Re: Suspension losses

Zen Cycle <funkmaster@hotmail.com> 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)
>

 
One can put some bounds on the spatial frequencies of interest. Those
longer than the tire radius likely don't make it past the tire. Likewise,
those shorter than the contact patch don't either, though they might
still excite internal flexural losses in the tire. Neither of those
constraints is exactly true, but true enough for practical purposes.

Delta function inputs, like hitting a sharp edge, are physically
relevant (it happens) but not relevant to analysis of efficiency,
the first rule of efficiency being "don't crash". 8-)

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 a lightly damped system, yes. in an overdamped system I'd put that
use case in the same category as hitting a curb. More trouble than it's
worth apart from avoiding it.
 

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. I've personally witnessed an
electronic assembly quite nearly disintegrate with the right frequency
and energy input. The task then was to redesign the piece such that the
resonance was damped.
 
It's easy to see how this can translate to the rumble strip test. Under
the right conditions, one might actually see a speed _increase_ as a
result of a sympathetic resonance.
>

I'm not sure of that. Energy injected from the strip must be reflected
back on the rebound to be recovered. Since there are losses at every
step of the excitation process I think the rolling resistance will
always be elevated at resonance, though some modes could have lower
losses than others. Either way, the rider won't like it.

>
A pair of series RLC circuits (one for the road-tire interface
and a second for the suspension-rider interface) would be a good
start. I'm not skilled enough to do the calculations, but others
on this group likely are. The hardest part is apt to be finding
an equivalent circuit for the rider, who isn't a rigid mass but
rather a dissipative blob....8-)

 
In the old days, we had to do reiterative tests on massive vibration
tables. These days, FEA software removes the vast amount of guesswork.
The last few times I've had to conduct these tests I only had to do one
test twice, and the problem turned out to be an assembly specification
error rather than inherent design.
 
 

Thanks for reading,
>
bob prohaska
>

 
Clever.
 
I take from that, you think the actual impact/height change/velocity
change etc from various irregular surfaces can be quantified for any
given random gravel (or road) experience and used to compare efficiency
for other iterations.

 
"Real-world" would simulate a more stochastic environment with larger
"impact" events rather than a regular "sinusoidal" spectrum like a
rumble strip. Currently, for example, we use this for our truck-mounted
electronics:
 
https://cvgstrategy.com/wp-content/uploads/2019/08/MIL-STD-810H-Method-514.8-Vibration.pdf
 
Refer to page 514.8C-5 (Page 58 in the PDF).
 

 
I hadn't thought of that, but if that's true then the rumble strip test
isn't necessary for comparison. Which assumes sensors have adequate
sensitivity across whatever range and that software for that data truly
derives actual impedimenta values.

 
Even low-cost accelerometers have incredible accuracy, sensitivity, and
repeatability across spectrum they're designed to operate these days. We
have two 3-axis units accurate to .01G that we paid like $25 each for -
coupled to a mid-range oscilloscope they give more than adequate results
for our "warm fuzzy" testing before we send of to a testing lab for 3rd
party analysis.
>

The notion of mounting accelerometers on axles, seatpost and rider is a
good one if somebody is motivated to do it. It might shed light on the
importance of frame stiffness as well.
 

 
*"Noise" being a broad term meaning quasi-random frequency and amplitude
components within limits.
 

Thanks for reading!

bob prohaska