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Understanding an important part of your motorcycle—the tires

Using a wrench on a front motorcycle tire

Motorcycles provide an exciting combination of acceleration, braking, and nimble handling. Of all the parts on a motorcycle, the most critical to achieving the best performance are the tires.

Tires can seem like a mystery when it’s time to replace them. Just remember, what you need to know is printed on the sidewalls. You just have to be able to read codes like this: 120/70 R 17 (58W).

Understanding your tires

  • Tire size: Tire size is expressed as two numbers separated by a “/”. The first number is the width of the tire measured in millimeters. The second number is called the aspect ratio, which is the height expressed as a percentage of the width.
  • Construction: A letter that indicates the construction type of the tire follows the size. Underneath the tire thread is the tire carcass composed of layers of fabric called plies that wrap around both tire beads extending up the sidewalls and under the thread. The two most common types are based on the angular relation of the plies to the bead:
    • B: Bias-ply tires have the plies laid diagonal to the bead. Usually the plies are laid in a crisscross pattern. A variation of this type is the bias belted tire, which has layers of fabric laid around
    • R: Radial tires have the plies laid perpendicular to the beads at 90 degrees. Radial tires can also have bias belts under the thread. Some also have a 0 degree belt that’s parallel to the beads.
  • Wheel size: This is the diameter of the wheel.
  • Load index and speed rating: Usually these are in parenthesis after the size code. Charts to decode them can be found on most tire manufacturers’ websites.
    • Load index: The load index is expressed as a number and represents the maximum safe load for a properly inflated tire. Most tire manufacturers show both the max load and the maximum cold inflation pressure.
    • Speed rating: The speed rating is a letter that represents the maximum safe speed the tire was designed to handle.
  • Inflation: Inflation is usually the maximum cold inflation shown as psi (pounds per square inch).
  • Direction of rotation: An arrow shows the way the tire should rotate on the wheel, which is important since rain sipes are designed to work in a certain direction.
  • Front vs. rear: This indicates which wheel the tire is designed for.
  • Manufacture date: This is a four-digit code. The first two digits are the week the tire was made. The last two are the year. It’s usually found next to the DOT tire identification number.

Each motorcycle is designed to work with particular tire sizes and ratings. Always refer to your owner’s manual for size, inflation, and load carrying specifications.

Till next time, ride safe!

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