[CAUT] restrung D

[David Porritt]

John:

You stated "Friction <http://en.wikipedia.org/wiki/Friction> is proportional
to (a) the coefficient of friction of the materials and (b) the normal force
between the surfaces."  

It is also proportional to amount of surface contact involved.  That's why
racers have big tires.  It's why pushing a large sanding block on wood is
harder than pushing a small block.  That's why big trucks have big brake
pads.  Strings burried into soft iron will have more surface contact and
result in more friction than a string touching only at one point.  

dave
____________________
David M. Porritt, RPT
dporritt at smu.edu
-----Original Message-----
From: caut-bounces at ptg.org [mailto:caut-bounces at ptg.org] On Behalf Of John
Delacour
Sent: Monday, April 16, 2007 5:39 PM
To: College and University Technicians
Subject: Re: [CAUT] restrung D

At 3:43 pm -0600 16/4/07, Fred Sturm wrote:

>...It is conceivable that wear and tear from tuning are greater from 
>a hard termination than from a softer one, but I wouldn't bet on it. 
>If the hard termination is more polished and less abrasive (which is 
>likely), it might well cause less wear. The softer material is 
>likely to have troughs/grooves, hence greater area in contact with 
>the string, hence greater friction.

That is not so, though the falsehood is oft repeated.  Friction 
<http://en.wikipedia.org/wiki/Friction> is proportional to (a) the 
coefficient of friction of the materials and (b) the normal force 
between the surfaces.  There would be no greater friction if the wire 
were embedded for half its diameter in a grooved cast-iron bridge 
than if its surface contacted the bridge for only a tiny fraction of 
its diameter.  No amount of polishing is going to reduce friction.

What the effect of cast steel wire rubbing against titanium is I have 
no idea, and the hardness of the titanium is irrelevant -- what 
matters is the coefficient of friction -- but friction is low in the 
case of steel against cast (grey) iron owing to the graphite in the 
iron.

JD