The rim is the outer edge of a wheel, holding the tire. It makes up the outer circular design of the wheel on which the inside edge of the tire is mounted on vehicles such as automobiles.

In the 1st millennium BC an iron rim was introduced around the wooden wheels of chariots.

Diameter (effective)
Distance between the bead seats (for the tire), as measured in the plane of the rim and through the axis of the hub which is or will be attached, or which is integral with the rim.

Width (effective)
Separation distance between opposed rim flanges. The flange-to-flange width of a rim should be a minimum of three-quarters of the tire section width. And the maximum rim width should be equal to the width of the tire thread.

Depends on the type of vehicle and tire. There are various rim profiles, as well as the number of rim components.

Modern passenger vehicles and tubeless tires typically use one-piece rims with a safety rim profile. The safety feature helps keep the tire bead held to the rim under adverse conditions by having a pair of safety humps extending inwardly of the rim toward the other tire bead seat from an outer contoured surface of the rim.

Heavy vehicles and some trucks may have a removable multi-piece rim assembly consisting of a base that mounts to the wheel and axle. They then have either a side ring or a side and lock ring combination. These parts are removable from one side for tire mounting, while the opposite side attached to the base has a fixed flange.

Vehicle performance
Because the rim is where the tire resides on the wheel and the rim supports the tire shape, the dimensions of the rims are a factor in the handling characteristics of an automobile. For example:

Overly wide rims in relation to the tire width for a particular car may result in more vibration and less comfortable ride because the sidewalls of the tire have insufficient curvature to flex properly over rough driving surfaces. Oversized rims will cause the tire to rub when turning.

Overly narrow rims in relation to the tire width may cause poor handling as the tire may distort sideways under fast cornering.


A standard automotive steel wheel rim is made from a rectangular sheet metal. The metal plate is bent to produce a cylindrical sleeve with the two free edges of the sleeve welded together. At least one cylindrical flow spinning operation is carried out to obtain a given thickness profile of the sleeve ó in particular comprising in the zone intended to constitute the outer seat an angle of inclination relative to the axial direction. The sleeve is then shaped to obtain the rims on each side with a radially inner cylindrical wall in the zone of the outer seat and with a radially outer frusto-conical wall inclined at an angle corresponding to the standard inclination of the rim seats. The rim is then calibrated.

To support the cylindrical rim structure, a disc is made by stamping a metal plate. It has to have appropriate holes for the center hub and lug-nuts. The radial outer surface of the wheel disk has a cylindrical geometry to fit inside the rim. The rim and wheel disk are assembled by fitting together under the outer seat of the rim and the assembly welded together.

One-piece rim and wheel assemblies may be obtained by casting or forging.

The terms wheel and rim are often used synonymously, as in decorative wheels being called rims. Some authors are careful to use rim for only the outer portion of a wheel, where the tire mounts, just as the rim of a coffee cup or a meteor crater does not refer to the entire object. Others use rim to mean the entire metal part to which the tire mounts, because the rim and the wheel are often cast or stamped from a single piece of metal instead of being distinct as with wire wheels. At the same time, wheel may refer to the entire rotating assembly, including the tire.

Alloy wheel
Alloy wheels are automobile (car, motorcycle and truck) wheels which are made from an alloy of aluminium or magnesium. They are typically lighter for the same strength and provide better heat conduction and improved cosmetic appearance than normal wheels. The earliest light alloy wheels made were made of magnesium alloys. Although they lost favor for common vehicles they remained popular through the 1960s albeit in very limited numbers. In the mid to late 1960s aluminum casting refinement finally started to allow manufacture of wheels that were safe. Until this time most aluminum wheels suffered from low ductility, usually ranging from 2-3% elongation. This meant these earlier aluminum alloy wheels were quite brittle, and as light alloy wheels at the time that were often made of magnesium and referred to as mags these early wheel failures were later attributed to magnesium's low ductility, when in many instances these wheels were poorly cast aluminum alloy wheels. Once these aluminum casting improvements were more widely adopted, the aluminum wheel took its rightful place as low cost high performance wheels for motorsports.

Lighter wheels can improve handling by reducing unsprung mass, allowing suspension to follow the terrain more closely and thus improve grip, however not all alloy wheels are lighter than their steel equivalents. Reduction in overall vehicle mass can also help to reduce fuel consumption.

Better heat conduction can help dissipate heat from the brakes, which improves braking performance in more demanding driving conditions and reduces the chance of brake failure due to overheating.

Alloy wheels are also purchased for cosmetic purposes although the alloys used are not corrosion-resistant. Alloys allow the use of attractive bare-metal finishes, but these require to be sealed with paint or wheel covers. Even if so protected the wheels in use will eventually start to corrode after 3 to 5 years but refurbishment is now widely available at a cost. The manufacturing processes also allow intricate, bold designs. In contrast, steel wheels are usually pressed from sheet metal, and then welded together (often leaving unsightly bumps) and must be painted to avoid corrosion and/or hidden with wheel covers / hub caps.

Alloy wheels are prone to galvanic corrosion, which can cause the tires to leak air if appropriate preventive measures are not taken. Also, alloy wheels are more difficult to repair than steel wheels when bent, but their higher price usually makes repairs cheaper than replacement.

Alloy wheels are more expensive to produce than standard steel wheels, and thus are often not included as standard equipment, instead being marketed as optional add-ons or as part of a more expensive trim package. However, alloy wheels have become considerably more common since 2000, now being offered on economy and subcompact cars, compared to a decade earlier where alloy wheels were often not factory options on inexpensive vehicles. Alloy wheels have long been included as standard equipment on higher-priced luxury or sports cars, with larger-sized or exclusive alloy wheels being options. The high cost of alloy wheels makes them attractive to thieves; to counter this, automakers and dealers often use locking wheel nuts which require a special key to remove.

Most alloy wheels are manufactured using casting, but some are forged. Forged wheels are usually lighter, stronger, but much more expensive than cast wheels

Aftermarket wheels
A sizeable selection of alloy wheels (sometimes called mags) are available to automobile owners who want lighter, more visually appealing, rarer, and/or larger wheels on their cars.

Aftermarket brands
Most aftermarket wheels are cast, while only a few above are forged, such as Vellano, and Weld. Many companies have been formed over the years (some recently) due to the increasing demand from street racing enthusiasts and the rising demand for larger diameter wheels.

Cast aftermarket wheels have also been oversaturated due to the vast influx of inexpensive chrome wheels from China. They manufacture products to global scale due to primarily cheap but highly skilled and qualified labor.

A recent trend in the industry includes joint venture partnerships being formed between offshore manufacturers and local importers/distributors such as PDW Wheels which started in Australia in 2006, amongst a few others. Most wheel brands are ultimately sold through dealers such as First Choice Wheels and Tires.

Some aftermarket are/were also available as original equipment manufacturer (OEM) fitments.

Magnesium alloy wheels
Magnesium wheels were the first die-cast wheels produced, and were often referred to as simply mag wheels. However, true magnesium wheels are no longer produced, being found only on classic cars. Magnesium suffered from many problems. It was very susceptible to pitting and corrosion, and would start to break down in just a few months. Cracking was a common problem, and the wheels were very flammable. Magnesium is used for flares and early flash lamps. Magnesium in bulk is hard to ignite but, once lit, is very hard to extinguish, being able to burn under water or in carbon dioxide, which are common extinguishing materials. Tires that caught fire could soon ignite the magnesium, creating difficulties for fire responders. Magnesium wheels required constant maintenance to keep polished. Alloys of magnesium were later developed to help alleviate some of the problems.

Magnesium wheels were originally used for racing, but their popularity during the 1960s lead to the development of other die-cast wheels, particularly of aluminum alloys. The term mag wheels became synonymous with die-cast wheels made from any material, from aluminum alloy wheels to plastic and composite wheels.

Magnesium alloy wheels are sometimes used on racing cars, in place of heavier steel or aluminum wheels, for better performance. Magnesium wheels can be produced through various methods.

Forging can be done by a one or multi-step process forging from various magnesium alloys, most commonly AZ80, ZK60 (MA14 in Russia). Wheels produced by this method are usually of higher toughness and ductility than aluminum wheels, although the costs are much higher.

High pressure die casting (HPDC)
This process uses a die arranged in a large machine that has high closing force to clamp the die closed. The molten magnesium is poured into a filler tube called a shot sleeve. A piston pushes the metal into the die with high speed and pressure, the magnesium solidifies and the die is opened and the wheel is released. Wheels produced by this method can offer reductions in price and improvements in corrosion resistance but they are less ductile and of lower strength due to the nature of HPDC.

Low pressure casting (LPDC)
This process usually employs a steel die, it is arranged above the crucible filled with molten magnesium. Most commonly the crucible is sealed against the die and pressurized air/cover gas mix is used to force the molten metal up a straw like filler tube into the die. When processed using best practice methods LPDC wheels can offer improvements in ductility over HPDC magnesium wheels and any cast aluminum wheels, they remain less ductile than forged magnesium.

Gravity casting (permanent mold and sand casting)
Gravity cast magnesium wheels have been in production since the early 1920s. This method offers wheels with good ductility, and relative properties above what can be made with aluminum casting. Tooling costs for gravity cast wheels are among the cheapest of any process. This has allowed small batch production, flexibility in design and short development time.

Novel methods for wheel production

Raffles process
This is a process invented by a prominent South African metallurgist named Noel Raffle. The process consists of a means of casting aluminum or magnesium without the need for holding molten metal. This reduces operating costs significantly and most important retains the highest mechanical properties and structural integrity of any casting system. The process has yet to find commercial support, yet samples tested show properties to be comparable to those resulting from forging similar alloys. It being such a fundamental shift in technology has meant industry is slower to adapt.

T-Mag process
This is a process developed by a group of companies. The process is a refinement of a tilt casting machine and offers much of the benefit of LPDC, without the risk of pressurized molten metal. Properties appear to be midway between gravity cast and Raffles casting wherein the ductility remains but the strengths are defined by limits of normal micro-structure resulting from standard solidification.

The mass of a typical magnesium automotive wheel is about 5Ė9 kg (depending on size).

Wheel sizing
The wheel size for a motor vehicle or similar wheel has a number of parameters.

Bolt pattern
The bolt pattern is the number of lug nuts or wheel studs on the wheel hub. As the bolts are most often evenly spaced, the number of bolts determines the pattern. For example: smaller cars have three (CitroŽn 2CV, Renault 4, some Peugeot 106s and CitroŽn Saxos, and the Tata Nano). Compact cars may have four bolts. Most United States passenger cars have five bolts. Pickup trucks and large SUVs can have as many as six or eight.

Bolt circle
The bolt circle is the notional circle determined by the positions of the bolts. The center of every bolt lies on the circumference of the bolt circle. The important measurement is the bolt circle diameter (BCD), also called the pitch circle diameter (PCD).

The BCD may be expressed in millimeters or inches, and is usually given with the number of bolts. For example, a 1974 MG B has a 4/4.5 inch (4/114.3 mm) wheel hub, meaning it has a 4-bolt pattern with a 4.5 inch (114.3 mm) bolt circle diameter.

The most common BCD values are 100 mm (3.94 inches) and 4.5 inches (114.3 mm). Many old British cars use 4 x 4"

Special BCD values are: 95.25 mm (for MGF and Rover Metro), 98 mm (for Fiat group automobiles), 101.6 mm (for Mini, MG, Austin Metro), 100 or 105 mm (for some GM vehicles), 108 mm (mostly for PSA, Ford and Lincoln), 110 mm (for some GM vehicles), 112 mm (for Mercedes-Benz and some VW group vehicles), 115 mm (for some GM vehicles), 120 mm (mostly for BMW), 130 mm (for Porsche), 170 mm (for Saab 96). Old British Triumph cars used 4 x 3.75" using 3/8" studs. (7/16" or M12 studs are advisable for track use).

Determining the bolt circle
For a 4- or 6-bolt wheel, this measurement is merely the distance between the center of two diametrically opposite bolts. In the 4-bolt picture to the right, this would be the distance between holes #1 and #4, for example.

Some basic geometry is needed to find the center of a 5-bolt pattern. In practice, the BCD can be found by multiplying the center distance between any two adjacent holes by 1.701.

The easiest way to calculate bolt circle is to take callipers and measure the hole size in the centre of the wheel (note this dimension). Next measure the distance between the edge of the centre hole and the centre of one stud. Double this measurement and add it to the first. Job done! This method works with any number of studs.

Lug nuts or bolts
Wheels must be fitted with the correct type of lug nuts on wheel studs, or bolts. Lug (aka wheel nuts in British English) nuts are usually either flat, tapered (generally at 60 degrees and referred to as conical seat), or ball seats, meaning the mounting surfaces are flat, tapered, or spherical respectively.

Most Mercedes have ball lug seats from the factory while most aftermarket wheels have a tapered lug design. Wrong lug nuts for the wheel will not properly center it and cause wobble. Some manufacturers (e.g. Toyota and Lexus) have used taper lug nuts for steel wheels and flat seated lug nuts for alloy wheels.

Some aftermarket wheels will only fit smaller lug nuts, or not allow an ordinary lug nut to be properly torqued down because a socket will not fit into the lug hole. Tuner lug nuts were created to solve this problem by utilizing a special key to allow removal and installation with standard lug wrench or socket. The design of tuner lug nuts can range from bit style to multisided or spline drive, and are sometimes lightweight for performance purposes.

A variation is the locking wheel nut, which is almost universally used for alloy wheels in the United Kingdom. One standard lug nut on each wheel is replaced with a nut which requires a special and unique key (typically a computer-designed, rounded star shape) to fit and remove the nut. This helps to discourage theft of wheels. However, universal removal tools are available which grip the head of the locking nut using a hardened left-hand thread. The success of these depends on whether there is room to use it in the lug hole, and whether the manufacturer has incorporated a free-spinning outer casing to the lock. Keeping an appropriate tool to lock and unlock aftermarket nuts, and a spare set of nuts, with the spare tire in the boot of the car is recommended by manufacturers.

The offset, measured in millimeters, can be negative or positive, and is the distance from the hub-mounting surface to the rim's true centerline. A positive offset means the hub-mounting surface is closer to the outside edge of the wheel, i.e. the wheel wraps around the hub and brake hardware more deeply; a negative offset means the hub-mounting surface is closer to the inside edge of the wheel and wheel sticks outwards more than inwards.

When selecting aftermarket wheels, a wheel with too little positive offset will be closer to edge of the fender, and one with too much positive offset will tuck inside the fender and be closer to the suspension components. Wheel width, offset and tire size all determine the way a particular wheel/tire combination will work on a given car. Offset also affects the scrub radius of the steering and it is advisable to stay within the limits allowed by the vehicle manufacturer. Because wheel offset changes the lever-arm length between the center of the tire and the centerline of the steering knuckle, the way bumps, road imperfections and acceleration and braking forces are translated to steering torques (bump-steer, torque-steer, etc.) will change depending on wheel offset. Likewise, the wheel bearings will see increased thrust loads if the wheel centerline is moved away from the bearing centerline. In other words using the stock wheel offset number as the starting point, a lower off set number makes the wheel stick out more and a with higher number it sticks out less. (Lower is out more, higher is in more.)

Wheel size
The wheel size is the diameter of the wheel in inches where the beads of the tire sit on the wheel. This measurement does not include the rim flange. Modern road tires have several measurements associated with their size as specified by tire codes like 225/70R14. The first number in the code (e.g., "225") represents the nominal tire width in millimeters. This is followed by the aspect ratio (e.g.,"70"), which is the height of the side wall expressed as a percentage of the nominal width. "R" stands for radial and relates to the tire construction. The final number in the code (e.g.,"14") is the rim size measured in inches. The overall circumference of the tire will increase by increasing any of the tire's specifications. For example increasing the width of the tire will also increase its circumference, because the side wall height is a proportional length. Increasing the aspect ratio will increase the height of the tire and hence the circumference.

Off-roading tires may use a different measurement scheme: outside diameter times(by) tread width, followed by rim size (all in inches) - for example 31x10.50R15. The size of the disc however is denoted like 8.5" X 20.0". This shall mean that the width (of thickness) of the wheel is 8.5 inches and the diameter is 20 inches.

A dub is a custom wheel that has a wheel size of 20 inches or greater; the term dub is a slang term for twenty.

The centerbore of a wheel is the size of the hole in the back of the wheel that centers it over the mounting hub of the car. Some factory wheels have a centerbore that matches exactly with the hub to reduce vibration by keeping the wheel centered. Wheels with the correct centerbore to the car they will be mounted on are known as hubcentric. Hubcentric wheels take the stress off the lug nuts, reducing the job of the lug nuts to center the wheel to the car. Wheels that are not hubcentric are known as lugcentric, as the job of centering is done by the lug nuts assuming they are properly torqued down.

Centerbore on aftermarket wheels must be equal to or greater than that of the hub, otherwise the wheel cannot be mounted on the car. Many aftermarket wheels come with "hubcentric rings" that lock or slide into the back of the wheel to adapt a wheel with a larger centerbore to a smaller hub. These adapters are usually made of plastic but also in aluminum.

Caliper Clearance (X-factor): The amount of clearance built into the wheel to allow for the vehicle's disc brake and caliper assembly.

Load capacity
Load capacity is the amount of weight a wheel will carry. This number will vary depending on the number of lugs, the PCD, the material used and the type of axle the wheel is used on. A wheel used on a free rolling trailer axle will carry more weight than that same wheel used on the drive or steering axle of a vehicle. All wheels will have the load capacity stamped on the back of the wheel.

This is the Gross Vehicle Weight Rating. In the United States this information is required to be on the vehicle's door placard. The load capacity of the total number of wheels on the vehicle combined must meet or exceed the vehicle's GVWR.

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