From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: mavic 220 rim cracks
Date: 16 Nov 1997 22:35:24 GMT

Charles Beristain writes:

> I don't think the rims are anodized.. silver colored and the brake
> pad area looks machined.

There are plenty of silver anodized rims, although I haven't looked at
the 220's.  MACHINED RIMS!  This has got to be the stupidest idea
since hard anodizing.  My rims are machined the first time I ride in
the rain, and who needs it.  Rims have been made without machining
certainly for more than 50 years with imperceptible joints.

The only excuse I have found for machining is a sloppy joint, and how
sloppy can a joint be if the extrusion dies aren't cast offs from some
curb-stone shop.  The solution to that problem is to fix the joint,
not machine the entire rim faces.  What this does is to make the side
walls of these rims, no thicker than usual before machining, thinner
and non uniform, so the rim is paper thin in some places.

I realized that something was amiss way back when Fiamme, the premiere
rim maker of old, riveted the joints of their rims so they wouldn't
come apart in use.  They reinforced the idea that the spkes at the
joint had to cross to keep the wheel from opening up in use.  They, as
Mavic, had some serious lapses in engineering education concerning rim
joints, anodizing, machining and a bunch of other subjects.  The bike
industry survives on a thin veneer without much science.  That's why
it takes Darwinian time to sort out what's good.  There are always new
moths to fly into the flame of the candle of research, as they crash
in flames aka FiberFlight et al.  Who?  Rolf!

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: mavic 220 rim cracks
Date: 16 Nov 1997 22:51:50 GMT

Matt O'Toole writes:

> The problems with "hard" anodizing are well known, but what about
> "soft" anodizing, the light or thin variety found in brightly
> colored bike parts?  Is this as harmful, or even harmful at all?

This may be "well known" but Mavic doesn't want to hear of it and still
makes claims that anodizing strengthens rims, a claim can be said to
be true albeit with a serious mental reservation.  That being , that
structurally it is brittle although more abrasion resistant.

Most rims are made from straight bar extruded forms, that are first
anodized and then rolled into hoops before drilling.  They can't be
drilled when straight because they would form facets instead of a
smooth curve when the bar is curled.  That means the material is
curved beyond the yield point and the entire skin, if hard anodized,
cracked, before the wheel is even spoked.  It is these cracks that
grow into the base metal.  I have found these cracks in brand new
rims.  On sectioning and polishing a used rim, the propagation of
surface cracks is clearly visible in the aluminum where they were
originally only in the anodizing.

> What about nitriding?  This was common in lightweight racing
> sailboat parts several years ago.  It was believed to be less
> crack-inducing than anodizing, but I think it fell out of favor due
> to its gold color.

These are all hard coating or surface hardeners that do not enhance
the structure because its strength is in the metal, not in a surface
coat that is less than 1/10mm thick.  In fact the disparity in elastic
modulus and hardness means that all the strength must come from the
surface, and because it cannot carry the load, the surface cracks into
what looks like stretch marks.  These cracks then propagate into the
metal and cause failure.  The color anodizing if kept thin is too weak
to make a difference but it is not good.  The classic light blue or
pale gold hydraulic fittings have benign coatings, but they also come
off with steel wool in a jiffy.  That would probably be a good test.

The only surface treatments that do any structural good are polishing
and shot peening.  The first of these removes cracks and crack
initiation, the second prestresses the surface into compression,
further preventing crack initiation.  Neither of these changes the
elastic modulus or the homogeneity of the metal.

Jobst Brandt      <jbrandt@hpl.hp.com>