Steel Frames and "Flexible" rear triangles?

Pinnah

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?
_______________________________________________
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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
==============================================

Jim Beam

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.

pinnah wrote:

> 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 Touring@phred.org 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
> ==============================================