Don't use this spoke-length formula

[a_dam]

I've found a few different online spoke length calculators which seemed to produce good results. The free download "Spocalc.xls" was accurate and easy to use. But sometimes I want to find spoke lengths on a computer without MS Excel installed. I also don't like having to be online to do calculations. With a dial-up web connection and an old, lean machine/system, I have no need or desire to use online calculators on slow-loading web pages bloated with jquery, flash, activex, etc..

So I'm making my own calculator with only simple html elements, script, and an image with a hub diagram to aid in entering the correct measurements. The hub/rim database in Spocalc.xls is nice, but I end up measuring things myself anyways just to be sure, so databases are never very useful for me.

Searching for spoke-length formulas, I found one at http://www.pijin.co.uk/spokecalc.html. I spent quite a bit of time testing it because the results seemed incorrect (by about 10mm) and I naturally assumed I was making some mistake.

After lots of wasted time and frustration, I found other examples of formulas - and could see that they were not the same. There is a good formula with a thourough explanation of its geometry at http://www.wheelpro.co.uk/spokecalc in the file "spoke-length-proof2.pdf".

In the image below, you can see that both formulas are basically the same except for the part in the parentheses after the cosine.

I couldn't get the calculator at pijin.co to work. Maybe it produces proper spoke lengths, but the mathematical formula they show is wrong. Don't trust everything you find online!

After I finish making my simple calculator, I will figure out a way to make it available to anyone here that is interested.

 

[a_dam]

OK. It figures.

Failing in the past to find on the internet a bare-bones calculator, now 15 min. after starting this thread, I found one at http://goto.glocalnet.net/mabe/mltn-orbit/files_moo/Spokalkulator.html. It is small and easily saved to disk to use offline anytime.

Not crazy about that name "Spokalkulator" BTW, but it seems pretty much what I was about to make myself. Good explanations and illustrations.

Can't vouch for its accuracy yet, but a few quick tests seemed to give proper results. Plus since it is so basic, any tweaking of the code will be easy.

In the script is the formula:

Math.sqrt(hfd*hfd+hld*hld+afr*afr-2*hfd*hld*Math.cos(360/as*ak/57.2956))-3

the "cos(360/as*ak/57.2956))-3" part I'm going to have to investigate more.
I'm no mathematician by any means, and if the results seem right, then I'll assume the math is proper.

Hope you find this useful - I will - and sorry for wasting a lot of words. But the point above still applies... Don't use that formula posted at Pijin.co.uk.

 

[a_dam]

Replying to my own posts again (talking to myself). Oh well - I've never denied being psycho.

After looking over that last one (Spokalkulator), I've decided to make my own. I could use a little refresher practice with math, web design, programming... But mostly I don't agree with a few things the Spokalkulator does.

The rim measurement should be given as ERD, or Effective Rim Diameter. This is almost always provided by rim manufacturers (but is sometimes hard to find). If you have to measure things yourself, ERD is basically the diameter of where you WANT the spoke ends to be. A quick Google search will show many fine examples explaining how to determine ERD. Spokalkulator says "A typical difference between the Effective Rim Diameter [ERD] and the actual, average inside rim diameter [RID] of about 8 mm is also inferred." But the calculator is asking you to input the inside diameter, then giving a spoke length that clearly extends beyond that. The 8mm difference may be "inferred", but that difference should be derived with an extra, adjustable variable, not by fudging the basic formula.
You should determine your ERD, then input that.

This ties into the "spoke nipple length" and "nipple protrusion" figures. Don't get me wrong. I really like the terms "nipple length" and "nipple protrusion", but can't understand why they affect the calculation of spoke length so much in this calculator.
Determine your ERD, then input that, and quit fooling with your nipples.

The spoke length formula should be purely based on math/trigonometry. The wheelpro PDF gave a good explanation of their formula, which seems to concur with the wikipedia definition.

There is no explanation of "cos(360/as*ak/57.2956))-3" in the Spokalkulator script. Why "57.2956", and why "-3"?

However, I did appreciate the simplicity and small size of the script and html. The GIF images were also very small, clear and descriptive, although the final product didn't utilize them well.

I want to make a free calculator that is smaller, simpler, and easier to understand, use, and modify than anything else I've found. I hope you guys will test it and give feedback.

 

[a_dam]

There seems to be not much interest in this thread.

I can understand that. This isn't a wheelbuilding site. I've built less than a dozen wheels in my life, and I seem to go out of my way to DIY!

But for anyone interested, I made a (html) spoke-length calculator so simple that anyone can understand it and edit it with Notepad.

There is one small diagram of a hub to aid in entering the correct measurements, but here is a shot of the calculator without the hub image.

Can I post the page code? How?

 

[a_dam]

Just built a rear wheel. New 700c Rhyno Lite rim - old Schwinn hub - reused Wheelsmith/Dt spokes.

Internet searches mostly agree with a factory-stated 612 ERD for the rim. I still want to measure it myself after the fact.

The other hub measurements I obtained with calipers. Any slight variation/error in those figures would result in a spoke length difference of less than one millimeter.

My homemade calculator agrees with other online calculators. They recommend 291mm and 293mm spoke lengths for this build. I had 290 and 292 spokes, so I used them.

The wheel came out great. The spoke ends are all at the bottom of the nipple screwdriver slots.

If I had to buy new spokes for this wheel, I would choose 289-291 mm (one mm shorter). That's 2mm shorter than my homemade (and other) spoke calculators recommend.

Still pleased with the outcome and still trying to find a way to share the 4k html file, if anyone is interested.

 

[bluegoatwoods]

For what it's worth, I appreciate your efforts.

I'm also not advanced enough to make any use of this. I can true a crooked wheel to some degree. But I've never laced a wheel and have never had the need to calculate spoke length.

I've had some thought, at times, of replacing 14 gauge spokes on a cheap wheel with 12 or 11s. But then I'd just measure the existing spokes.

Though I might never do it, I find the subject of building my own wheels interesting. So feel free to keep on posting anything you have to say on the subject.

 

[a_dam]

Thanks for the reply, bluegoatwoods.

Good to see someone here from our area, though not good riding weather lately.

I believe everyone should learn to do most of the work on their bikes. Cables? I've seen about a thousand people with "bike problems" due to cables needing lubrication. Even around here, bikes are left outside in all kinds of weather.

I'd better stop ranting now; mistreatment of bikes riles me almost as much as mistreatment of animals.

Anyway, building a wheel takes a lot of time and patience. But when you do it right yourself, the result is better than anything you can buy.

I ride old Schwinn "10-speeds"; Continentals and Varsities, because I grew up with them and collected a bunch of free bikes/parts curbside.

Old Schwinn parts are mostly high quality, but the old spokes Suck!
That's why I'm taking the time and abuse to write this. You might not need thicker spokes. Bigger spokes would require bigger holes in the hub and rim and bigger nipples.

Good quality 14ga spokes will make a strong wheel.

 

[bluegoatwoods]

I recognize Momence, in fact. But I'm having a hard time placing just where your town is. Southern IL, perhaps? Maybe in that area east of St Louis?

I'm also quite fond of the Continental and Varsity. As a kid I never had one. But I had a couple as a grown up. Also a Japanes built Traveller. It was a fine, fine bike.

That was in the early 80s. Right when mountain bikes were a new thing. I had a look at those straight handlebars and switched my old dropbars out to straight.

That Traveller handled off-road just as well as the mountain bikes of the time. I still remember it fondly.

I might be a Continental/Varsity/Traveller freak even today if it weren't that I've come to think wider wheels are the way to go.

 

[a_dam]

Momence is a suburb of the huge metropolis of Kankakee!
So many people who grew up in the ghetto (Chicago) move this way (south). I wish it was southern Ill. The weather is completely different just a few hundred miles south in KY, TN, but we are up here by the frigid "Great Lakes".

 

[bluegoatwoods]

Oh, yes. I remember now.

And your winters are one notch or so tougher than ours. It's always seemed a bit strange to me. Areas like yours are not all that far North of me, yet the difference is noticeable.

The same thing works in the other direction. That area East of St Louis.....Effingham, for instance...... is measurably less harsh than our area.

I suppose you could drive there in three hours. And that small geographic difference has a large climate difference.

Not that they can't ever have snow or frigid temps. But, on average, it doesn't last as long and it warms up more in between.