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A standard measurement for a wheel is given using several figures, for example: 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 diameter.
- 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:
6-stud - Length 'X' x 2
5-stud - Length 'X' x 1.7012
3-stud - 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
Offset is the measurement in millimeters from the wheel centre line to the wheel mounting face. More often than not, offsets are positive(+) where wheel styles tend to be flush or convex. Negative (-) offsets tend to have concave styling.
WARNING: Incorrect offset can seriously affect vehicle performance.
Fig. Rim Measurements
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 above:
Stud length = 22mm
Nut = M12x1.5
Bolt Hole Depth = 10.5mm (22 minus 11.5)
Thread Engagement = 7.6 turns (10.5/1.5)
The following points should be adhered to BEFORE fitting any wheel and tire:
Check 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 tires are fitted.
Read the maintenance and care procedure supplied by the wheel manufacturer to ensure the maximum life of the rims can be obtained.
Check the vehicle hubs for hub protrusion before mounting the tires 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.
Once the tires have been fitted, check for air leakage between the tire 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 tire pressure stated in the vehicle handbook or recommended by the tire manufacturer. Incorrect tire pressure will result in poor tire wear.
Ensure that the correct torque setting is applied. 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.
Note: 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.
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.
When increasing the wheel diameter, it is necessary to alter the aspect ratio to match the overall gearing within the tolerance of *ETRTO.
TIRE PART NUMBER EXPLAINED
1) Tire width in millimeters
2) Tire aspect ratio (percentage of tire width)
3) R = Radial construction
4) Tire diameter (also known as applicable rim diameter)
5) Load index
6) Speed symbol
*ETRTO or E.T.R.T.O. is a European organization founded in October 1964 whose principal objectives are to further align of national standards and ultimately to achieve interchangeability of pneumatic tires, rims and valves in Europe as far as fitting and use are concerned; To establish common engineering dimensions, load/pressure characteristics and operational guidelines and to promote the free exchange of technical information appertaining to pneumatic tires, rims and valves.
Modified by svander at 10:03 PM 11-14-2006