ERD Calculate
ERD: Effective Rim Diameter
ERD Calculate
To determine the proper spoke length for a wheel build, start with the rim’s ERD. ERD is short for effective rim diameter, it is the maximum distance that spokes bridge going from one end of the rim to another.
How do we calculate the ERD of our rims? Below are calculation equation and sketch for your reference:
Please note that we measure the ERD from the nipple bed on one side of the rim to the nipple bed on the other (refer to the graphic below), which is different from a nipple head-to-head ERD measurement.
ERD=Rim outer diameter-(rim depth)*2+(Rim nipple bed thickness)*2
For example：
ERD of RD28-22C rim=633mm-22mm*2+3.5mm*2=596mm
Most people prefer ERD including the length of the nipple head.
However, customers may use different nipples and components for building.
Please add the length of the nipple head (2.5mm~3mm generally) for your spoke calculation.
If the length of the nipple head is 3mm, then ERD of your RD28-22C rim is 596mm+3mm*2=602mm
When using different nipples minor changes may need to be made to the calculated spoke lengths. If using an online spoke calculator, round calculations up to the nearest mm.
(1) External nipples with a standard head: most online calculators can use our ERD to get an accurate spoke length.
(2) Internal nipples: add 8mm to the results if using 10mm-long nipples, 10mm for 12mm nipples, and 12mm for 14mm nipples.
(3) External nipples with significantly raised heads (such as DT Swiss Squorx, or Double Square): add 2mm to the results for correct spoke lengths.
