ERD and Spoke Length

I laced up a hub motor to a Rim that was 604 mm ERD a few months ago,

However

I just got a new rim, same outside diamater, but the ERD is 605 mm

Does that mean that the new spokes need to be longer or shorter ?

( I forgot all the other measurements to use a spoke calculator, like the Hub Flange Diameter, and Hub Center to Flange center, Hub Spoke Hole Diameter,
I still have the hub laced onto the rim that is 604mm ERD so I cannot measure these ) , the hub is the Q100c CST.

the Number of spokes and cross pattern are the same .

604 to 605 ERD could use an extra 1mm length spoke. Personally, I don’t worry about that much short. In most cases, I’d rather have spokes 1-2mm too short than 1-2mm too long.

I just spent the last hour researching, and calculating and came up with 250.4 mm spoke length , for the Q100c CST laced to a 605 ERD 700c Rim , but that is eyeballing the measurements .
I went back to another thread, and read that you use 251 mm spokes on your Q100 builds .

Here are the instructions I used for calculating the spoke length .

https://leonard.io/edd/howtomeasure

http://www.bikeschool.com/tools/spoke-length-calculator

Ykick , after my last build / lacing up spokes to the little Q100c motor, I agree with you about it being better to have the spokes 1-2 mm shorter , since my last spokes of 252mm for one side and 253 mm for the other side on the Q100c CST hub and 604 ERD 700c wheel / one cross, was just a little long , the spokes threaded up about 1mm to 1.5 mm out through the top of the nipple , it worked out OK , only because of the rim being double wall.

Is there another place besides Dan's Comp , that sells 13/14 gauge spokes ?
( 13 gauge at the hub and 14 gauge at the nipple )

Ykick said:

604 to 605 ERD could use an extra 1mm length spoke. Personally, I don’t worry about that much short. In most cases, I’d rather have spokes 1-2mm too short than 1-2mm too long.

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ERD is diametral; spoke length is radial. 1mm difference in ERD makes 0.5mm difference in spoke length.

You should be fine, lace it up and see. If the rim spins true and is properly tensioned, the possibility exists that one or several spokes will have the threaded tip poking out above the nipple just a hair. I would not hesitate to grind down that protrusion with an abrasive wheel on a dremel. That's probably not the proper way to do it, so...

If there is one or several nipples that are spun all the way down, and the spoke is still too loose, I'd order a new set of spokes.

Chalo said:

ERD is diametral; spoke length is radial. 1mm difference in ERD makes 0.5mm difference in spoke length.

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Of course, great point and voice of experience! Thanks Chalo...

'curious, do you have much problem using spokes 1-2mm shorter than ideal calculations?

I used the Grin calculator twice. They didn't have my motors in the pull down menu, but how hard can it be to measure the flange width and the space between opposite holes? ERD isn't hard either. Maybe a 1mm tolerance on the measurement Still, my spokes were about 2 mm off the first time. I couldn't even get them tight. Wonder if it was the ebay vendor in China. I re-ordered them 2mm shorter from the same guy. Chalked it up to a learning experience.

Second time, I didn't have the rim yet, so I used the ERD in the vendor's website. Still about a mm too long on the 13G-14G spokes, which I bought from Yojimbos in Chicago. Sapim spoke washers fixed that. Later, I found out 14G nipples come in three lengths, and maybe I bought the wrong size.

Ykick said:

'curious, do you have much problem using spokes 1-2mm shorter than ideal calculations?

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I use the wrong spokes for a rim quite a lot (reusing old spokes for new wheels with differnet hubs or rims) and as long as I can get them started and at least most of the threaded area within the nipple, then I don't usually have problems with them even with the abuse I put them thru.

Sometimes I have to use spokes that are too long by a few mm, and as long as there's enough thread for tensioning properly, they still work, and I grind off the extra that sticks out of the rim so I don't shred my tubes.


Of course, I dont' recommend any of this but if you don't have the money for the right spokes/etc., it *can* work out just fine if you have the experience to deal with the issues (or are willing to fiddle until you do).

Ykick said:

'curious, do you have much problem using spokes 1-2mm shorter than ideal calculations?

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If the spoke doesn't expose thread at either end of a 12mm nipple, I consider its length to be correct. If it shows a bit of thread above the nipple, I'll use 16mm spoke nipples if I don't have a more appropriate length spoke available.

If it pokes out of the bottom of the nipple a thread or two, I usually ignore it and make sure to use a good rim strip. Most of the wheels I build have double walled rims of sufficient depth that it doesn't matter anyway.

However, it is a pleasure to get the length just right, barely filling all the thread when at correct tension. That's what I prefer.

Chalo said:

Ykick said:

'curious, do you have much problem using spokes 1-2mm shorter than ideal calculations?

Click to expand...

If the spoke doesn't expose thread at either end of a 12mm nipple, I consider its length to be correct. If it shows a bit of thread above the nipple, I'll use 16mm spoke nipples if I don't have a more appropriate length spoke available.

If it pokes out of the bottom of the nipple a thread or two, I usually ignore it and make sure to use a good rim strip. Most of the wheels I build have double walled rims of sufficient depth that it doesn't matter anyway.

However, it is a pleasure to get the length just right, barely filling all the thread when at correct tension. That's what I prefer.

Click to expand...

Right on brother, perfect is best! And good tip about the longer nipples - thank you....

Chalo said:

Most of the wheels I build have double walled rims of sufficient depth that it doesn't matter anyway.

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Looking for a rim with lots of distance between the tires tube and the nipple head. Anyone got any suggestions?

The one I currently have is a 31mm wide and I am unsure the depth, Crystalyte Downhill rim from ebikes.ca with lots of depth.

Looking through Alexrims website, these were the only ones that had depth to them.
Alex XD2800 has depth of 28mm instead of the usual 21-22 for most others ------------ and 23mm wide
G6000 30mm depth 22mm wide and XD-Sport 25mm depth 24mm wide

markz said:

Looking for a rim with lots of distance between the tires tube and the nipple head. Anyone got any suggestions?

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How deep is deep enough? How wide is wide enough? What wheel diameter are you using, and what are you trying to accomplish with greater rim depth? In the context of this discussion, eventually you'll run out of spoke threads if you keep screwing the nipples on farther and farther.

Velocity Chukker is the rim that comes to mind for medium sized MTB tires in 26" or 29". 32mm deep by about 24mm wide if I recall correctly.

Once the rim gets more than about 25mm deep, you have to use special long valve tubes.