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Tire Hieroglyphics by John Goux
images by Matt Crawford
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he Valley of the Kings, Egypt, 1922: Torch in hand, intrepid archaeologist Howard Carter enters King Tut’s tomb and... sees walls covered in indecipherable hieroglyphics. This is pretty much what most of us see when we try to understand the coded information on the sidewall of a modern automobile tire. Driving in an F1 race, winning the Indianapolis 500, or stopping safely in the rain all require the right tire. I’m here to act as your personal Rosetta Stone to the mysteries of tire sidewalls. Adjust your pith helmets, and let’s begin. To demonstrate, I’ll use the OEM tires for Mercedes’s highest volume seller, the GLC300. It comes fitted with Pirelli’s Scorpion Verde, All Season 235/60 R18 tires. First, 235 millimeters ( 1 ). Why? John Boyd Dunlop, 19th-century Scottish veterinary surgeon, inventor, and maker of rubber devices, invented pneumatic “tyres” for his daughter’s tricycle. Thus, tires were invented
March 2020
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using the Queen’s metrics: 235mm is the width of the tread and 60 is the percentage of the tread width for sidewall height. Doing the math, 235x.60 = 141mm tall sidewall. Now you know a quick method to find the height of any tire. Yay, math! Next, R ( 2 ). More than just a pirate’s favorite letter in the alphabet, it stands for “radial.” Radial means the plies (cords) under the tread are perpendicular to the tire’s centerline and bead. The centerline is, well, the centerline. The bead is the part of the tire shaped to fit the rim and is what holds the tire to the wheel. Don’t hurt your brain trying to picture the geometry. Just trust me on this one. And 18 ( 3 ), the 18 of R18 is inches. Yes, 18 inches of inner-tire diameter. Why inches and not millimeters? Because wheels made for American cars have their diameters in inches like God intended! Followed by 103H ( 4 ). This grouping describes tire load and speed rating; 103 means each tire can 35