Originally Posted by Bouak
Dave, I believe they are load ratings and have to do with the weight of a bike.
Hi Mark, Dave,
The load factor of a tire is expressed by a number on the tire together with the speed rating, and in general is at the end of the tire size. It is called a service description.
For example the rear Metzeler Roadtec Z6 is a 180/55 ZR 17 (73W). The ZR indicates it is a Radial tyre with a speed designation above 240 km/h or 149 mph. The actual speed limit is designated by the W in 73W, meaning 270 km/h or 168 mph. When the service description is enclosed in brackets, this means that the tire has been tested with speeds in excess of the speed factor. If no brackets are there it is tested until the speed limit. So those Hayabusa, R1 or F4 riders most likely will have tires with a service description like (73Y), meaning tested above 300 km/h or 186 mph.
For tires having a maximum speed capability above 240 km/h (149 mph), a "ZR" may appear in the size designation. For tires having a maximum speed capability above 300 km/h (186 mph), a "ZR" must appear in the size designation. If there is no service description on a ZR tire one must inquire with the manufacturer what the actual speed limit is (unless your vehicle does not go over 240 km/h or 149 mph of course).
The 73 is the load factor, in this case 365 kg or 805 pounds. The front has 58W but I could not find directly the associated load limit for that one.
Coming back to the C and the K on the Z6s: these are designations that the tire sidewalls have been reinforced and are not directly related to the load rating! In general those tires are a bit more expensive than those without the C or K, and are designed for the heavier bikes like our K12GTs. The older K12RS also needed the reinforced version of the tires.
Sorry for being a bit long here, but I hope it clarifies...