The wall thickness is actually .1mm and .2mm less for French and Swiss respectively because the difference in diameters is twice the wall thickness.
Also the real issue is the minor thread diameter and the root form. If the Metric threads have a shallow form that minimizes the stress riser the steerer may be just as resistant to fatigue or even better.
I have ridden many miles on the Swiss 25.0 OD 22.2 ID and it seems okay (at least up to now!). I would speculate the biggest threat to the upper part of a steerer is overtightened stems.
Joe Bender-Zanoni Great Notch, NJ----- Original Message ----- From: "jerrymoos" <email@example.com> To: "Fred Rafael Rednor" <firstname.lastname@example.org>; <FujiFish1@aol.com>; <email@example.com> Sent: Thursday, June 03, 2004 8:27 PM Subject: Re: [CR]Question about 1972 Motobecane Grand Record /Stronglightheadset
> The "opposite" arrangement, i.e. French threaded headset with 22.2 stem is
> often seen on Swiss bikes, including my 1968 Allegro. This gives one a
> little concern about the wall thickness of the steerer. French steerer with
> 25mm OD and 22mm ID are already (nominally) 0.2 mm thinner than English
> steerers with 25.4mm OD and 22.2mm ID. The "Swiss" steerer with 25mm OD and
> 22.2mm ID is 0.4 mm thinner than English. The other problem is that with
> some brands of headsets, it is difficult to insert a 22.2mm stem through a
> French HS top nut.
> Jerry Moos
> ----- Original Message -----
> From: "Fred Rafael Rednor" <firstname.lastname@example.org>
> To: <FujiFish1@aol.com>; <email@example.com>
> Sent: Thursday, June 03, 2004 3:59 PM
> Subject: Re: [CR]Question about 1972 Motobecane Grand Record /
> > Mark,
> > If the steerer is built to French specifications, it's 25mm
> > in diameter rather than 1 inch (i.e. 25.4mm), although people
> > sometimes also refer to the French spec steerer as "1 inch".
> > I would say that it's _nearly_ certain that the headset on
> > this bicycle is French threaded. The reason I would not say
> > it's 100% certain is because of an astounding experience I had
> > this past weekend.
> > I was working on a lightweight Peugeot from the same period
> > as your Motobecane and found that the steerer tube was threaded
> > for an English headset. Nevertheless, the inside diameter of
> > the steerer was 22mm - i.e. the French standard. This was the
> > opposite of my previous experience with "mixed-spec" steerers.
> > That is, in the past I've worked on French bikes whose steerer
> > tubes had French threading although the inside diameter would
> > take a 22.2mm (i.e. "rest of the world") stem.
> > I still have not fully recovered from this experience.
> > Cheers,
> > Fred Rednor - Arlington, Virginia
> > --- FujiFish1@aol.com wrote:
> > > Hey,
> > > I currently have listed on Ebay a 1972 Motobecane Grand
> > > Record frameset,
> > > including a Stronglight Competition headset. Being a deeply
> > > rooted
> > > Italianophile, I cannot answer a couple of questions asked my
> > > about the items, so I
> > > humbly bring them to y'all ...
> > >
> > > 1) Is the headset definitely French threaded?
> > > 2) Is the steerer 1" like English and Italian?
> > >
> > > View the Motobecane in question here, at Ebay #368115051,
> > > or try this URL:
> > >
> > > %3AMESE%3AIT&rd=1>
> > >
> > >
> > > Thanks very much for the info kids!
> > >
> > > Ciao,
> > > Mark Agree
> > > Southfield MI