Spoke lengths - round up, or round down

rb58

Enigma
Location
Bexley, Kent
Roger Musson says round down, which is what I plan to do. But what about when the calculated length for my build is just below the standard length? Is there ever a case to round up to avoid over tensioning? I'm still working out my options, but the calculator suggests I need:

292.7 - that seems fine to round down to 292.
291.3 - is 1.3mm too much to round down?
293.6 - likewise, is 1.6mm too much to round down?

And if I round up, do I run the risk of the end of the spoke fouling the rim tape, or are the tolerances not that tight?

This is my first build completely from scratch, so please excuse the novice questions.

Cheers
 

raleighnut

Legendary Member
Location
On 3 Wheels
Depends if you have single wall or double walled (chambered) rims, one of the wheels John just laced up for me used spokes that came 2 mil or so through the nipples but they were still inside the double wall of the rim.
 
Location
Loch side.
Roger Musson says round down, which is what I plan to do. But what about when the calculated length for my build is just below the standard length? Is there ever a case to round up to avoid over tensioning? I'm still working out my options, but the calculator suggests I need:

292.7 - that seems fine to round down to 292.
291.3 - is 1.3mm too much to round down?
293.6 - likewise, is 1.6mm too much to round down?

And if I round up, do I run the risk of the end of the spoke fouling the rim tape, or are the tolerances not that tight?

This is my first build completely from scratch, so please excuse the novice questions.

Cheers
Always round down, never up.

On 292.7, go down to 291.
On 291.3, go 290
On 293.6, go 292.
 

Venod

Eh up
Location
Yorkshire
An extract from Roger Mussons book.

" Another example, say you calculate 263mm but can only obtain even length spokes, you can easily go + or - 1mm giving you the option of 262 or 264. If you go down 1mm or more you will be getting tight spokes early in the build process"

I would be very careful how much you deviate from the calculated length either way.
 
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John the Monkey

Frivolous Cyclist
Location
Crewe
Sadly, I think the answer is "depends". I remember that rounding down on Exal LX17s / Deore gave me a ridiculously tight back wheel, and I ended up changing the drive side spokes, as I remember. Round down is a good rule of thumb, but read the blurb accompanying the calculator you're using, and if possible, seek the advice of people who've build using the combination of components you plan to use.
 
Location
Loch side.
Sadly, I think the answer is "depends". I remember that rounding down on Exal LX17s / Deore gave me a ridiculously tight back wheel, and I ended up changing the drive side spokes, as I remember. Round down is a good rule of thumb, but read the blurb accompanying the calculator you're using, and if possible, seek the advice of people who've build using the combination of components you plan to use.
I think this is good advice. Some calculators, such as the one on the DT Swiss website, asks you for the type of spoke you will be using and then rounds down for you, roughly as I explained in the post in the link.
My example is for a calculator that uses the exact mathematical formula for a line in space and that calculation does not take stretch and compression into account. I would imagine that an experienced, professional wheelbuilder would not want a spreadsheet to make decisions for him.
Also, small errors in measurement could have caused your problem with the Exal rim. Some manufacturers measure and state ERD differently from others, My advice is to always measure the ERD yourself. And then, in two orientations and average the sum. This is because new rims are not round.
 
Depends if you have single wall or double walled (chambered) rims, one of the wheels John just laced up for me used spokes that came 2 mil or so through the nipples but they were still inside the double wall of the rim.
I see what you are saying but I wouldn't advise it. In time I've seen the tube getting through to the tip of the spoke, when the rim tape is old. I saw this with Velox cloth rim tape.

Sadly, I think the answer is "depends". I remember that rounding down on Exal LX17s / Deore gave me a ridiculously tight back wheel, and I ended up changing the drive side spokes, as I remember. Round down is a good rule of thumb, but read the blurb accompanying the calculator you're using, and if possible, seek the advice of people who've build using the combination of components you plan to use.
RM wrote his own spoke calculator and he has listened to people and tuned it over time. His advice is worth following I think. Some.6 or below and round down, above that and round up. His guidelines have been spot on... in a couple of occasions I couldn't find the right size and I went for 1+ longer and that showed.

Other spoke lengh calculator I have used always suggest longer spokes though.
 
OP
rb58

rb58

Enigma
Location
Bexley, Kent
Thanks for the advice folks. The consensus seems to be to round down, which is what I shall do. If it helps, I'm looking at Mavic Open Pro rims on XT hubs (T780), 36 spoke, 3 cross. Anyone got experience of that combination?
 

raleighnut

Legendary Member
Location
On 3 Wheels
I see what you are saying but I wouldn't advise it. In time I've seen the tube getting through to the tip of the spoke, when the rim tape is old. I saw this with Velox cloth rim tape.



RM wrote his own spoke calculator and he has listened to people and tuned it over time. His advice is worth following I think. Some.6 or below and round down, above that and round up. His guidelines have been spot on... in a couple of occasions I couldn't find the right size and I went for 1+ longer and that showed.

Other spoke lengh calculator I have used always suggest longer spokes though.
TBH if the rim tape went 'down to the tip of the spoke' then the inner tube will have gone ages ago, unless the tape had rotted away.
 

Smurfy

Naturist Smurf
I'm afraid the suggestion to 'Always round down' is poor advice, and as pointed out by Afnug, is not what Roger Musson describes.

My latest wheel build came out good, based on the following:
Calculated Actual Wheel
265.3 266 Front
263.8 264 RDS
265.4 266 RNDS

There is no way that I would round down more than 1mm, so in my case I must round up, but by less than 1mm.

I used spocalc express and the Roger Musson measuring method, with the end of the spoke threads level with the bottom of the nipple slot, as should be done. My measurements are very accurate, as I measure both rims, take 16 measurements (32 holes) from each rim and average the results.
 
Location
Loch side.
I'm afraid the suggestion to 'Always round down' is poor advice, and as pointed out by Afnug, is not what Roger Musson describes.

My latest wheel build came out good, based on the following:
Calculated Actual Wheel
265.3 266 Front
263.8 264 RDS
265.4 266 RNDS

There is no way that I would round down more than 1mm, so in my case I must round up, but by less than 1mm.

I used spocalc express and the Roger Musson measuring method, with the end of the spoke threads level with the bottom of the nipple slot, as should be done. My measurements are very accurate, as I measure both rims, take 16 measurements (32 holes) from each rim and average the results.
Well, then, explain why it is poor advice and tell me when it is OK to round up.

Telling me that so-and-so says so is not a good enough argument. You have to find a flaw in my reasoning. The actual spoke calculator is irrelevant in the matter, it is an understanding of how much a spoke stretches at 1000N of tension, that dictates the starting length and last time I looked, not a single spoke went shorter under tension.
 

Venod

Eh up
Location
Yorkshire
Well, then, explain why it is poor advice and tell me when it is OK to round up.
The OP seems to be following Roger Mussons excellent book, an extract I have quoted above saying its OK to round up no more than 1mm, as this is his 1st build he doesn't have the experience to sort out any build problems shorter spokes may introduce, at 1mm or less longer they will not prove a problem, experienced builders know your spoke lengths would be fine as spokes do stretch & rims compress slightly but I always find it an easier to build if I stick close to the measurements from Roger Mussons calculator, when the op has a bit more experience he will get a feel for how much he can deviate from the norm.

I wouldn't say its bad advice to round down (slightly), but its also not bad advice to round up (slightly).

Roger Mussons calculator is designed to knock 0.3mm of the theoretical spoke length.
 
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I'm afraid the suggestion to 'Always round down' is poor advice, and as pointed out by Afnug, is not what Roger Musson describes.

My latest wheel build came out good, based on the following:
Calculated Actual Wheel
265.3 266 Front
263.8 264 RDS
265.4 266 RNDS

There is no way that I would round down more than 1mm, so in my case I must round up, but by less than 1mm.

I used spocalc express and the Roger Musson measuring method, with the end of the spoke threads level with the bottom of the nipple slot, as should be done. My measurements are very accurate, as I measure both rims, take 16 measurements (32 holes) from each rim and average the results.
I also follow Roger Musson..... I guess Yellow Saddle must be wondering who the hell is this Roger Musson that keeps coming up in nearly every thread :smile: Well, Roger Musson is a guy who spent many years building mainly MTB wheels and he used to support riders in down hill competitions. He has a Engineering background and he wrote a book on how to build wheels, from the practical point of view. He does mention things like stress relieving but not in the way Jobs Brandt does. The important thing for a beginner to understand is that something like stress relieving is important in the process of wheel building and at that stage the beginner doesn't need any more information. Jobs Brandt book can be a bit overwhelming with too much information for a beginner I think. Undoubtedly it is a great book but, for me at least, it was too much to start building wheels :smile: not patient enough I guess, I just wanted to get on with it and build a wheel. The second part of JB book is the practical part and if I'm honest I think RM book is better in that aspect.

Now, going back to the point I wanted to make before I tried to explain a little bit who Roger Musson is.

I don't have RM book here with me but I'm pretty sure he would have rounded off more like this:

265.3 266 Front - down to 265 and up to 266 if no spokes found in 265
263.8 264 RDS - down to 263 if no spokes found in 264
265.4 266 RNDS - down to 265 and up to 266 if no spokes found in 265


Well, then, explain why it is poor advice and tell me when it is OK to round up.

Telling me that so-and-so says so is not a good enough argument. You have to find a flaw in my reasoning. The actual spoke calculator is irrelevant in the matter, it is an understanding of how much a spoke stretches at 1000N of tension, that dictates the starting length and last time I looked, not a single spoke went shorter under tension.
I have followed RM advise and used RM spoke calculator for over 200 wheels and it has been spot on. I guess RM allows for the spoke stretching under tension. He does say that the lengths of his spoke calculator should be treated as a maximum and not to round up or down more than 1mm, So faced with something like 265.6 then down to 265 but a 265.7 up to 266 BUT as you know, sometimes you just cannot find the correct spoke length so 265 or 266 is fine. If I had to choose a spoke because 265 and 266 aren't available then I would go with a 264 rather than 267. I would need to alter the first stage of the building process slightly but I would be fine.
 
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