Re: Maintenance Manuals

carlfogel@xxxxxxxxxxx wrote:
On Tue, 09 Oct 2007 10:11:15 -0400, Peter Cole
<peter_cole@xxxxxxxxxxx> wrote:

[snip]

No sarcasm, no condescension, just a frank opinion of his methods. We've discussed this at length before. Carl is always attempting to (dis)prove theory with actual measurements. The problem with this is it generally takes an even better grasp of theory just to set up the measurement. I don't mean to slam Carl, I appreciate the difficulty of what he's trying to do (I've worked in the precision measurement field) -- unfortunately, he often seems not to.

Dear Peter,

We're often at odds, but I think that you're doing a better job here
than I usually give you credit for.

That's not saying much, if anything, but do go on.


In the same fashion, I don't mean to slam you. I appreciate that you
have considerable engineering skills and experience.

The problem that I see is that--and you're not alone in this--we on
RBT unfortunately tend to apply theory to a practical situation
without testing or even considering the possibility that our
theoretical, untested model may not include crucial real-world
factors, which by their very nature never how up in our purely
theoretical reasoning.

Sure, the selected model may not be a good match to reality, or it may be parameterized wrong. That happens, but what is much more frequent is arguments based on a lack of understanding of the model or its underlying science.


(We don't know what's wrong with our theories until reality rubs our
noses in our misunderstandings.)

Even more unfortunately, we then defend our theoretical position with
more theory and whatever ad hoc observations support us, which is more
debating than science.

(That's why we have the phrase "Back to the drawing board" to describe
what happens when theories are not tested.)

If you say so, but I'm not finding your argument convincing.

Here's about as plain and simple a bike example as I can think of,
where perfectly valid theory drew an obviously impractical conclusion:

http://groups.google.com/group/rec.bicycles.tech/msg/a14af7c4e0dc2fb0

When a closed chain is worn enough, you can easily invert it, despite
theory, turning it inside and out like a rubber band.

Sure, but it was a sucker's bet, given the amount of wear on the chain.


Similarly, I found to my surprise that the spoke tension on a cheap
MTB wheel dropped when I let the air out of the tire, instead of
rising as theory and even testing on 700c wheels showed it should do.

Again, the "prediction" had to include casing angle, there's no promise that inflation will compress of expand the rim without it.


And I found that the contact patch refuses to expand or contract
nearly as much as RBT theory predicted when tire pressure is raised or
lowered. It turns out that a pressing on the outside of an inflated
canvas toroid is more complicated than a rigid steel piston rising or
falling in a rigid steel chamber.

I don't know what "RBT theory" is. The weight of a bike and rider is borne by the sidewall, which has a complex deflection -- how that translates into footprint is not simple.

My favorite non-bicycling example of theory blinding us to reality is
the Mpemba effect,

Interesting, but no news & OT.

A sound theory by definition leads to a sound conclusion, but in the
real world things are often so complicated that describing the
situation takes more sound theories than we first imagine.

That comes as no news to an engineer. That's pretty much what our professional lives are all about.


My vote is for testing and measuring, particularly when there are
disagreements. Squeezing spoke pairs, for example, has lots of
plausible theory and plenty of enthusiasts behind it, but we still
have no tests confirming that it prevents broken spokes, much less
demonstrating how.

If something can't be proved, it's religion, by definition. To conduct a valid experiment, you must prove the experiment. That's the hard part, both in design and realization.
.

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