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Motocycle tyre knowledge
Sep 18, 2017


At the time of its introduction, the GL1800 was the largest, heaviest and most powerful motorcycle ever to be equipped with radial tires. Thinking back to our discussion of sidewall stiffness and tread compounding specific to a particular bike’s weight and intended use, we can understand why, though radial tires existed at the time that fit the new GL’s rims, new tires specific to the GL1800 had to be developed. That is why it’s important when purchasing new tires that the selections be listed by the manufacturer as designed specifically for fitment to the GL1800. The correct sizes are 130/70R 18M/C 63H inflated to 36 pounds per square inch cold for the front and 180/60R 16M/C 74H inflated to 41 psi for the rear. What do all those letters and numbers mean? Read on.

Moving on, the R indicates radial construction. The next four digits (e.g. 16M/C) indicate the correct rim diameter for fitment and the fact that the tire is motorcycle-specific. Of the last three digits, the two numbers indicate the tire’s load index, and the letter indicates the speed index. This is an explanation of the Metric Designation scheme applied to GL1800 tires. Two other schemes in common use are the Inch Designation and Alpha-numeric Designation. These most often apply to tires designed for different styles of motorcycles (cruisers) and older motorcycles (GL1000, 1100, etc.), but they contain the same basic elements. Charts showing the relationship of load index to load rating and speed index to maximum speed rating are included at the end of this article.


We’ve discussed tire date codes and the six-year maximum recommended use interval. Now let’s talk about tire wear. Underinflation, overloading and severe braking can all cause irregular tire wear. If it affects handling or ride noticeably, the tires should be replaced. Otherwise, replace your tires at or before the time when the wear bars become flush with the tread surface, noting that wear will accelerate after that point, and the tread sipes won’t be deep enough to be effective in channeling away water.

What are wear bars? Looking at the point on your tires where the sidewall meets the tread, you’ll notice small arrows marked TWI (tread wear indicator). Looking straight across the surface of the tread at those points, you’ll find raised portions down in the sipes. These are the wear bars.