# Bicycle Mechanics - novice wheel building question - spoke length

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02-18-09, 03:26 PM
i'm embarking upon a first time wheel building project. i've measured all my parts to calculate my needed spoke length, and came up with front(L/R): 296.4mm, rear(L): 296.5mm, rear(R): 294.8mm

i'm just curious how much play is in the calculated spoke length? are the lengths determined so they end up with the end of the spoke in the middle of the nipple, or closer to the outside end of the nipple?

my curiosity is driven by wanting to just get a box of spokes at the same length since they seem really close to me (but then again, this is my first time). can i get away with using 296mm spokes for all? or will the 1/2mm short on the fronts and on the rear-lefts make them too short? will i have to grind down the extra on all the rear right ones?

thanks for any help.

Panthers007
02-18-09, 05:27 PM
You need to read up much more on building wheels and correct spoke-lengths.

I'll start you off here:

http://sheldonbrown.com/wheelbuild.html

And for a spoke-calculator:

http://vocabforbreakfast.railsplayground.net/edd/

Wanting to use the same (rounded-off) spokes shows you need more information before you proceed. I'm sure others will chime in shortly. I'm fighting a FD on a triple - to those who know...

DannoXYZ
02-18-09, 05:49 PM
The 0.5mm too-short won't be a problem. But the 1mm too-long spoke on the right-rear may bottom out the nipple.

I'd also recommend measuring the rim and hub yourself and plug into Damon Rinard's spoke calculator. I've seen numerous errors from the manufacturer's specs and DT and Wheelsmith just publish the erroneous info without verification.

operator
02-18-09, 09:41 PM
i'm embarking upon a first time wheel building project. i've measured all my parts to calculate my needed spoke length, and came up with front(L/R): 296.4mm, rear(L): 296.5mm, rear(R): 294.8mm

i'm just curious how much play is in the calculated spoke length? are the lengths determined so they end up with the end of the spoke in the middle of the nipple, or closer to the outside end of the nipple?

my curiosity is driven by wanting to just get a box of spokes at the same length since they seem really close to me (but then again, this is my first time). can i get away with using 296mm spokes for all? or will the 1/2mm short on the fronts and on the rear-lefts make them too short? will i have to grind down the extra on all the rear right ones?

thanks for any help.

Do you like to gamble?

Drive/non-drive side spokes usually differ by about 2mm. Unless you are talking about rounding down/up on both length spokes it's entirely likely that you will screw up one side, if you use the same length spoke. Get the proper length spoke. Don't buy into a stupid problem. It doesn't cost that much.

Penny wise, pound foolish comes to mind.

02-19-09, 01:32 PM
thanks all for the help. just fyi, i did measure my hub demensions using a 1/2-way decent caliper. granted i'm using the rim's erd from the manufacturer, but everything i checked/double checked and is accurate. the spoke lengths i referenced were using a couple online calculators (one from the mentioned sheldon brown link).

my concern was mostly based on said link, where this was mentioned: The length is not super critical. Most spoke calculators give results to the tenth of a mm, but spokes are usually sold in 1 mm size increments (some brands only in 2 mm increments.) Generally, I round upward to the nearest available larger size.

i'm a science geek by trade, and lots of the numbers i deal with +/- 0.5mm would be huge. so i wasn't sure what was concidered critical or not. i figured the 0.5mm short thing wouldn't be too much of a problem, since all the spokes i looked at were rounded to the nearest 1mm. i just wasn't too sure about the drive side rear #.

so no, i'm not too interested in the gamble, and that's why i asked before i bought. it seemed like a pretty good deal to get a box, but i'm not interested in dealing with it coming out crappy either.

thanks again!

Panthers007
02-19-09, 01:36 PM
Go low, young man, always go low.

Lawrence08648
02-19-09, 03:03 PM
The too long may not be a problem if you have deep depth rims and the nipple sits down in the hole a bit. It's a problem if you have shallow depth rims and the spoke comes through the nipple and pokes a hole in the rim tape and tube.

interested
02-19-09, 04:45 PM
i'm embarking upon a first time wheel building project. i've measured all my parts to calculate my needed spoke length, and came up with front(L/R): 296.4mm, rear(L): 296.5mm, rear(R): 294.8mm

i'm just curious how much play is in the calculated spoke length? are the lengths determined so they end up with the end of the spoke in the middle of the nipple, or closer to the outside end of the nipple?

The target length for spoke calculation is that the spokes are flush with the "groove" /screwdriver trench* on top of the nipple when in tension and using standard 12 mm nipples. The distance between this "groove" and a similar nipple groove on the opposite site of the rim is the ERD (Effective Rim Diameter).

There are several reasons why this length is the target length, but an important one is, that when the spoke is flush with the nipple groove then there is still 1 mm unengaged thread left on the bottom of the spoke thread. This gives a much needed margin of error or allows further tensioning of the spokes which can be useful at times.

The standard spoke calculation formula is a geometric calculation that gives an "ideal" spoke length. The problem with this is that spokes actually elongate during tensioning (and rims compress), so one has to subtract this elongation from the ideal spoke length or else the spokes will be too long in actual use. At least 0,5 mm must be subtracted if the spoke calculator used gives raw, ideal spoke lengths. IMHO just round down aggressively like removing all digits after the separator: 288.9mm becomes 288 mm etc.

Too long spokes means that they will run out of thread before the correct tension is reached, not much can be done about that. Too short spokes is much less of a problem since eg. 2 mm too short spokes still have plenty of thread engagement left, or one can use 14 or 16 mm DT Swiss nipples instead of 12 mm nipples since these nipples effectively reduces the needed spoke length. A spoke that is 2 mm too short for a 12 mm nipple, is the perfect length for a 16 mm DT Swiss nipple.

my curiosity is driven by wanting to just get a box of spokes at the same length since they seem really close to me (but then again, this is my first time). can i get away with using 296mm spokes for all? or will the 1/2mm short on the fronts and on the rear-lefts make them too short? will i have to grind down the extra on all the rear right ones?

thanks for any help.

IMHO, 296 mm is too long (if this is an "ideal", raw spoke length number). 295 mm might, just might work if you measurements are really good. I would however use both 296 mm and 294 mm spokes.
Regarding grinding down too long spokes; the problem isn't really that the spokes may stick out, but that they run out of thread before the correct tension can be reached.

A really good investment in making good wheels are either Musson's e-book:
http://www.wheelpro.co.uk/wheelbuilding/book.php
or Jobst Brant's "The Bicycle Wheel" 3. ed. They are both cheap and will help you make really good wheels.

*not sure what the English word for this is.

--
Regards

Al1943
02-19-09, 08:46 PM
Too long spokes means that they will run out of thread before the correct tension is reached, not much can be done about that.

+1

This is the real issue. Never round up. When using double butted spokes I always round down at least 1 mm to account for possible spoke stretch. The DT Swiss calculator will automatically round down the rear driveside spokes for their Revolution which are double butted 2.0-1.5-2.0.

BigBlueToe
02-19-09, 09:04 PM
When I was researching this, Harris Cyclery had an online spreadsheet (in Excel, I think) where you entered the model of your rim and hub and it told you what length spokes to buy.

When I built my wheels I bought the rims and hubs and spokes from Harris and they took care of selecting the proper spokes.

operator
02-19-09, 10:37 PM
The target length for spoke calculation is that the spokes are flush with the "groove" /screwdriver trench* on top of the nipple when in tension and using standard 12 mm nipples. The distance between this "groove" and a similar nipple groove on the opposite site of the rim is the ERD (Effective Rim Diameter).

There are several reasons why this length is the target length, but an important one is, that when the spoke is flush with the nipple groove then there is still 1 mm unengaged thread left on the bottom of the spoke thread. This gives a much needed margin of error or allows further tensioning of the spokes which can be useful at times.

The standard spoke calculation formula is a geometric calculation that gives an "ideal" spoke length. The problem with this is that spokes actually elongate during tensioning (and rims compress), so one has to subtract this elongation from the ideal spoke length or else the spokes will be too long in actual use. At least 0,5 mm must be subtracted if the spoke calculator used gives raw, ideal spoke lengths. IMHO just round down aggressively like removing all digits after the separator: 288.9mm becomes 288 mm etc.

Too long spokes means that they will run out of thread before the correct tension is reached, not much can be done about that. Too short spokes is much less of a problem since eg. 2 mm too short spokes still have plenty of thread engagement left, or one can use 14 or 16 mm DT Swiss nipples instead of 12 mm nipples since these nipples effectively reduces the needed spoke length. A spoke that is 2 mm too short for a 12 mm nipple, is the perfect length for a 16 mm DT Swiss nipple.

IMHO, 296 mm is too long (if this is an "ideal", raw spoke length number). 295 mm might, just might work if you measurements are really good. I would however use both 296 mm and 294 mm spokes.
Regarding grinding down too long spokes; the problem isn't really that the spokes may stick out, but that they run out of thread before the correct tension can be reached.

A really good investment in making good wheels are either Musson's e-book:
http://www.wheelpro.co.uk/wheelbuilding/book.php
or Jobst Brant's "The Bicycle Wheel" 3. ed. They are both cheap and will help you make really good wheels.

*not sure what the English word for this is.

--
Regards

This is already compensated for by DT champion spokes out of the box. For example 275mm spokes are actually 274mm. If you go by the length of spocalc and what it says on the box you won't go wrong.

Mike T.
02-20-09, 05:47 AM
*not sure what the English word for this is.
The word to use instead of you "trench" would be "slot".

To the original poster - get the Roger Musson wheelbuilding e-book that was already mentioned. The tolerance on spoke lengths is +/-1mm. The ideal length of spoke is where the top of the spoke sits at the bottom of the screwdriver slot. Whether to round up or down depends on what length of spoke pops out of the calculator compared to the closest available length. Most places only stock even length spokes.

Roger Musson's spoke calculator, accessed from his website, is a great one. He wrote it himself.

interested
02-20-09, 10:02 AM
This is already compensated for by DT champion spokes out of the box. For example 275mm spokes are actually 274mm. If you go by the length of spocalc and what it says on the box you won't go wrong.

I am not convinced you are right. First off all, notice how DT Swiss' own online spoke calculator gives two different numbers, the raw value and a "corrected" value. Below is an example from one of my calculations using the DT Swiss spoke length calculator:

Spoke length :
precise le. 288.30 mm ri. 286.70 mm
rounded (incl. Corrections) le. 288 mm ri. 286 mm .

Notice how it rounds down. I find it strange that DT Swiss would do so if they already had subtracted a whole millimeter from their spoke lengths.

A more reasonable explanation for why you measure DT Swiss spokes to be 1 mm shorter than advertised on the box could be that you measure them in a "wrong" way.

The word on the net is that you measure spokes from the end of the thread to the elbow. So people tend to hold a ruler parallel to the spoke and press it against the elbow. That way you are measuring how long the spoke is from the _bend_ of the elbow. But that method doesn't make sense, since the bend is not where the spoke is supported. What you really want to measure is the longest possible distance between the end of the spoke thread and where the spoke elbow support it self on the hub flange. And that place is at the end of the spoke elbow, just before the "head". So instead of holding the ruler parallel to the spoke, you need to angle it a bit so it rests against the elbow/head so that spoke and ruler forms a triangle. This way of measuring spoke length ensures that one is measuring the actual geometric length of the spoke, which again corresponds with the geometric spoke length formula.

If you try to measure your DT Swiss spokes that way, you will find that they are as long as claimed by the manufacturer. I don't have any Sapim spokes handy, but I am sure that they follow the same standard as DT Swiss.
So I, like so many others, would still say that the safest thing to do is to subtract at least 0.5 mm or round down to compensate for spoke elongation when using "raw" data from spoke calculators like Spocalc.

--
Regards

Mike T.
02-20-09, 10:21 AM
Spokes are correctly measured by dangling them from a dedicated spoke ruler. Then no-one has to guess where they measured to and from.

interested
02-20-09, 11:00 AM
Spokes are correctly measured by dangling them from a dedicated spoke ruler. Then no-one has to guess where they measured to and from.

I disagree. When using my dedicated spoke ruler I can vary the measured spoke length by 1 mm (the same as with my steel ruler) depending on whether the spoke is pressed flat against the spoke ruler, or whether the spoke head is pressed flat against the spoke ruler's backside so that spoke and ruler forms a triangle (the correct way). So using a dedicated spoke ruler changes nothing. One still need a methodology for measuring the spoke length, since different methods yields different measurements.

Having the spoke head pressed firmly against a slot or steel ruler roughly the same dimensions as a hub flange mimics how the spoke will be positioned in the final wheel, while dangling in a slot with the spoke head perhaps 1.5 mm from the flange doesn't.

Regarding the precision of a dedicated spoke ruler, then I'd rather trust a quality steel ruler made by a tool company than some random dedicated spoke ruler. My spoke ruler has some slots for measuring spoke thickness with, but it is hopelessly wrong in its figures compared to eg. my digital caliper.

--
Regards