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Airless Tires
Of all the inventions that came out of the bicycle industry, probably none is as important and useful as Dr. Dunlop's pneumatic tire.
Airless tires have been obsolete for over a century, but crackpot "inventors" keep trying to bring them back. They are heavy and slow. They give a harsh ride and poor high-speed cornering on rough surfaces. They are also likely to cause wheel damage, due to their poor cushioning ability. A pneumatic tire uses all of the air in the whole tube as a spring, while foam-type "airless" tires/tubes only use the air in the immediate area of impact.
Airless tires are made either of elastomers (rubbery materials) or closed-cell foams, which are rubbery materials with lots of tiny air bubbles. The better ones are foam type, because solid elastomers have hardly any shock absorbency.
This sort of material has a non-linear response to compression loads: as a compressive load is applied, the stiffness of the material increases as it gets squashed thinner and thinner. The beauty of pneumatic tires is that the compression is nearly linear.
A basic fact of physics is that pressure is inversely proportional to volume.
Imagine a pneumatic tire that is divided into lots of little segments so that each inch or so of tire is effectively a separate "balloon." Let's say it's 1 inch thick, and when a rider sits on the bike, the tire compresses 1/4 inch. That means the volume of the localized "balloon" is now 75% of what it was before the rider got on, so the pressure in the bubble is going to become 133% of what it was.
If the rider hits a bump that compresses the tire another 1/4 inch, the volume will be half the static value, so the pressure will be double the starting pressure.
If the rider hits a bump that compresses the tire 1/2 inch (plus the static 1/4 inch) the volume will have been reduced to 25% of the base volume, and the pressure will now be 4 times the base pressure!
"Airless" tires that use foam derive their resiliency from the bubbles in the foam, so this describes their general functioning. The bubbles are only part of the mix, though, so a 1 inch thick tire doesn't actually have an inch of air to play with before the bubbles are all compressed as far as they can go. You can only compress the bubbles so much, and the more you compress them, the harder they press back, in geometric progression.
Contrast this with a pneumatic tire, where the whole volume of air in the tire is being compressed as a unit. When you sit on the bike, the bottom part of the tire flattens out, say 1/4 inch, but this only reduces the total air volume by a fraction of a percent. Thus the pressure is nearly constant under all conditions, and the tire can be equally shock-absorbent for the full "travel" of its thickness.
It is this property of providing nearly linear response to external pressure that is the unique feature of pneumatic tires. It is not possible for any system that doesn't have this feature to give as good ride as pneumatics do. This is why every vehicle designed for road use in the last hundred years has used pneumatic tires.
The near-linear response of pneumatic tires is not just a matter of comfort. It also improves traction at higher speeds, because the tires don't tend to bounce as much as harder tires do. Bouncing can cause loss of traction in high speed corners, because when the tire is airborne, it can't have any traction.
A peneumatic tire can compress nearly all the way to the rim without any risk of damage either to the rim or to itself.
Airless tires do have their applications. They can work well either where speeds are very slow, or where surfaces are very smooth. Thus, they're pretty satisfactory for wheelchairs, especially those mainly used indoors, and also for railroad trains, roller skates, furniture casters, children's riding toys and wagons.
