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· Monkey with a Football
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I had just finished answering a PM question about this same thing when I saw this so I will post what I sent out a minute ago...

As a wheel spins it creates G forces straight out from the center of the spin. Just as if it were wet and spinning off water. A straight rubber stem is in the center of this pressure so it only sees pressure straight down from the cap. But right angle stems have mass off to one side and that mass wants to travel in the G force direction as well.
So as the wheel spins, the right angle part of the stem bows down toward the rim and bends the straight part of the stem at it's base. You don't see this because when you stop, that G force pressure is gone and the stem straightens back up again. The rider is oblivious to this effect but the result is, every time you take off and stop, or even slow down and speed up, you are bending the base of the stem as if you were trying to break it off.

Eventually rubber cracking starts and the bend becomes a rapid tear and you have almost instant deflation, and almost always at riding speed where the tendency to tear would be greatest. This is also a cause for those mysterious pressure leaks when the bike is running but not when it stops.

The steel/chrome stems feel this pressure as well but do not flex at that point. They still have pressure on the rubber washers that seal them but they are rigid enough to not have this pressure affect them in a negative way.

Honda engineers knew this and added the support bar and a catch for it built into the rim. Even those need to have a ty-wrap added to keep the rubber stems from flexing inside the hole for them and to keep the support bar from getting loose and adding yet more side mass to the problem.

Some dealers and owners want to ignore or poopoo the physics involved. That is their prerogative. I prefer to pay attention to important details like that.
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