[Author Index] [Date Index] [Thread Index]
[Date Prev][Date Next][Thread Prev][Thread Next]

Re: [ST] New Member



John wrote:
>
> You are correct in that the shape of a a member in tension or
> compression does not change the reactions at the end.  A force applied
> at one end is resisted by opposite forces at the other end, if the
> member is stable and not moving.

This much I agree with :-)

> The shape of a member in torsion does effect the reactions, though.

Why? Apart from the forces that will occur due to the off centre weight of
the swinging arm (plus any due to the slop in the spindle bearings etc) I
can't see that there will be any force there at all. As long as you can
assume the spindle and it's bearings are perfectly rigid and it's rigidly
supported in the arm there will be no external twisting force applied to the
frame.

> An offset between the load and any part of the member creates a moment
> that must also be resisted by the other end.

The only offset that will matter is the point of application of the load and
the point of support of the member. The swinging arm could stick out 3
metres to the left as long as it loops back so the support point is on the
centre line (other than the off centre weight as previously mentioned).

> Using your paper clip, in the
> shape of a square question mark, a force applied to the top of the "?"
> will make the tail twist in your fingers.

If by the top you mean the part of the loop directly in line with the tail
then it doesn't. Try it. However if you push on a point not in line with the
tail then the point of support (the tail) and the point of application of
the force are no longer in line and it twists (as it should).

> Obviously Honda, Ducati, BMW, and Triumph have the capability of
> building bikes capable of withstanding the forces.  I prefer not to
> have a SSSA.

SSSA's definitely have disadvantages, I'm not denying that for a second.
They have to be built much more substantially to resist the internal
twisting forces and try to maintain acceptable levels of rigidity. They
inevitably end up heavier and sometimes have more flex too into the bargain.
But they just look so damn cool :-)

Phil
'02 S3


     *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *   *
      The ST/RS Mailing list is sponsored by Jack Lilley Ltd.
          http://www.TriumphNet.com/st/lilley for more info
   http://www.TriumphNet.com/st for ST, RS and Mailing List info

=-=-=-= Next Message =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=