to quantify the influence of sag on suspension performance across a wider range of bump conditions.

Table 1. Alternative rear suspension configurations compared during on-road testing

Results of the study show quantifiable improvements in both suspension comfort and road holding when using increased suspension displacements and increased sag (more available rebound stroke), though degradations to chassis control were noted during aggressive braking events. The increase in available extension displacement of rear suspension Layout #2 enabled a reduction in rear shock rebound damping (Layout#3) without increasing the occurrence of topping. This reduction in rear shock rebound damping improved road holding by allowing the rear tire to better follow the profile of negative road events and improved comfort through reduced forward pitching of the vehicle over discrete positive events (Cao, 2011). Additionally, the increase in available extension displacement reduced the spring preload force near full extension and enabled improved comfort by reducing the abruptness of discrete positive events encountered while at the extended portion of suspension stroke. Analysis of the measured vehicle data enabled the creation of objective suspension performance metrics and provided the necessary information to understand these alternative rear suspension layouts using multibody modeling. Simulation results further expanded on the performance trade-offs between the suspension displacement and target sag by enabling the analysis of additional bump configurations. Ultimately the study provided systematic guidance on the nominal suspension displacement and target sag for the specific road-going motorcycle studied and highlights sag as a critical tuning parameter often neglected in the literature.

Figure 1. Left: Example of surrogate on-vehicle data over discrete negative event (step-down) including rear suspension displacement and velocity (solid black and grey dashed respectively) as well as pitch rate and pitch angle (solid green and magenta dotted). Right: Summary of compression and extension displacement usage over a discrete negative event (step-down) at both 20 and 30 mph comparing two surrogate rear suspension configurations differing in extension displacement and target sag.

References

Cao, D., Song, X., & Ahdmadian, M. (2011). Editors’ perspectives: Road Vehicle Suspension Design, Dynamics, and Control. Journal of Vehicle System Dynamics. https://doi.org/10.1080/00423114.2010.532223.

Cossalter, V. (2006). Motorcycle Dynamics (2nd ed.). Lulu.com.

Lot, R., & Sadauckas, J. (2021). Motorcycle Design: Vehicle Dynamics Concepts and Applications. Lulu.com.

Thede, P., & Parks, L. (2010). Race Tech's Motorcycle Suspension Bible. Motorbooks.