# Bicycle Mechanics - Check my math (ERD)

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View Full Version : Check my math (ERD)

urbanknight
03-28-08, 09:52 PM
Following the late Sheldon Brown's guide to measuring ERD of a rim, I came up with 624mm. That's 598mm nipple to nipple, 12mm per nipple, and 1mm per nipple washer. This is a very low profile Martano rim from the 70's, but I just wanted to check, does that sound like a possible number to come up with? At this low of profile, having a spoke more than 2mm too long would risk poking the tire. Thanks in advance.

Rowan
03-29-08, 06:06 AM
Yes, it sounds right -- 598+12+12+1+1 = 624. And probably you should credit Damon Rinard with the spoke calculator that just happens to reside on Sheldon's site.

Round down your final spoke result, too, but just remember you might need to do down another 1mm to get an off-the-shelf spoke of the right length.

urbanknight
03-29-08, 08:30 AM
Thanks (and sorry to Damon Rinard for not giving him credit, his component weights list is awesome too!), but I'm still confused as sources say a 700c rim can't have an ERD greater than 620. Do I have a 27" tubular rim? Anybody know what the outter diameter of a 27" tubular should be? I'm measuring 25" almst exactly.

Scooper
03-29-08, 09:32 AM
All tubular rims are 700C. Some manufacturers used to call them 27" just to confuse things. Since the bead seat diameter of 700C wheels is 622mm, I think your measurements are off.

Sheldon and Damon Rinard suggest measuring with the end of the spoke flush with the end of the nipple, while Roger Musson suggests measuring with the end of the spoke flush with the bottom of the nipple slot. I use Roger's method.

IMHO the best and most accurate way to measure ERD is to do it the way Roger Musson (http://www.wheelpro.co.uk/wheelbuilding/book.php) suggests:

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Wheels/MeasuringERD.jpg

vindicator
03-29-08, 11:52 PM
Since the bead seat diameter of 700C wheels is 622mm, I think your measurements are off.
Yeah I agree. 624 would put your nipples 1 mm inside your tube. Generally road rims are somewhere near the 600 mark. Less if they have a deep section and more if they are tubular but even those won't go higher than 615. 617 is the highest I've ever seen.

urbanknight
03-30-08, 02:21 PM
Thanks for the suggestions again. I tried the Roger Musson method with the measuring tape flat on the table, and measured it with the washers to be as accurate as possible. I came up with 622 ERD.

I'm actually starting to think this could be correct as a tubular rim doesn't really have a bead seat, so BSD is an imaginary term on a tubular. I measured the overall diameter of the rim at 634mm which seems about 4mm larger than the outside of a normal clincher. It also looks about 4mm larger than my clincher wheel when holding them side by side. Doesn't the rim diameter shrink a little when tensioned up anyway?

Keeping in mind this is a very shallow profile rim (the entire profile is the size of the braking surface on my clinchers), does this sound plausible?

Scooper
03-30-08, 02:59 PM
Thanks for the suggestions again. I tried the Roger Musson method with the measuring tape flat on the table, and measured it with the washers to be as accurate as possible. I came up with 622 ERD.

I'm actually starting to think this could be correct as a tubular rim doesn't really have a bead seat, so BSD is an imaginary term on a tubular. I measured the overall diameter of the rim at 634mm which seems about 4mm larger than the outside of a normal clincher. It also looks about 4mm larger than my clincher wheel when holding them side by side. Doesn't the rim diameter shrink a little when tensioned up anyway?

Keeping in mind this is a very shallow profile rim (the entire profile is the size of the braking surface on my clinchers), does this sound plausible?

Hmmm.. I don't know what you have, but you're right about tubulars not having a bead. I guess I should have said tubulars have the equivalent of a bead seat diameter of 622mm. 27" clinchers do have a bead seat diameter of 630mm, or 8mm larger diameter than 700C.

EDIT - This is from Sheldon's website:

"Tubular tires are mainly used for racing. A tubular tire has no beads; instead, the two edges of the carcass are sewn together (hence the term "sew-up") with the inner tube inside. Tubulars fit only on special rims, where they are held on by cement.

Tubulars existed in 6 different sizes, but only two of them are readily available these days.

- Full-sized tubulars fit rims of the same diameter as 622 mm (700c) clinchers. This size is sometimes referred to as "28 inch" or "700". It is also, confusingly, sometimes referred to as "27 inch." The "27 inch" designation is inaccurate and obsolete, but you'll sometimes run into it in older printed material.

In clincher tires, there is a real difference between "700c" and "27 inch" sizes, but for tubulars this is a false distinction. Whenever you see mention of "27 inch tubulars" the writer is actually referring to standard full-sized tubulars, as used on most racing bikes.

- "26 inch" or "650" tubulars are smaller, mainly used on time-trial or motorpacing track bikes.

- "24 inch", "22 inch" "20 inch" and "18 inch" tubulars are sizes formerly used for children's racing bikes, but pretty much extinct these days.

Tubulars are also sometimes called "sew-ups" or "tubs" (British usage.)
If you want to sound like an ignorant yahoo, call them "tubies" or "tubeless tires."

Exactly what make and model rims are you measuring?

urbanknight
03-30-08, 03:20 PM
The only marking is a label that says Martano in in a red bow tie shape with a multi-colored globe behind it. They closely resemble these rims in shape and color.
http://cgi.ebay.com/NISI-Campione-del-Mondo-Tubular-RIMS-36h-4-PAIRS-NOS_W0QQitemZ360038041181QQihZ023QQcategoryZ56197QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/NISI-Campione-Del-Mondo-tubular-rims-700C-36-hole_W0QQitemZ220217719187QQihZ012QQcategoryZ36144QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Sheldon Brown's site states that the outer diameter of a clincher rim will be approximately 8mm more than the 622mm bead seat diameter, so it sounds reasonable that 700C tubulars will measure around 630mm. That also allows an ERD greater than 622 since the nipples could come in where the bead would normally be. Add in the washers, and we're getting pretty close to that 630mm diameter anyway.

I'm just hoping someone can tell me it makes sense or that I'm missing something before I order the spokes.

urbanknight
03-30-08, 03:21 PM
This listing also makes me believe I'm doing something right since the spoke length calculator tells me I need 302, 301, and 300 spokes for the build.
http://cgi.ebay.com/DT-stainless-steel-spokes-NEW-for-nisi-martano-fiamme_W0QQitemZ350038759930QQihZ022QQcategoryZ36144QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

edit: And this looks exactly like the rims I have, with a slightly different label.

Scooper
03-30-08, 03:43 PM
If you accurately measured the rims you have using the Roger Musson method and got an ERD of 622mm, then that's what I'd use, but looking at Damon Rinard's rim database I don't see any tubular rims with ERD greater than 618mm. Mavic's Reflex tubular rims, for example, have an ERD of 605mm.

I dunno...

urbanknight
03-30-08, 04:04 PM
Thanks. I guess I'll just hope I don't have to return the spokes! Thinking about calculating it for 618 and ordering extra long nipples just in case.

Scooper
03-30-08, 04:33 PM
Thanks. I guess I'll just hope I don't have to return the spokes! Thinking about calculating it for 618 and ordering extra long nipples just in case.

That's probably a reasonable plan. Just remember that longer nipples means you'll need more threads on the spokes. I usually measure everything as accurately as I can, plug the numbers into spocalc, then round down to the nearest millimeter for spoke lengths. I'd rather have them a little shorter than any amount too long. If they're too long, the risk is that you'll bottom out on the threads before achieving the desired tension. In my experience, that's much more likely to happen than having the spokes poke holes in the tube.

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Wheels/SpokesandNipples.jpg

urbanknight
03-30-08, 04:37 PM

urbanknight
03-30-08, 04:43 PM
While I've got people's attention, does anyone know where to find recommended tensions for these old rims? I learned how to build by feel, but with 24 spokes on an older, lightweight rim, I don't want to screw it up. The rider weighs 145lb so it shouldn't be a problem, but I know what happens when you overtension low spoke count wheels.

Scooper
03-30-08, 04:59 PM
I know you're using washers, but do the spoke holes have eyelets?

urbanknight
03-30-08, 06:05 PM
Nope, no eyelets.

Scooper
03-30-08, 07:47 PM
Nope, no eyelets.

I would guess the spoke tension would be fine in the 90-105 kgf range for the front wheel and the rear wheel drive side. Depending on the rear wheel dish, the non-drive side tension will be ~70% of the drive side tension.

This is the range Mavic recommends for the 32 spoke Open Pro, which admittedly is probably a little stronger rim than your older tubular rims, and the OP has eyelets.

Attached are some typical spoke tensions from a chart on the Park Tool website (http://www.parktool.com/repair/readhowto.asp?id=128).

urbanknight
03-31-08, 12:17 PM
Thanks. I poked around on the Park site and didn't catch that file. I was figuring somewhere around 100 anyway, but wanted to be sure.

urbanknight
04-02-08, 11:17 PM
Update: I used the 622 I measured and rounded each number down, then ordered the spokes. They came in today and I did a dry build to check. They are just right!