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. LOADED SECTION HEIGHT indeed feature a 100mm bolt circle. Excessive negative offset (placing the The loaded radius, minus half of the tire further outboard) can increase the nominal rim diameter. loads experienced by wheel bearings and ball joints due to an extended ASPECT RATIO This refers to the tire’s section height in leverage effect. relation to its section width, as a percent- Too much negative offset can also age. For example, a 60-series tire features result in increased steering wheel ef- a sidewall that’s 60 percent as tall as the fort and steering wheel “kickback” re- tire’s section width. A 50-series tire will sulting from increasing the front axle feature a shorter sidewall, at 50 percent track width. In the majority of cases, of section width. A 35-series tire will it’s always best to adhere to the OEM feature an extremely short sidewall The bolt circle caliper gauge in use. wheel offset as closely as possible. (only 35 percent of section width), etc. When using this specific tool, measure It’s feasible to deviate offset by as two bolt holes that are immediately much as 5mm, while increasing or Aspect ratio = Nominal section height adjacent to each other (the tool has divided by nominal section width been calibrated to use these locations). decreasing offset by a much as 8mm times 100 Insert carefully and allow the gauge to or so may be acceptable in some Example: If section height is 3 inches slide while the tapered dowels find true cases. For example, if the OEM off- and section width is 10 inches, 3/10 x center. Always insert from the hub face set is +35mm, moving to a +40mm 100 = 30, which would mean that this since the holes at the front of the wheel offset should pose no problems, but tire features an aspect ratio of 30 will typically be too far recessed to use it’s always best to measure first for (a 30-series tire). this tool. clearance. If rim width is altered from OEM TREAD WIDTH This is the distance measured from the where it touches the underside of the (most performance enthusiasts will inner tread shoulder to the outer tread horizontal straightedge. Remember, tend to move to a wider wheel/tire shoulder. Tread width isn’t to be confused you’re trying to measure the distance package), offset will likely change with section width, as section width is from the hub face to the rear outer in order to maintain proper inboard always greater. edge of the rim. and outboard clearances. Always be WHEEL & TIRE ARTICLE

TIRE TREAD TERMS ASYMMETRIC TREAD LONGITUDINAL GROOVES This a tire where one side of the tread These are the grooves that run the cir- has a different design than the other cumference of the tire and channel wa- side, rather than a mirror image from ter from between the tire and the road side-to-side. With differing inner and outer treads, engineers can design a tire for maximum performance. The outer half of the tread is designed for high cornering stresses and the inner tread is designed for straight-line stabil- ity and good water dispersion. Blocks Dimples Grooves Ribs BLOCKS Sipes Shoulder These are the individual sections of rub- Longitudinal grooves in the tread allow ber defi ned by the grooves surrounding for immediate water passage. them. It’s the faces of the blocks that make contact with the road. to prevent hydroplaning and possible loss of vehicle control. The new breed CONTACT PATCH of super rain tires often have a deep This is the total area of the tire that longitudinal channel down the center shoulder. This is the area responsible contacts the road surface at any given of the tire. for lateral grip in cornering maneu- time. Often this area is about the size of vers. It’s usually slightly rounded to a small book, but the size and shape of RIBS give a progressive steering response. the contact patch depends on the speed These are any easily recognized pattern and tire dynamics in any given condi- of tread blocks that make up the contact SYMMETRIC TREAD tion. The patch is different during cor- band around the circumference of the A tire has symmetric tread when the nering than traveling straight ahead, for tire. treads on both sides of the centerline instance. Typical passenger car tires with of the tire are mirror images of one a 60 aspect ratio usually have a con- SIPES another. tact patch that’s longer than it is wide. Very narrow slits in the tread are called Low-profi le performance tires usually sipes (pictured below). They improve UNIDIRECTIONAL have a contact patch that’s wider than it traction in water, snow, ice and loose If the tire is designed to rotate in one is long. At very high speeds, the vehicle dirt. Basically, they act like squeegees direction only, it’s referred to as uni- tends to lift, making the contact patch that squirm to help move water from directional. The tread design will move narrower, which is why performance the tread blocks. water well in only the proper direction. tires are usually so wide. Infl ation These tires will have arrows or other pressure can also change the area of the markings on the sidewall describing the contact patch. proper operating rotation. If mounted in the incorrect rotational direction, you DIMPLES won’t damage the tire but you won’t be These are the indentations in the tread able to take full advantage of the tire’s blocks or ribs that help cool the tire. performance.

LATERAL GROOVES VOID RATIO The amount of space between the tread These are the grooves that lead from the The smallest cuts in thread blocks are center of the tread to the outer edges. referred to as sipes. They provide added block surface compared to the space They can be straight or curved. Lateral traction on wet or icy surfaces. taken up by the grooves (or voids) is grooves are generally 3mm or wider. the void ratio. A low void ratio has less Narrower than 3mm, these grooves may SHOULDER groove area and more tread area. A typi- be called sipes (see defi nition). Larger The area of transition between the cal rain tire will have a larger void ratio lateral grooves also help direct water sidewall and the tire’s tread (both than a high-performance tire designed from under the tire. inboard and outboard sides) is the for dry driving. WHEEL & TIRE ARTICLE

aware that offset not only affects The bolt pattern refers to both the Discs are available that feature multiple TIRE TREAD TERMS appearance, but handling, straight- number of fastener locations and the hole patterns with each hole identified line tracking and steering effort as diameter of the hole pattern. A 5 x for bolt circle. Simply place the disc well. Always maintain as close to OEM 100 bolt pattern refers to a wheel that against a wheel hub face or onto the offset as possible when dealing with a features five fastener holes laid out in a vehicle hub until all the holes align. The production chassis. 100mm diameter path. A 5 x 4.75 pat- holes that align to your wheel or hub Measuring offset is fairly simple. tern designation refers to a pattern with indicate the bolt circle. This is the difference between the five holes running along a 4.75-inch Another specialty tool that allows wheel’s hub face location and the diameter. In the old days, we could ge- quick measuring of a wheel’s bolt circle center of the rim width. Refer to the nerically refer to a 5 x 4.5-inch pattern is a sliding caliper. This tool features backspace measurement taken earlier as a Ford pattern or a 5 x 4.75-inch as a two tapered dowels that slip into the and mark this on the rim’s drop-cen- Chevy pattern. wheel’s bolt holes. Insert the dow- ter well surface. Next, place a mark on In order to measure a bolt circle els into the two holes that are closest the rim’s center well area that rep- using a ruler, remember that we’re together. The graduated gauge face of resents the center point between the simply trying to find the diameter of the tool indicates the bolt circle, based wheel’s front and rear rim faces (total an imaginary circle that runs through on the number of bolt holes (the gauge wheel width). The difference between the center of each fastener hole. If features multiple pointers for 4-, 5-, 6- these two marks represents offset. If the wheel or hub features directly or 8-bolt). the hub face is positioned behind the opposing holes (as found on a 4, 6 or The sliding caliper tool is offered by rim center (toward the wheel back), 8 wheel or hub), simply measure from Excalibur Wheel Accessories, though I the wheel features negative offset. If the center of one hole to the center sourced mine through American Tire the hub face is located forward of the of the hole directly across the pattern Distributors. rim center (toward the front of the (or from the outside of one hole to the If you don’t want to bother measuring wheel), the wheel has positive offset. inside of the opposite hole). bolt or stud shanks or nuts, it’s a good Use this formula to determine back- If the wheel or hub features a fivebolt idea to keep a reference set on hand space distance and whether a wheel pattern, measure from the center of one attached to a small board. has positive or negative offset: hole to the outside edge of the hole Backspace – furthest away. Understanding Tire Dimensions (Wheel Overall Width/2) = Offset If you don’t want to be bothered by The sidewall of a tire offers an measuring with a ruler, tape measure abundance of information, including If you’re considering wheels that are or dial caliper, you can cheat by using the tire’s size, maximum inflation, identified by the maker in millime- a round plastic bolt circle template disc serial number, manufacturing location ters, you’ll need to either use a metric (Made 4 You Products offers this tool). and tread wear rating. In essence, this ruler or convert inches to millimeters. Here are two handy inch/metric con- version formulas:

1 inch = 25.4mm In order to convert inches to millimeters, multiply inches by 25.4 Example: 4 inches x 25.4 = 101.6mm In order to convert millimeters to inches, divide millimeters by 25.4 Example: 40mm/25.4 = 1.575 inches

Bolt Circle & Bolt Pattern The bolt circle refers to the spacing of the wheel’s fastener holes, using diameter, or circle, as the designated dimension. If we draw a circular line that runs through the center of all of the bolt holes, this circular diameter measurement indicates the bolt circle. For instance, if a 4.5-inch circle can be positioned at the center of the bolt hole group, this is a 4.5 bolt circle. WHEEL & TIRE ARTICLE information serves somewhat as an How Tires Handle Water create a larger footprint, which may owner’s manual for that specific tire. Traction grades (featured on the aid in dry traction (taking advantage of Since much of this sidewall infor- sidewall markings) provide a general more rubber on the road for increased mation can be confusing, I’ll walk you indication of a tire’s ability to con- grip). However, in wet weather, this through a scenario to help you under- trol and stop a vehicle on a wet road stand the data provided that applies surface, with testing performed in a to a tire’s dimensions. straight-line braking scenario on a Three sizing systems are employed controlled wet-surface test pavement, today for passenger tires—P-Metric, including both concrete and asphalt. European Metric and Alpha-Numeric. The ratings that result from these tests The most common system is P-Metric. are performed in straight-line testing The size designation indicates cross and may not indicate cornering trac- section in millimeters, aspect ratio, tion. The tire manufacturers give each type of construction and appropri- tire a rating of A, B or C, with tires Directional tires will feature a mark ate rim diameter. For example, a rated “A” having the highest traction. on the sidewall indicating direction of P235/50R16 describes a tire that’s in- These traction ratings are estab- rotation. You won’t damage the tire if tended for passenger use (P) and fea- lished on skid pads that are designed mounted backward but you’ll greatly tures a cross section width of 235mm, following federal government stan- diminish the performance advantages an aspect ratio of 50, is of radial dards. Twenty measurements are that the tire offers. construction, and is to be mounted to taken using an industry standard a 15-inch diameter wheel. “control” tire on both an asphalt European Metric (also called Metric) surface and a concrete surface, with is similar. No letter “P” is used as a the results from each surface test prefix. The three-digit number at the averaged. The same scenario is then beginning indicates cross section in performed with the specific tire being millimeters. The next letter indicates tested and the results are then com- the speed rating. An “R” following pared with the data obtained from this indicates that the tire is of radial the control-tire tests. The rating grade construction and the final two- is then assigned. digit number indicates rim diameter Three “grade rating” numbers will Angled grooves, referred to as lateral in inches. If the aspect ratio is low- typically appear on the tire sidewall— grooves, direct water out from the er than 82, a slash, followed by the the treadwear rating, traction rating longitudinal grooves to aid in water aspect ratio number, will follow the and temperature rating. The phenom- evacuation. Water is collected in the section width number. enon called “hydroplaning” occurs main grooves and directed out of the A European 155SR13 metric tire when a tire encounters a wedge of tread area via lateral grooves. with an 82 aspect ratio indicates water between the tread and the road that it has a 155mm section width, surface. This can cause temporary an S speed rating, radial construction, directional pull and loss of control. and is intended for a 13-inch wheel. While virtually any vehicle and tire Alpha-Numeric tires originated in combination has the potential to the 1960s. This sizing system features hydroplane at various vehicle speeds a load-based identification approach (wherever the limit is reached where where the first letter designates water can’t evacuate quickly enough the tire’s load carrying capacity. For between the tire and road), it’s im- example, a BR60- 13 tire features a portant to understand that a number “B” load rating, radial construction, a of factors can combine in order to Shoulder blocks may vary in size be- 60-series aspect ratio and a 13-inch experience hydroplaning, including tween outboard and inboard shoulders, rim requirement. The alpha (letter) vehicle speed, tread design, tread depending on the specific design. A tire character can range from A–N, depth, tread compound, tire width, that is marked “this side out” or “this side in” will feature larger shoulder depending on load capacity. The vehicle weight and water depth. tread blocks on the outboard side. If higher the letter, the higher the load A tire contacts the road in what’s in doubt, always install the tire to the rating. Alphanumeric sizes are in called the “contact patch.” This is vehicle with the larger shoulder blocks popular demand for muscle car the footprint that a tire creates as it facing outboard. The larger-footprint restorations where the original rolls down the road. Larger-profile/ shoulder blocks are designed to accom- size/type tire is desired. lower-aspectratio performance tires modate cornering (lateral loads). WHEEL & TIRE ARTICLE larger footprint can potentially degrade features that will enhance the traction, speed has been met. traction because of the larger water steering response, lateral control and Beginning in 1991, speed ratings wedge that can be created (a narrower braking of the vehicle at legal highway have been placed immediately follow- tire may “slice” through water better speeds. The design and construction of ing the load index number, for example than a very wide tire). the tire has been enhanced, offering the 225/50R16 89S. When Z-rated tires In response to this trait, tire manu- side benefi t of high-speed capability. were fi rst introduced, they were thought facturers go to great extents to design Speed ratings are based on laboratory at the time to refl ect the highest tire tread patterns and compounds that tests where the tire is loaded against a speed rating that would ever be re- will resist hydroplaning. large-diameter metal drum to refl ect its quired, with the Z rating indicating appropriate load and run at increasing that the tire was capable of speeds “in Speed Ratings speeds in 6.2 mph steps in 10-min- excess of 149 mph.” Speed ratings, in addition to the ute increments until the tire’s required Eventually, the tire industry added W speed at which a tire model has been and Y speed ratings. A Z rating may still tested and verifi ed, indicate the tire’s appear on the tire on its own (indicating performance characteristics. Just be- a rating of 149 mph+), but may also cause a tire is rated at 149 mph, that appear in addition to a W or Y rating doesn’t mean that you should attempt symbol (with W indicating a rating of to operate the vehicle on a public 168 mph; and Y denoting 186 mph). roadway at that speed. If you want to Some tires may feature a rating symbol play, rent some time on a race course. of Y (following the load index), with the You shouldn’t select a tire by focusing load index and speed rating encased on the mph speed number because in parenthesis. Example: 285/35ZR19 you’re probably not going to drive at (99Y). If the Y is seen within parenthe- those speeds while on public roads sis, this indicates a speed rating “in anyway (at least, you shouldn’t). excess of 186 mph.” The high-speed rating is a clear indi- Tires referred to as “Ultra High Per- cation that the tire has highperformance formance” include those featuring V, design, material and construction Z, W, Y and (Y) speed ratings. MAM

Thank you to Travis Weeks for allowing us permission to reprint this fantastic article, which originally ran on www.hotrodandrestoration.com.