Index Home About Blog
Subject: Re: Headset cheap fix - works
Message-ID: <SjMzb.743$>
Date: Thu, 04 Dec 2003 19:56:34 GMT

Pete Biggs writes:

> Thank you for the rest of your reply.  I'm still not convinced so I'll
> have to read up more on fretting, steerer flex/movement and what other
> factors can damage headsets.

You may not find much on fretting, considering that even many
tribologists do not understand the nature of liquids in thin films
such as they are in rolling element bearings where steel runs on
steel.  Much of what can be found on the web is done outside of the
USA and most of the references are about test machines rather than an
analysis of the cause.

Tedric Harris has diagrams of relative motion of bearing balls that
show hoe they slide as they "roll" on a curved race.  Geometrically it
should be obvious that a ball in a matching groove must slide,
backward in the center and forward at the edges of its contact with
the race.  This requires a lubricant.

My first law of tribology states that no two materials can slide over
one another without a lubricant.  If there is no lubricant, one or
both elements must melt locally to furnish the liquid interface.  This
sounds a bit harsh but it is supported by all research that I have
seen.  A liquid in a thin film of several mono-molecular layers does
not flow and does not act as we know liquids to act in bulk.  They are
in essence trapped solids that when reunited with their bulk remain
unchanged because they are shapeless liquids.  Those who work with
vacuum know how solidly liquids, like water, attach themselves to the
surfaces n a vacuum chamber.

I know there appear to be exceptions to the friction model I proposed
but even these under careful scrutiny fit.  Teflon is one of these.
Flash temperatures well above melting temperatures have been measured
in sliding interfaces by researchers.  It is this evidence that made
clear to me why carbon is an elegant substance for friction
interfaces.  It produces no wear debris, its residue being CO2.  When
I proposed this in wear testing of hard disk sliders, it was rejected
by the old guard upon which I invited a PhD thesis to be done that
proved this phenomenon.

Fretting is a mechanism that follows my first law of sliding friction
admirably because it is capable of displacing any lubricant between
the mating surfaces.

That this is still a misunderstood phenomenon is show by these
descriptions that assume that there need be induced motion in a spline
shaft for instance.  Just elastic deformation is adequate to make two
surfaces squirm at their interface.  What tribologists must do to
visualize this is to see all materials as various hardnesses of
rubber.  Just pushing a car across a shiny smooth floor causes
squealing sounds from elastic deformation of the tire as it "rolls".

The best examples of fretting are rouge in the interface between
aluminum crank and steel spindle, damaged surfaces on pedal cranks
where the pedal shaft shoulder bears, and head bearing failures.  We
don't need no steekin test machines, we ride one every day.

Jobst Brandt

Index Home About Blog