SON Rim ERD Gauge

Take the guesswork out of spoke length calculations with SON’s easy-to-use ERD Gauge. To calculate the correct spoke lengths, it’s essential to know the Effective Rim Diameter (ERD) as well as various dimensions of the hub. Unfortunately, manufacturers’ specs are not always reliable and should be double-checked. The hub’s dimensions can be measured with calipers, but the rim diameter is more difficult to establish. There are different methods of measuring ERD, using spokes and a tape measure or a ruler, but it can be tricky to obtain the required accuracy. 

The SON Rim ERD Gauge makes this easy and quick: Simply insert the long rod into one spoke hole and through the opposite spoke hole until it bottoms out. (One end is shaped like a spoke nipple.) Slide the gauge onto the rod and read off the ERD. 

The tool also works for small rims: Markings on the rod make it easy to subtract 100, 200 or 300 mm from the measurement to obtain the correct ERD.

Note: The tool works for most rims. For large rims, including many classic 700C rims, use the Large-Rim Adapter, which extends the measuring rod by 10 mm.

Pro Tip: Some online spoke length calculators assume you’ll input the actual measurement for the ERD, while others require adding 1-2 mm for the thickness of the nipple head. Similarly, some rim specs include the nipple head in the ERD measurement, while others don’t. When in doubt, use SON’s calculator, which is designed for the actual measurement of the ERD—the value you read off the tool. The calculator is intended for front wheels with generator hubs, but it works for rear wheels, too: nabendynamo.de/en/service/spoke-length/