On Sun, 27 Jun 2004 00:37:43 GMT, Kinky Cowboy
<
[email protected]> wrote:
>On Sat, 26 Jun 2004 17:17:20 -0600,
>
[email protected] wrote:
>
>>On Sat, 26 Jun 2004 21:48:57 GMT, Kinky Cowboy
>><
[email protected]> wrote:
>>
>>>On Sat, 26 Jun 2004 20:49:46 GMT,
>>>
[email protected] wrote:
>>>
>>>>Russell Seaton writes:
>>>>
>>>>> 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.
>>>>
>>>>Jobst Brandt
[email protected]
>>>
>>>
>>>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 BB shell, chainstays
>>>and rear axle form a trapezium, which is deformed by
>>>axial loads from the rear wheel. 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 attemting 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.
>>>
>>>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)
>>>
>>>
>>>Kinky Cowboy*
>>
>>Dear Kinky,
>>
>>I think that I see your point about the chain-stay
>>bridge being different that the seat-stay bridge because
>>the chain-stays don't converge to a point but instead
>>remain a little separated (roughly a tire-width or 0.8
>>basset noses).
>>
>>Where I'm not following you (yet) is how this modified
>>tetrahedron (trapezium?) is distorting. That is, what
>>parts will bend less which way with the bridges?
>>
>>(I realize that this kind of thing is hard to describe in
>>words, so I appreciate any explanation that you attempt.)
>
>Very difficult without a drawing, but without a bridge,
>pushing the hub axle sideways will cause the angle between
>the chainstay and BB to change. With the bridge, this still
>happens, but in addtion, the angles between the bridge and
>the stays also have to change, and the bridge either has to
>change length or the stays have to bend. There is also some
>neding of the dropouts in both cases. All other things
>being equal, the same force wil result in less deflection
>with the bridge in place.
>
>>I'm not sure about the motorcycle analogy. Aren't most
>>modern street bikes a trailing fork back to the axle, with
>>a heavy reinforcement as you describe, but with a only
>>little triangle whose top goes to a monoshock and whose
>>"seat-stays" arrive about half-way down the "chain-stays"
>>instead of at the axle?
>
>True, a motorcycle has to do without seatstays, and the
>chainstays are much further apart, making the structure
>less like a triangle than a bicycle
>
>>To be truly similar, wouldn't a bicycle frame have to
>>dangle its axle on a foot-long pair of horizontal
>>trailing struts?
>
>As many suspension designs do
>
>>I'm not sure at all about this, modern street motorcycles
>>being purely theoretical to me, so take a deep breath
>>before pointing out any gaping holes in my question. I'm
>>really just asking if the axles are mounted well outside
>>the stiff triangular frame on a motorcycle, but almost
>>inside it on a bicycle.
>>
>>Thanks,
>>
>>Carl Fogel
>
>Kinky Cowboy*
>
>*Batteries not included May contain traces of nuts Your
>milage may vary
Dear Kinky,
Okay, now I'm thinking of the bottom-bracket, chain-stays,
and axle as a long, two-rung ladder lying flat on the floor,
with a narrow rung at one end and a wide rung at the bottom.
If the narrow end of the ladder is nailed to the floor,
pushing sideways on the ladder's wide end will deform the
whole rickety trapezoidal arrangement.
But a third rung near the narrow end will stiffen the
ladder.
Is this roughly the idea?
Carl Fogel
