In article <[email hidden]>, [email hidden]
(JP) said:Ryan Cousineau <[email hidden]> wrote in message
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Quoted message said:In article <[email hidden]>, [email hidden]
(JP) said:[email hidden] wrote in message
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Quoted message said:Quoted message said:> Oops! The strength of a round cross section is given primarily by its outside diameter. That
> is why bicycle frames are made of thin walled tubing instead of solid bars. Most of the
> aluminum inside the stem is just excess weight. Meanwhile consider the thin skin of an
> aircraft.
Maybe we should make stems, and bicycles for that matter, from rolled aluminum foil, then.
We would, except for the "beer-can effect" of too-easy denting. Really light aluminum road
frames and some MTB monocoque frames (the Norco VPS bikes come to mind) already push the limits
of frame wall thinness against the limits of reasonable dent resistance.
So you're saying that ultra thin wall aluminum frames aren't strong enough? At this extreme
strength is not simply derived from the diameter of the tubing, it's based on frame design, the
way the tubes are arranged, just as an airplane's aluminum skin would be inadequate without its
internal skeleton. Try folding an empty beer can. Its weakness is not just observable as proneness
to denting.
No, I'm saying that thin-wall aluminum frames are quite strong in the ways that matter for a
cyclist: they're stiff and resist being compressed, flexed, or twisted. The arrangement of tubes on
a UCI-legal bicycle is largely fixed: there is little leeway for one design to have a more rigid
arrangement of tubes than another, for the same material in the same dimensions.
The biggest constraint on making tubes as thin and large-diameter as the normal stresses of a bike
frame would prefer is that at some point you would build a bike with featherweight, huge-diameter
tubes, but the wall thickness (being so much less important than the outside diameter for
determining rigidity) would be so low that the tubes would have too little dent resistance. Dent
resistance isn't part of what happens to a bike when it gets ridden, but bikes need some
dent-resistance so they don't get dented when not being ridden, especially since dents in round
tubes (or in the wrong place on any tube) greatly compromise strength.
Empty beer cans do fold easily, but they don't need dent resistance at that point in their life, and
their walls are less than .013" thick. Beer cans are remarkable structures, being very carefully
analyzed and costed, and a beer can is effectively a double-butted tube, with thicker material at
the top and bottom of the can. They are tremendously strong for their weight, and a very optimized
design. Internal gusseting can make any design stronger, but for a given weight, it's not the way to
go, since you can get more strength (at least against torsional and bending loads) by making the
cylinder diameter larger.
Notes on aluminum cans: psc.eduALCOA light.htmlOpen ↗
Dent-resistance in aluminum cans: psc.eduALCOA model.htmlOpen ↗
An aircraft's skin is a stressed structure, a "monocoque" design. The frame inside the skin is part
of the structural strength of a plane, but not as much as you would think. One important function
the inner frame provides is a place to mount interior fittings. Meanwhile the important job of
keeping the aircraft from twisting, flexing, or folding is done mostly by the exterior skin, not the
inner frame.
Quoted message said:Quoted message said:The other limit is UCI regulations, which give a maximum diameter for tubes, IIRC.
Right. When you reach the UCI limit for tube diameter, you also reach a limit for wall thichness
beyond which strength will be inadequate.
Strength is a function of both tube diameter and tube wall thickness, not to mention alloy and
heat treatment properties.
It is, but the former completely dwarfs the latter, and the last two aren't part of this discussion.
Let me put it to you another way: given the same amount of aluminum (that is, the same weight, and
of course the same alloy/heat treatment) in two different round tubes spanning the same distance
(say, 60 cm, the top tube length of a large but typical bike), the one with the larger diameter
will always be stronger against torsion and bending, and Jobst will correct me, but I think
compression too.
The other tube will of course have thicker walls, but it won't be as strong because diameter trumps
wall thickness.
Now, "your" tube will take less damage if I take a hammer and try to put a dent in the side of both
tubes. And in the real world, I eventually make "my" tubes so thin that they get crumpled by passing
breezes, aluminum foil style. At the point where the tube doesn't have enough wall strength to
resist ordinary wear and tear (you know, people touching the frame 🙂, it will dent easily, and once
it is dented, it will no longer be a nice round tube, and its strength will be badly compromised.
Going back to the beer can analogy, it's easy to dent an empty beer can, and a dented beer can is
much easier to crush than an undented one, because it collapses around the dent, causing more
folding and "dents", and then the thing loses all resemblance to a tube and falls down.
Quoted message said:That's not to say that I disagree with Jobst's conclusion that the wider diameter stem will be
stronger for a given weight, but I'm guessing that the difference in strength between typical
threadless and quill stems is not a meaningful consideration, unless you are pushing the design to
its limit for weight savings. The bottomline in the real world of off the shelf components is
weight (and business costs) versus adjustibility.
It's a positive attribute of a design which has other more important advantages: threadless stems
have no quill to seize or invisibly bulge the steer tube. As several have already noted, these
advantages are not enough that most of us need to rush out and change our headsets, but all things
being equal, threadless is preferable.
As for the adjustability issue, stem reach is at least as important as stem height for establishing
a proper fit, but nobody seems to mention that if anything, it is much easier to change out a
threadless stem for one of different reach than it is to change many threaded stems, since a
threaded stem without much excess cable may require detaching cables to fully remove the stem. Not
to mention that while removable-cap threadless stems are nearly universal, removable-cap threaded
stems are at best only moderately available. If you're trying to get your handlebars into the right
position by adjusting the height without changing the reach, you're just trying to get an
ill-fitting bike into the least-worst position!
--
Ryan Cousineau, [email hidden] sfu.ca~rcousineOpen ↗ President, Fabrizio Mazzoleni Fan Club
