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From: Frank Krygowski <frkrygow@sbcglobal.net>
Newsgroups: rec.bicycles.tech
Subject: Re: Suspension losses
Date: Thu, 2 Jan 2025 12:17:22 -0500
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On 1/2/2025 11:06 AM, AMuzi wrote:
> On 1/2/2025 9:42 AM, bp@www.zefox.net wrote:
>> 
>> But one can observe that in the case of smooth pavement,
>> suspension losses vanish, while hysteresis losses persist.

I think that would be true only if the smooth pavement were as smooth as 
a linoleum floor. Or a wooden track. IIRC, what got Jan Heine started on 
investigations of rolling resistance vs. tire width was coast-down tests 
on a Soapbox Derby track. I suspect that was quite smooth. Soapbox cars 
have hard tires and no suspension, AFAIK.

>> 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.
>>
>> 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.

>> 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-)

I actually think physically modeling that dissipative blob might be 
valuable for the tire industry. Using such a blob to apply weight during 
a rolling drum test might give better information than what those tests 
give now.

> 
> 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.
> 
> 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.

There are ways of quantifying roughness, with varying scales, varying 
tools. I'm most familiar with roughness measurement of machined parts, 
with tools varying from sample cards for "fingernail" test comparisons, 
to RMS readers akin to phonograph needles or laser scattering devices.

https://en.wikipedia.org/wiki/Surface_roughness

ISTR reading about systems for evaluating pavement fairly crudely, as in 
whether it should be repaved or not. I don't know of a system actually 
used for measuring pavement roughness at a scale affecting bike tire 
choice.

-- 
- Frank Krygowski