Session: 08-01-01: General Dynamics, Vibration, and Control I

Paper Number: 165450

Design of a Fixture for Vibration Testing of Mountain Bikes 

Mountain bikes play a vital role in both sporting and recreational industries, with rider safety and comfort at the core of their design and development. Agility is another essential factor, which is often challenging to balance with stability and comfort. Striking the right equilibrium between these competing demands makes mountain bike design both intricate and demanding.

The design process involves fine-tuning front and rear shock absorbers for riding experience, optimizing the frame’s hard points for better handling, and testing performance across diverse terrains. These aspects are not merely sequential steps but rather interconnected elements that must be addressed simultaneously for an effective design. Testing is conducted both at the component level—examining shocks, forks, and dampers in isolation—and at the system level, evaluating their combined impact on overall structural response and durability. Additionally, mountain bikes are tested to assess rider comfort in both real-world terrain conditions and in controlled laboratory environments using test rigs, fixtures, and shaker tables.

The success of vibrational testing of any system is predicated on the development and implementation of an appropriate test fixture. Specifically, test fixtures must provide necessary mounting or support capabilities while simultaneously avoiding interference with the natural frequencies of the test specimen. After analyzing the preexisting test structure through frequency response analysis using ANSYS (modal), it was found that the natural frequencies of the fixture overlapped with the typical frequency range of the bounce and pitch modes of mountain bikes. The ANSYS results confirmed visual observations that the test structure exhibited at least one mode within the bike’s frequency range of interest. Subsequently, experimental modal analysis was performed on the test structure to validate the simulation results. The experimental findings confirmed mode shapes observed in the simulation, as well as the approximate natural frequencies associated with those shapes. The agreement between simulation and experimental results demonstrates that the current test fixture indeed has natural frequencies that would interfere with testing and analysis of mountain bike dynamics.

This interference leads to a higher mass participation factor when testing is conducted using such structures which, in turn, pollutes the data being collected for the actual test specimen. Several mitigation strategies exist to address this issue, including introducing modal damping to reduce displacement amplitudes, adjusting the stiffness of the mountain bike, and increasing the fixture’s natural frequencies. The latter is a practical approach explored in this paper by estimating modal participation factors and mode shapes for various test fixture designs to determine an optimized version. The analysis also considers the spatial constraints of the test facility, with the final design incorporating large portions of the existing structure while enhancing fixture stiffness to push natural frequencies beyond the range of interest. This adjustment effectively lowers the fixture’s modal participation factor, ensuring more accurate vibrational testing of mountain bikes.

Presenting Author: Matthew Clontz Western Carolina University

Presenting Author Biography: Dr. Matthew Clontz is an Assistant Professor in the School of Engineering + Technology at Western Carolina University. He joined WCU after several years in various industry roles in fields such as automotive research, electric vehicle, and industrial equipment. He earned his BS and PhD from Virginia Tech in Mechanical Engineering where his dissertation was focused on the application of structural dynamics techniques to automotive suspension systems. At WCU, Dr. Clontz has turned his background towards the dynamics and vibrations of mountain bikes. He partners with regional companies on projects of both academic and industrial relevance in the mountain biking space.

Authors:

Matthew Clontz Western Carolina University
Chaitanya Borra Western Carolina University