(Pitch Circle Diameter, ) and spigot size. The PCD is easy to
match as this relates to the number of studs you need to hold the wheel on
the car. The ACTUAL meaning is the diameter of the studs from the centre
of the wheel.
Spigot size is a bit
trickier....the spigot is the bit in the centre of the hub that you rest
the inside centre of the wheel on whilst aligning the studs and screwing
back the wheel nuts. On generic after market wheels, the spigot hole
inside the wheels is a lot bigger than the spigot on the car. So what you
need to do in this case is fit spigot locating rings. These are just rings
of aluminium or hard plastic, that fit over the spigot on your car and
then have a proper fit with the spigot hole on the wheel. If you don't
have the spigot taking all the weight of the car, chances are you'll break
one or more studs when you drive the car hard or have to brake hard.
Remember the wheel nuts are simply there to hold the wheel on, NOT support
the weight of the car. Also, as there is nothing to centre the wheel,
you'll notice the wheels go in and out of balance because as you drive
around, they'll move around on the hub.
It is quite normal to
alter the size of wheel when fitting alloys. When changing to alloys - you
can replace with the same size of wheel, and keep the same size tyre, or
move up to 13" wheels. The larger the wheel and the smaller the tyre
profile the more impact the wheel chosen will have on the overall look of
the car.
The important thing is to keep the same overall tyre
diameter. This is done by using a lower profile tyre. Increasing the tyre
width and reducing the profile height will give the vehicle different
handling characteristics. The car will be a lot more responsive when
cornering, but a lot more sensitive to bumps and irregularities in the
road.
To give you an indication of what can be done if you want to
change to a bigger wheel.Take the first number of your tyre size (this is
the width of the tread) and multiply it by the second number (this is the
profile height expressed as a percentage) and then divide by 100. You now
have the height of one sidewall in millimetres.
Multiply it by 2
(for top and bottom sidewalls) and divide this by 25.4 to convert to
inches and add it to the third number of your tyre size (the rim diameter
in inches) to give the overall diameter in inches.
If you want to
convert back to millimetres multiply by 25.4
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:
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 below.
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 Pitch Circle Diameterof 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.Just because a
wheel from one vehicle has the same PCD and offset as the wheel from
another does not mean they are interchangeable - the centre bore of the
wheel and hub must also be the same to ensure centralisation of the wheel,
and the shape of the spokes must ensure clearance of the brake calipers.
Many manufacturers use the same wheel fitments as others, but some are
unique.
Wheel Location on
Hub
Adaptor / Spigor Ring Arrangement 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 diagram.
As an example, TSW Spigot
rings should always be fitted without the 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
Legal and illegal
wheel/tyre combinationsVehicle 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’.
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.
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.
When you have bought your
new alloy wheels, you will need the special nuts to fit them to the hub.
Stud threads vary, so dont rush out and buy the first set of wheel nuts
you see at a bargain price because they may not fit and will strip the
thread on the studs. Then you'll need to buy new studs and either fit them
yourself or pay a garage to do it for you.
Alloy wheels need
special studs so that they fit inside the indent in the wheel and tighten
up onto the stud properly. Normal nuts used on steel wheels are not
suitable, as the nut will probably only go halfway down the stud, and when
you brake suddenly or with normal driving over a period of time the nuts
will work loose, or wear away the alloy hole in the wheel making the wheel
useless, and the worse scenario would be the studs breaking and the wheel
coming off completely.
Rover Alloys use Rover nuts, many other
manufacturers use either generic nuts or special nuts to fit their own
brand of wheels. These nuts are not interchangeable, you can't use Rover
nuts on some alloy wheels, and vice versa.
You should also ensure
that if using wider wheels that they do not protrude outside the line of
the car, as in the diagram below.
Check you have the
right specification alloys and buy locking wheel nuts so you hold on to
them!
