spoke length calculator

Don't have the exact measurements with me right now (will edit the thread when I get home), but I have been using roger musson's calculator to determine spoke length for my factory built Giant PR-2 wheelset.

I understand the spoke length varies depending on how you lace the wheel, and I'm assuming that my rear wheel was laced 3x (spoke moves over 3 holes from hub to rim, yes my terminology is way off!)

And the calculator is throwing out measurements that are way off the spokes that I have previously removed from my wheel (5+ mm). The reason I'm re-building the wheel is because I kept losing spokes when the wheels had done less than 800 miles. How likely in your experience is it that these factory built wheels might be using the wrong sized spokes or have I messed up my own measurements (I'm 99% sure these are accurate within 1-3mm)?
 
Location
Loch side.
If the old spokes were too short, they would have shown thread below the nipple. Had they been too long, they would not have been able to tension up properly because the nipple would have been out of thread to run on. In other words, they were right.

If you had made a 3mm error on the ERD, measurements, your spokes will be way off. Why not just use the length you had in there?

If you post the various dimensions I'll calculate the length for you.
 

Ian H

I am an ancient randonneur, & I stop often for tea
Location
East Devon
Unless you could see the thread under the nipple, or there was 3mm or more poking out above the nipple, the spoke length was fine. 3x means the spoke crosses 3 others. That means two adjacent spokes that cross by the flange would have 9 holes between them at the rim. Lacing means taking the outside spoke under the last crossing (inside) spoke (so for 3x over the first two and under the last one).
Surprisingly large inaccuracies in spoke length can result from inaccurate measurements of hub and rim.
 
OP
confusedcyclist

confusedcyclist

Veteran
Why not just use the length you had in there?

I could do that, but wasn't sure if the broken spokes was because of the incorrect sizing. There was no issue with tension or exposed threads on the spokes previously, ideally though I'd like to get the same measurements myself before heading out for replacements, just as a learning experience as I feel just using giant's spokes as a reference would be "cheating".

I was having a tough time measuring using a flexing tape measure so i'll chalk it down to my inaccuracy, I'll try get hold of a proper steel ruler instead before posting my measurements.
 
Location
Loch side.
I could do that, but wasn't sure if the broken spokes was because of the incorrect sizing. There was no issue with tension or exposed threads on the spokes previously, ideally though I'd like to get the same measurements myself before heading out for replacements, just as a learning experience as I feel just using giant's spokes as a reference would be "cheating".

I was having a tough time measuring using a flexing tape measure so i'll chalk it down to my inaccuracy, I'll try get hold of a proper steel ruler instead before posting my measurements.
Spokes don't break because they're the incorrect length. They break in fatigue - metal fatigue. Unless of course someone chucked a broomstick into the wheel. You can't measure the ERD with a tape measure. You need to make yourself a little tool for the job or understand the concept and Heath Robinson it with a nipple and vernier. Cheat and get the job done. It is no fun buying spokes for the fun of it and you'll do lots of that if you don't get the calculations right.
 

albion

Veteran
Location
South Tyneside
I would say that they mainly break because of lack of tension (loose spokes). Too much tension does the same too but it is lack of tension that is the main culprit.
Its why 2 or 3 close by might go shortly after one breaks breaks, caused by a ride home on loose spokes.

Re- spoke length. You will have the wrong rim or hub. I bet brand wheels chop and change their supplier.
 
Location
Loch side.
I would say that they mainly break because of lack of tension (loose spokes). Too much tension does the same too but it is lack of tension that is the main culprit.
Its why 2 or 3 close by might go shortly after one breaks breaks, caused by a ride home on loose spokes.

Re- spoke length. You will have the wrong rim or hub. I bet brand wheels chop and change their supplier.
No, spokes don't break from lack of tension or excess tension for that matter. The fatigue limits of steel in tension but below yield is a well understood science. Besides, a loose spoke receives no cyclical stress in anyway since the nipple will just lift from the rim and the spoke will do no work in the cycle.
Further, the adjacent spokes to a broken one do not break because of a ride home on loose spokes.
 
Don't have the exact measurements with me right now (will edit the thread when I get home), but I have been using roger musson's calculator to determine spoke length for my factory built Giant PR-2 wheelset.

I understand the spoke length varies depending on how you lace the wheel, and I'm assuming that my rear wheel was laced 3x (spoke moves over 3 holes from hub to rim, yes my terminology is way off!)

And the calculator is throwing out measurements that are way off the spokes that I have previously removed from my wheel (5+ mm). The reason I'm re-building the wheel is because I kept losing spokes when the wheels had done less than 800 miles. How likely in your experience is it that these factory built wheels might be using the wrong sized spokes or have I messed up my own measurements (I'm 99% sure these are accurate within 1-3mm)?
5+ mm difference does not equate to 1 to 3mm error in your measurements. A very small error on A and B flange distances makes hardly any difference and you can check that for yourself. 3mm error on the ERD measurement will give you about 1.5mm difference either way from the correct spoke length. So my guess is that you are making bigger errors. I wonder how you are measuring ERD.

The other point is why are you not using the same length spoke that you are replacing? Measure booth sides in case you have a disc hub, different lacing or it's a rear wheel.

I don't think the factory wheel has the wrong size spokes, much more likely that the spokes were under tensioned.

Finally, if you don't know what you are doing then you will still be braking spokes after a while. My advise is to learn first and then rebuild your wheel. Roger Musson books is excellent. If you are pressed for time and can't read the book then I suggest you at least learn about pre lacing spoke lubrication - spoke stress relieving - spoke twisting - balanced spoke tension and you will have a good chance of rebuilding a good wheel :smile:

Good luck and have fun.
 
OP
confusedcyclist

confusedcyclist

Veteran
What were you measuring with a tape measure?
I measured the gap between two 200mm cut down spokes on their nipples, just like in roger musson's book. I even took several measurements and averaged them out, the ERD was 595, pretty confident about this one tbh as this was the easiest to measure. The hub was tricker due to its shape. I'll have another bash at this with a steel ruler shortly. Previous measurements as it stands...

X was 31mm
Y was 50 mm
Z was 70 mm

based on the guidance in the book that left me with

A = 55
B = 55
C = 39
D = 20

I think the tape measure is perhaps to blame for inaccuracy, its the firm retractable kind, not the tailoring type.
 
I measured the gap between two 200mm cut down spokes on their nipples, just like in roger musson's book. I even took several measurements and averaged them out, the ERD was 595, pretty confident about this one tbh as this was the easiest to measure. The hub was tricker due to its shape. I'll have another bash at this with a steel ruler shortly. Previous measurements as it stands...

X was 31mm
Y was 50 mm
Z was 70 mm

based on the guidance in the book that left me with

A = 55
B = 55
C = 39
D = 20

I think the tape measure is perhaps to blame for inaccuracy, its the firm retractable kind, not the tailoring type.
So, your hub is 140mm wide, locknut to locknut, right?
 
Location
Loch side.
I can't relate to X Y ad Z without the book in front of me but a 140 OLD is not right. Mountain bikes are 135, road bikes 130, tandems 145 and some downhillers 150mm.
595 ERD is possible. That means it is a reasonably large profile 700C rim.
 
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