Tech Guide for Rims and Wheels

This will be the first in a series of tech reference articles for the end user to utilize in researching aftermarket wheels. We will first begin by listing a glossary of commonly used terms in the wheel industry. The objective of this glossary will be for the average tuner to easily understand terms that wheel experts use on a daily basis. I will add to this glossary every month along with tech articles which refer to a consumers selection process of a wheel.


Bead Seat
The Bead Seat is the area of a rim where the tire will be sealed to prevent air leakage.

Often times a wheel can have a slotted or knurled bead to reduce slippage in high performance applications such as rally racing.


Cast Wheels (One Piece)
A cast wheel is made by pouring molten aluminum ingot into a mold; the metal then takes the mold's shape as it cools and hardens. There are several types of casting methods which include low pressure/gravity, counter pressure, and high counter pressure molding (HCM); each method has its place in today's market. A wheel manufacturer will select a particular method according to the weight, strength and finish that they have specified for that design.

The benefit of casting wheels is the relative low cost for set up and manufacturing. However cast wheels are typically heavier and less robust compared to a wheel manufactured using a forging method.

Forged One-piece Wheels
A forged wheel is made by forming a wheel under extreme high pressure. The material used is an aluminum billet—a solid piece of dense aluminum. A forging press, a huge machine that exerts thousands of tons of pressure on the metal, basically presses or rolls the billet into its fundamental form. This forging is then machined to final finish. This process allows wheels to be built with much less material, allowing for lighter weight and unique designs, because the process creates a much stronger wheel than other methods.

Many premium wheels are made using forging technology.

Multi-piece Wheels
The processes mentioned above can be combined to produce a wheel of a particular strength and weight. For example, some wheels have a cast aluminum center, welded to a steel outer. Another type of two-piece construction features a billet center welded to a rolled outer (extruded aluminum rolled into a hoop). Still other wheels feature a split outer so that widths and offsets can be made to custom specifications. This last method is the three-piece type construction. In this type of wheel, the center which can be cast, billet or forged, is usually attached to the outer assembly by special screws or rivets.

Bridge
The bridge of a wheel is the portion of the wheel between the spokes.

Disk
The disk of a wheel can be also referred to as the face of a wheel. This is the portion of a wheel that is most visible when installed on a vehicle. The disk is composed of spokes, mounting surface, and lug holes.

Rim
This rim of a wheel is the “barrel” of a wheel that is the first point of contact from road forces from the tire during driving.

Size
The size of a wheel is determined by the diameter and width of a wheel. The diameter of a wheel is measured from the inside flange of a wheel. The width of a wheels is measured again from the inside part of the flange of a wheel on each side.

Wheel size is measured in inches. For example – 18x10 means that a wheel is 18inches in diameter and 10inches in width.

Rim Flange
Rim Flange is the portion of a wheel where the disk meets the rim.

Offset
Wheel offset is the way for a wheel manufacturer to determine how a wheel is placed on the car’s wheel well in relation to the suspension components and fenders. By taking into consideration the width and diameter of a wheel, a wheel technician can utilize offset to determine fitment. The offset indicates the distance from the center of the rim to the mounting point of the wheel to the car. Offset is measured in millimeters. A positive offset is indicated when the mounting surface extends beyond the central line of the rim and a negative offset results when the mounting surface falls within.

Basically a positive offset means that the rim will go further towards the suspension and a lower offset means that the rim will go further towards the fenders. Make sure to check with your wheel tech to make sure you are using the proper offset when matching up wheels for your car.

P.C.D.
P.C.D. (Pitch Circle Diameter) This is also referred to as “Bolt Pattern” – Cars will have a designated number of mounting bolts which car vary from 3 to 8 bolts to mount the wheel to the car. Each bolt will be spaced in a circle diameter accordingly to car manufacturer’s specification. This is usually measured in millimeters or sometimes in inches.

For example – “ 5-114.3 “ will mean that the vehicle has 5 bolts which are spaced 114.3 millimeters apart in diameter.

Again make sure you know the P.C.D. of your vehicle to determine which wheel will work with your car.

Center Bore
Center bore of a wheel is the measurement (in millimeters) of the diameter of the hole opening in the center of the wheel which is placed on the hub of the vehicle.

A center bore of a wheel is usually oversized to accept different vehicle applications. A hubcentric ring is then used to make the wheel hubcentric.

Hubcentric Ring
This is used to make a wheel hubcentric to a car where the centerbore of a wheel matches the pilot bore of a car.

This is an important condition to eliminate potential vibration and to ensure a perfect exact fit.

Hub
The hub of a car is a section that protrudes out of the mounting surface for the wheel of a vehicle. The measurement of the hub can also be referred to as the pilot bore.

Valve Stem
The Valve Stem is a valve that is used to maintain air pressure of the tire. Usually mounted on the rim section of the wheel, a valve stem is necessary to inflate a tire to the rim.


Pad Height / Disk Height
Depending on the wheel manufacturer, the Disk or a Pad height is determined to satisfy brake caliper overhang clearance. Although each manufacturer determines their own Disk / Pad heights, they will specify certain heights to specify brake caliper clearance with notations such as High Pad/ Disk, Mid Pad / Disk, and Low Pad / Disk. A High Pad or Disk is for cars with larger oem calipers or for applications that will use upgraded big brake systems.

As a rule of thumb when comparing 2 wheels of a same size and offset, a High Pad wheel will have a smaller rim depth (lip size) since the mounting surface is raised to clear brakes. A low disk will have a larger lip size since the mounting surface is shorter, which then allows the lip size to be lengthened. Low disk is usually for the rear of cars which have small brake calipers.

Chrome
This is a popular finish on wheels which resembles a bright mirrorlike finish. A wheel is stripped to its bare metal and different layers of metals are applied to the wheel which is then finished off with a nickel layer, which creates the shiny finish. A chrome wheel is usually heavier and can be weaker than a painted wheel due to the addition of metal and depending on the stripping process, acid may be used which will affect the surface of a wheel.

Rotating Weight
During the driving of a vehicle, there is a pressure that is applied to the tires and wheels of the vehicle. This is called the Gyroscope effect. Especially during high performance driving conditions - acceleration, turning, and braking is all effected by these rotating forces. Reducing weight while retaining superior strength results in success in hi-performance driving.

Unsprung Weight
The weight from the Suspension apparatus which includes the Tire and Wheel combination is called “Unsprung Weight”, and reducing the weight of this section is crucial to the performance of the vehicle. If 1kg of weight is saved from the suspension system, the value of a 15kg weight loss from the vehicle’s overall weight can be saved. This means if an aluminum wheel which is 1kg lighter than a stock steel wheel is installed, 60kg of total of body weight can be saved.

jwl
Many may have noticed a marking on wheels with a visible logo “jwl”. Japanese wheels must have a "jwl" logo on the wheel and "jwl" (Japan Light Wheel Alloy) is a compilation of standards defined by the Japanese Government to ensure the vehicle's safety for aluminum road wheels. Every wheel put to market must be tested to meet jwl standards before a wheel can be put out to market in Japan. These standards are generally accepted worldwide as acceptable for most road conditions. That is why you will see these marks on European and other Asian country wheels.

jwl TESTS:

Cornering Fatigue Test (Drum Endurance Test)
The purpose of this test is for checking wheel rim strength and durability. Place a wheel assembly with tire on test equipment. Radial stress is then added to simulate vehicle weight.

Test run 500,000 cycles
After 500,000 cycles inspect for cracks, deformation and any nut loosening.

Radial Load Fatigue Test (Rotary Bending Test)
jwl pass criteria: 100,000 cycles
The purpose of this test is for checking durability of the wheel disc. The spokes and the hub are the first point of weight distribution from the vehicle and spoke strength is gauged by this test.

Place a wheel on testing equipment and by adding bending stress to center of disc –

Test run 100,000 cycles
After 100,000 cycles, inspect for cracks, deformation and any nut loosening.

13 Degree Impact Test
jwl pass criteria: 594 kg

The main purpose of test is for checking strength between rim and disc cross section. The “joint” or the area where the spoke and the rim meet is an important structural area that is subject to great stress. The horizontal weight of the rim and the vertical stress of the spokes meet at this joint and proper structural stability is crucial for a proper wheel.

The wheel assembly is placed with tire tilted 13 degrees from horizontal plane on testing equipment.

A preset weight is dropped on the outer sidewall of tire, thus giving impact to the wheel/tire. Then inspect for cracks, deformation and air leak.

This test is very important for the recent trend of a larger/wider wheel with lower profile tire combination, because there is less air volume than OE wheel/tire and more stress to inner rim section. This test simulates actual usage condition such as hitting corner curb or objects from side.

VIA (Vehicle Inspection Association)
Third party organization (In Japan) that enforces the jwl standards. They make sure that companies are adhering to governmental standards.

Shot Blasting
Some higher end wheel manufacturers will shot blast or shot peen a wheel in preparation for the finishing treatment. The Shot-blasting process involves thousands of tiny industrial grade ball bearings pounding the wheel in a controlled environment to further harden the surface of the material. This helps to lock in surface toughness and is also used in many other automotive and industrial applications.

Plus Sizing
Plus sizing is installing a wheel with a larger diameter with a lower profiled tire to match the size of the stock vehicle wheel and tire combination. The increase in size of wheels and tires is the best way to improve both performance and appearance of the vehicle. However, it is important to maintain the overall diameter of the tire because it is possible to change odometer and speed meter readings and adversely affect handling. By using a low-profile tire, quickness in steering response and better lateral stability is gained. At the same time, the car becomes visually impressive.

Stainless Lip
Some wheel manufacturers will utilize stainless lips for their one piece and multi piece design wheels. This allows a lip to have a very high luster without having to machine, polish or chrome the lip of a wheel. Often times, a stainless lip will be removable in case of curb damage instead of having to repair or replace a wheel.

Lug Nut
A nut that is used to fasten the wheel to the hub of a vehicle. Usually hexagonal in design, lug nuts are offered in a variety of sizes, shapes, and patterns to work with different cars and wheels. Make sure to check with the wheel and vehicle manufacturer to make sure which lug nut and wheel combo will work best for a desired application.

Popular Lug nut materials:

Steel: Mostly used for oem applications. You will mostly see chrome, zinc plated, or painted finishes in today’s market.
Aluminum: Used for aftermarket applications or racing applications. Cost is much higher than Steel lugs. You will see mostly painted or anodized finishes. Aluminum alloy nuts are light weight and reduce rotating mass and unsprung weight. However, due to aluminum’s inherent softness compared to Steel, make sure to re-torque aluminum lugs from time to time.
Chrome Vanadium: Used for aftermarket applications. Cost is similar to aluminum lugs. Chrome vanadium is a material found in several automotive applications such as engine valve springs. Although heavier than aluminum, chrome vanadium lugs are very hard and durable. A good compromise between aluminum and steel.
Titanium: Used for high end aftermarket applications. Most expensive lugs you can buy. Light weight and tough, titanium lugs can be found in high end aftermarket applications for exotic cars.
Japanese Car Lug Nut Thread Pitch Guide:
12 x 1.25 Thread Pitch –
Nissan, Infiniti, Subaru

12 x 1.5 Thread Pitch –
Toyota, Lexus, Honda, Mazda, Mitsubishi.

Wheel Lock
A type of lug nut that is used to prevent theft of a wheel. A wheel lock will feature a pattern that is not normally used on typical lug nuts.

Popular Wheel Manufacturing Materials:
Steel: Generally used on lower cost oem vehicles (and nascar!) , Steel wheels are a typically not very complex in shape.

Aluminum: The most common material used in alloy wheels in the world. Aluminum exhibits desirable characteristics such as relative light weight, malleability, resistance to corrosion and cost. Please note that different grades of aluminum are used in wheels of different manufacturing processed. For example, forged wheels will use higher grade aluminum such as 6061 aluminum due to its high tensile strength and purity.

Magnesium: Used primarily for high end wheels and racing applications, magnesium alloy wheels exhibit superior light weight characteristics and up to par strength characteristics. Only premium wheel manufacturers with high technical backgrounds are able to manufacture magnesium wheels due to its difficulty in processing and its reaction to extreme heat.

Other manufacturing materials: Carbon and Titanium materials are being researched to use for wheel applications in the pursuit of lightness and strength.

Terms Used in Wheel Design:
5 Spoke – A classic approach to a wheel. Often viewed to the eye as both Racy and Elegant, the 5 spoke design, although less efficient than other designs in terms of strength and lightness still remain as the most popular wheel design base for aftermarket wheels.

Mesh – A design pioneered by European Wheel manufacturers, the mesh design incorporates multiple spokes that intersect each other at various points to offer high strength and durability properties.

Fin – A design used in many touring car applications. A fin design is usually referenced to a wheel that features more than 12 thin spokes. The idea of a fin design is to reduce weight without sacrificing strength by distributing road forces and weight through multiple thin distribution points.

Dish – A design element that incorporates small, short spokes with a large full center disk. Often used for larger sedans. Not very efficient in weight, but high on elegance.