Automotive Wheels

Wheels are one of the most common upgrades for a car, the main reason is
aesthetic improvements, but performance can be greatly improved simply
by the wheel selected. There are three main factors in performance that
can be helped with wheel selection, unsprung weight, track changes and
tire performance. A light weight wheel can greatly improve the
functionality of the suspension improving performance and ride comfort.
Also difference sizes of wheels allow different tires to be used which
can improve the performance of the tire. Changing the track, or the
distance between the wheels from side to side, can help balance the car
and improve handling.


Contents


    * 1 Wheel Size
    * 2 The Offset
    * 3 Stud/Bolt Patterns
    * 4 Wheel Construction and Quality
          o 4.1 One-Piece Cast Wheels
          o 4.2 Gravity Casting
          o 4.3 Positive Pressure Casting
          o 4.4 Spun, Flow Forming or Rim Rolling technology
          o 4.5 Forged or Semi-solid Forged
          o 4.6 Multi-Piece Wheels
    * 5 Chrome
    * 6 The Components of a Wheel
          o 6.1 The Hub and Centre Bore
          o 6.2 The Disk
          o 6.3 The Rim/Barrel

Wheel Size

Wheel size is probably the most important part of choosing the right
wheel, the overall dimensions of the wheel consists of 2 main
measurements, the diameter and the width; both will affect clearance
under the guards, as well as tire choice and general performance and
ride comfort. Wider wheels allow you to run wider tires, so selecting
the right width is important and should be considered when deciding what
tire to use. Also having a larger diameter means to keep a similar
overall diameter (with the tire and wheel) means running a lower profile
tire, this has its advantages and disadvantages such as response in
steering, harsher ride comfort, which once again means this must be
taken into consideration when making the decision.

Staggered sizes can also be used, depending on the desired outcome,
whether it be performance or looks, staggered size is surprisingly
popular, especially width. For RWD cars it is very common to see a lot
wider wheels at the rear to allow much wider tires so that they can put
the power down through the rear. Also staggered diameter is sometimes
used, often seen in drag racing as well as used for presentation
purpose.


The Offset

The offset of a wheel is the measurement from the centre of the wheel
(half the total width of the rim) to the mounting surface. A neutral or
0 offset, means the mounting surface is right in the middle of the rim,
a positive offset means the mounting surface is positioned forward
(towards the outside of the wheel) of the centre. This in effect brings
the wheel and tire in from the guard near the inside more; it will mean
more clearance from your guard, but less from suspension components. A
negative offset means the opposite; the mounting surface of the wheel is
behind the centre of the rim. This will cause the tire and wheel to
stick out away from the vehicle closer to the guard but less inner
clearance for the suspension components. When offset is displayed, it's
usually described like +32 meaning positive 32mm offset from the centre,
or -21, meaning negative 21mm offset from the centre.


Stud/Bolt Patterns

Different cars have different stud patterns, this means wheels are made
to fit different cars.

Most Bolt Patterns are represented in the following manner: 5/114.3

The "5" indicates the number of holes in the wheel for the bolts to
enter and mount the wheel onto the car. The "114.3" indicates the PCD
(Pitch Circle Diameter), the diameter of the bolt circle measured in
millimetres or inches. 4 & 6 bolt wheels are measured from the centre of
one bolt hole to the centre of the bolt hole directly across from it.

Wheel Construction and Quality

One-Piece Cast Wheels

This is the most popular type of wheel where the whole wheel
construction is one piece made with a cast mold. The process involves
getting molten alloy into a mold.

Gravity Casting

Gravity casting is the most basic technique of casting by using gravity
fill the mold. Gravity casting keeps production cost down and is usually
a popular method for wheels that are visually oriented and when
producing a lightweight wheel is not very important. Since the process
uses low pressure to fill the mold, the alloy isn't very densely packed
in the mold leading to low strength, this means more metal is needed
meaning more weight to achieve a safe amount of strength within the
wheel.


Positive Pressure Casting

Positive pressure casting uses pressure to compress the molten alloy
into the mold more forcefully compact the alloy in the mold more. This
increase in density improves the strength over the gravity casting
technique leading to better results. The only downside to pressure
casting over gravity casting is it has a higher production cost. Low
pressure is the most popular process used for alloy wheels especially in
the OE market as it provides good strength whilst still keeping down
production costs. Some manufacturers produce wheels that use higher
pressure when casting with special equipment, this is of course the most
expensive pressure casting technique but it provides great lightweight
properties.


Spun, Flow Forming or Rim Rolling technology

This specialized technique starts with a pressure type of casting and
then spins the casting, the outer portion of the casting is then heated
and then steel rollers are pressed against the rim area to pull the rim
to a final width and shape. The process used a combination of heat,
pressure and the spinning motion to create a rim with the strength
properties similar to a forged wheel without the higher production cost
of forging. This is sometimes used for the OE market in high performance
cars, and is also popular for aftermarket wheels. Spun rims are often
used with forged faces or disks as it saves a lot of production costs
but still achieves similar results.

Forged or Semi-solid Forged

Forging is the ultimate in wheel production, it is the process of
forcing or pressing a solid billet of alloy into the forging dies under
massive amounts of pressure. This obviously creates a very dense wheel,
which produces maximum strength which means less metal can be used which
means less weight. Of course as you would expect the production costs is
massive, so it's not a favoured process unless lightweight is high on
the agenda. Semi-solid forging is a similar process but instead heats
the billet of special alloy to an almost molten state and then the alloy
is forced into the mold at a very high rate. The final product offers
mechanical properties very similar to a forged wheel but has a lower
production cost. SSR (Speed Star Racing) currently owns the license to
this process and are the only ones to use it.


Multi-Piece Wheels

This type of wheel uses 2-3 major components secured together to create
a full wheel. This means that the different components can utilize
different processes of manufacturing. Using a forged disk or face means
maximum strength, and using a spun rim means similar strength but it
keeps the overall production cost down.

The 2-piece wheel is a more common wheel these days; it does not have
the same range of applications as the 3-piece but it is usually a lot
cheaper to buy on the market. The different parts are often held
together by bolts connecting the face/disk onto the rim, or sometimes
they are simply pressed together and welded.


Chrome

This is a popular finish on wheels which results in the surface to be
bright and reflective like a mirror. The process involves the wheel
being stripped to its bare metal and several different layers of metals
being applied which is finished off with a nickel layer that creates the
shiny finish. Chrome wheels are usually heavier and can be weaker due to
the extra metal and sometimes the use of acid in the stripping process.



The Components of a Wheel

The components of a wheel all play a critical part in the function of
the wheel, they sometimes take on different names depending on where
you're from, but there are some common names for most components of the
wheel.


The Hub and Centre Bore

The hub of a car is a small ring that protrudes out of the mounting
surface for the centre bore of the wheel to fit onto. The measurement of
the hub can also be referred to as the pilot bore.

The centre bore is the hole in the centre of the wheel that centres the
wheel on the hub of the car. Since most wheels are mass produced, they
have a large centre bore to accommodate several different vehicles.


The Disk

The disk of a wheel is also known as the face of a wheel as it is most
visible looking at a wheel fitted on the car. The disk includes the
spokes, mounting surface, and lug holes.

There are different types of disks, referred to as high and low disk, or
A, B and C. A high disk or a disk type A, will tend to have more space
behind the spokes where it mounts to the mounting surface, this also
means that the disk will stick further out away from the car relative to
the barrel or the rim. A low disk or disk type C will mean the opposite;
it will have less space behind the spokes, and will stick in further
relative to the rim. A high disk type is usually used on cars with big
brakes that need to be cleared by the spokes, and low disk's are usually
used on smaller brake cars and is often preferred for styling reasons as
a low disk often means more dish or concave spokes.


The Rim/Barrel

The rim is the barrel of the wheel which basically connects the tire to
the disk. Its main job is to hold the tire in place and keep pressure
sealed under the tire to keep it inflated. The design of this can
actually be surprisingly important, especially the bead seat. The Bead
Seat is the area of a rim where the tire walls sit and seal to prevent
air leakage. A rim can have a slotted or knurled bead to reduce slippage
in high performance applications. Different designs can be easier and
harder to bead a tire which can be important when a smaller tire is
stretched over a wider wheel making it hard to bead.