On Sun, 27 Jun 2004 02:05:20 GMT,
[email protected]
wrote:
>Kinky Cowboy snipes anonymously:
>
>>>> How does not having a chainstay bridge affect the frame? I have a
>>>> "loaded" touring frame without a chainstay bridge.
>
>>> Depending on frame size and geometry, the bridge doesn't even serve
>>> that purpose. It is never structurally necessary, the rear triangle
>>> actually being a tetrahedron, the stiffest solid body available.
>
>> We've been here before. The rear triangle isn't a tetrahedron,
>> because if it was the chainstays would meet in the centre of the BB
>> shell.
>
>The virtual intersection of these tubes is guaranteed by the massive
>BB shell reinforced by BB cups that make this an equivalent of a
>truncated tetrahedron. Even a well meaning constructor of such a
>figure using bicycle tubes would not extent the tubes to a sharp point
>intersection as a mathematical line model would. Stop splitting
>hairs.
But the pin jointed model, while manifestly a gross
oversimplification, does serve to illustrate the likely effect of
adding the extra element, by making it easy to visualise which angles
or lengths need to change to accomodate the deflection
>> The BB shell, chainstays and rear axle form a trapezium, which is
>> deformed by axial loads from the rear wheel.
>
>So? Even a perfect tetrahedron would be deformed. This is a case of
>"lies of the second kind" to support a semantic rather than a
>mechanical problem. Metals are, after all, an elastic medium. It is
>not the truncated tetrahedron that is deforming as you imply but don't
>say, but rather the elements of the tetrahedron.
There is a deformation of the trapezium, in the sense of the angles
changing, as well as deformation of the elements by bending and/or
stretching
>> Bracing this trapezium at some point distant from, but parallel to,
>> the BB axis will cause this deforming tendency to be resisted by a
>> bending moment in the chainstays, whereas without the bridge the
>> entire load must be resisted by the BB/Chainstay junction attempting
>> to change it's angle. For this reason, bridgeless designs usually
>> rely on reinforcement of this junction, such as gussets, extended BB
>> shell or, in the case of lugged steel designs, longer and heavier
>> points on the BB shell. Admittedly, the seatstays (also braced, by
>> the brake bridge) also resist this load, but anybody who thinks the
>> chainstay bridge serves no structural function should have a look at
>> the swingarm of the nearest high performance motorcycle.
>
>You might as well take the lugs of a Rene Herse frame as proof of
>stresses at certain junctions when in fact much of this is artistic
>license or in response to an unrelated frame failure. Similarly, you
>could cite tying and soldering spokes at their crossings.
But I didn't; I carefully cited the similar trapezoidal structure of
the motorcycle swingarm (technically, a swinging fork), which has been
subject to a great deal of investigation as to the best method of
bracing against the large axial and torsional loads fed into it by the
rear wheel, and which also has it's direct bicycle equivalent on many
suspended frame designs.
>> It is possible to properly design a bicycle without a chainstay
>> bridge, and I do not presume to know which approach is ultimately
>> "better", by whatever test you happen to mean by better; this will
>> vary according to geometry and material choice, but in general if a
>> designer has chosen to use a bridge, it's likely to be there for
>> structural reasons; it certainly isn't required for tyre restraint
>> or mudguard mounting on my track or dirt-jump bikes, both of which
>> enjoy the structural benefit of substantial chainstay bridges (and,
>> for that matter, "brake" bridges, even though one is braked by the
>> sprocket and the other by a disc)
>
>Well that's a great disclaimer after all that dogmatic explanation of
>the "rear triangle".
>
>Jobst Brandt
>[email protected]
Why thank you! I suspect you were indulging in the lowest form of wit,
but, ignoring your sarcasm, all bicycle design (as in any other branch
of engineering) is a series of compromises, and there are entirely
legitimate reasons why a bridgeless design might be the best
compromise for one application while a bidged design is equally
appropriate for another. While I'm being pragmatic (a quality you
might like to acquire) I am equally sure that a great many features of
bicycle design are continued because they've always been done that
way, and that as a result there are probably some redundant chainstay
bridges out there.
Kinky Cowboy*
*Batteries not included
May contain traces of nuts
Your milage may vary
