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 two sets:
700mm pair—2 rods, one marked from 700mm to 400mm—for 700c, 650c, and 26".
500mm pair—2 rods, one marked from 500mm to 270mm—for smaller 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
