Named the Tweel, Michelin's invention in an all-in-one wheel and tire: deforming resin spokes provide the give that air would've previously, and an outer tread gives the same grip as rubber. The benefits are pretty obvious: no air means no pesky punctures, and the slight deformity means the wheel rolls over bumps more smoothly.



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Now Bridgestone has one:

 

That's actually pretty cool technology, that'd be great for off-roading (or on roading for that matter).