THIS IS A NO FLAT SOLID PUNCTURE PROOF TIRE SIZE 9X3.5-4. FITS PART NUMBERS 72410088, VKXWHEEL, AND 10816. COMPATIBLE WITH THE TWO WHEEL VELKE SYSTEM, NOT FOR THE SINGLE WHEEL SYSTEM. INCLUDED INSIDE THE RIM IS ONE ROLLER BEARING AND TWO BEARING RETAINERS. THE ROLLER BEARING FIT PART NUMBERS 77410031 AND VKXRLBRG AND THE BEARING RETAINERS FIT PART NUMBERS 77410030 AND VKXBRGRTNR. HUB LENGTH: 2 1/4" OFFSET, TREAD: TRACTION, TYPE: SOLID FOAM, NO FLAT, COMES WITH A GREASE ZERK. FLAT FREE TIRES PROVIDE THE BOUNCE AND LOAD HANDLING CHARACTERISTICS SIMILAR TO AN AIR-FILLED TIRE WITH THE BENEFIT OF NEVER GOING FLAT.