What is ERD (Effective Rim Diameter)?

ERD is the diameter of the circle formed by the spoke nipple seats in the rim. It is measured from the bottom of one nipple seat across to the opposite side. Accurate ERD measurement is critical — a 1mm error in ERD produces a 0.5mm error in spoke length. Measure by inserting two spokes and measuring the distance between their tips.

What is a 3-cross lacing pattern?

In a 3-cross pattern, each spoke crosses over 3 other spokes from the same hub flange before reaching the rim. This is the most common pattern for touring and bikepacking wheels — it provides good strength, even tension, and shock absorption. Radial (0-cross) is lighter but less strong.

How accurate do spoke lengths need to be?

Spokes should be within 1-2mm of the calculated length. Too short and the nipple will not engage enough thread. Too long and the spoke protrudes through the nipple and can puncture the tube. Most spokes are available in 1mm increments. Round to the nearest available size, preferring slightly shorter.

What spoke count should I use for bikepacking?

For bikepacking with heavy loads, 32 or 36 spokes per wheel is recommended. 28 spokes is the minimum for loaded touring. More spokes distribute the load better and make the wheel more durable. If one spoke breaks, a 36-spoke wheel is still rideable; a 24-spoke wheel is not.

CampCalcs

Spoke Length Calculator

Calculate spoke lengths from hub and rim measurements for wheel building.

Rim ERD

Effective rim diameter

Hub Flange Diameter (PCD)

Centre to Flange

Hub centre to flange face

Spoke Hole Diameter

Total Spoke Count

Cross Pattern

How We Calculate This

Spoke Length Formula

L = sqrt((ERD/2)² + (FlangeD/2)² + Offset² - ERD x FlangeD/2 x cos(Cross x 720/Spokes)) - HoleDia/2

This geometric formula calculates the straight-line distance from hub flange to rim nipple seat, accounting for the crossing pattern angle.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify conditions and use proper safety equipment outdoors.