Hello Folks
I have a set of old rims that I am going to build several wheels from. I am attempting to calculate erd (effective rim diameter) so I can then calculate the spoke lenth. The numbers that I keep coming up with are around 506.4 mm. Is it likely that these are 507mm rims considering the tires that were on there were 24 X 1.75 in size?
thanks,
jake
How do you calculate the ERD? I would measure it: take two spokes, shorten them to 20cm, screw them into nipples such that the end sits where the nipple would be seated in the rim and glue them to that position. Then, use the two spokes and a ruler to measure the ERD, at least twice around the wheel.
Oh, and if you can measure the ERD within 1/10 of a millimetre with any method, please tell me how (not that this is important, many shops only stock 2mm increments in spoke length).
Jack/Jake: see arrows in the pic below:
[attachment=4339]
It is the surface that inner diameter of the tire seats on.
Nigel
Jake;
Those rims look a bit scary. The original plating and the rust need to be removed, which may not leave much metal thickness on the rims. Oxalic acid, available from auto parts stores for rust removal will take the rust off; not sure about the plating. Alternately, you could take them to a plating shop to discuss your options.
For ERD; measure from the BSD; only need one spoke nipple. PM or e-mail me to remind me; and I'll send you a sketch on how.
Nigel
[attachment=4341]
Tools required: calipers (Harbor Freight; $9.99 with coupon) and small flat piece (metal, plastic, wood, etc) with flat and parallel sides.
Place flat across edge of rim, measure down the bead seat surface; record number (or zero the caliper), next measure to the end of the nipple dropped in a spoke hole. The difference between the two measurements is equal to (BSD-ERD)/2
Example: if your BSD is 507mm, and the difference between the two measurements above is 7mm; then your ERD is 493mm.
Repeat the measurement several times around the rim, and take an average.
Nigel