inner tube 
noun
A separate inflatable tube inside a pneumatic tyre.Early next morning we took the inner tubes and tyre to the garage to remove some bits of metal that had become embedded, and to patch the tyre again....
- The robber stole a few cartons of cigarettes and some bicycle tyre inner tubes from their shop.
- Instead of an inner tube the tyre has a rubber lining and the pressure of the beading against the rim of the wheel is claimed to give a seal which is proof against any escape of air.