What is E.R.D.?

The uni.com description for the 24" BFR rim shows the following.

406mm-394 E.R.D., 507mm-492 E.R.D.

510 grams (406mm), 635 grams (507mm)

Can anyone turn this into something I can understand. Why the different weights if this is supposed to be a 24" rim?

Thanks.

ERD is the Effective Rim Diameter. This is the actual diameter used to calculate spoke length. It is measured from the outside of one spoke nipple to the diametrically opposite spoke nipple.

406mm-394 E.R.D., 507mm-492 E.R.D.

The 406mm-394 E.R.D. refers to a 20 inch rim, which actually has a Bead Seat Diameter (where the tire clamps to the edge of the rim) of 406mm. The 507mm-492 E.R.D. refers to a 24" rim. A 26" rim is 559mm, a 29er is 622mm.

510 grams (406mm), 635 grams (507mm)

The 20 inch rim weighs 510 grams, the 24 inch rim weighs 635 grams.