ERD. What is it? Can I trust it? How do I measure it?Updated 7 months ago
ERD: Can I trust it?
Unfortunately, there are plenty of rims from different companies with “ERD” on the label that report only the diameter of the rim’s nipple support surface, without accounting for any nipples. So sadly, no, you can’t trust ERD from all rim makers.
Stan’s rims with labeled ERD accounts for nipples in addition to the diameter of the rim’s nipple support surface. When building Stan’s rims using Sapim Polyax or Double Square nipples (or similar), Stan’s ERD can be input directly into common spoke length calculators to calculate accurate spoke lengths.
Background
“ERD” stands for “Effective Rim Diameter” and is useful for calculating spoke length. ERD is defined as the diameter of the imaginary circle that goes through the ends of all the spokes when they are of the perfect length.
Because most nipples work best with some spoke length threaded inside the nipple head, the perfect spoke length extends a bit beyond the rim’s nipple support surface and into the nipple head. That means ERD is not a rim dimension, it’s a wheel dimension. Adding the appropriate extra length for the spoke to extend properly into the nipple means ERD depends on the dimensions of both the rim and the nipple.
The rim dimension that affects ERD is the diameter of the Nipple Support Surface or NSS. This is the diameter, on the rim, of a circle that goes through the areas that support each nipple.
The nipple dimension that affects ERD doesn’t have a specific name, and it varies depending on the details of the nipples selected. In practice, common nipples like Sapim Polyax or Double Square or similar need about 1.5mm extra spoke length to adequately engage the nipple head. Adding this to the rim’s nipple support surface gives about 3mm more in diameter.
Tip: When in doubt, measure the ERD on your individual rim before choosing spokes. One way to measure your own ERD is to use two spokes with nipples as measuring devices stretched across the rim diameter, similar to the way many of us used to use Rim Rods. For 29er wheels, 250mm is a convenient spoke length for this purpose. Follow these steps:
Identify opposite spoke holes across the diameter of the rim. Count to be sure.
Install your two spokes with nipples in these rim holes.
Engage each spoke’s threads in its nipple, just the way you want it to be (even with the bottom of the slot, or even with the top of the nipple, or however you want the spokes to be engaged).
Bring the spokes’ J-bend ends toward the center of the rim as if radially laced to an imaginary hub, and measure (with calipers or similar) the gap across the J-bends.
Add it all up: 250 + 250 + caliper reading. This is your ERD, with your nipples, engaged just the way you want.
Because rims aren’t always perfectly round, we recommend measuring at several places around the rim.