Which of the following is not classified as a solid tire for stackers?

High-speed tires are specifically designed for certain applications where increased speed is required, particularly on smooth surfaces. They are not typically classified as solid tires for stackers, which are more associated with stability and durability for heavy loads.

In contrast, polyurethane tires, pneumatic tires, and rubber tires can all serve different purposes within the category of solid tires or tire substitutes for stackers. Polyurethane tires are often used because they offer a good balance of durability and traction. Pneumatic tires, although they contain air and have a cushion effect, are not considered solid tires in the traditional sense, as they are designed for off-road conditions where shock absorption is crucial. Rubber tires, while they can be solid, usually refer to the base material rather than the specific configuration used in stacker applications.

Thus, identifying high-speed tires as not being classified as a solid tire for stackers is correct, as it highlights a specific category that does not fit within the typical solid tire classification used in the context of stackers.