What do think of mass damper on DH bikes?
Mass damper on bike
Picard
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What would it do in the road biking application that you mentioned before? Would it counterbalance up/down leg mass movement (oscillation)?
These guys talk about the DH application this interview:
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A lot of compliance and comfort are related to time domain effects - Ie. Vibration
Roadies are sitting there in a fairly narrow range of RPM and road speeds on a rigid structure. They're much more likely to satisfy conditions for resonance to be a contributing factor for comfort.
They're also doing it for hours at a time in a fixed position so comfort matters more in their situation.
You could in theory use a tuned mass damper to change how that rigid bike vibrates. Similar to how a particular high pitch sound can be super annoying - you can change the frequency to something that is more pleasant. Leg movement is probably too heavy and too slow to be a factor, but you could reduce road buzz and maybe affect general fatigue.
The video raises more questions and answers for me.
If the mass damper reduces movement - why is it attached to the suspension and not the frame? Surely a frame that moves less is more comfortable/more stable/more predictable?
Reducing the sensitivity of the suspension would transfer more force to the frame and then the rider.
Maybe you could argue that on the rear suspension in particular the damper itself has too much hysteresis. The leverage ratio is working against it. The mass damper could reduce some movement and some work for the damper. But that weight is far too small and would only work for a particular frequency.
How often are you hitting the one exact sized bump and the exact same speed over and over again?
I guess you are married
It is typically tuned to cancel the natural frequency of the original mass-spring-damper system i.e. the mass of the bike/rider, and the stiffness and damping of the suspension and frame compoenents. These will all have a single resonant freqency, although in reality, they will amplify across a wide range. The filter (the mass-damper) will also have a certain Q factor for its resonsance, so will attenuate across a certain frequency range as well, depening on how high that Q factor is.
It's not a gimmick, it has been used in all sorts of systems. It's not meant to cancel the driving or input frequency - the suspension system is meant to do that. It's meant to cancel the vibrations resulting from exciting that system. The closer you drive it to the natural frequency, the higher energy those oscillations will be, but even in a wide bandwidth beyond that, you will still excite them to some degree. And this is meant to cancel those out. It does however introduce a secondary natural frequency.
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It sounds like teams have been experimenting with different mounting locations. Loris Vergier's bike had it attached to the frame.
Brake bumps might qualify for that. Whistler bike park in late summer is a decent example, especially the high traffic blue trails like Una Moss.
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At World Cup pace I bet the suspension is getting pushed to the limit so any little bit extra can be advantageous. At MTBR pace not so much.
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How do you interpret this working on the bike/rider as a system?
I can intuit how it works where there is a single degree of freedom. I also didn't know that a lower Q factor corresponds to a higher bandwidth. That's surprising and kinda cool.
But my mind scrambles when I think of the bike and the rider together. Surely the variability of how closely coupled they are would add so much noise to the analysis. How do you determine the natural frequency of something like that?
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What a @Nat-tarded comment!
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At first I thought that sounded silly but then when you consider that the top places are separated by hundredths of a second it sounds less silly.
Bike/rider is pretty tricky because like the ground, the rider can also input variable frequency force.
But that's why I think these kind of "filters" are for free, or natural vibrations. Your suspension system is underdamped, and acts to absorb energy from a wide bandwidth of vibrations, but in turn, it creates a natural frequency of its own (actually, if you look at spectrum analyses you'll see all the individual mass/spring systems ring individually, and the lower stiffness ones tend to ring the highest - that would be your suspension itself). And because it's underdamped, you'll never be able to attenuate that natural, or free, vibration as much as you could by adding a second overdamped mass-damper to the system.
When I read the literature, this is how these systems work. Think about one in a building. You'd never be able to tune it for all the forced vibration frequencies from wind and ground vibrations, so you tune it to the natural frequency of the building, typically the least stiff mode with the highest energy. When the wind or ground vibration matches the natural frequency, that's when you'll have the highest energy transfer, and that's when it's most dangerous, so that's the one you want to try to cancel. But, as I said, all filters have some Q factor, and will attenuate beyond their "tuned" frequency.
I recall reading books on car suspension design, long ago, that talked about setting your natural frequency to certain frequencies that weren't as noticable by passengers. But that's a pretty simplistic design approach and maintaining comfort we try to damp out that, and other frequencies that may resonate the cabin.
You could also tune them to one specific forcing frequency, or as I was speculating on Dak Norton's bike, for the unsprung mass of his rear end (he had his right on the chain stay). But I think beyond the unsprung mass, they'd generally be more effective as I decribed above rather than some narrow band track frequency (the suspension should handle that), but it's just another knob in tuning the suspension and they could use that mass-damper to knock something out that the suspension may miss with a certain tune that they like.
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Ask the wives.
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