WallbrownF

Okay...so, I'm googling how to replace spokes and measure spokes and spoke this and spoke that and I still can't figure out how to find out what the length of my spoke is to buy a new spoke to replace a spoke on my bike.

From where to where do you measure from?

FBinNY 

spokes are measured from the inside of the elbow to the end.

dabac

Easiest is to remove a spoke from the wheel, measure it loose. I prefer to use a millimeter graded ruler, with the grading starting at the end. Just hang the bend over the end, see where the spoke stops.

WallbrownF

K, I figured it was something like that, but I just wanted to make sure.

I figured that it would be best to measure it loose and not stretched. I just wish I had a millimeter graded ruler...

Bill Kapaun

IF you are replacing ONE spoke, just take off a good one and head down to the LBS.

FBinNY 

But you can multiply by 25.4. (though you'd also have to convert fractions to decimal)

for the mathematically impaired use the LT 1" conversions to convert fraction to decimal, add the rest of the length, and enter into the automatic converter. 1e. 11-19/32 = 11.5938" = 294.482...mm

dabac

I'd be more concerned about measuring between the right spots than stretch errors. A standard, 2.0 mm spoke really won't stretch that much.
The big advantage of measuring a loose spoke is that it makes "inside elbow" and "end-of-spoke" much more defined spots, and you'll have a straight run between them.
Another concern, particularly on an older wheel/bike, is previous history/quality of repairs. Worked on a wheel that didn't make sense until I figured out that it had 3 spokes of two other lengths lurking among the others on one side.

FBinNY 

Spokes stretch less than 1/2% of their length under tension. So the most you'd be off is 1mm, and I don't think it's that much.

WallbrownF

No LBS.

I found a cm ruler.

I am worried. I didn't eve think to measure the width. Indeed.



Do I measure from where I circled? That would make it 292mm, right?

FBinNY 

Yes to both (assuming the other end is at "0")

Al1943

I'd say that depends on the spoke. I've had DT Revolutions stretch quite a bit, certainly more than 1/2%. That's why I quit using them on the drive side rear.

FBinNY 

Let's define terms. At proper working tension (within the elastic limit of the spoke) a spoke will elongate from tension, and return to it's original size if the tension is relieved. The most that a spoke can stretch this way staying within the elastic limit is about 1/2%.

OTOH, if you stress the spoke enough to stretch it beyond it's elastic limit, it will stretch and no longer return to it's original length. Essentially, you've cold set the spoke to a new longer length. Many builders forget to adjust the working tension to the spokes' capacity when working with light spokes, taking them beyond the elastic limit, and later coming back and re-adding more tension, repeating the process until they run out of thread.

It's fairly simple, if you permanently elongate any spoke, it's because it was stressed beyond the yield point, ie. built too tight for the strength of the spoke.

While I'm here, a bit of spoke strength math. If you reduce a spokes gauge by 25%, as in going from 2mm to 1.5mm you'll reduce it's load capacity not by the same 25%, but by 44%, since the strength is proportional to the square of the diameter. Knowing that, you can see how easy it would be to overload (and stretch) light spokes if building to the same range of tension you're used to using with 2mm spokes.

Asi

Throw away that "cm ruler" back to grandma's knitting box.
That thing has more stretch than any spoke in the world even abused ones.
Use a metal ruler, use a woodworking tape measure, use some long calipers (ok, this is hard to find at 300mm), even a plastic ruler used in school (or better yet used in technical hand drawings, like stuff made by Rotring).

I'm just when I see stuff like this. I don't say it's not 292mm, it may well be (or anything between 290 and 294), just use the proper tool for the job, that in this case is any metal ruler regardless of units of measure as long as it has units and proper resolution. After that is just a lightning quick calculus for converting to mm.

FBinNY 

Don't get so worked up. A quality tape measure from Grandmas box has surprisingly little stretch. I have a nice roll up 6 foot cloth/plastic? tape that I've had for eons. I checked over 36" compared to my steel yardstick, and it took considerable tension, far more than I'd ever accidentally apply to get it to stretch more than 1mm.

Of course these come in a variety of materials and qualities, but odds are that when the OP takes your advice and gets a steel rule, he'll still need a 292mm spoke.

Drew Eckhardt 

That's not relevant in wheels with conventional spoke counts and quality spokes.

Tension in the lowest spokes drops to support the load, with minimal increases in the rest of the wheel. Load in an assembled wheel is therefore limited by the point at which spoke tensions becomes 0 leaving the rim laterally unsupported or the point before that at which tension drops low enough that the nipples unscrew which in turn comes from the spokes' initial tension.

With conventional spoke counts and shallower rims you get to the rim's elastic limit before the spokes' so it doesn't matter. With conventionally thickness extrusions you get enough tension in the spoke bed for radically reduced fatigue life in the rim before you get to the spokes' elastic limit or point where they'd have unacceptable fatigue life so it doesn't matter either.

Hardly.

Measured yield strength on DT 2.0mm spokes is somewhat past 3250 Newtons or 330kgf with the failure in the elbow.

Measured yield strength on DT 1.8/1.6mm spokes is somewhat past 2250 Newtons or 230kgf with the failure in the thin section.

Extrapolating, one would expect 1977 Newtons or 200kgf out of a swaged spoke with a 1.5mm midsection like a DT 2.0/1.5m Revolution.

I used DT 2.0/1.5mm Revolutions on the last wheel I built with average drive side tension of 124 kgf with a 129 kgf maximum.

The spokes didn't stretch and loose tension following the severe overload produced by stress relieving.

After getting a tension meter I found that I'd built my five-day-a-week front wheel with the same 110kgf tension (32 spokes in a Mavic Reflex Clincher) using 2.0/1.5 DT Revolution spokes that I'd try for with the tension meter. I'm pushing 16 issue-free years on those spokes (I bought into conventional wisdom about thicker rear drive side spokes at the time so those are 2.0/1.8 and not a test case).

Asi

I'm not the "right tool for the right job" man, but a "right precision on any tool that fits the job". Resolution and precision here are about 0.5mm (plus/minus) rounded to nearest (or to smaller if permanent stretch of the spoke is accounted). Well since I'm in Europe and my mother and (past) grandmothers are avid knitters I have a lot of this cloth/rubbery/plastic tape measures and they suck. I can stretch half a meter of them by more 5-20mm (depends) with very light and modest pressure to keep it straight (if I strain myself, before breakage as much of 50% is off so 500mm becomes somewhere 600 to 800mm). With bad quality plastic rulers (chinaware for 0.05$) I put my calipers on and found 0.5mm short over 175mm - that is unacceptable, impov.

Yesterday I saw someone to measure geometry of the wheels on a car by strings. - the method worked well for many years for those in this trade, and the talent to do with that is unquestionable (yet no better than a monkey brain instructed on how to use laser align tools), but NOT WITH PLAIN ROPES that are soooo stretchy - so a rant came and swore not to touch a foot in that shop anymore. - conclusion: I gave a beer to whom held the key to the laser align lab and did it myself.

FBinNY 

Thanks for the correction, but a quick question. Are the figures you used, tensile strength or yield strength. The point is that yield strength in Stainless steels is typically less than 1/2 the tensile strength. I did a very cursory calculation using your numbers, and I suspect the values cited were tensile strength. Otherwise, unless I flubbed the math, (not impossible) DT has somehow come up with a steel more than twice as strong as any alloy available here in the USA.

The difference is key. I wasn't posting about spoke breakage, but about spoke elongation resulting from being loaded beyond the yield, not tensile strength. The spoke will hold up fine, but now being a bit longer in a structure which defines it's length it will be under reduced tension. The phenomenon Al1943 experienced, and to which I was responding.

Also, while your analysis of stresses in a rolling wheel is correct, it doesn't describe all the stresses to which spokes are subjected. In the real world wheels can suffer fairly high side loads. We all see pictures of bikes leaned over when sprinting, but road hazards also beat up wheels. A common one is the stone or small piece of tree limb which the wheel climbs halfway up before pinging it sideways. Landing from this creates a large transverse shock load. Then there's my favorite, hitting wet leaves in the fall while negotiating a curve, The bike begins to slide out since the leaf slips on the pavement. When the leaf wears through the tire bites. I've seen this taco a wheel instantly.

So while smooth pavement is unlikely to take a spoke beyond it's elastic limit (not the breaking point), the real world can take a wheel whose dead load is near the spokes' yield, beyond it, eventually slackening (not necessarily breaking) light spokes.