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Old 01-01-09, 08:28 PM   #1
jakub.ner
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Spoke Length Calculators and 20" Wheels

Up until now I've been building non-dished wheels for my bikes. Managed this by using single speed hubs and internal gears. I'm in the process of building my first dished 20" wheel that will use a 7 speed cassette.

With the non dished wheels calculating spoke length was simple. Any spoke length calculator on the Web gave me good resuts, albeit almost always the spoke was a wee bit too long (a millimeter or so).

For th dished wheel the results tell me I need 191 mm on the non drive side and 188 mm on the drive side. When verifying these spoke lengths (with the 6 spoke method) I've determined that on a 20" wheel this will give me significantly less dish then I need to center the rim. The spoke sizes I determined I need are indeed 197 mm and 178 mm. That's a 19 mm difference in length! I want to ping this off of you whether this is a reasonable result.
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Old 01-02-09, 04:50 PM   #2
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^ That's a huge discrepancy jakub.ner I wonder what happened there?

When I build special (or unusual) wheels, I never trust spoke calculators because I can't "see" how the calculations were derived. I almost always do my calculations by 1:1 scale compounded geometry drawings. It's as simple as measuring the final length with a ruler. I used to do it on a drafting board with pencil and paper, but I find it much quicker to do on my computer using Adobe Illustrator.

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Old 01-02-09, 05:37 PM   #3
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It is eh? OK I will verify again. Thanks!

Interesting way of visualizing and calculating. Gives me an idea.

- Jakub
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Old 01-02-09, 11:34 PM   #4
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I build many wheels of different sizes. I always used online calculator and so far so good. Jakub, I suggest you double check your measurements of both rims and hubs. I am very satisfied with this calculator.

http://www.bikeschool.com/spokes/

When I measure the effective rim diameter, I don't take into consideration the extra thickness of the rim's aluminum wall.

Furthermore, you have a very detailed spoke length calculator from Harris Cyclery with all the details thoroughly explained.

http://sheldonbrown.com/rinard/spocalc.htm
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Old 01-03-09, 10:23 AM   #5
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I would go with theHArris spke calc as a base.... than check www.sapim.be and DT' spoke calculator..maybe check with spokecalc as well..... than u have 3 or more ..( lol ) measuremensts and they should somehow all arrive pretty close ...

1 mm difference is nothing to be conceerned

long and short nipples are NOT making any difference as the thread inside and the position is always the same

I tend to like them a mm longer rather than a mm shorter if ther is a choice ...

when ready you should not be able to see the threadso the spokes no more and they shouldnt poke out of the nipple either

if every 4th spoke is sticking out and the other 3 are too short you need to rotate the spoke holes one forward( or backwards )

have fun ...its a huge accomplishment to have built your own wheels

for small wheels use spokes with a 90 degree bend rather than the usual bend ... and use polyax nipples from Sapim * they have a rounded SEAT and therfroe dont BEND the spoke in from straight to where the spoke goes ...


All the best Thor
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Old 01-03-09, 06:16 PM   #6
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Cao, Thor, thanks for confirming that something is amiss in my endeavour.

Indeed I do use the excel spreadshets from Harris Cyclery. The results from this tool are reasonable.

I've rechecked the lacing and looks good. I rechecked all dimensions on the hub and the ERD as well.

I am beggining to question the 6 spoke length verification method. Is lacing 6 spokes to verify their length something one can depend on? Maybe this method is faulty? It's described in The Bicycle Repair Book by Rob van der Plas, this is what yielded me the 19mm difference in results.
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Old 01-04-09, 11:12 AM   #7
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Quote:
Originally Posted by brakemeister View Post

for small wheels use spokes with a 90 degree bend rather than the usual bend ... and use polyax nipples from Sapim * they have a rounded SEAT and therfroe dont BEND the spoke in from straight to where the spoke goes ...


All the best Thor
That is interesting Thor. I never heard of those nipples. Thanks for mentioning it.
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