 |
Home
|
| 
Technical Aspects of Alloy Wheels
|
Wheel Size
Two measurements, the Rim Diameter and the Rim Width,
normally determine the wheel size. This is best explained
with reference to the diagram shown on the right:
|
Cross Section of Wheel |
| Offset |
The ‘Offset’ of a wheel measures the distance
between the wheel centreline and the wheel mounting face
and is measured in millimetres. It is extremely important
that wheels of the correct offset are used in order to
maintain the correct track of the vehicles.
Offset can be Positive or Negative and is
best illustrated in the two diagrams on the left. Most
Modern vehicles are front-wheel drive, and as such generally
require positive offset wheels. The main exception to
this rule is 4x4 vehicles, which often use negative
offset wheels.
|
| Clearance |
|
It is, of course, extremely important that the clearance
 of
both the vehicle body work and steering/suspension components
are maintained. Failure to do so would most likely cause
damage to both the tyre/wheel assembly as well as the
body of the vehicles. Clearance is achieved by selecting
a wheel of the correct size and type of tyre. Quality
alloy wheel suppliers can give such information.
Pitch Circle
Diameter (PCD)
The P.C.D. can be defined as the diameter (in millimetres)
of an imaginary circle drawn through the centre of the
stud holes on the wheel and/or the vehicle wheel hub.
When new wheels are required, it is essential that
they have the correct P.C.D. for the vehicle concerned.
|
| Wheel
Location on Hub |
Motor vehicle manufacturers use a central location collar
on the stub axle hub in order to accurately locate the
wheel. The wheel collar diameter varies, depending on
the make of vehicle.
Many replacement allow wheel manufacturers use an adapter
(spigot ring) to vary the diameter of the locating hole.
In this way, a particular wheel can, by changing the
spigot ring, be used on a variety of vehicles.
The adapter/spigot ring arrangement is illustrated
in the diagrams to the left.
As an example, TSW Spigot rings should always be fitted
without he tapered edge facing the vehicle hub. This
is to allow easy and accurate location of the ring onto
the hub collar.
|
| Upstepping
to High Performance |
 Vehicle
performance can be maximised by selecting a lower profile
tyre and larger wheel diameter combination. Care must
be taken to select the appropriate replacement wheel /
tyre combination to avoid problems.
For example, it is a legal requirements that the tyre
and wheel assemblies stay within the body of the car.
The fitting of
larger diameter replacement wheels is referred to as
‘up-stepping’.  ‘Plus
One’ or ‘Plus Two’ denotes how much larger
the wheel diameter is.
This approach of ‘up-stepping’ allows the
tyre section width to be increased whilst maintaining
the correct overall diameter. This allows the optimisation
of cornering force and grip whilst maintaining the original
gearing of the vehicle. One example of this is:
|
|
|
195/60
R 14
|
14
inch diameter
|
Original
size
|
|
205/50
R 15
|
15
inch diameter
|
Plus
One
|
|
205/45
R 16
|
16
inch diameter
|
Plus
Two
|
|
205/40
R 17
|
17
inch diameter
|
Plus
Three
|
All of the above have an overall rolling
diameter which is within an acceptable tolerance.
|
|
 Aspect
Ratio
Aspect ratio is the ratio of section height to section
width. It should be noted that in general, tyres with
a lower section height (i.e. where the aspect ratio
is lowered) have a higher cornering force and therefore
improved performance-handling characteristics.
Source: NTDA
|
|
|
|
|
