Rim Rods are still the most practical way to calculate ERD (Effective Rim Diameter). We (Wheelsmith) introduced the first many decades ago. Today, the best set you can find is by BSC.
They are recommended on Cycling Tips here. We offer the larger set—2 x 350mm rods, one plain rod, one with ruler markings from 700mm down to 400mm. Suitable for 700c, 650c, and 26" wheels.
Features
no calculation required, just read off the ruler scale where the rods overlap
can still measure straight for ERD on offset rims
stainless steel for minimal rod flex and long life
two clips supplied so you can secure the rods together - you can measure the overlap manually if it does happen to be longer than the marked ruler
