Wheels
Wheel Composition
Alloy Wheels
Alloy metals provide superior strength and dramatic weight reductions over ferrous
metals such as steel, and as such they represent the ideal material from which to
create a high performance wheel. In fact, today it is hard to imagine a world class
racing car or high performance road vehicle that doesn't utilize the benefits of
alloy wheels.
The alloy used in the finest road wheels today is a blend of aluminum and other
elements. The term "mag wheel" is sometimes incorrectly used to describe alloy
wheels. Magnesium is generally considered to be an unsuitable alloy for road
usage due to its brittle nature and susceptibility to corrosion (Flammability
doesn't help either !)
While many people choose alloy wheels for their beauty, there are equally
important performance benefits to be derived including...
- Reduced Unsprung Weight: Compared to Steel Wheels: This is one of the most critical
factors affecting a vehicle's road holding ability. Unsprung weight is that portion
of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes)
and therefore most susceptible to road shock and cornering forces. By reducing unsprung
weight, alloy wheels provide more precise steering input and improved "turning in"
characteristics.
- Improved Acceleration and Braking: By reducing the weight of the vehicle's
rotational mass, alloy wheels provide more responsive acceleration and braking.
- Added Rigidity: The added strength of a quality alloy wheel can significantly
reduce wheel/tire deflection in cornering. This is particularly critical with an automobile
equipped with high performance tires where lateral forces may approach 1.0g.
- Increased Brake Cooling: The metals in alloy wheels are excellent conductors of
heat - improving heat dissipation from the brakes - reducing risk of brake fade under
demanding conditions. Additionally, alloy wheels can be designed to allow more cooling
air to flow over the brakes.
Wheel Construction
One-Piece Cast Wheels
This is the most common type of aluminum wheel. The casting of wheels is the process of
getting molten aluminum inside a mold to form a wheel. There are different ways this
can be accomplished and although it sounds simple, this is truly an art when done properly.
Gravity casting: Gravity casting is the most basic process of pouring molten
aluminum into a mold utilizing the earth’s gravity to fill the mold. Gravity casting
offers a very reasonable production cost and is a good method for casting designs that
are more visually oriented or when reducing weight is not a primary concern. Since the
process relies on gravity to fill the mold, the aluminum is not as densely packed in
the mold as some other casting processes. Often gravity cast wheels will have a
higher weight to achieve the required strength.
Low pressure casting: Low pressure casting uses positive pressure to move
the molten aluminum into the mold quicker and achieve a finished product that has
improved mechanical properties (more dense) over a gravity cast wheel. Low-pressure
casting has a slightly higher production cost over gravity casting. Low pressure is
the most common process approved for aluminum wheels sold to the O.E.M. market.
Low-pressure cast wheels offer a good value for the aftermarket as well. Some
companies offer wheels that are produced under a higher pressure in special casting
equipment to create a wheel that is lighter and stronger than a wheel produced in
low pressure. Once again in the quest for lighter weight, there is a higher cost
associated with the process.
Spun-rim or rim rolling technology: This specialized process begins with a low
pressure type of casting and uses a special machine that spins the initial casting,
heats the outer portion of the casting and then uses steel rollers pressed against
the rim area to pull the rim to its final width and shape. The combination of the
heat, pressure and spinning create a rim area with the strength similar to a forged
wheel without the high cost of the forging. Some of the special wheels produced for
the O.E.M. high performance or limited production vehicles utilize this type of
technology resulting in a light and strong wheel at a reasonable cost. BBS has used
this technology for several years in their production of racing wheels for Formula One
and Indy cars. The BBS RC wheel uses this same technology to produce a light and
strong wheel for the aftermarket.
Forged or Semi-Solid Forged: The ultimate in one-piece wheels. Forging is the
process of forcing a solid billet of aluminum between the forging dies under an
extreme amount of pressure. This creates a finished product that is very dense,
very strong and therefore can be very light. The costs of tooling, development,
equipment, etc., make this type of wheel very exclusive and usually demand a
high price in the aftermarket. Semi-solid forging (SSF) is a process that heats
a billet of special alloy to an almost liquid state and then the aluminum is forced
into a mold at a very high rate. The finished product offers mechanical properties very
similar to a forged wheel without the high production and tooling costs of a forged wheel.
When low weight and performance are on your priority list, the SSF technology offers an
excellent value. Currently only SSR (Speed Star Racing) from Japan is licensed to use
this process for the production of wheels.
Multi-Piece Wheels
This type of wheel utilizes two or three components assembled together to produce
a finished wheel. Multi-piece wheels can use many different methods of manufacturing.
Centers can be cast in various methods or forged. The rim sections for 3-piece wheels
are normally spun from disks of aluminum. Generally, spun rim sections offer the ability
to custom-tailor wheels for special applications that would not be available otherwise.
The rim sections are bolted to the center and normally a sealant is applied in or on the
assembly area to seal the wheel. This type of 3-piece construction was originally
developed for racing in the early 1970s and has been used on cars ever since. The
3-piece wheels are most popular in the 17” and larger diameters.
There are now many options for 2-piece wheels in the market. The 2-piece wheel
design does not offer as wide a range of application that a 3-piece wheel allows,
however they are more common in the market and the prices start well below the
average 3-piece wheel. Some 2-piece wheels have the center bolted into a cast or
cast/spun rim section and other manufacturers press centers into spun rim sections
and weld the unit together. When BBS developed a new 2-piece wheel to replace the
previous 3-piece street wheel, they used the special rim-rolling technology (originally
developed for racing wheels) to give the rim section the weight and strength advantages
similar to a forged rim. On the high-end of the 2-piece wheel market you can find wheels
using forged rims and forged centers. Since these are only sold in small volume and due
to the high development and production costs associated with the forging process, they
tend to be on the high end of the price scale.
Upgrading Your Wheels
Muscle cars came with what we would consider narrow wheels and tires. Many
cars came with 14"x6" or 15"x7" wheels with 70 or 78 aspect tires. When considering
upgrading your wheels and tires, it is important to remember that wheel height affects
the readings on your speedometer and odometer and can adversely affect the handling
and suspension. Therefore, be sure that the overall height of the wheel stays the
same, especially if you are putting on larger wheels and tires. You can use the
chart below to check your before and after wheel height. The optimal wheel width
depends on the aspect ratio of the tire that will be mounted on it. A 75 or 70 series
tire has high sidewalls which will curve in when mounting them on a narrow rim. This
will result in poor handling as the sidewalls deform sideways significantly before the
car even begins to turn! However, the high sidewalls allow you to use a narrower wheel
as this bending allows the tire to remain flat on the ground. The optimal wheel width
on 75 series tires is about 70-85% of the width of the tire. Lower profile tires such
as 60 or 50 series, result in stiffer sidewalls. Although they improve handling due
to less flexing, they need to be mounted on narrower wheels, otherwise the tread itself
will flex and lead to less contact with the road. The optimal wheel width on 60 series
tires is about 85-100% of the width of the tire. See the chart below for recommended
tires on a given width wheel.
| Optimal Tire Sizes |
| Width of Wheel |
75 or 70 series Tires |
50 or 60 series Tires |
| 5.0" |
150 - 185 |
125 - 150 |
| 5.5" |
165 - 195 |
145 - 165 |
| 6.0" |
185 - 215 |
155 - 185 |
| 6.5" |
195 - 235 |
165 - 195 |
| 7.0" |
215 - 255 |
175 - 215 |
| 7.5" |
225 - 275 |
195 - 225 |
| 8.0" |
245 - 295 |
205 - 245 |
| 9.0" |
275 - 325 |
225 - 275 |
| 10.0" |
305 - 365 |
255 - 305 |
