So now we’ve discussed the concept and hypothesis and early development of a Tune Mass Damper for a Mountain Bike and we’ve delved into the math required to achieve the critical damping of our system, it’s time to take a closer look at the design of the Rimpact TMD. But first let’s take a look at how the design has changed from early concept to current working prototype.
The overall concept has always been to produce a Single Degree of Freedom (SDoF) TMD where a mass is suspended between two springs and is able to oscillate freely, out of phase with the vibrational frequencies that pass through the bike as well as the displacement of the front end. The basic design saw a central shaft inside a cylinder that was closed on each end. The central shaft had a mass that operated in a linear vertical motion along the shaft and ran on bearings that allowed it run friction free.
The Mass is the centre point of our design. Getting the right weight, shape and size was key to gaining the right performance characteristics given the design restrictions. Before we made the mass out of steel (and before we had applied any math) we initially designed, and 3D printed a shuttle that could be adjustable which we had hoped would allow us to test
multiple weights. We definitely over engineered this system, hoping to use steel rods encased in the plastic to increasingly apply weight but we couldn’t cram in enough weight or stability to the plastic part and the retainer tabs kept breaking. So instead of spending time reworking this we moved straight to steel. After experimenting a little with steel, we quickly found that the density of the material was insufficient to achieve the mass required in the size restraints we had. Steel has a density in the region of 8kg/cm³ which inherently means we weren’t going to squeeze enough into the system even with smart design and so we looked to other materials. Tungsten is the densest material that is safe to handle and easily obtainable which lends itself perfectly to this project. At a density in the region of 19g/cm³ it is over twice as dense as steel allowing us to achieve a higher weight in a smaller package. Initially we machined the Tungsten Mass to a fine tolerance with the outer casing to achieve a hydraulic fit, allowing it to force air past itself and between the walls of the tube. This action was intended to provide a small amount of friction damping however in practice the difficulty in hitting these tolerances was not cost effective or repeatable.
Typically, in the cycling world, weight is given an over inflated importance and the idea of adding weight to your bike is met with cynicism. However, this has been proven time and again to be false logic and multiple teams are even running solid weights on the downtube of their race bikes to find more performance despite manufacturers putting resources into engineering ever lighter frames and components. As such we don’t believe the total system weight of around 400g (in the case of this prototype) will have a negative effect on the bike.
Picture comparing an early steel mass vs a tungsten mass. Later iterations of the design would see this increase up to nearly 300g allowing the mass to exert further change over the system.
The Mass was originally machined to house linear roller bearings completely as seen in the images. However, to up the total mass in the damper we began machining the bearings to sit at half depth allowing us to retain a higher weight to footprint and retain the total achievable travel of the damper. As the springs have a given solid length, the bearing could nestle into this gap when compressed fully. We also wanted to achieve a wider stance with the two bearings on the central shaft as early versions had some tendency to twist so the wider spacing offered smoother actuation. We started off with steel ball bearings as we hoped to make the damper as service free as possible, but soon found the bearings scored the shaft and disintegrated causing vibrations and noise to be experienced on the trail. This didn’t occur until many hours of ride time, but we weren’t happy with the degradation so looked to other options.
We tested with solid polymer bearings like the ones in this image. After some research we learned that steel ball bearings take time to spin up and tend to slide at high accelerations which is the standard use case for our damper. We were sceptical about friction of such a simple plastic on metal interface however the data proved this was unfounded and the static polymer bearings provide better characteristics for this use case:
“During high acceleration or deceleration, ball bearings can also be prone to skidding: Their inertia must be overcome, so the balls first slide and push against one another (or their cage) before speed or motion become constant, and the balls begin to roll. Because this sliding can cause scoring on the shaft, it sometimes forces engineers to lower cycle times.”
The next update to the design saw a change from thick polymer bearings to sleeve bushes as the friction levels remained acceptable but allowed for more material to be retained within the restricted space.
Here you can see that the plastic offers a lower operation noise as well. Noise is friction so the lower the better. In this case -20db is achieved with plastic over conventional bearings.
During our research on Mass Dampers, we learned that the individual parts of the damper system could be changed to produce different frequencies of damping. There isn’t a ‘correct’ set up but a range of correct set ups for a desired result. We decided to ensure our design allowed for a range of springs to be installed that would give the user the opportunity to fine tune the performance of the damper. We began by trying a range of springs but found the ratio of free length to solid length needed to be within a fine window to work in the confined space inside the headtube. By adjusting the formulae used to ascertaine the correct balance between mass and stiffness, we zeroed in on 3 different spring rates that targeted different frequencies and damping ratios and had 3 springs made to allow us to move forward with testing and adjust from there if necessary.
The central shaft of the mass has seen a change, moving from a 6mm bolt to an 8mm increased the stiffness of the shaft by over 3x. We needed to do this to ensure the shaft would not flex or snap from high strain the opposing springs were exerting on them. The top and steerer caps also had a structural improvement to handle this expansion force.
Finally, we upgraded to Die Springs from standard Stainless Steel ones. The move allowed us to control the spring rate more accurately as these springs are made to a higher degree of precision. It also allowed for colour codes that would enable the user to differentiate between springs when fine tuning their own set up. We were also looking to up the stiffness of the system slightly as our field testing was showing that we were at times achieving bottom out of the springs adding feedback to the rider. A recalibration of the Mass, stiffness and design kept the function correct whilst reducing this risk.
In the next post we will begin to show some test results!
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