A standard measurement
for a wheel is given using several figures, e.g.
8 x 18, 4 x 100, 35
This can be broken down as follows:
8 x 18 represents a wheel which is 8 inches wide and 18 inches in
4 x 100 represents the number of studs/bolts and the PCD in millimeters (explained later).
35 represents the
Offset (sometimes referred to as the ET).
PITCH CIRCLE DIAMETER (PCD)
The number of bolts used to install a wheel varies from 3 to 8 studs depending on
the vehicle make and model. The pitch is the diameter (mm) of the circle that intersects the stud centres. To fix a wheel
correctly, the number of studs and the pitch should be the same on the vehicle and the wheel.
The following calculations can be used to work out the PCD:
||Length 'X' x 2|
||Length 'X' x 1.7012|
||Length 'X' x 1.154|
Common Stud Patterns:
|3 x 98
||4 x 95.25
||4 x 98|
|4 x 100
||4 x 108
||4 x 114.3|
|5 x 98
||5 x 100
||5 x 108|
|5 x 110
||5 x 112
||5 x 114.3|
|5 x 120
||5 x 127
||5 x 130|
|5 x 139.7
||5 x 150
||6 x 139.7|
OFFSET (ET) EXPLAINED
THREAD ENGAGEMENT RULES
For European nut and bolt engagement approvals, 6 turns thread
engagement is required for M12x1.5/M14x1.5 and 9 turns for M12x1.25
Example shown opposite:
Stud length =
Nut = M12x1.5
Bolt Hole Depth = 10.5mm (22 minus 11.5)
Thread Engagement = 7.6 turns (10.5/1.5)
|The following points must be adhered to BEFORE fitting any wheel and tyre:|
with the wheel manufacturer that the intended wheel style has sufficient caliper clearances, the correct offset/PCD and accessories
have been supplied for the intended vehicle. Wheel manufacturers will rarely credit if tyres are fitted.
the maintenance and care procedure supplied by the wheel manufacturer to ensure the maximum life of the rims can be obtained.
the vehicle hubs for hub protrusion before mounting the tyres on the wheels, e.g. location pins, temporary fixing screws and
hub balance weights. In the event of this, check with wheel manufacturers for the correct wheel application.
the tyres have been fitted, check for air leakage between the tyre and rim flange and check the air valves. Quite often valves
are supplied for the rims. These valves must be used in the fitting procedure.
Apply the correct tyre pressure
stated in the vehicle handbook or recommended by the tyre manufacturer. Incorrect tyre pressure will result in poor tyre wear.
that the correct torque setting is applied. This procedure must be repeated within 50 miles (80 km). Over-tightening of the
nuts/bolts can deform the wheel stud holes. Lack of torque will result in the wheel separating from the vehicle hub.
There are several types of wheel securing nut/bolt seatings. 60-degree cone seating being the most common (radius seatings
are applied by German vehicle manufacturers and flat seatings are often applied by Japanese manufacturers). A greater percentage
of aftermarket wheel applications use the 60-degree type of seating. Note: mixing of seating is highly dangerous.
It is against the law to drive a vehicle with a wheel and tyre combination the protrudes outside outside the vehicle body.
When upstepping it is important to retain the rolling diameter
as specified by the manufacturer. The tolerances for the rolling diameter are governed by ETRTO.
the wheel diameter, it is necessary to alter the aspect ratio to match the overall gearing within the tolerance of ETRTO.
TYRE PART NUMBER EXPLAINED
15 86 H
||1) Tyre width in millimeters|
||2) Tyre aspect ratio (percentage of tyre width)|
|165 S R
||3) R = Radial construction|
|31 x 10.50 R
||4) Tyre diameter (also known as applicable rim diameter)|
||5) Load index|
||6) Speed symbol|
||7) Tyre outer diameter in inches|
||8) Tyre rating (tyre structure-ply rating)|
||9) Tyre internal diameter in millimeters (millimetric tyres only)|