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.

The bolt pattern refers to both the number of fastener locations and the diameter of the hole pattern. A 5 x 100 bolt pattern refers to a wheel that features five fastener holes laid out in a 100mm diameter path. A 5 x 4.75 pattern designation refers to a pattern with five holes running along a 4.75-inch diameter. In the old days, we could generically refer to a 5 x 4.5-inch pattern as a Ford pattern or a 5 x 4.75-inch as a Chevy pattern.

In order to measure a bolt circle using a ruler, remember that we’re simply trying to find the diameter of an imaginary circle that runs through the center of each fastener hole. If the wheel or hub features directly opposing holes (as found on a 4, 6 or 8 wheel or hub), simply measure from the center of one hole to the center of the hole directly across the pattern (or from the outside of one hole to the inside of the opposite hole).

If the wheel or hub features a fivebolt pattern, measure from the center of one hole to the outside edge of the hole furthest away.

If you don’t want to be bothered by measuring with a ruler, tape measure or dial caliper, you can cheat by using a round plastic bolt circle template disc (Made 4 You Products offers this tool).

Discs are available that feature multiple hole patterns with each hole identified for bolt circle. Simply place the disc against a wheel hub face or onto the vehicle hub until all the holes align. The holes that align to your wheel or hub indicate the bolt circle.

Another specialty tool that allows quick measuring of a wheel’s bolt circle is a sliding caliper. This tool features two tapered dowels that slip into the wheel’s bolt holes. Insert the dowels into the two holes that are closest together. The graduated gauge face of the tool indicates the bolt circle, based on the number of bolt holes (the gauge features multiple pointers for 4-, 5-, 6- or 8-bolt).

The sliding caliper tool is offered by Excalibur Wheel Accessories, though I sourced mine through American Tire Distributors.

If you don’t want to bother measuring bolt or stud shanks or nuts, it’s a good idea to keep a reference set on hand attached to a small board.

Understanding Tire Dimensions

The sidewall of a tire offers an abundance of information, including the tire’s size, maximum inflation, serial number, manufacturing location and tread wear rating. In essence, this information serves somewhat as an owner’s manual for that specific tire.

Since much of this sidewall information can be confusing, I’ll walk you through a scenario to help you understand the data provided that applies to a tire’s dimensions.

Three sizing systems are employed today for passenger tires—P-Metric, European Metric and Alpha-Numeric. The most common system is P-Metric.

The size designation indicates cross section in millimeters, aspect ratio, type of construction and appropriate rim diameter. For example, a P235/50R16 describes a tire that’s intended for passenger use (P) and features a cross section width of 235mm, an aspect ratio of 50, is of radial construction, and is to be mounted to a 15-inch diameter wheel.

European Metric (also called Metric) is similar. No letter “P” is used as a prefix. The three-digit number at the beginning indicates cross section in millimeters. The next letter indicates the speed rating. An “R” following this indicates that the tire is of radial construction and the final twodigit number indicates rim diameter in inches. If the aspect ratio is lower than 82, a slash, followed by the aspect ratio number, will follow the section width number.

A European 155SR13 metric tire with an 82 aspect ratio indicates that it has a 155mm section width, an S speed rating, radial construction, and is intended for a 13-inch wheel.

Alpha-Numeric tires originated in the 1960s. This sizing system features a load-based identification approach where the first letter designates the tire’s load carrying capacity. For example, a BR60- 13 tire features a “B” load rating, radial construction, a 60-series aspect ratio and a 13-inch rim requirement. The alpha (letter) character can range from A–N, depending on load capacity. The higher the letter, the higher the load rating. Alphanumeric sizes are in popular demand for muscle car restorations where the original size/type tire is desired.

How Tires Handle Water

Traction grades (featured on the sidewall markings) provide a general indication of a tire’s ability to control and stop a vehicle on a wet road surface, with testing performed in a straight-line braking scenario on a controlled wet-surface test pavement, including both concrete and asphalt. The ratings that result from these tests are performed in straight-line testing and may not indicate cornering traction. The tire manufacturers give each tire a rating of A, B or C, with tires rated “A” having the highest traction.

These traction ratings are established on skid pads that are designed following federal government standards. Twenty measurements are taken using an industry standard “control” tire on both an asphalt surface and a concrete surface, with the results from each surface test averaged. The same scenario is then performed with the specific tire being tested and the results are then compared with the data obtained from the control-tire tests. The rating grade is then assigned.

Three “grade rating” numbers will typically appear on the tire sidewall— the treadwear rating, traction rating and temperature rating. The phenomenon called “hydroplaning” occurs when a tire encounters a wedge of water between the tread and the road surface. This can cause temporary directional pull and loss of control. While virtually any vehicle and tire combination has the potential to hydroplane at various vehicle speeds (wherever the limit is reached where water can’t evacuate quickly enough between the tire and road), it’s important to understand that a number of factors can combine in order to experience hydroplaning, including vehicle speed, tread design, tread depth, tread compound, tire width, vehicle weight and water depth.

A tire contacts the road in what’s called the “contact patch.” This is the footprint that a tire creates as it rolls down the road. Larger-profile/ lower-aspectratio performance tires create a larger footprint, which may aid in dry traction (taking advantage of more rubber on the road for increased grip). However, in wet weather, this larger footprint can potentially degrade traction because of the larger water wedge that can be created (a narrower tire may “slice” through water better than a very wide tire).

In response to this trait, tire manufacturers go to great extents to design tread patterns and compounds that will resist hydroplaning.

Speed Ratings

Speed ratings, in addition to the speed at which a tire model has been tested and verifi ed, indicate the tire’s performance characteristics. Just because a tire is rated at 149 mph, that doesn’t mean that you should attempt to operate the vehicle on a public roadway at that speed. If you want to play, rent some time on a race course. You shouldn’t select a tire by focusing on the mph speed number because you’re probably not going to drive at those speeds while on public roads anyway (at least, you shouldn’t).

The high-speed rating is a clear indication that the tire has highperformance design, material and construction

features that will enhance the traction, steering response, lateral control and braking of the vehicle at legal highway speeds. The design and construction of the tire has been enhanced, offering the side benefi t of high-speed capability.

Speed ratings are based on laboratory tests where the tire is loaded against a large-diameter metal drum to refl ect its appropriate load and run at increasing speeds in 6.2 mph steps in 10-minute increments until the tire’s required speed has been met

Beginning in 1991, speed ratings have been placed immediately following the load index number, for example 225/50R16 89S. When Z-rated tires were fi rst introduced, they were thought at the time to refl ect the highest tire speed rating that would ever be required, with the Z rating indicating that the tire was capable of speeds “in excess of 149 mph.”

Eventually, the tire industry added W and Y speed ratings. A Z rating may still appear on the tire on its own (indicating a rating of 149 mph+), but may also appear in addition to a W or Y rating symbol (with W indicating a rating of 168 mph; and Y denoting 186 mph). Some tires may feature a rating symbol of Y (following the load index), with the load index and speed rating encased in parenthesis. Example: 285/35ZR19 (99Y). If the Y is seen within parenthesis, this indicates a speed rating “in excess of 186 mph.”

Tires referred to as “Ultra High Performance” include those featuring V, Z, W, Y and (Y) speed ratings.