On a Roll
What You Need to Know When Selecting Wheels and Tires
Of all the elements involved with street rod styling, the combination of wheels, tires, and stance may very well be the most critical. Use the wrong combination of rolling stock and it’s difficult, if not impossible, to ignore and may cause all the other cool elements of the car to go unappreciated.
Although the look of the wheel and tire combo is important performance is part of the package as well. There are growing numbers of builders who want more rubber to meet the road to enhance their car’s grip, but regardless of what you’re after, traditional bigs ’n’ littles or the latest in fat and sticky low-profile rollers, the challenge is figuring out what will fit before buying wheels and tires that won’t. One of the best methods to do that is with a tire-mounting fixture that Dean Livermore, at Hot Rods by Dean uses called the Tire Mount Mate.
Available from WheelWorks, this clever tool allows tires to be
testfit by simulating a wheel’s diameter, width, and backspacing. Kits are available for a variety of vehicles, including eight-lug trucks, however, the most popular kit for street rodders simulates wheels from 14- to 20-inch diameter up to 16.5 inches wide with up to 12 inches of backspacing.
Once you’ve figured out what will fit in the space available there is some basic information about wheels you should be familiar with before making what is often a substantial investment.
• Bolt Patterns
This consists of two numbers, such as 5-on-4.5. The first number is the number of mounting holes; the second is the diameter of the circle the holes are laid out on. In this example there would be five holes in a 4.5-inch circle. Some wheels will have two sets of boltholes, 5-on-4-.5 (often referred to the late Ford/Mopar pattern) and 5-on-4.75 (found on many Chevrolets).
• Wheel Width
This simple measurement can be confusing. It’s the width of the wheel from bead seat to bead seat and does not include the flanges that are outside the tire.
Like the width of a wheel, the diameter is from bead seat to bead seat and does not include the outer flanges on each side.
• Wheel Offset
This is the distance from the mounting surface of the wheel and the wheel’s centerline. Zero offset means that the wheel’s mounting surface and the centerline are the same. Positive offset means the mounting surface is toward the outer edge of
the wheel. Negative offset means the mounting surface is toward the back of the wheel. (A classic example is a “reversed” wheel.)
• Center Register
Wheels must be centered on the hubs—this is done by two different methods. With hub-centric designs (most often used with OEM steel wheels) the center hole in the wheel fits tightly on (and is supported by) the hub as well as the studs. Lug-centric wheels have a larger center hole than the hub (or axle protrusion) and are located by the lug nuts.
• The X-Factor
The X-Factor is the amount of clearance between the wheel and the disc brake caliper. This has become increasingly important with the popularity of large-diameter aftermarket brakes.
• Lug Nut Style
There are several different types of lug nuts available and it is very important to match them to the wheels being used. Conical seat lug nuts are available with 60- or 45-degree taper with 60 degrees being the most common—they are found on most OEM and aftermarket wheels. What’s called an ET-style lug nut also has a 60-degree conical seat with a short extended shank to allow for more thread engagement. Because 45-degree lug nuts have a wider surface contacting the wheel some racing organizations (such as NASCAR) require them.
Shank-style lug nuts, often referred to as mag wheel nuts, have an extended portion that fits into straight holes in the wheel and uses flat washers. When using mag wheel lug nuts make sure the shanks are not so long that they bottom out against the hub. Also, when using any lug nut with a closed end, make sure the stud does not bottom out in the nut, as in both cases the wheel will be loose.
Ball seat lug nuts look similar to the conical style, but their seating surface is rounded rather that straight— they are normally found on import vehicles. The important thing to remember is the lug nuts and wheels must be compatible.
• Interpreting Tire Speak
At one time about all there was on the sidewall of a tire was the brand name and size. When numeric tire sizing was used a 6.50-15 tire was 6.5 inches wide and fit a 15-inch wheel. Today there is much more information in the tire code found on the sidewalls: The first letter in the size code indicates the intended use of the tire. P stands for passenger vehicles; LT means light truck tire, vehicles towing trailers or have 3/4- and 1-ton
load capacity; ST stands for Special Trailer, as the name implies they are for trailers. If there’s no letter before the first number the tire is a metric or European load–rated tire.
• Tire Size
Using a P225/50R17 as an example, the P stands for passenger car; 225 is the tire’s section width from sidewall to sidewall in millimeters. The next number, 50 in this case, is aspect ratio or the percentage of the tire’s sidewall height compared to its width—50 means that the tire’s section height is 50 percent of the tire’s section width. The larger the aspect number, the taller the tire’s sidewall.
A single letter indicates the internal construction of the tire: R is for radial tires, D is for tires built with diagonal plies (bias-ply construction).
• Wheel Diameter
This two-digit number specifies what size wheel the tire fits. Load Index and Speed Rating Example: P225/50R17 98H
The load index and speed rating come after the tire size. Load indicates the weight the tire can carry and is represented by a number that refers to a load index chart. In this case 98 indicates 1,653 pounds.
Speed ratings are represented by letters, with the highest rating being Z. At the time this rating was devised 149 mph seemed adequate. But thanks to some supercars higher ratings of W and Y are now available (those letters had not been used before).
• Tire ID
The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) requires that tires be identified by the letters DOT, followed by letters and/or numbers that identify the manufacturing location, tire size, and manufacturer’s code, along with the week and year the tire was manufactured. Since 2000, the last four digits of the code indicate the week and year the tire was produced—the first pair represents the week, the second pair indicates the year. Tires manufactured before 2000 used a three-letter code at the end of the id numbers; two numbers to indicate the week followed by a single number for the year.
• Tire Grading
NHTSA developed tests to grade tires in three areas:
Treadwear: This is the wear rate of the tire compared to other tires offered by the same manufacturer, with 100 being the baseline number. A tire with a wear rate of 200 should last twice as long.
Traction: Traction grades are AA, A, B, and C (with AA being the highest grade)—they represent stopping distance on wet pavement.
Temperature: The temperature grades are A, B, and C from the lowest to the highest and indicate the tire’s ability to dissipate heat.
• Replacement Due to Age
Over the past few years many tire shops have begun refusing to mount, balance, or repair any tire that is over 6 years old. Although there is no law or regulation we have found that addresses this, the reason given for the policy is liability since a number of auto companies have suggested that tires be replaced after six years. Ironically several major tire manufacturers tell us that tires should be good for up to 10 years if they are not damaged.
Before removing the fixture and tire, the X-factor was doublechecked. Tire clearance is checked at full steering lock full left. 8
With the assembly in place and fender clearance verified the wheel offset is measured. 6
Backspacing can be checked on the floor with the fixture removed. 7
With support under the lower A-arms the car can be “jounced” to check fender clearance. 5
This is one of the four clamps that hold the tire in place. The fixture is adjusted to the suggested width for the tire, then the beads are clamped in place. 4
With the suspension at ride height the fixture is bolted to the hub and adjusted for the diameter of the wheel. 3
The popularity of large disc brake rotors and calipers requires careful measurement of the “X-factor” to ensure the wheels will clear.
The perfect street rod stance is difficult to describe, but you’ll know it when you see it. This Ford Fairlane was built for the Road Tour by Hot Rods by Dean. We wanted as much rubber under the Fairlane’s fenders that would fit so HRBD used a Wheel...
11 Using the correct lug nuts is vital. This vintage aluminum wheel uses straight holes with flat surfaces for washers. Typical “mag wheel” lug nuts have straight shanks. It’s critical that the shank does not bottom out on the hub before the wheel is...
When determining the width of a wheel the measurement is made inside the flanges. Diameter is also measured from the tire bead surface not the od flange. 10
Here tire clearance is checked with the steering at full right. 9