It is layer with steel belts, fiber layers, rubber, and so on. (humblemechanic.com)
A Space Saver spare tire was standard when DeVilles were ordered with optional white sidewall steel belted radial tires. (en.wikipedia.org)
Tread separation occurs when the rubber outer layer of the tire peels away from the underlying structure, which is usually comprised of steel belts. (mcarthurlawfirm.com)