Airless Urethane Inserts
A great idea but bad execution. No Mor Flats has been around for years. At one time, manufactured allegedly in Colorado, this fairly good concept offered a limited number of sized inserts that could fit a decent number of bicycle rims. In case you didn't know, No Mor Flats is a urethane "doughnut" that fits inside of a rubber bicycle tire and once mounted to the rim of a bicycle, will render it flat proof for practically forever. It is true, No Mor Flats will allow the rider to go over a bed of nails and still keep going. In reality, several dynamics are at work that make the No Mor Flats almost impossible to dislodge once properly mounted, making ideally suited for wagons or carts that go off road or even EMT off road gurneys.
Read more and see more pics
But..... the problem is the urethane the No Mor Flats is made of is very dead. Meaning if this stuff was rolled up into a ball it wouldn't bounce but one inch off the ground when dropped from ten feet. It is squishy, squirmy and devoid of jiggle, but it is also dense and ...dead. What really gives this system trouble is weight. Since the No Mor Flats comes in only one density (dead), a person who weighed 35lbs might enjoy it, but a person who weighs over 200lbs would feel like they were riding in sand with the air in the tires half let out.
Inserts are a good idea and some are made for the wheelchair industry that have a bit more spring in them but again, there aren't many sizes available. Manufacturers should make inserts in all sizes and several different urethane densities so that people with different needs and weights can choose one appropriate for their needs. The molds needed to make inserts are inexpensive to make.
Manufacturers who do make inserts are aware of the limitations and have been traditionally reluctant to get in to cycling where the demand is greater but so the need for performance. I don't think that inserts could perform better than high quality race tires, but inserts could perform better than they do now with minimal investment and resources. Just simply going to another urethane could improve the ones made now.
A good insert and rubber tire combo in my mind would be where the insert has been made for a specific rim and tire so that the whole system works together. Rubber tires are everywhere, so design an insert to fit them. Then once the insert is designed, use a real springy urethane so the tire has life. I need to emphasis that inserts do work and they last forever and are very safe but..... that damn urethane they use is dead and even they know (the manufacturers) that.
People have reported using inserts in hunting carts and having great success in bringing home Bear and Elk, and people have been using them for years on wheelchairs with out problem, but typically when used on a bicycle, they don't roll very well because of the fit sometimes, but also because the urethane isn't bouncy