WHEEL & TIRE ARTICLE UNDERSTANDING BASIC WHEEL & TIRE DIMENSIONS, TERMINOLOGY & DESIGN ASPECTS Wheels and tires impact the styling, handling, performance and safety of a vehicle. To ensure you’re making the proper wheel and tire choices for your customers, you have to understand the basics, such as wheel dimensions and tire sizing. — BY MIKE MAVRIGIAN o you know how to measure instance, a wheel that’s designed to wheel diameter? How is offset accept a 17-inch tire may feature an Ddetermined? What’s the speed outside diameter of 18-19 inches. rating of the tire your customer’s The wheel diameter (the number that looking at? How can you fi gure out a indicates tire mounting diameter) is tire’s aspect ratio? measured at the “fl oor” area where the If you’re unsure how to answer these tire bead seats. Since you may not own questions, this article will provide you a large measuring caliper, here’s an easy with the answers, as well as additional method to measure wheel diameter. information on wheel and tire basics that will make you better informed Wheel overall diameter- bead seat height x 2=rim diameter. and more prepared to answer any wheel or tire questions your customers To utilize this formula lay the bare may have. wheel down on a clean, fl at surface. Using a straight ruler or tape measure, Wheel (Rim) Diameter measure the outer diameter of the Wheel diameter refers to the wheel’s wheel, making sure that you posi- bead seat diameter and isn’t mea- tion the ruler so that it intersects the sured at the outside edge of the rim center of the wheel. Note the outer itself. Since the wheel must feature an diameter. For example, let’s say this is outboard rim lip at its outboard and 18.5 inches. Next, place a ruler onto In order to measure wheel diameter, fi rst inboard sides to act as a stopping point the fl oor of the bead seat adjacent to measure the overall diameter as shown to secure the tire bead, the outside the side that you measured for out- here. This wheel measures 18.5 inches diameter of the wheel will always be side diameter, with the ruler placed overall. Remember, this isn’t the rated wheel diameter relative to tire size. larger than the bead seat diameter. For 90 degrees to the beat seat fl oor. WHEEL & TIRE ARTICLE Measure the depth from the bead seat recommended rim width for a partic- Measuring Backspace floor to the edge of the wheel’s rim ular tire. If a tire manufacturer notes Backspace refers to the depth of the lip. For this example, let’s say this is that a 235/45R17 tire has an ac- rear of the wheel, measured from the 0.750 inches. In order to determine ceptable rim width range of 7.5-9.0 rear of the wheel rim outer edge to the bead seat diameter, we simply inches, don’t deviate. If you mount the hub mounting face. To find this, subtract the total height of the rim lip that tire onto a 7-inch-wide rim, lay the wheel face-down on a clean, area from the outer wheel diameter. the beads will be squeezed together level surface. Position a ruler verti- Since we need to subtract the rim lip beyond the tire design spec, making it cally onto the hub face. Lay a long height from each edge of the wheel difficult to mount the tire and posing straightedge horizontally across the (at the two spots you used when you an increased chance for bead damage outer edges of the rear rim. The point measured overall diameter), multiply during mounting. where the horizontal straightedge the rim lip height by 2 and subtract intersects with the vertical ruler is the that total from the overall wheel backspace distance. Be sure to refer- diameter. ence the section of your vertical ruler Example: Outer wheel diameter (at largest area) = 18;5 inches Rim lip height is 0.750 inches x 2 = 1.5 inches Outer diameter 18.5 inches - 1.5 inches = 17.0 inches Rim width is measured between the In this example, the actual rim bead walls, not at the overall width. diameter, where the tire seats, is 17 This wheel measures 7 inches between inches, which is correct for a 17-inch the bead walls. tire (for example, a 225/45R17). The tight fit may also cause the Rim Width tread/shoulder area to deform, which Offset is the difference between the center of the rim and backspace. I may lead to increased wear at the In order to measure wheel width, marked the center of the rim. Knowing we again need to measure the area center tread area, in addition to po- that backspace is 5.75 inches, I marked between the bead seat walls. The tentially decreasing the tire’s design this, measuring from the wheel rear overall width of the wheel will vary performance. If you attempt to mount edge. The difference between the two depending on rim material thickness. that same tire onto a 10-inch wide marks (center and backspace) in this For example, if a particular tire is rim, you’ll run into bead seating case measures 1.5 inches or 38.1mm. recommended to mount to a 7-inch difficulties and possible bead damage As a result, I’ll note this wheel as wide rim, the 7-inch dimension during mounting. In addition, you run having an offset of 38mm. doesn’t indicate overall wheel width, the risk of increasing shoulder wear but rather the distance between the and losing some of the tire’s perfor- rim’s bead seat lips where the tire mance potential due to the carcass bead actually makes contact. deformation on a too-wide rim. With the bare wheel positioned In other words, if you must run upright on a clean, level surface, a 235/45R17 tire that requires a measure the distance from the front 7.5-9.0 inch rim, then you’ll simply rim’s inner lip flange to the rear rim’s need to find a wheel that offers that inner lip flange. For instance, a wheel recommended rim width, even if that that’s designed with a 7-inch rim means moving to a different brand I measured a five-bolt wheel’s bolt width may actually measure 8 inches or style from the original brand/style circle by measuring from the center of that the customer may have selected. from the front rim face to the rear rim one hole to the outer edge of a hole face. Remember, we only want the Always stick to the manufacturer’s rim furthest away from the first hole. In this width between the bead seat flang- width recommendations. example, we see a measurement that’s es. When identifying rim width, the This is especially important when just under 4 inches. However, it can be overall width of the wheel (outside to dealing with low-profile, stiff-side- difficult for a novice to accurately place a outside) is immaterial. wall performance tires that don’t like tape measure or ruler at the exact center When selecting a tire and wheel being stretched or squeezed into an of the first hole. In this case, the circle is combination, always refer to the unnatural shape, which will result in minutely less than 4 inches (it’s actually a 100mm circle or 3.9937 inches). tiremaker’s specifications regarding a compromised thread contact patch. WHEEL & TIRE ARTICLE TIRE DIMENSION TERMS Finding Offset Wheel offset refers to the location of OVERALL DIAMETER the wheel’s mounting surface (where The outside diameter of the tire when the wheel mates to the hub) in re- mounted and inflated, but with no vehicle lation to the centerline of the over- load all wheel width. If the center of the SECTION WIDTH wheels overall width aligns with the Also called “overall width,” this is the hub contact surface, the wheel has maximum width of the cross section of an zero offset. If the hub mounting face unloaded, mounted and inflated tire (the is positioned closer to the rear wheel widest point of the tire when mounted and rim, the wheel features negative off- inflated, but with no vehicle weight). set. If the hub contact face is posi- tioned closer to the front of the wheel FREE RADIUS rim, the wheel has positive offset. The radius of the tire/wheel assembly that isn’t affected by load. This is the distance A negative offset wheel has a from the wheel axle centerline to the top of “deep-dish” appearance, which the tire tread face. moves the tire further outward in the wheelwell. A positive offset wheel LOADED RADIUS moves the tire center further inboard The distance from the wheel axle centerline (less rim depth at the wheel face). to the ground, drawn vertically. This is the Front-wheel drive applications typ- distance from the vehicle hub centerline to ically require a positive offset due to the ground when the tire is inflated and the placement of the vehicle hub. when the tire supports vehicle weight. The offset dimension is critical for a SECTION HEIGHT In this example, I used a bolt circle number of reasons, including out- The distance from the bead to the template. Once I find a series of five board (fender) and inboard (sus- tread face holes that align with the holes in the pension/brake) clearance, as well as wheel, I’ll note that this wheel does handling and braking performance.