Simple, because no rim is without internal stresses. When an additional force acts on the rim things will change. It's difficult to see, but when you watch carefully the read outs on the video with and without tire aren't identical. Also, no tire is perfect. The casing thickness and shape uniformity varies and thereby effects how it acts on the rim. If you have a pair of accurate calipers you'll also find that clincher section of a rim will slightly spread open as you pump up the tire. (note, were talking very small variances here. Not something you would normally consider actually "going out of true".)tidelag said:How can somebody believe that the wheeltrueness decreases when one add a tire? Eric killed a myth with his video?
Aside from durability, being able to maintain tolerances after it has been ridden, is proof of a properly built wheel.Recently I asked for help here about my crappy 24 spokes wheel. Got ~7% difference in tension, which seemed okay. But it did recently go out of trueness recently so my suspection is that this wasn't stress relieved good enought by me. dammit!
That's why I searched more about stress relieving.
When I have got more time, I'll try to do it more carefully.
Or I'll build a new wheel, I does not know yet.
BTW, there's an easy bench test that will confirm whether your wheel will stay true during use. Put axle on ground (preferably a piece of carpet so you don't damage it), press on opposite sides of the rim. Go all the way round at every spoke of the same hub side and do both sides of the wheel.
When your wheel has been built well you'll only have to make minor corrections after this procedure. Repeat until wheel stays with-in the same tolerance.....a badly built wheel will never stay the same and you'll be repeating this procedure forever.
BTW2, make sure you also correct for optimal spoke line. Without it your build won't be good either.