Though we're not sure how'd they'd hold up to some serious thrashing, these airless bike tires are certainly interesting. No, they aren't the cheesy solid rubber or foam tube replacements that you can buy at Walmart... countless hours of engineering and testing has been done to get the tires to the point they're at today. How amazing would it be to ditch the pump, spare tube, and patch kit from your pack, let alone always knowing your tires were ready to rock? The Energy Return Wheel (ERW) concept was originally developed for cars and has since been brought to mountain bikes by Brian Russell from Boulder, Colorado.
As shown in the photo above, tires are mounted (glued?) on top of the ERW system, which looks to be bolted to rim. It does provide some give, just like a traditional tire, and the amount of give can be adjusted depending on terrain and traction needs. Note that a thin sidewall will fully enclose the sides of the wheel. What you see here is a few generations into the prototype phase, and new concepts are being worked on and refined. Road and BMX prototypes are also in the works.