January 01, 2004
Custom Wheel Dimensions
Knowing wheels inside and out eases the critical identification process and boosts customer satisfaction
By: Mike Mavrigian
DETERMINING FASTENER SIZES
All too often, wheel fasteners are treated as mere afterthoughts, when in fact they´re one of the most critical components on any vehicle. Wheel fasteners provide the only means of attaching the wheel/tire package onto the vehicle, whether that involves nuts threading onto hub studs or bolts threading into hubs. If wheel fasteners loosen or come depart, the wheel and tires will fall off of the vehicle while it´s being driven down the highway.
Here are the basics of how to identify various nut, stud and wheel bolt sizes.
1. Fractional pitch. The pitch number refers to the number of threads found in one inch of that fastener´s length. For example, a 1/2-inch x 20 bolt would be a 1/2-inch diameter with 20 threads per inch of the bolt length (1/2-inch USS coarse would have 13 threads per inch). A 9/16-inch x 18 would be a 9/16 inches diameter with 18 threads per inch (9/16-inch USS coarse would have 12 threads per inch).
When dealing with fractional fasteners, the higher the pitch number, the finer the threads. Common fractional-sized wheel fasteners include 7/16-inch x 20, 1/2-inch x 20 and 9/16-inch x 18.
2. Metric pitch. The pitch number refers to the distance between threads (a 1.25 thread features threads spaced 1.25mm apart). It´s just too difficult to measure the small distance between two individual threads, so it also can be determined by the number of threads within a 10mm length, divided by 10. For example, if a bolt has eight threads within a 10mm length, it´s a 1.25 pitch.
When dealing with metric fasteners, the lower the pitch number, the finer the threads. If you want to look at metric fasteners in a "fine to coarse" manner, 0.75 pitch would be very fine, with pitch numbers running up the scale (1.0, 1.25, 1.5, 1.75, etc.) toward more "coarse" thread. Most metric wheel fasteners will feature a 1.25 or 1.5 pitch.
Considering that most metric wheel studs or bolts will feature a diameter of 12mm to 14mm, a 1.5 pitch would be considered relatively "fine" to "medium" (once again, pitch "fineness" is relative to shank diameter). Common metric-sized wheel fasteners include 10mm x 1.25, 12mm x 1.25, 12mm x 1.5, 12mm x 1.75, 14mm x 1.5 and 14mm x 2.0.
YOU DO THE MATH
The formula for determining rim, or bead seat, diameter is as follows:
Overall wheel diameter - (bead seat height x 2) = rim diameter
The formula for determining backspace distance and whether it´s positive or negative is as follows:
Backspace - (wheel overall width/2) = offset
WORDS TO WORK BY: Quick-reference wheel dimension definitions
Wheel diameter: diameter of the bead seat area of the rim (not measured at the overall rim diameter).
Rim width: width measured from the front bead seat wall to the rear bead seat wall. Not measured at the overall wheel width.
Backspace: Distance from the wheel hub mounting face to the rear outer edge of the rim.
Offset: the difference between the true center of the rim width to the location of the hub mounting face. If offset pushes the tire outward relative to the hub face (deep-dish appearance on the outside), this is negative offset. If offset locates the tire inboard relative to the hub face, this is positive offset.
Bolt pattern: This identifies the number of fastener holes along with the diameter path of the hole placement. A 5 x 100 bolt pattern indicates five mounting holes, with hole centers oriented along a 100mm diameter.
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