Tools
- Metric Calipers
- Wheelsmith Rim Rods
- Ruler
- Screwdriver
The rim ERD can be measured simply with the Wheelsmith Rim Rods but placing the two rods in either side of the rim so that they meet and cross in the middle. The total overall length of the rods is 700mm. what you would do then is measure the over lapping portion of the rod and minus it from 700 to give you ERD. if you don’t have the wheelsmith Rim Rods then follow the next 3 steps.
- Carefully measure the outside diameter of the rim, from outside edge to outside edge, being careful to measure exactly in the center across the rim. To be sure the rim is round, measure in at least two locations 90 degrees apart. If the two measurements are different, measure in a third location and average the three measurements.
- Next, drop a spoke nipple into any hole in the rim. Using the depth gauge of the measuring caliper, measure from the top edge of the rim to the base of the screwdriver slot of the nipple. This represents where the end of the spoke should be when the wheel is fully tensioned.
- Using the measurement obtained in step 2, multiply the result by 2 and subtract that number from the measurement obtained in step 1 (the outside rim measurement). The result will be the Effective Rim Diameter (ERD).
This is the measurement in millimeters from the center of one spoke hole to another on the opposite side.
Step 3. Enter the Hub Center To Flange Center Dimension
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You must first measure the overall axle length; a standard Mountain bike will be 135. Take that number and divide it by 2 giving you your center of 67.5. Now from the center of the hub measure to the left flange. This will give you the left center to flange. Repeat for the right side. This method is good to use because you can adjust the dish of the wheel. Some manufactures are using a dishless rear wheel and they achieve this by making the rear end of the bike out of symmetry. Giving you a stronger rear wheel. All you would need to do for this is compensate in you center point of the hub.
Another way to easily measure the center to flange is by measuring from the axle to the center of the flange then subtract that number from half of the overall axle length. for example your hub has an axle length of 135mm divide that by 2 gives you 67.5 and if we measure from the center of the flange to the axle and get say 25 we would then minus 25 from 67.5 giving us 42.5
This is the number of holes that are in the hub or the rim even if you are finding the length of only one side of the hub.
This is how many times a spoke will intersect another spoke. The standard mountain bike wheel is a 3 cross were as the standard BMX wheel is a 4 cross. More crosses create a longer spoke, more tangent spoke, and more torsionally efficient wheel. Less crosses require a shorter spoke, but is less torsionally efficient.