This is a complete set of wheels (4 drive wheels) with two matched tracks. You need 1 of these for the HD2. If desired you can specify a custom length. These are Generation 3 tracks with a standard tread pattern.
Heavy duty robot wheel with traction lug Tiller tread. It does not have a bearing. Size: 3.5-4. It's 10.5 inches in diameter, 3.5" inches wide. Its Pneumatic with a 4 bolt pattern so you can easily mount it to a flanged hub.