Bicycles, like cars, have a number of dimensions that are important to the accurate construction and maintenance of the bike. One such dimension is the effective rim diameter, or ERD, measured on the rims of your bike tires. Knowing the ERD and how it is used is just another part of bicycle maintenance.
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Bicycle rims are the metal circles that form the shape of your wheels. The tire, with tube inside, fits securely onto the rim. The rims are connected by thin rods, called spokes, to the hub at the center of the wheel. All of the spokes must be aligned properly and tightened evenly to the rim to ensure that your wheels do not wobble as they spin.
The effective rim diameter, ERD, is the length measured as the diameter across the rim, minus the length of two of the nipple heads that connect the rim to the spokes. Most road bike rims measure 26 or 27 inches across, while the nipples measure about 12 to 16 mm, making the ERD of most road bikes somewhat less than 26 or 27 inches.
Calculating Spoke Length
To calculate spoke length, you need to know the ERD and several other dimensions for the rim, the hub and the spokes. Since the measuring of these dimensions accurately can be difficult, there are online spoke length calculators that have databases of wheels. You can select your make and model, and the calculator will automatically enter your dimensions. You may wish to seek instruction before measuring your wheel.
Rims used to be made of steel but are now almost always constructed of extruded aluminum. The extruded piece is formed into a circle, then welded or otherwise joined together. Spoke holes are poked in the rim and sometimes reinforced with eyelets. A wheel will have anywhere from 16 to 40 spoke holes, depending on the kind of bike and whether it is a front or rear wheel.