bm0p700f wrote: ↑
Sat Oct 26, 2019 7:05 pm
Air pressure does act in all directions and that why pressure barely has any affect on spoke tension. I have a good tension gauge and measure these things. 250N DS tension drop is what you see on most tu eless installs. If you dont get your stress relieving right then girting tyre and seating knocks it out of true. Also for dishes wheels seating the turn changes the dish by about 1/4 turn of the NDS nipples.
Let the easy pressure test talk first.
The tension meter reading dropped from 27, ~ 85 kgf to 25, ~73 kgf, when the old pro one inflated from 15 psi to 65 psi.
Then the physics.
Air pressure does act in all directions, correct; thus pressure barely has any affect on tension, not correct.
. The bold font means vector. The pressure does act on the rim and the tyre respectively. What forces are doing act on the rim iteself? Let's make it simple by only considering the force inward and outward. Those are the air pressure force inward and, the pushing force outward on the hook due to the tyre or the friction outward between the tyre and the rim if hookless. The former is easy, simply Fin = P*S1. Then the latter. If the tyre was ideally rigid, there would be no pushing force or fiction outward, only the air force inward. In this impossible situation, the tension would drop linearly due to the pressure. That's not reality. The tyre will be pushed and stretched by air in fact, but it is still a solid, and thus the force outward Fout < P*S2. I've said only considering the force inward and outward, so the area S1 and S2, is the projection, which means S1 = S2. Now we can easily get Fin > Fout.