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The Precision Rim Diameter System

Measure your own ERDs (Effective Rim Diameters)!

Never rebuild a wheel again because you started with a bogus ERD. Our system allows you to measure the ERD yourself quickly and accurately. The system comes with a patent-pending rim diameter tape made of long-lasting dimensionally stable plastic, and a high quality depth gauge.

Measure an ERD in 3 steps!

1 Measure the distance form the outside edge of the rim to the bottom of the screwdriver slot in the spoke nipple (12.5 mm here)
2 Wrap the tape around the rim.
3 Find the 12.5 mm mark through the overlapping tape, and match that to 548 mm. 548 mm is the ERD.

Want to see it in action?

Watch as its creator, Howard Sutherland, measures an ERD with his Precision Rim Diameter System:

Published ERDs aren't reliable because...

  1. A rim model may be updated without changing the name. A change in shape may change the ERD.
  2. When the rim was manufactured was the die used for extruding the rim new or was it worn and near the end of its life cycle? A worn die can cause the rim to be thicker thereby needing a slightly longer spoke.
  3. Are the dimensions of the nipple you are using exactly the same as the one used to measure the published ERD?
  4. Is the ERD to the top or the bottom of the screwdriver slot in the nipple?

The math behind the Precision Rim Diameter System

(optional, for math junkies. You don't need to understand this to use Sutherland's Precision Rim Diameter Tape.)

The Effective Rim Diameter (ERD) is used by spoke calculators to determine the correct spoke length. The ERD is the diameter of the rim measured between the ends of two spokes on the opposite sides of the rim.

Sutherland's Precision Rim Diameter System determines the ERD by measuring the outside circumference of the rim and then subtracting the distance from the outside edge of the rim to the place you want the end of the spokes of the built up wheel to be. The tape measures the circumference but reads in diameter. You may remember from geometry that the circumference of a circle divided by pi gives you the diameter. Each unit on the rim diameter scale is π (3.14159265) millimeters apart. When you wrap the tape around the rim the scale reads in millimeters of diameter at the vertical line where the zero points overlaps. The spoke end diameter scale goes in the opposite direction from the zero point and subtracts the distance from the outside edge to the spoke end two times (once for each side of the rim). The ERD is then read on the rim diameter scale. Each unit of the spoke end correction scale is (6.2831853) millimeters apart. To sum it up the rim diameter scale measures the outside of the rim and the spoke end scale subtracts the distance from the outside of the rim to where you want the end of the spoke to be. The result is the ERD.