Originally Posted by Jonathandavid

When I pluck the upper spokes like strings, the sound is the same regardless of whether I am sitting on the rack or not. When I do the same with the lower spokes, the sound is lower when I sit on the rack. This is because the increased load on the bike, which puts compressive stress on the lower spokes.

If you keep a rubber band between two fingers and pluck it, moving the ends of the rubber band towards another will produce a lower sound. This is basically the same mechanism.

If you draw the forces on a stationary wheel on a piece of paper, you will see that the downward force acts at the hub, while the upward force acts at the base of the wheel. This way you can easily see that compression of the lower spokes is inevitable. Empirical testing shows that the tension in the upper spokes does not change (see above). Therefore, the hub rests on the lower spokes.