From: cbm@athena.mit.edu (Charles Bruce Musgrave)
Newsgroups: rec.bicycles.tech,rec.bicycles.misc,rec.bicycles.racing,
	rec.bicycles.off-road
Subject: Re: frame flex and power loss?
Date: 8 Feb 1996 16:43:22 GMT

In article <DMDDvz.4tF@hpl.hp.com>, jbrandt@hpl.hp.com (Jobst Brandt) writes:
|> Ted Haskell writes:
|> 
|> > Several questions come to mind: 1) Is there relevant research on the
|> > topic? 2) Did the use of lightweight and excessively flexible frames
|> > actually pay off in placings? (In other words did we see riders
|> > winning races against similarly specialized riders on stiffer,
|> > slightly heavier bikes?) 3) Isn't it true that most riders have
|> > given up on these whippy frames? I seems like the lightest bikes
|> > riders use are much stiffer these days.
|> 
|> As I stated, professional racing teams had the most flexible aluminum
|> bicycles for their hill climbs.  They would not have done this if
|> there had not been substantial success with it.  Today, equally light
|> carbon fiber reinforced bicycles are used by these same people.  The
|> flexible stuff is collecting dust.  That is why the riders no longer
|> ride such bikes.
|> 
|> > The issue of the frame storing energy like a spring is interesting,
|> > and it undoubtedly happens. Most of the rbr postings have talked
|> > mostly about whether the rebounding energy can be effectively used
|> > by the rider due to the mechanics of riding a bike.
|> 
|> How about looking at it another way.  It works, so let's figure out
|> why.  If that isn't good enough, then how about considering where the
|> "lost" energy is going.  It must be producing heat somewhere by a
|> frictional hysteretic process.  I don't find one in a bicycle frame.

What about the these ones?

The potential energy of atomic interaction for metallic and covalent
bonds is nearly quadradic with interatomic seperation near the 
equilibrium bond distance.  As you move away from the equilibrium
interatomic distance you increase the amount of anharmonic interaction.
It is the anharmonic interaction which is capable of producing energy
loss through phonon generation.  Past this intermediate range further
atomic seperation leads to bond dissociation and the possible generation
of lattice defects in the material (dislocations, stacking faults, 
vacancies, etc.) and macroscopic defects (necking, voids, etc.).

In other words, frame materials behave as perfect springs for
small deformations.  Intermediate deformations introduce interactions
which can produce loss modes which are lattice vibrations (for
example, sound waves in the crystal lattice).  Those modes 
eventually dissipate as heat.  Even larger deformations of
the materials lead to plastic, or irreversible deformation,
which lead to defects that store a lot of energy.

So for a given force, stiffer materials are more likely to remain 
perfect springs.  Flexible designs have a proportionately higher
amount energy loss since they can't keep the material out of
the anharmonic regime.  And once the material reaches the
non-harmonic region, it is generally less able to withstand
more deformation, that is the spring constant is now a function
of the amount of deformation and becomes weaker with larger
stretches of the material.  

The question is then: how much heat is generated through
these mechanisms?  If you were in the plastic deformation
range, it would be a lot.  But the regime is probably
in the nearly perfect spring range so it is probably 
not obviously noticable.  Maybe an expert in this area
might have some numbers.

|> 
|> > Another issue is that some energy must be lost in the conversion
|> > process as heat (one of the laws of thermodynamics says that
|> > whenever there is a conversion of energy, some is converted to less
|> > useful heat). The question is how much. Does anybody know?
|> 
|> Where?

Phonon generation, probably mostly accoustic.

|> 
|> Jobst Brandt      <jbrandt@hpl.hp.com> 

Charles

From: cbm@athena.mit.edu (Charles Bruce Musgrave)
Newsgroups: rec.bicycles.tech,rec.bicycles.misc,rec.bicycles.racing,
	rec.bicycles.off-road
Subject: Re: frame flex and power loss?
Date: 8 Feb 1996 16:53:15 GMT

|> In article <DM20x5.rz@hpl.hp.com>, jbrandt@hpl.hp.com (Jobst Brandt)
|> writes:
|> 
|> >Steven Andrawes writes:
|> >
|> >> How do you know if any of your energy is being lost due to frame
|> >> flex?  I was wondering this because I felt like something was
|> >> weighing me and my bike down when I was riding a trail that had a
|> >> lot of pine needles on it.
|> >
|> >You aren't unless the bicycle is so weak that it fails in fatigue in a
|> >few hundred miles.  Frame flex is more a nuisance for handling than

The fatigue life of a material, and the energy loss due to deformation
isn't always closely related in many cases.  It is possible
to include many fatigue life increasing mechanisms into the microstructure
of the material, without increasing the stiffness.  For example,
alternating layers of material with different compliences will tend
to deflect the progression of the crack tip.  Particulates can do the
same thing, while not adding much to the frame stiffness.

|> >for power loss.  This is apparent from the use of aluminum Alan frames
|> >(with standard steel tube diameters) that were used professionally for
|> >hill climbs almost exclusively.  These frames were so flexible that
|> >they descended poorly and rode a wavy line compared to other frames in
|> >a straight TT.

Yeah, it isn't so great to have imprecise steering because your
bike's alignment is temporarily out of wack.  Especially down
a twisty descent.  The noises of misalignment are also annoying
(and energy dissipating too! Hey, some of use need to conserve
every bit we have.).

|> >
|> >There is little doubt that a torsionally elastic frame is not
|> >desirable, but I'm fairly sure from the evidence, that it is not
|> >because energy is lost.  Otherwise such frames would not have been so
|> >widely used for climbing hills.

The amount of energy required to lift an object against gravity 
is so large that the weight savings outweighs the loss mechanisms.
I am not sure I'd base too much the proof on how widely used
something is...otherwise Jobst might waste you for using cycling
tradition and myths rather than engineering or science to explain
why something is better. 

|> >
|> >Jobst Brandt      <jbrandt@hpl.hp.com> 
|> >

have a good one,

Charles