Originally Posted by
rpenmanparker
Would an engineer care to explain what higher air volume in wider tire has to do with being able to run lower pressure? I will tell you that while thinking about this recently, I came to the conclusion it has nothing to do with contained air volume. Assuming you want a uniform 15% tire depression ("drop") when the bike is loaded, two things happen when the tire is depressed. First the pressure increases inside the tire just s tiny bit. But the contact patch area increases significantly as the tire is squashed. It seems to me the reason you can lower pressure on a wider tire is that the contact patch with the ground is bigger on wider tires. The force at the contact patch has to support the bike. So a lower pressure times a larger area will give the necessary force (F=P X A).
Think about this as well: a lower profile tire would contain less air but have a larger contact patch. You should be able inflate it to lower pressure than a narrower, taller tire with a higher air volume.
Comments? I really want to know if I am making an obvious mistake.
I re-read your post now that I have a little more time.
I think I am understanding what you are asking, and with all variables aside, I agree with your "assessment".
At the same inflation pressure, a wide and a narrow tire have the same contact area.
A wide tire is flattens over its width. A narrow tire has a skinnier but longer contact area, which means as you lower pressure proportionally, the narrow tire will move the rim closer to the ground than the wider tire.
This is assuming the same width rim and same tire casing.
So... wider tires can be run at a lower pressure without risk of bottoming out a rim, because of the shape of the contact patch. If that answers what I think your question is.
