The correct spoke length is one that penetrates the nipple just right — not below the screwdriver flats and not past the top of the nipple. You don’t want to go too short because spoke threads reinforce nipples internally. Nipples, particularly aluminum nipples, have higher failure rates when paired with short spokes. You don’t want to go too long either. I define too long as past the end of the nipple. This is strictly true for some nipples however the nipples sold here can tolerate a couple turns past the end without issue. Either way turning past this threshold causes nipple threads to grind into the unthreaded spoke shank. This causes weakness in the spoke and is associated with spoke breakage.
The rounding part
Spoke length calculators output lengths to the tenth of a millimetre, which must be rounded to match available supply. There’s no universal rounding algorithm because it depends on how you calculate spoke lengths in the first place. It depends on ERD measurement technique.
In my process I use measuring tools based on theoretical spoke penetration to the bottom of the nipple screwdriver slot. I don’t want my actual spoke penetration to be short of this mark, so I’m cautious about rounding down. I generally round up to the next millimetre although rounding down half a millimetre is fine when building to high tension (e.g. 125kgf / 1200N). Under tension you can expect spokes to stretch a fraction, notably high tension with lightweight spokes.
(Hopefully you can see this rounding logic doesn’t make sense if your theoretical spoke length targets the end of the nipple. In that case you would rarely round up.)
Traditionally spokes are stocked in two millimetre increments, which invites compromises depending on your calculated lengths. You can be forced to reckon with the the risks of going too short or too long. Maybe the uncertainty steers you to brass nipples instead of aluminum. Maybe it causes you to pick up nipple washers as an insurance policy. Bottom line: it’s not doing you any favours and to build the best wheels you ought to shop in one millimetre increments.
About this series
This is a series about spoke length. These aren’t introductory articles but rather a collection of practical hints. The focus is correct inputs to spoke length calculations — correct input leads to correct output — and how to use outputs. For more general information, see my books list.
This is a reminder for those who may have missed the announcement on Twitter: SpokeService will close on December 24 for the remainder of the year. The time is needed for shop updates, tool service, inventory and reams of government paperwork. On a best-effort basis some orders may ship during this period but no guarantees — place orders now to avoid disappointment.
Happy holidays from SpokeService. Wishing you tailwinds and round wheels into 2019!
Some parts of Canada call for a different approach to winter riding. These wheels are the full fat approach, marrying Industry Nine fat hubs to DT Swiss BR710 rims. The hubs are 142×15 in the front and 177×12 in the rear. These wheels are going to be rippers in the snow.
These rims are a good example of ERD issues discussed in my last blog. The DT Swiss rim decals include technical information stating ERD to be 549mm. I like that but it doesn’t change my process — I measured them anyway getting 547 on the front and 546 on the rear. To add intrigue the DT Swiss spoke length calculator lists 547 for this rim. Going my own way resulted in perfect penetration, a hair below the top of the nipple in each case. With single wall rims you want to be more fussy about spoke length, particularly avoiding penetration past the top of the nipple.
Because spoke lengths depend on ERD, I always measure my rims. People tell me they’ve ordered spokes for use with XYZ rim and sometimes it leads to conversations like this:
Wheelbuilder: These spokes are for my build with XYZ rim.
SpokeService: Did you measure your XYZ rim?
Wheelbuilder: No I used the numbers from the website.
SpokeService: Sorry your spoke lengths are wrong — the site is confusing.
There are two ways ERD goes wrong.
First there are variances in rim manufacturing. Some manufacturers are better than others and to some extent you get what you pay for. But I always measure and frequently find rims out of spec or find differences between pairs of identical rims. If you know what you’re working with, variances are not a big deal. If you input correct dimensions, you’ll get correct spoke lengths.
The second issue is differing definitions of ERD. Typical definitions include penetration of the spoke into the nipple head, which is important for maximum strength. But how far the spoke extends beyond the nipple seat (into the rim) is unclear. It depends on nipple geometry and encodes some preference for how much penetration is ideal. In other words ERD is relative to an undisclosed nipple and opinions regarding its use.
Typically the magnitude of these errors is pretty small, maybe 0.5mm each. Additional error is introduced when you round spoke lengths to match availability (why buying spokes in 2mm increments is rough). Maybe these errors add up to a problem and maybe they cancel each other out.
This sounds like a pain but it’s not at all. You simply make your own tools and take your own measurements. The parts cost next to nothing, assembly takes 10 minutes and you’re left with tools you can use forever. Maybe it seems like a pain because you want your spokes and rims to arrive at the same time. That’s fair but getting it right the first time is always fastest. SpokeService is here to help with spokes in 1mm increments, shipped daily from Canadian soil for five bucks.
(If you like this idea but don’t have the time to put the parts together, I’ll do it for you. Click here to order the spokes and nipples for your measuring tool. They’ll ship completely assembled. Use the checkout comments box to indicate which nipple you intend to glorify with use on your wheels.)
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