Re: Suspension losses

On 1/3/2025 10:41 AM, Zen Cycle wrote:

On 1/2/2025 11:06 AM, AMuzi wrote:

On 1/2/2025 9:42 AM, bp@www.zefox.net wrote:

AMuzi <am@yellowjersey.org> wrote:

>
I don't have a coherent argument either way but a rumble
strip test introduces a repeatable experience so that
various data may be compared.  Each rider on a dirt or
gravel path, and each ride experience by any given rider, is
an unique set of impedimenta such that data cannot be as
readily compared.

>
AMuzi <am@yellowjersey.org> wrote:

>
I don't have a coherent argument either way but a rumble
strip test introduces a repeatable experience so that
various data may be compared.  Each rider on a dirt or
gravel path, and each ride experience by any given rider, is
an unique set of impedimenta such that data cannot be as
readily compared.

>
But one can observe that in the case of smooth pavement,
suspension losses vanish, while hysteresis losses persist.
>
In the end a bike is an overdamped resonator excited by the
pavement and damped by hysteresis, separately in the tire and
suspenesion. In that limit, suspension would be faster if used
with very hard tires on very smooth surfaces. In the limit of
hard tires and no suspension, the dissipative element becomes
the rider whose elastic properties are apt to be poor, perhaps
accounting for the apparent slowness of solid tires.
>
Use of a rumble strip for testing is equivalent to selecting
a particular excitation spectrum. Choice of spectrum will affect
dissipation depending on internal resonances of the bike/rider
system. A real road likely corresponds to a 1/f spectrum, but
a rumble strip will likely be something else. How much difference
that makes isn't clear but it could be estimated using a mechanical
analogy equivalent circuit of the kind used to model loudspeakers.

This is a great analysis and reveals a highly problematic aspect of
the "rumble strip" test. As Bob notes, it's essentially limiting the
noise input into the system to a somewhat narrow spectral component
(though the 1/f assumption for real-world is way to broad)
The idea of using the rumble strip test seems adequate at first, but
is prone to misleading results. Since the rumble strip sets up a
regular frequency component, there's a possibility that a resonance
or cancellation effect can occur which can dramatically skew the
results.
In the world of environmental testing, physical vibration analysis
is typically broken up into three different stimuli - swept
frequency, noise*, and environmental specific (usually a combination
of noise with higher energy components around certain frequencies).
It's nearly impossible to simulate all the possible real-world
conditions, which is why the testing regimen includes a sweep - the
intent being to see any resonances.

>
Interesting. I suppose the rumble strip test could do a "sweep" by
riding repeatedly at a wide range of speeds.