Most mountain bikers wouldn't want shock absorbers that were simply springy air chambers with no damping. According to the guys at CushCore, however, that's exactly what tires are. Their solution? An energy-absorbing foam insert that sits inside a tubeless tire, acting as a damper while also adding flat protection and improving cornering.
The CushCore insert only occupies half of the space inside the tire, leaving an air pocket underneath the tread – the air pressure is set by the rider using a standard pump. That pocket acts as a tuneable spring, while the closed-cell foam insert serves as a damper. The setup reportedly makes the tire less likely to bounce off of obstacles such as rocks and roots, resulting in a smoother ride.
Additionally, the insert protects against pinch flats. In fact, riders can improve traction by running their tires at a lower air pressure than would ordinarily be possible. What's more, if the rider does get a flat, the insert reportedly provides enough support for them to still limp home with no air in the tire. The system is sealant-compatible.
Finally, the insert is claimed to add lateral stability to the inner portion of the tire, where it hooks onto the rim. This is said to keep the tire from squirming sideways relative to the rim while cornering, resulting in increased sidewall stability and lessening the chances of the tire "burping" right off the rim.
But yes, there is a bit of a weight penalty. An insert for a 27.5-inch tire tips the scales at about 250 grams, while a 29-inch insert weighs 265 g.
A set of two will cost you US$149. By contrast, Schwalbe's Procore system, which utilizes a high-pressure inner tube instead of a foam insert, costs about $230 per set.
There's more information in the video below.