Spoke lengths - round up, or round down

[Spoked Wheels]

It would help any builder to understand exactly what the calculator is doing or not doing.

It occurred to me to try some numbers on two different calculators and one of them being RM calculator.....

Pro Wheel Builder Spoke Length Calculator
ERD = 602
3 X - 32 spokes
Flange diam L = 44 and R = 46
Center to flange LS = 36 and RS = 18

Suggested lengths = Left 295.5 and Right 293.5

Now, the calculator I always use is the RM and that suggests:

Suggested lengths = Left 294 and Right 292.1

I'm quite happy to go with those suggested lengths

 

[Yellow Saddle]

It occurred to me to try some numbers on two different calculators and one of them being RM calculator.....

Pro Wheel Builder Spoke Length Calculator
ERD = 602
3 X - 32 spokes
Flange diam L = 44 and R = 46
Center to flange LS = 36 and RS = 18

Suggested lengths = Left 295.5 and Right 293.5

Now, the calculator I always use is the RM and that suggests:

Suggested lengths = Left 294 and Right 292.1

I'm quite happy to go with those suggested lengths

This makes the point about understanding what the calculator is doing very valid.

Here's a mathematical length calculation.

Under 3X row in the Output Data column you'll see that the absolute (geometric/mathematical) lengths are 292.6 and 294.0. Look at the red figures.
Way out from the first calculation above. My only assumption is a spoke hole dia of 2.4mm. The ERD is 602, Crosses 3, right flange width 18mm and left flange width 36mm. Flange dia's are 44 and 46 respectively. We may differ in the way we measure flange width. I measure mine centre to centre (of the flange).

Nevertheless, these are not suggested lengths but actual lengths. You talk about suggested lengths, which if it isn't something you inadvertently used, means that some black box inside the calculator is making decisions for you. I don't like that. I'm a control bunny. Based on these figures, I now make an additional decision based on whether they are Revolution/Laser spokes and their length - 26" or 29" .


But if it works, it works. Horses for courses.

 

[Spoked Wheels]

This makes the point about understanding what the calculator is doing very valid.

Here's a mathematical length calculation.

View attachment 81967

Under 3X row in the Output Data column you'll see that the absolute (geometric/mathematical) lengths are 292.6 and 294.0. Look at the red figures.
Way out from the first calculation above. My only assumption is a spoke hole dia of 2.4mm. The ERD is 602, Crosses 3, right flange width 18mm and left flange width 36mm. Flange dia's are 44 and 46 respectively. We may differ in the way we measure flange width. I measure mine centre to centre (of the flange).

Nevertheless, these are not suggested lengths but actual lengths. You talk about suggested lengths, which if it isn't something you inadvertently used, means that some black box inside the calculator is making decisions for you. I don't like that. I'm a control bunny. Based on these figures, I now make an additional decision based on whether they are Revolution/Laser spokes and their length - 26" or 29" .


But if it works, it works. Horses for courses.

The word "suggested" is being used inadvertently really. The matter of any rounding and decisions when the sizes are not available are still down to me. I don't make any decisions on how thin or thick the spokes I'm using are. I have to say you are the first wheel builder I have heard that adjust the length of the spokes to the thickness of the spoke. I think the difference must be so small that it's not even worth considering.

 

[Yellow Saddle]

The word "suggested" is being used inadvertently really. The matter of any rounding and decisions when the sizes are not available are still down to me. I don't make any decisions on how thin or thick the spokes I'm using are. I have to say you are the first wheel builder I have heard that adjust the length of the spokes to the thickness of the spoke. I think the difference must be so small that it's not even worth considering.

The difference is bigger than you think. For a 290mm Revolution spoke (DT Revolutions and Sapim Lasers are 1.5mm rather than 1.8mm and starting length is proportional to increase in length under tension), the increase in length under tension is about 2mm. On a 26" wheel it is about 1.5mm. That's over a 1.8mm spoke. The difference in strain between a straight-gauge 2mm and Revolution is even greater.

The Young's Modulus of stainless steel is known, as is the diameter, length and tension. Therefore you can calculate the strain (increase in length). Based on the observation that with these spokes I end up with rather too much length peering past the nipple, I've created myself a rule of thumb based on the actual strain. Hence my little table of suggested rounding down values earlier on in this thread. However, I've now discovered that most people aren't as anal as me, so I'll just look the other way when they round up. I used to own a workshop where we churned out high-quality hand-built wheels for the MTB market and I installed the rounding down by spoke type rule so that I could put out consistent products. By knowing exactly how the spoke lengths were calculated and adjusted, I could just look at a wheel, peer through the spoke holes, notice the position of the spoke in the nipple and conclude accurately that the wheel is within an acceptable tension range. We offered a life-time warrantee on spoke life and an unlimited free tune-up for trueness. Just by looking at the spokes I could also tell whether someone else had fiddle with the wheel or not. Occasionally I'd find an odd spoke (too long or too short in the nipple) and instantly know that this wheel had been worked on elsewhere. But like I say, this isn't for everyone.

 

[Spoked Wheels]

The difference is bigger than you think. For a 290mm Revolution spoke (DT Revolutions and Sapim Lasers are 1.5mm rather than 1.8mm and starting length is proportional to increase in length under tension), the increase in length under tension is about 2mm. On a 26" wheel it is about 1.5mm. That's over a 1.8mm spoke. The difference in strain between a straight-gauge 2mm and Revolution is even greater.

That I find interesting. Certainly something to bear in mind. Thanks.

By knowing exactly how the spoke lengths were calculated and adjusted, I could just look at a wheel, peer through the spoke holes, notice the position of the spoke in the nipple and conclude accurately that the wheel is within an acceptable tension range.

That I can believe.

1) The spoke stretches under tension. A thicker spoke stretches more than a thinner spoke.
2) The rim compresses under tension. Big rims compress more than skinny box section rims.
3) The nominal length almost always includes a fraction and spoke sizes aren't available in fractions.

We have two constraints in our flexibility to change the length.
1) Ideally the spoke should not protrude past the slot in the nipple. This is so that it can be removed with a screwdriver if the flats stripped. A secondary reason is that if you use a speed driver, a too-long spoke ejects the driver before you've wound up much tread and it slows the job down.
2) The variance we have in how far in or out the spoke can be screwed in is small. At the upper end is in point 1 above. At the lower end it is thread showing below the nipple once the wheel is tensioned.

Reading through your post I cannot help it but to conclude we take different routes to build a wheel. I haven't tried this in the practice but I think I might have to adjust my method to work with shorter spokes like you do. I do most of the truing, laterally and vertically, dishing, initial spoke tension equalizing while the spokes aren't yet too tight. At this point, without actually trying it, I feel that, with shorter spokes like you suggest, with my method the spokes would be too tight at this stage, RM calls this the stage where the wheel is stable.

Now, if you tell me there is up to 2mm of spoke stretching then I would consider reviewing things but I'm not really convinced by the reasons stated in point 1.

 

[e-rider]

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

depends on spoke calculator used and nipple length. The DT Swiss calculator usually works best rounded down

 

[Yellow Saddle]

That I find interesting.

Now, if you tell me there is up to 2mm of spoke stretching then I would consider reviewing things but I'm not really convinced by the reasons stated in point 1.

Just remember that I corrected myself on that quoted post. A THINNER spoke stretches more than a thicker spoke. I think I'll edit the text itself, actually.

 

[Spoked Wheels]

Just remember that I corrected myself on that quoted post. A THINNER spoke stretches more than a thicker spoke. I think I'll edit the text itself, actually.

Oh yes, I thought you had made a mistake not to worry.

 

[Spoked Wheels]

If you round down by the number I've quoted, you won't find problems with the last set of spokes at all. I manage to wind up all the nipples right up to the end of the spoke's thread (on the hub side), using just a speed driver. As you know, a speed driver exerts almost no torque.

Well I put your suggestions to the test... I was more interested in your spoke stretching figures and I felt that it was worth trying.

I laced a new wheel with spokes that were 1mm shorter than I would have normally chosen and it was pretty difficult to lace I had to go back and reduce the number of turns I was using which resulted, as I suspected, in one nipple coming loose.... luckily it didn't take too long to fish it out. I used all the tricks in the book to complete the last two sets of spokes. Flatenning the spoke elbows wasn't enough. Using a 14mm nipple and pulling the hub enough to be able to screw a 12mm spoke next was the only way.
The time wasted was far longer than any gains I might get later on when I use the nipple driver.

The fact I was using a Dura Ace hub, with narrow holes and thick flanges didnt help either.
Anyway, the wheel is now laced and all looks OK.

I know that with practice I'll get better at it but I've laced enough wheels to know there are no time gains to be made with shorter spokes. I'm just glad I didn't go 2mm shorter

I'm looking forward to seeing the result of putting 120kgf of tension and see if I can see a 1.8mm of spoke stretching. I mean, I know where I should expect to see the end of the spoke, more or less, with a 1mm longer.

At this point I can't help it but ask you with all respect, have you actually tried this yourself on a regular basis or is it just theory, with the physics book under the arm?

 

[Yellow Saddle]

Well I put your suggestions to the test... I was more interested in your spoke stretching figures and I felt that it was worth trying.

I laced a new wheel with spokes that were 1mm shorter than I would have normally chosen and it was pretty difficult to lace I cut cut cut cut

That's a meaningless reference. I have no idea what you would normally have chosen. However, if you give me the specifics, I'll analyse what went wrong. I
l'll need ERD (and preferably rim type, I have no idea how the ERD was measured), hub model and lengths that you actually used.

I'm looking forward to seeing the result of putting 120kgf of tension and see if I can see a 1.8mm of spoke stretching. I mean, I know where I should expect to see the end of the spoke, more or less, with a 1mm longer.

Well, why not just measure a wheel that you've built and see for yourself how much the sproke stretches. Then, compare that against a calculated value. If you tell me what spokes you used I'll show you how the calculation works.

At this point I can't help it but ask you with all respect, have you actually tried this yourself on a regular basis or is it just theory, with the physics book under the arm?

Naah, I just make stuff up as I go along.

 

[Spoked Wheels]

That's a meaningless reference. I have no idea what you would normally have chosen. However, if you give me the specifics, I'll analyse what went wrong. I
l'll need ERD (and preferably rim type, I have no idea how the ERD was measured), hub model and lengths that you actually used.

I didn't feel the need to draw you a picture to be honest. I thought that from my previous post you could see the kind of numbers I was getting from my favorite spoke length calculator. If you remember your numbers and my numbers were nearly identical, from which you said there would be a 1.8mm of spoke stretching on a road wheel to allow for. That particular bit of information alerted my curiosity, hence my little experiment.

I completed the wheel last night and there is some stretching, I see the end of the spoke more or less where I would expect to see, maybe just a tiny little less not enough to doubt there was some stretching.

What could be wrong? I was thinking about this and I think the problem is when lacing, the spokes are not stretched so the extra millimetre is the difference between the nipple just touching the spoke or actually screwing on the spoke.

Naah, I just make stuff up as I go along.

Well, you said it, who am I to argue otherwise?

Conclusion, Although, allowing a 1mm for spoke stretching will give good results, the hassle and added difficulty to lace the wheel make me think there is no reason to change the way I have been rounding up/down my spoke lengths.

So let's leave it here because you are not going to persuade me otherwise.

.

 

[S.Giles]

I've only built one wheel (twice, though!) and was very concerned that I may have ordered the wrong length spokes, it being the first time I'd done the calculation. Everything worked out fine though, despite the need to use the same spoke-length for both sides of a disc-brake wheel (with considerable dishing necessary). I even re-used the spokes when I later replaced the hub with a different type.

This suggests (but doesn't prove) that spoke-length calculation is not quite as critical as it is sometimes made-out to be. Keep doing whatever has worked for you in the past, I would say.

 

[Spoked Wheels]

I've only built one wheel (twice, though!) and was very concerned that I may have ordered the wrong length spokes, it being the first time I'd done the calculation. Everything worked out fine though, despite the need to use the same spoke-length for both sides of a disc-brake wheel (with considerable dishing necessary). I even re-used the spokes when I later replaced the hub with a different type.

This suggests (but doesn't prove) that spoke-length calculation is not quite as critical as it is sometimes made-out to be. Keep doing whatever has worked for you in the past, I would say.

I think you will find that 100% accuracy while measuring the hub is not crucial. ERD has more impact on the final length of the spoke. Then, if you use RM spoke length calculator, you have up to 1mm up or down to play with, a bit more room can be considered but it's not free of consequences.

 

[S.Giles]

I think you will find that 100% accuracy while measuring the hub is not crucial. ERD has more impact on the final length of the spoke.

I agree. A slight change in hub diameter on a wheel with 'crossed' spokes just slightly changes the angle at which the spoke enters the rim.

(Those wheels with (truly) radial spokes are a different matter. They seem to cause several problems without actually achieving anything, and personally I'm not even keen on the way they look. Is there some point to them that I'm missing? I would appreciate it if someone could enlighten me!)

 

[Spoked Wheels]

I agree. A slight change in hub diameter on a wheel with 'crossed' spokes just slightly changes the angle at which the spoke enters the rim.

(Those wheels with (truly) radial spokes are a different matter. They seem to cause several problems without actually achieving anything, and personally I'm not even keen on the way they look. Is there some point to them that I'm missing? I would appreciate it if someone could enlighten me!)

Other for the way they look, nothing of real importance. I have one wheel and about to build another though