January 01, 2004
Custom Wheel Dimensions
Knowing wheels inside and out eases the critical identification process and boosts customer satisfaction
By: Mike Mavrigian
WHEEL DIAMETER
Any custom wheel already should be identified -- either by a stamping in a steel wheel or by a cast-in I.D. in an alloy wheel. These usually appear on the rear of the center section or rim, but could also be found on the inside well of the rim.
For instance, a designation of "16x7" would indicate a rim diameter of 16 inches and a rim width of seven inches. But what if a size designation can´t be found, or if you want to verify the rim diameter? Here´s what you need to know.
Wheel diameter refers to the wheel´s bead seat diameter, and is not measured at the outside edge of the rim itself. Since the wheel must feature an outboard rim lip (at the front of the rim and at the back of the rim) to act as a stopping point to secure the tire bead, the outside diameter of the wheel will always be larger than the bead seat diameter. For instance, a wheel that is designed to accept a 17-inch tire may feature an outside diameter of 18.5 to 19 inches.
The wheel/rim diameter (the number that indicates tire mounting diameter) is measured at the "floor" area where the tire bead seats. It should not be confused with the overall, or outer, wheel diameter.
Since you may not own a large measuring caliper, here´s an easy method to measure wheel diameter.
First, lay the bare wheel face up or face down on a clean, flat surface. Using a straight ruler or tape measure, measure the outer diameter of the wheel, making sure that you position the ruler so that it intersects the center of the wheel. Note the outer diameter. Let´s say this is 18.5 inches.
Next, place a ruler onto the floor of the bead seat (adjacent to the side that you measured for outside diameter), with the ruler placed 90 degrees to the bead seat floor. Measure the depth from the bead seat floor to the edge of the wheel´s rim lip. Let´s say this is 3/4-inch (or 0.750 inch).
In order to determine the bead seat diameter, we need to subtract the total height of the rim lip area from the outer wheel diameter. So measure the rim lip height from each edge of the wheel (at the two spots you used when you measured overall diameter) and multiply by two. When you subtract the total height of the rim lip area from the outer wheel diameter, you get the rim, or bead seat, diameter.
Example:
Outer wheel diameter (at largest area) = 18.5"
Rim lip height is 0.750" x 2 = 1.5"
18.5" - 1.5" = 17"
In this example, the actual rim diameter where the tire seat is 17 inches, which is correct for a 17-inch tire (for example, a 225/45R17).
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