writes:

David Damerell writes:

I think you (and many other people) are missing the fact that Simon
(and Guy) are disputing the terminology only. No-one is claiming
anything other than that there is a tension change in the bottom
spokes.

Because this is not a semantic difference but a technical one, the
book shows an aluminum a common die-cast moped wheel that looks as
though it might have wire spokes.

http://mopedarmy.com/photos/brand/6/1681/

In such wheels, knowing that they are not tensioned, evokes the
response that "of course, this wheel stands on its bottom spokes."
However, by selective cooling in the die cast process, these spokes
can be tensioned, and the answer becomes unclear. Visually it appears
to be between a wire spoked wheel and a wooden wagon wheel.

This is a poor example. The spokes of this wheel are not free to slide
at either end, either in or out. The spokes of a cart wheel sit in
mortices both at the hub and the rim end. Under tension, they would
slide out of their mortices. Consequently they physically cannot
transmit a tensive load. The rim of a cart wheel is pushed outwards by
its spokes.

The spokes on a bicycle wheel slide through holes in the rim and are
retained by nipples screwed on from the outside. Under compression[1]
they would push the nipple out of the hole in the rim. Consequently
they physically cannot transmit a compressive load. The rim of a
bicycle wheel is pulled inward by its spokes.

The spokes in the example cast wheel are physically part of a unitary
structure. There are no sliding joints in it anywhere. Consequently
it's physically possible that the spokes work under either tension or
compression, or even alternate between tension and compression as the
wheel rotates.

Does prestress of a spoke change its function and that of the wheel?

Clearly it must.

[1] if they did not simply bend (because, unlike cartwheel spokes,
they are not shaped to remain in column under compression)

--
(Simon Brooke) http://www.jasmine.org.uk/~simon/

'Victories are not solutions.'
;; John Hume, Northern Irish politician, on Radio Scotland 1/2/95
;; Nobel Peace Prize laureate 1998; few have deserved it so much