From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: "tired" springs
Date: 17 Mar 1998 18:51:32 GMT

Mark McMaster wrote:

>> springs do not get "tired." I suppose increased friction elsewhere
>> in the mechanism could give the illusion of a "tired" spring.

> Than why do automobile suspensions sag with long, hard use?  I've
> had some coil springs noticeably shorten with long use.

This was a common occurrence until automobile design was driven to its
current state by Japanese competition.  It was caused by a reasonable
layout ignoring that all car designs tend to end heavier than
originally conceived.  Thus the final car had the choice of being a
low rider or the springs were jacked up.  The second option was the
only one so such cars go into yield at the first hard, fully loaded
bump.  On older cars the bumper pads collapse making the low end of
spring travel even greater and causing more yield.

My first encounter with this was the Porsche 356 that used VW torsion
bars even though the car had a larger fuel tank and carried the
battery in the front instead of the rear.  On top of that the car was
heavier and shorter than the VW Beetle.  All 356 Porsches sloped
forward shortly after leaving the showroom as is visible by the
formerly horizontal trim strip beneath the door.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: fatigue of steel frames
Date: 13 Nov 1998 19:28:17 GMT

Marty Wallace writes:

> A piece of steel left by itself for a while will loose the internal
> stresses it has.  Steel is the only metal that does this.

Just thermal cycling from night to day reduces stress slightly but it
does not significantly affect structural materials used in frames.  As
an example, spring steel holds its rebound for many years without
measurable loss as in watch springs, weight scales and auto valve
springs that operate in a wider temperature range than any bicycle.

Once again, an immeasurable effect that exists practically only in
theory has been applied to the wrong place to give the wrong
impression.

> If you want to demonstrate this, go to a frame repairer and ask him
> the ratio of repair jobs for Aluminium versus Steel frames.  A steel
> frame will last as long as you will!!

And how does this demonstrate stress reduction by aging?

Jobst Brandt      <jbrandt@hpl.hp.com>