As avid riders and racers, here at Rimpact we are always looking to optimise and develop our bikes, components, and skills to eke out any available performance. Most mountain bikers in general are the same. As riders push themselves increasingly fast through rough terrain, jumps, and drops, the impact forces and vibrations take a toll on the body. Over a day of riding long descents, hands forearm, and brain fatigue can creep in reducing participation time and reducing the enjoyment of the sport. Trail obstacles create vibrations through the bike that can be uncomfortable and even dangerous for the rider. Here at Rimpact we’ve been making tyre inserts for 6 years that aim, in part, to reduce these vibrations and thus the fatigue that riders experience. Whilst the inserts we produce are extremely effective we felt we could go further and produce a mechanical device that could build upon these improvements to ride quality. This is where the development of a tuned mass damper can be useful. Research has shown that the vibrations a rider experiences can match that of a jack hammer and go beyond some recommended exposure limits.
Further to the health factors though we are considering a system that can reduce the oscillations and high frequency movement of the front-end system of a bike to damp the effect it has over the rider, allowing them to focus at higher speeds and control the bike more effectively. Over the past couple of years, we’ve been intermittently developing a Tuned Mass Damper, dubbed TMD, to solve this problem and we’ve reached a point where we are happy to share this development process, the learnings and talking points around the project. In Part 1 of this development blog, we will cover the concept and testing methodology of the component.
Picture by Guillom -Taipei 101, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2516993
A Tuned Mass Damper is a mechanical device that is designed to reduce the effects of vibrations in a structure by attaching a mass suspended with springs and allowing it to oscillate out of phase with the structure. This graph taken from Wikipedia depicts how the red line (a system with mass damper attached) experiences less extreme displacement than the blue line (baseline). The Y axis is force amplitude and X axis is time. We believe this concept can be applied to a mountain bike to improve the comfort and safety of the ride but also has potential to provide chassis stability benefits that in turn would allow the rider to achieve more composure and thus better results. Mass Dampers have been used to great success in other sports, such as the infamous Renault F1 car, and are a staple of engineering, most well known for their use in Earthquake prone locations by stopping buildings from crumbling, so adapting it to a mountain bike is not a huge leap.
Image Credit: CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=18539914
To develop a tuned mass damper for use in mountain biking, several factors need to be considered. The mass of the damper needs to be carefully chosen to ensure that it provides effective damping without causing additional problems. The frequency and amplitude of vibration that the damper must counter needs to be measured, and the damper's natural frequency needs to be determined. This point is likely to be our biggest hurdle in development as the frequency of vibrations is subject to change, rider to rider, track to track, bike to bike. However, there are some assumptions, considerations and generalisations that can be made that would allow us to circumnavigate this issue and still produce an effective piece of equipment for the majority of riders and skill levels. A mass damper’s effectiveness can be altered by adjusting values such as spring rate, mass and damping coefficient meaning adjustability shouldn’t be an issue if the design is considered.
The stiffness of the damper also needs to be optimal, as it will determine the force required to displace the damper’s mass. Finally, it’s placement on the bike will limit the design and impose restrictions in its operation we may need to design around. Once these factors have been considered, a prototype can be designed and tested.
To get some sort of idea what the system might look like we whipped up a basic enclosure that suspended a mass between two springs. The mass could be adjusted to tune the damper to the natural frequency of the mountain bike, and we relied on friction to act as the damping mechanism whilst we honed in on the desired level of damping. As the rider is attached solely to the sprung mass of the bike, the mass damper needs to be affixed to this part of the bike to be able to help exert change in this area. If it was attached to the un-sprung mass of the bike, two separate dampers would be required one for each end of the bike and then the suspension would still cause movement in the frame reducing the damper’s effect. We will look to try multiple dampers on different parts of the bike during development but initially the best location for the damper would likely be the headtube of the bike due to its location and hollow nature making it perfect for a system like this.
We whipped up a very basic concept in a few minutes with acrylic tube and spring rates arrived at with some napkin math, to get a visual representation of what would happen if we suspended a mass inside in this approximate area. We used a steel mass and polymer bushes to allow the mass to move under small impacts. Recording its motion in slow-mo allowed us to learn a few key points that would help us flesh out the first prototype design. We will discuss more on the design in part 2.
Once the prototype was built, it could be tested on various terrain to determine its effectiveness. If the damper provides a slight improvement in the comfort and safety of the ride, it could be further developed to provide added benefit in racing applications. We conducted some vibration analysis on the handlebar and crown, prior to fitting the damper and again with the damper fitted to ascertain its effectiveness and further tune its capability.
To test the prototype, we devised several tests that would adequately identify the change that the TMD would exert on the bike and attempt to validate the theory that such a device could be useful in the sport. Firstly, an impact test in which the bike was mounted to a jig and struck with a measurable and consistent force. The vibration passing through the headtube would be measured at crown and bar and compared, with and without the TMD fitted, to see what affect the damper was having. We would expect to see a lower overall amplitude of initial force followed by a quicker return to static levels vs the same test without the TMD fitted.
A second test we conducted was one where a straight section of very rough trail was used to exert continuous but repeatable impacts on the front wheel. The rider would start from a single position lent against a tree and release the brakes, putting zero input into the bike, with straight arms and legs. This allowed us to record the average frequencies the bike experienced over this stretch of trail and we repeated the test many times with and without the TMD fitted to get an average difference. We compared the vibration frequencies and amplitudes to determine if there was a gain in performance with this early prototype.
In conclusion, the development of a tuned mass damper for use in mountain biking could improve the comfort and safety of the rider by reducing the effects of vibrations as suggested by our early testing. There could also be a speed gain from the device whereby the rider is able to push themselves harder thanks to a more composed ride. If future upgrades to our prototype are successful, a tuned mass damper could become a valuable addition to every mountain biker’s bike, not just at the high end of the sport. Future posts will begin to shed light into the specifics of the design and challenges we have had to overcome.
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