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From: Frank Krygowski <frkrygow@sbcglobal.net>
Newsgroups: rec.bicycles.tech
Subject: Re: Suspension losses
Date: Sat, 4 Jan 2025 13:02:24 -0500
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On 1/3/2025 11:46 PM, Jeff Liebermann wrote:
> On Thu, 2 Jan 2025 12:17:22 -0500, Frank Krygowski
> <frkrygow@sbcglobal.net> wrote:
> 
>>>> 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.
>>
>> I _may_ have been able to do such calculations 50 years ago, but I'm not
>> sure. I certainly can't do them now.
> 
> Actually, the analogy between a mechanical system and RLC (resistance,
> inductance and cazapitance) calculations are fairly simple.  For
> example:
> 
> "Mechanical-electrical analogies"
> <https://en.wikipedia.org/wiki/Mechanical%E2%80%93electrical_analogies>
> 
> "Electrical Analogies of Mechanical Systems"
> <https://www.tutorialspoint.com/control_systems/control_systems_electrical_analogies_mechanical.htm>
> 
> "RLC circuit: Analogy with mechanical systems."  (From Brazil)
> <https://proceedings.sbmac.org.br/sbmac/article/download/134486/3384/0>

I'm pretty familiar with the RLC analogy to mechanical vibrations.

Actually, I remember sitting in a Vibrations class taught by my least 
favorite professor on the day he brought in a large, demonstration-sized 
Analog Computer (Remember those?) to model something we had been 
calculating. To the amusement of the students, he was not able to get 
the thing to work properly.

> 
>>>> 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-)
> 
> If you're going to build a computer simulation, there are cut-n-paste
> mechanical models of various human bodies available.
> 
> "A mechanical model to determine the influence of masses and mass
> distribution on the impact force during running"
> <https://pubmed.ncbi.nlm.nih.gov/10653036/>
> "Simple spring-damper-mass models have been widely used to simulate
> human locomotion. However, most previous models have not accounted for
> the effect of non-rigid masses (wobbling masses) on impact forces."
> 
> Ok, a running model is not going to work well on a bicycle.  So, look
> around for something that's a better fit.  I'll admit that I've never
> done anything like this, but I can see how it might be possible to
> model a wobbling blob on a bicycle.
> 
> Also, modeling is NOT the hardest part of the problem.  In my never
> humble opinion, the most difficult part is dealing with the large
> number of significant figured necessary to maintain accuracy.  I human
> or bicycle model might work accurate to maybe 1/10th of a watt, while
> the power produced by a road bump powered energy harvesting system
> might be on the order of fractions of a milliwatt.  This forces the
> human model to be accurate well beyond reasonable limits.

My idea wasn't to model a human body for computation purposes, although 
others are probably interested in doing that. I'm thinking more of 
coming up with a physical device, perhaps for weighting a tire during a 
rolling drum test, so the test would more accurately reflect behavior of 
tires when ridden by a human pedaling a bicycle. Maybe 75 pounds of raw 
meat?


-- 
- Frank Krygowski