spocalc calls for range between 294 and 297 -- can I cheat?
 

building two wheels. according to spocalc:

front wheel needs 297.4mm spokes

rear wheel needs 296.1 on the left and 294.4 on the right (6spd freewheel)

can I get away with using 296mm for the front and 295mm (only) for the back?

I'm trying to save some $$, I've already got the 296mm in my shop and 295 is easier for me to find at good price.

thx

I'd personally err on the side of too long. I reused spokes (>30 year old French spokes, btw) when converting from a geared hub to a singlespeed flip-flop. The former nondrive side spokes were long enough that I ground them down even with the inner faces of the nipples and have never had a problem with them.

I worry that going a mm or so shorter than called for means about a thread and a half less engagement in the nipples. In other words, I'd go with all 296, rather than 295, but I don't think it will matter in the long run. Of course, if you use 295 and the spokes are tight after just a turn or so, you may want to consider spending the money on longer spokes before you end up needing a new rim and all after they fail catastrophically.

It's amazing how often this question comes up.

Always round down. Never try to use longer spokes. Nothing worse than running out of threads before reaching your target tension.

You could use the 296 mm spokes on the front.

You could probably use 296 mm on the non-driveside rear since they are normally the lowest tension spokes so running out of threads shouldn't be an issue there.

Buy 294 mm spokes for the driveside rear. Thise will be the highest tension and have the highest risk of running out of threads (bottoming out the nipples). If you are using double butted spokes there is also the possibility of the spokes stretching.

Al

Is that your experience with Spocalc? I've had the opposite experience: For three wheels builds, Spocalc has recommended that I use spokes that are too short - even after rounding up.

Build #1:

Spocalc calculated 267.9. I built this front wheel using 268's but should have used 269's. Spokes to the insides of the flanges are OK at 268. However, spokes to the outsides of the flanges ended up about half a millimeter below the slots in the nipples.

Build #2:

Spocalc calculated 265.6 and 266.8 using the hub data in it's database. After fixing a grievous hub dimension error, it calculated 265.2 and 266.8. 267's were out of stock so I rounded down and purchased 265's and 266's. Upon assembly, it was quickly obvious that the spokes were too short. Therefore, I moved the 266's from the NDS to the DS, and substituted 268's for the NDS. These lengths worked out just fine.

My more recent calculations show an ideal length of 268 for the left flange outside (ie heads in) spokes, 267 for the left inside, and 266 for both sides of the right flange. These numbers jibe with how the wheel actually went together. These same calculations give a minimum length for the left outer spokes of 267. The Spocalc 266.8 length rounded down to 266 would have left threads visible above the nipples!

Build #3:

Spocalc calculated 290.6 and 291.6. I was going to build the wheel with the rounded up sizes of 291 and 292. Luckily the co-op was out of 291's. I went longer to 292 and (to keep a left/right difference) 293. These lengths worked fine. More recent calculations confirm the 292 and 293 although 291 would have been OK for the inside of the right flange.

Reasons why Spocalc gives lengths that are too short:

Spocalc calculates only one spoke length for each flange. My calculations for several examples give approximately 0.7mm difference in length to the outside of the flange (heads in) versus to the inside (heads out). Further, from measuring a few actual hubs, it is obvious that Spocalc's hub database dose not contain consistent flange data. The dimensions for some hubs are of the outside of the flange. Others are of the inside of the flange. Yet others give dimensions somewhere within the flanges, perhaps an eyeballed midpoint. And a few are just plain wrong.

Spocalc calculates the distance from the hub to the ERD. The slot in a standard DT nipple seems to be 0.5mm beyond the ERD. (A bog standard DT Champion spoke can extend 2.0mm beyond the slot before the threads jam. The same spoke will show threads outside the nipple until it is within 1.5mm of the slot.)

Weaving spokes at the cross point requires both spokes to be about 0.1mm longer than the straight line calculation.

Tension differences in a dished wheel require the length delta to be increased by about 0.2mm: The NDS spokes have lower tension, so they stretch less, which means they need to start out longer. This is in addition to already being the longer spokes.

Adding, 0.7 plus 0.5 plus 0.2 plus 0.1 is 1.5mm. That's significant!


Reasons why Spocalc gives lengths that might be too long:

The flange position uncertainty may calculate a length that is too long (ie 0.7mm) for the flange inside (heads out) spokes.

Spokes stretch. If your spoke manufacturer doesn't account for that in the advertised length, your spokes will end up a bit long.


The executive summary: Spocalc is an imprecise tool. You need a more precise tool to use the same spoke lengths for the left and right of a dished wheel. Otherwise, you need to aim for the middle of the length ranges and buy two different lengths. Hopefully the correct lengths will fall within the range and things will work out.

Considering the margin of error for many of the calculators and real life applications, I try to get as close as possible because you never really know which way they go. I have never had a spoke come up too long, but I have had some come a little shorter than I prefer (still had 1-2 threads showing after tensioning). Still, by rounding as close as possible, I have yet to get stuck with the wrong spoke length using Spocalc or Dan Halem's online calculator. I usually run both and check for discrepancies, which usually come within .5mm anyway.

I don't think that difference is that great. Playing around with Spocalc by adding/subtracting 2 mm (flange thickness and therefore the difference between heads in or out) I only get around 0.2 mm for the hubs sizes I tried it on.
But according to Spocalc itself, it measures spoke length from the center of the flange, not from one side of the flange, so the difference between heads in or heads out spokes will be half of the the above figure, around 0.1 mm.

I also measured 2 pair of spokes spokes; I used my Cyclus nipple driver bit with adjustable pin. Inserted it in the spoke slot and extended the pin until it hit the top of the spoke. (The wheel was build with 14 mm nipples and therefore the ideal spoke length is 1 mm below the slot). I measured how long the pin was extended with a very good ruler, resetted the pin and measured again. I did 3 measurements for each spoke. The readings generally came out between 2 and 2.3 mm. I could not reliably measure any difference between heads in or heads out spokes, nor between weaved or just crossed spokes. I also checked with my Park Tools TMS-1 tensiometer that the spoke tension was very close.
Now, there are obviously limits to the precision of my measurements, but I feel confident that a whole 0.7-0.8 mm length difference could be measured by either the ruler or the tensiometer. Besides my measurements are in line with the calculations I performed with Spocalc.


No one has really taken charge of the Spocalc database since since Rinnard, so no one can correct the errors, but even DT- Swiss's online spoke calculator has lots of faulty data, and eg. Mavic doesn't provide ERD for their rims but "nipple seat diameter" which is something that only Mavic knows what is. The morale is; trust no one and make you own measurements.

ERD isn't a strictly defined term, but it is measured as the distance between two opposite spoke slots. The reason is that you are targeting a spoke length that terminates with the spoke end being flush with the nipple slot since that will give you around +-1 mm of thread on a standard 12 mm nipple. (yes you can extend it more than 1 mm by crushing the thread a little). So ERD is something you measure on the rim, but it is also the point where you want you spoke to end, and you want your ERD to be at the bottom of the nipple slot so the spoke end will be flush with the slot, not 0.5 mm before or beyond that point.
In short, there will be nothing in this that will make Spocalc calculate shorter or longer spoke lengths, so your 0.5 mm deviation is wrong.




That argument simply doesn't make sense to me. First of all NDS and DS spoke lengths and their tension really doesn't have anything to do with each other regarding calculating spoke length using the spoke length formula. In both cases you want to terminate the spoke at the ERD point and therefore use the spoke length formula in Spocalc to calculate an ideal length. The number Spocalc gives is a "raw", ideal distance in form of a number with one significant digit, without any regard to the fact that spokes elongate in tension and rims compress. None of the spokes "need to start out longer" in fact they need to be shorter than the Spocalc's raw number says. It is true that DS and NDS spokes elongate to different lengths because of difference in tension, but the difference is small, and it will only mean that the spokes will become longer, not shorter, so NDS, DS tension differences will not mean that you will end up with shorter spokes.

I only get a maximum of 0.2 mm (heads in/out+weave), insignificant, especially compared to how much spokes elongate.

To summon up, no factors will make spoke lengths shorter than their calculation to any significant degree, while spoke elongation will add significant length to a spoke in tension. Therefore one should always round down when choosing spokes. That is also the advice of all wheel builders I know of or have read about.

Spoke manufacturers never take account for spoke elongation when they advertise the length of their spokes.


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Regards

1.4 mm round down is safe to do on the front wheel. There just might be some thread showing, but as long as we are not talking several millimeters that should be safe.

You might get away with using only 295 mm spokes on the rear, but you can easily risk that they are too long and therefore that it will cost more money than if you had more suitable spoke lengths from start. I wouldn't do it myself.

--
Regards

Hyper correct spoke length should have just a tiny hair of thread showing at the top of the nipple anyways ...

Huh?

You're going to have to explain that one because I think it's crazy. My normal wheelbuilding process is to stick my thumbnail into the last spoke thread and spin the nipple until it my thumbnail stops it. Fron there it takes maybe 4 full turns to bring the wheel up to tension.

thanks all. I used 296mm for the front wheel and it came out fine.

Plan is to go with 294mm drive side and 295mm non-drive side to avoid too-long situation.

Re: spocalc database comments, I have not been able to find the actual measurements of any hubs I use in real life, so I measure them myself and add them to the spocalc xls, in effect creating my own database. If anyone wants the (approximate) measurements for a formula th21 flip flop hub or a circa 1985 maillard 6-speed freewheel, let me know. cheers

Try these numbers: ERD 598, 32 spokes, 3x. Left flange outside 35.9, left inside 32.45, right inside 18.85, right outside 22.3. (That's a 3.45mm flange thickness.) Keep in mind that the spokes lie _next_ to the flange which increases the angle and length difference, so add/subtract an extra 1.0mm (for 2.0mm spokes.) Thus calculate with 36.9, 31.45, 17.85, 23.3. I calculate a left side length delta of 0.65mm and right side of 0.40mm.

Large wheels, narrow hubs, thin flanges, and thin spokes (ie road wheels) will reduce the delta. The converse (ie mountain 26" wheels) will increase the delta.

Looking at its formulas, it simply calculates at the offset(s) entered. It does nothing to consider flange thickness.

I looked at one of the first wheels I built. Even amongst spokes that should be identical, the spoke ends are all over the place. That wheel was true with tension identical to within one unit on the Park gauge. Thus I think it is possible to true a wheel that measures fine but doesn't have the nipples turned to the "correct" spot. A small calculated length difference might be lost in the "noise".

When building the 598 ERD wheel above, I could _see_ the length differences as I turned the wheel: Long, long, short, short, long, ...

True, true!

Hmmm. I should bring a precision measuring caliper and measure the local bike co-op's ERD tool to see where on a standard nipple it places the ERD.

Using the formula from Jobst Brandt's book, for the above wheel, the NDS spokes stretch 0.27mm (@ 60 kg-force) and the DS spokes stretch 0.44mm (@ 100 kg-force). If one started with spokes with a 2mm length difference, the difference will be only 1.83mm when tensioned. (Example: 292mm NDS spokes stretching to 292.27, and 290 DS spokes to 290.44.)

If a 2mm tensioned difference is needed, then the wheel needs to be built with spokes that differ by 2.17mm. Of course, such sizes are not offered for sale. This may necessitate starting with a 3mm difference.

Every DT Swiss spoke (admittedly not many lengths) I've ever measured has been a fraction of a millimeter short of the "marked" length. Using a precision caliper, I measured the DT Alpine III spokes I'm currently building with. Both lengths I bought are 0.3mm short. At 70 kg-force, according to the formula, these spokes elongate ... 0.3mm.

Long, long, short, short is what happens when you get the side-to-side pattern between the hub flanges off by one hole. that's easy to do.

I agree. There's no reason to see threads because the nipple has about 2mm of unthreaded length to hide the threads anyway. To me, the ideal length can almost hide the threads before I start bringing up the tension.

(Trying to think how that pattern would happen... Hmmm, if left rim holes were laced to the right (and right to the left), that would result in all spokes a bit short. If, instead of lacing to the 3x holes, one side was laced with leading spokes to 4x and trailing to 2x - then I think the lengths would be too far off to even assemble the wheel.) Can you expand on your statement?

The actual pattern on the rim was more like:
OK,
OK,
a little further above the slot (yup, an inside spoke),
this spoke could'a been a mm shorter (the other inside spoke),
OK,
OK,
a little...

The holes in the hub flanges aren't directly across from one another. If you lace the first spoke in the second flange in the wrong hole, the spoke pattern on each side will be correct but the pulling spokes will seem too long and the spokes going the other way too short or vice versa. When you tension the wheel your spokes will seem long, long, short, short all the way around the rim.

That is because flange thickness really doesn't play any significant role, even with a whopper thick flange like yours. I think the problem is that you are trying to track the physical length of the spoke. Now this may sound like the obvious thing to do, but the snag is that spoke length is a little more complicated than that:

The length of a eg. DT Swiss spoke is measured from the top of the thread (obviously) to the inside of the the spoke head, not the spoke elbow. When measuring a spoke length with a ruler, the ruler and spoke therefore forms a triangle. Measuring the spoke length is akin to measuring "virtual top tube length" on sloping frames.
Here is a primitive diagram on from where one measures spoke length: from the X, where the spoke will support itself on the hub flange when the spoke head is pressed against the flange:

http://upload.wikimedia.org/wikipedi..._rectangle.png
http://en.wikipedia.org/wiki/Right_triangle

Imagine that the A to B is the length of the spoke from elbow to thread, and A to C is the length of spoke from the elbow to spoke head. Then C to B will be the "virtual spoke length" which is the spoke length as advertised by DT Swiss on the outside of their spoke boxes.

The length of the spoke as advertised on the box is the "virtual length", or the hypotenuse on a right angled triangle. So a 250 mm spoke will always terminate 250 mm above* where it is supported on the hub flange, regardless of elbow length and flange thickness. Therefore stuff like how close the spoke lies to the flange etc, really doesn't matter regarding spoke length.

*At a right angle. Of course, one side of the flange is slightly farther away from the rim than the other, and will therefore "travel" slightly longer to reach the ERD. But the difference between heads in/out spokes is at max the difference that flange thickness causes, and since Spocalc measures from the middle of the flange, then effectively this deviation is cut in half. Eg. with a 4 mm thick flange the difference will be like adding/subtracting 2 mm to WL and WR in the formula. And since spoke length is actually measured from a point inside the flange, this will be a max deviation.

Playing around and adding or subtracting 2 mm from WL and WR (a 4 mm flange thickness) on a 26" (540 ERD) high flange (74 mm) dynamo hub 3X, 36H front wheel only gives me differences around 0.2 mm, rarely 0.3 mm, and that is a pretty extreme case.


We can certainly agree that the noise level is pretty high when using crude instruments like the TMS-1, but still, it is not my experience that spoke ends are all over the place when relative spoke tension is close. My own measurements of 4 adjacent spokes plus the well built wheels I have seen seems to support that too.
One really ought to measure or even see a 0.5-0.8 mm length difference between heads in and heads out spokes if such a discrepancy existed. There can be several explanations for your observed data like spoke wind up, but Retro Grouch's explanation seems to be the best since there seems to be some kind of system in how your spokes deviate in apparent length.

That really wouldn't help, since all you would do is to measure that bike tools definition of what ERD is. The point is that ERD is a practical guideline that depends on the implementation of spokes and nipples. This works out in practice since most spokes and nipples are "standard" ones as defined by tradition. But take eg. DT Swiss's 14 mm nipples. With those nipples the wanted termination point for the spoke is 1 mm below the nipple slot, since ERD is the termination point, that will mean that the ERD for a given rim depends on the nipple implementation. So ERD isn't a fixed number for a given rim. In practice however, ERD is a fixed number assigned to a given rim, like when Rigida prints the ERD on the rim label, and people using 14 mm DT Swiss nipples just subtract 1 mm from the calculated spoke length.


Due to an unfortunate mislabeling accident I was forced to measure many dozens of spokes in many different lengths. :notamused: So I can say with some confidence that the many spokes I measured from both Sapim and DT Swiss came remarkably close to their advertised length as a general rule, and certainly not 0.3 mm to short. I suspect the discrepancy you measure are due to your measuring methodology. Here is IMO how to measure spoke lengths the correct way: take a metric quality steel ruler. Place the spoke head at the bottom of the ruler. With one finger press the spoke head "flare" up against the ruler, so that the spoke is lifted above the ruler and therefore forms a triangle together with the ruler when seen from the side. Measure the spoke length.

--
Regards

(Lets assume 36 hole 3x wheels for this reply.)

OK, I get it now. However...

Lets say that all the spokes on one side of a previously correctly built wheel are being replaced. The spokes get laced one hole (20 degrees) off from where they should be. The result is a 2x/4x lacing pattern as described here: http://www.terminalvelocity.demon.co...ld/crossed.htm Calculating spoke lengths for a typical small flange front road wheel shows that 3x spokes won't work in that 2x/4x pattern. They'll be either 6mm too long or too short.

Lets say that a fresh wheel was assembled with this error. The hub can turn to equalize the error across both sides. They'll now be just 10 degrees off. The lacing pattern on both sides could be called 2.5x/3.5x. The spoke lengths will now be just 3mm long or short. OK, such a wheel might make it to a truing stand before the error is caught.

Lets assume a rear wheel (numbers are for a Tiagra DeepV). The 3x spokes are between 5 and 8mm long or short for the 2x/4x pattern. Even for 2.5x/3.5x, the spokes are 3 to 6mm long or short. The error would be caught before this wheel is fully assembled.

Hmmm. (trying something) Aha!

Assume a symmetrically laced rear wheel assembled without error. Turn the hub just 2 degrees the "wrong" way. The lacing pattern on both sides could be called 3.1x/2.9x. All the spokes end up about 1mm long or short. They'll be in a LLSSLLSSL... pattern.

Now turn this hub just 0.7 degrees the other way. This could be called 2.965x/3.035x. Now all the spokes reach almost the same spot in the nipples! The only casualty is a 2kg-force tension difference. That's small enough to be unmeasurable.

If a wheel is not symmetrically laced (ie the Shimano rear disc wheel recommendation), then turning the hub either way causes the LLSSLL... pattern to show up and get worse the more the hub is turned.

So - a LLSSLL... pattern is most likely a nipple tightening error, not a lacing error.

You're telling me that you think that you mis-tightened all of the spokes the exact same amount and still achieved a true wheel with even tension?

And you think that's a more likely answer than a side-to-side laceing error that would affect all of the spokes the same?

Hey, it's not my wheel so I don't care.

Over/under is irrelevant to a certain extent. Over and you'll bottom out the spokes, under and you dont have enough thread engagement..

There is zero difference in tension if a spoke is 2mm below the slot or 2mm above it. As long as adequete thread engagement occurs anything else is merefly fluff and doesnt matter.

Before any of you go off about length being a determining factor in tension I'll agree it does. Length is a fixed value thats determined by ERD of the rim NOT the length of a loose spoke. Nothing more, nothing less.


A short time ago I determined that maximum strength against stripping the threads out of a standard DT nipple occurs when the edge of the nipple is even with the bottom of the threads. when the nipple is even with the threads the spoke end sits about 1mm below the slot. figure thats 2mm from the top of the slot. Threaded all the way down there's about 1mm that extends past the top. I figurte one can safely go -2mm and upto +1 possibly +2mm form what spocalc calculates.

A side to side lacing error would produce a bigger variation than the what I saw. All the spokes ended above the slot and below the top of the nipple - so about 1mm of range. By "long" and "short" I mean "this one's approaching the top of the nipple" and "this one's down near the bottom of the slot".

Laura, your problem may have to do with how your rim is drilled. If your wheel is lying flat in front of you with the valve hole nearest you, whats the orientation of the spoke hole immediately to the left of the valve hole?

Is it high or low? Hub lacing is dependant on this.

Perhaps this could explain what Retro Grouch meant:
http://www.bikecyclingreviews.com/fa...d_Problem.html

Basically it is all about getting the first (key) spoke right when lacing the wheel.

--
Regards

Grrr. When I used the words long and short, I did not mean too long and too short. I meant that all the spokes from the outside of a flange came to about the same level in the nipple. Spokes from the inside of that flange reached about one thread further. I'm calling the outside spokes short, and the inside spokes long. Ditto with the other flange, except of course to two different levels.

Centered. Actually, offset 4mm left of the center - but they all are. (Velocity Aerohead OC)

There is nothing wrong with the wheel being discussed. It trued to within the thickness of a piece of paper. Tension was equal (on a side - its a rear wheel) to within one unit on the Park tension gauge. Dishing may have been off by half a millimeter or so because of an out of calibration truing stand.


Lets end this thread. I'm sure the OriginalPoster has long since had his question answered. I'll build my wheels my way, and you all can build your wheels your way.