Steel Frames and "Flexible" rear triangles?

Steel Frames and "Flexible" rear triangles?

Post by pinna » Wed, 30 Jun 2004 12:10:26


Input and advice needed...

I'm researching an old steel framed bike that
I may purchase and ran across a touring oriented
comment in the Classic Rendezvous archives that
just doesn't jive with my understanding of things.

Somebody asked (in 2002) about about using a 1983 Trek 311
for touring and another poster made the following
comment:

Quote:
> A lot of the older Trek sport bikes have a pretty
> flexible rear end that doesn't handle too well when
> heavily loaded.

Now, this runs counter to my understanding on a
couple of points but was wondering if anybody
could deny or confirm that observation.

Here is my understanding of things:

TUBES - My understanding is that steel tubes'
stiffness is determined by the outer diameter
and thickness/butting, not on the different recipies
of steel (which effect strength and brazing).
Further, in that era, Trek primarily used Ishiwata
022 and Mangy X and Reynold 531 and 501 in their
touring and sport bikes and those tube sets were/are
pretty much the same in terms of thicknes/butting and
weight [http://tinyurl.com/2skhk]
So, I conclude that this wobbly rear triangle observation
probably doesn't come doesn't come from material selection.

GEOMETRY - The big difference in the rear triangle is
chainstay length and Sport and Touring Treks of that era
ranged in the 43cm to 47cm range, compared to things in
the 41cm range for road bikes.  While a longer chain
stay would presumably make for more bottom bracket
sway, I thought the common wisdom for touring was
longer was better for chainstays for increased comfort,
heel clearance and stability.  I'm not sure wobbly
can be equated with bottom bracket sway so I totally
don't get what somebody would mean about a sport
frame being wobbly in this regard, especially when
the touring ideal is even longer!

Am I missing something here?  My belief has been
that wobble and speed shimmy are related to panniers,
racks and load distributions (assuming a relatively
long rake).  Could somebody defend this guy's
assertion or should I just dismiss it

Ding, ding, ding!  Some of the Trek sport bikes had
relatively (for the day) steep steering angles in the
73.5 to 74 degree range and relatively short (for the
day) fork rakes of 4cm.  I've felt bikes with tight
rakes get speed shimmy, especially with load up
high on the rear rack.  Did I just answer my own
question?
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-- Dave      
==============================================
"It is impossible, or not easy, to do noble acts
without the proper equipment."
Aristotle, <<Politics>>, 1323a-b, trans Jowett
==============================================

 
 
 

Steel Frames and "Flexible" rear triangles?

Post by jim bea » Thu, 01 Jul 2004 07:58:55

this is a shimmy question, yes?

i doubt focusing just on the rear triangle will address your concerns -
you need to look at the whole system, not just one aspect.  you are
correct that steels are all pretty much the same modulus, so you can
only affect stiffness by size & thickness of tube.  similarly, yes,
shimmy is also affected by load & load distribution.

if you want recommendations, i'm not a tourist, so others can advise you
better on frame geometry, but if it were /my/ frame and i wanted to be
sure i'd not be wrestling with a shimmy problem, regardless of "knee on
the top tube" or other emergency issues to stop it once it starts, i'd
make sure i did what i could to best address the cause rather than try
to manage the symptoms.  i'd make sure i had the largest diameter main
triangle tube available, especially the down & top tubes - limits frame
torsion.  i'd also use a mountain hub on the rear to ensure the best
possible spoke bracing angle - highly dished wheels have much more
lateral flexibility than less dished ones - and shimano hubs have the
best geometry of all in this respect.  there are plenty of other
googleable suggestions, but the above are issues i've investigated and
which appear to work.

Quote:

> Input and advice needed...

> I'm researching an old steel framed bike that
> I may purchase and ran across a touring oriented
> comment in the Classic Rendezvous archives that
> just doesn't jive with my understanding of things.

> Somebody asked (in 2002) about about using a 1983 Trek 311
> for touring and another poster made the following
> comment:

>>A lot of the older Trek sport bikes have a pretty
>>flexible rear end that doesn't handle too well when
>>heavily loaded.

> Now, this runs counter to my understanding on a
> couple of points but was wondering if anybody
> could deny or confirm that observation.

> Here is my understanding of things:

> TUBES - My understanding is that steel tubes'
> stiffness is determined by the outer diameter
> and thickness/butting, not on the different recipies
> of steel (which effect strength and brazing).
> Further, in that era, Trek primarily used Ishiwata
> 022 and Mangy X and Reynold 531 and 501 in their
> touring and sport bikes and those tube sets were/are
> pretty much the same in terms of thicknes/butting and
> weight [http://tinyurl.com/2skhk]
> So, I conclude that this wobbly rear triangle observation
> probably doesn't come doesn't come from material selection.

> GEOMETRY - The big difference in the rear triangle is
> chainstay length and Sport and Touring Treks of that era
> ranged in the 43cm to 47cm range, compared to things in
> the 41cm range for road bikes.  While a longer chain
> stay would presumably make for more bottom bracket
> sway, I thought the common wisdom for touring was
> longer was better for chainstays for increased comfort,
> heel clearance and stability.  I'm not sure wobbly
> can be equated with bottom bracket sway so I totally
> don't get what somebody would mean about a sport
> frame being wobbly in this regard, especially when
> the touring ideal is even longer!

> Am I missing something here?  My belief has been
> that wobble and speed shimmy are related to panniers,
> racks and load distributions (assuming a relatively
> long rake).  Could somebody defend this guy's
> assertion or should I just dismiss it

> Ding, ding, ding!  Some of the Trek sport bikes had
> relatively (for the day) steep steering angles in the
> 73.5 to 74 degree range and relatively short (for the
> day) fork rakes of 4cm.  I've felt bikes with tight
> rakes get speed shimmy, especially with load up
> high on the rear rack.  Did I just answer my own
> question?
> _______________________________________________
> Touring mailing list

> Browse and search the archives: http://search.bikelist.org
> Unsubscribe or list settings:
> http://www.phred.org/mailman/options/touring

> -- Dave      
> ==============================================
> "It is impossible, or not easy, to do noble acts
> without the proper equipment."
> Aristotle, <<Politics>>, 1323a-b, trans Jowett
> ==============================================


 
 
 

Steel Frames and "Flexible" rear triangles?

Post by Benjamin Weine » Thu, 01 Jul 2004 10:59:20

Quote:

> TUBES - My understanding is that steel tubes'
> stiffness is determined by the outer diameter
> and thickness/butting, not on the different recipies
> of steel (which effect strength and brazing).
> Further, in that era, Trek primarily used Ishiwata
> 022 and Mangy X and Reynold 531 and 501 in their
> touring and sport bikes and those tube sets were/are
> pretty much the same in terms of thicknes/butting and
> weight [http://tinyurl.com/2skhk]
> So, I conclude that this wobbly rear triangle observation
> probably doesn't come doesn't come from material selection.

Tubing stickers don't always tell the whole story about
what is in a frame.  You are correct that steel tubes'
stiffness depends on diameter and thickness, not on its
composition.  But for example, a table at the site you reference

<http://www.desperadocycles.com/The_Lowdown_On_Tubing/Tubing_Propertie...>

Note the difference between Reynolds 531c (Competition)
and 531st (Super Touring).  The latter has thicker walled downtube,
fork blades, and seatstays.  That's going to make a stiffer frame
that holds up better to having a rack mounted to the seatstays.

People have toured on every possible kind of bike and
anecdotal observations sometimes apply, sometimes don't.