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Discussion Starter · #1 ·
I'm have trouble believing my spoke length calculation. Using various calculators I need a 176 mm spoke, That seems way too long to me. Maybe I'm missing something so I figure you guys can help. This is an electric hub going on a 20" bmx type wheel (front) for a scooter I'm building.
Here are the specs:
36 hole 3 cross lacing
ERD - 394.84mm (single wall rim from a 20" bmx bike)
120mm diameter hub flange (the circle diameter of the spoke holes)
2mm spokes
c/line of hub to left flange 27.94 mm
c/line of hub to right flange 26.67mm
distance flange to flange 54.61mm
I thin I need something like 140 -150mm
Any help is appreciated, Russ
 

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Discussion Starter · #3 · (Edited)
Here is another issue. I measured the spokes I pulled off the original assembly. the spokes are 8.375 long (212.725mm) WRONG !! mis read caliper actually 7.375! I then measured the hub diameter (spoke circle diameter) it was 1.690 (42.92mm) . I measured the rim OD 16.636 (422.57mm ) then measured the depth of well .546"(13.86mm). multiplied the well depth by 2 = (27.73mm) and subtracted the answer from the OD =ERD ( 394.84) -I used the same parameters in the spoke calculator and got an answer of 189mm spoke!! that is a difference of 23mm - almost 1 inch!! What gives? I challenge any one here to measure a 20" wheel/hub and use the calc then measure the actual spoke on the wheel -does it match the calc answer?
 

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Something is not right with your numbers. What brand / model of hub is it, and what brand / model of rim is it?

With your new numbers, and 3x I get 143mm.

With a 4x lacing I get 150mm.

What is the current lacing pattern? Radial, 2x, 3x, 4x, crows foot?

What is the distance from the center of one hole to center of another hole in the flange.

What are you using to measure the spoke, and what kind of spokes are they?
 

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Discussion Starter · #5 · (Edited)
Thanks for sticking with me nov0798! I AM doing something wrong! I misread my caliper by 1"--- the actual spoke length was 7.375 NOT the 8.375 as I first wrote. I did neglect 1 measurement in my last post- the hub width and flange distance. here they are: hub width: 2.430" (61.72mm) and flange distance 1.215" (30.86mm) I apologize for dropping those. These fore mentioned numbers AND the numbers in my previous post are from the standard BMX rim/hub/spokes I am using for the donor on the e bike motor set up

The bmx hub was/is a "K.K." brand -likely no info. on that
the rim is a "Sumo" 20 x 1.75- again likely no info. either.
I am replacing the KK bike hub with an e bike hub - this hub also says 8fun on it.

So here are the specs again in 1 list:

rim ERD 394.84mm
bmx hub diameter 42.92mm (the distance across 2 opposing spoke holes)
hub width 61.72mm
flange distance 30.86mm
36 hole 3x lacing
ACTUAL spoke length from ASSEMBLED rim: 187.325mm

Does it seems logical that a 10mm shorter spoke would be used on a hub that is roughly 78mm larger in diameter? Thanks rjm
 

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Using those numbers with a spoke hole diameter of 2.4, I get 188.9 for both sides, very close to your 187.325. It would still help to have manufacturer numbers, rather than your measured numbers. Im not saying that they arent correct. but some variations could skew the numbers.

Is this the hub you have?

http://www.szbaf.com/index.aspx

Lets start here and if it is, send them an email and see if they have the specs on this hub.
 

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rjm said:
Thanks for sticking with me nov0798! I AM doing something wrong! I misread my caliper by 1"--- the actual spoke length was 7.375 NOT the 8.375 as I first wrote. I did neglect 1 measurement in my last post- the hub width and flange distance. here they are: hub width: 2.430" (61.72mm) and flange distance 1.215" (30.86mm) I apologize for dropping those. These fore mentioned numbers AND the numbers in my previous post are from the standard BMX rim/hub/spokes I am using for the donor on the e bike motor set up

The bmx hub was/is a "K.K." brand -likely no info. on that
the rim is a "Sumo" 20 x 1.75- again likely no info. either.
I am replacing the KK bike hub with an e bike hub - this hub also says 8fun on it.

So here are the specs again in 1 list:

rim ERD 394.84mm
bmx hub diameter 42.92mm (the distance across 2 opposing spoke holes)
hub width 61.72mm
flange distance 30.86mm
36 hole 3x lacing
ACTUAL spoke length from ASSEMBLED rim: 187.325mm

Does it seems logical that a 10mm shorter spoke would be used on a hub that is roughly 78mm larger in diameter? Thanks rjm
Yes, the spoke length looks right. There is not a 1:1 change in spoke length with flange diameter changes in a tangentially laced wheel. With the spoke vertical, the head moves more laterally than radially when the hub flange diameter changes.

You may need to use 2x with the large diameter hub and 20" rim, though.
 

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Discussion Starter · #8 · (Edited)
yep, that is the brand and the model is the "jewel" on their website -the specs are on the site also. looks like I have to believe the calculator. between shiggy and nov0798 is probably more knowledge and experience with this stuff than I will ever have. Thanks for your input -I'll order up some spokes. I will post up some pics of the contraption when its done. rjm
 
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