The recommended amount of air in a tire for a particular application is dictated by desired effect, design of the carcass, and intended use. The evolution
of manufacturing capability certainly has a little to do with the design. If you look back many years ago... the automobiles had tubes and spokes which you mentioned.
You asked why dirt bike tires run 8-15 psi... desired effect in correlation with design. You can put much more or less psi in the dirt bike tire, and when you get to the extremes of each end (2 psi and 25 psi) you will see these are undesirable effects.
You asked... "[color=#00ff00]Do all dirt bike tire applications involve a tube?"[/color] No. [color=#00ff00]"Why do bike wheels have spokes but not auto wheels? Auto wheels once had spokes; and wagon and carriages had spokes." [/color]Once upon a time we had to do our business in an out house too. Basically comes back to intended use. Imagine an auto type wheel for a dirt bike... how heavy would it be?
[color=#00ff00]Why do bicycle tires require a tube & dirt bike tires, but not car or heavy truck tires?[/color] Technology. Tire design and wheel design eliminated the tube.
[color=#00ff00]Is it because auto and truck rims are strong enough to withstand the pressure, but aluminum dirt bike rims can't?[/color] No.
[color=#00ff00]Why don't auto manufacturers reintroduce tubes into aluminum rims to save weight, or wouldn't aluminum casted rims be strong enough to support weight of car (they use alum. alloy now)? [/color]Adding a tube would add weight.[color=#00FF00] [/color] They cast aluminum wheels now.
[color=#00ff00]Why is a dent in rim of dirt bike tire important to avoid (from what I've read on this forum) if the rim does not have critical function of containing air pressure like an auto rim? What matter a dented rim?[/color] It's not that it is important to avoid, it is undesirable. A force induced bend or dent after the manufacturing process in a mild form will at the least affect the concentric effect of the wheel and give you a wobble or out of balance effect in addition to compromising the strength. See earlier post on buckling. In most cases if the rim is mildly bent (no more than .125") and the spokes are kept with proper tension the rim will survive for most normal riders.
I responded to those questions to get me to this... a tubeless option for the dirt bikes. They are already stamping the holes and bevels in the rim. What if they designed the nipples into the rim via stamping or forging? I know you now have one less replaceable part but I think I could live with that for the advantages it may bring. I have not thought it through entirely but if possible now the rim would be sealed, no tube. (I know the shelf of the rim would need to be redesigned along with the tire to seal the bead at relatively low pressure.) Then the hub could be either threaded or replaceable nipples designed in.
I know this would be a daunting task for the manufacturers of the rims, motorcycles, and tires, but think of the advantages it would bring in grip and un-sprung weight savings for dirt bike applications.
Edited by kx450f63, May 04, 2012 - 06:03 AM.