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  1. #1
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    Rounding Down Spoke Length-- how far?

    First of all, I should come clean: I'm cross posting this here after not getting an answer in Bicycle Mechanics.

    I'm looking to build up a wheel with an internally-geared hub to occasionally use on my fixed gear Cross Check for light touring. I hope the mention of internal gears doesn't generate a lot of boos! I'm going to keep the bike fixed for everyday use, but this will allow me to ride the same bike on tours. I've been intrigued by this as an option for quite a while.

    My calculated spoke lengths from Damon Rinard's spocalc.xls spreadsheet are 283.8 and 283.2 mm for the left and ride sides, respectively. I've read and been told to round DOWN, which would ideally indicate 283 mm spokes for both sides.

    I got a good deal on spokes from Nashbar, but they only had even sizes and I rounded all the way down to 282 mm.

    Is that too far, i.e. will I not get enough thread engagement in the nipples?

    I plan to start building tomorrow for a multi-day ride starting next Saturday. If these spokes are likely to be too short I can get some 284s from my LBS. My guess is that a mm below ideal is still fine, but if I need to bail on these spokes I'd like make the decision before I open the packaging so I can return them.

    Thanks for your time,

    -D

  2. #2
    don't pedal backwards... MacG's Avatar
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    Interestingly enough, I built a set of wheels yesterday with an internal gear hub and used spocalc, too. The building (and the tubes and tires) are a mothers day gift towards my ma's new bike that is slowly materializing.

    I've noticed that when using spocalc, the length it comes up with seems to be a tad on the long side. One wheel I built exactly according to the specifications has the threading extending through the back of the nipples by about 1 to 2mm, which doesn't leave much breathing room on the threading. The wheels I built yesterday came out pretty close, but still wouldn't have suffered terribly if the spokes had been a mm shorter.

    As a general rule of thumb, I would say to subtract about 1mm from what spocalc gives you. Either 283 or 282 should work without too much trouble. I guess I would be inclined to go with the 282s myself based on what I have seen spocalc do.

    What pattern are you using? Judging by the spoke length, it sounds like a pretty big shell. With a big hub and deep (or just small) rim, you should avoid patterns with many crosses due to the sharper than normal angles that will be formed at the rim. I built the Nexus/CR18 wheel with a 2-cross pattern on Jim's recommendation, and it looks great. I think 3-cross would be okay, but starting to push it a bit. 4-cross is right out.

    I also laced the front wheel (another Sun CR18 but laced to a Shimano Deore low flange hub) 2-cross because the spokes worked out better that way and it doesn't need a high cross pattern like a rear wheel (or a wheel with some kind of hub brake) does. For what it's worth, 2-cross is ligher than 3-cross or 4-cross because the spokes are physically shorter.

    If you decide on a spoke length that you can't get premade, call up Jim at the Hiawatha Cyclery in Minneapolis. He has a Phil Wood spoke cutting and thread rolling machine at his shop. Basically he can cut blank spokes to any length you want down to the millimeter. When he builds wheels for people, I think he charges $0.75 for straight spokes and $1 for double-butted. I don't know if the prices would be different for just spokes alone or not. They are all DT Swiss spokes as far as I know; good stuff. Not as cheap as Nashbar, but this would get you exact lengths if you need them.

    Good luck with the build!
    from Minneapolis, with bike love

  3. #3
    Curmudgeon Wil Davis's Avatar
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    If the length of the spokes (per side) is consistent, and the length of the threaded portion of the spokes is also consistent, then it is much easier to get the wheel to the point at which it is approximately true radially (i.e. start by tightening all the nipples so there's one thread exposed) - this sounds obvious but imagine how much more difficult it would be if there was a spread in (1) the lengths of the spokes, and (2) the length of the threaded part of the spoke.

    I would not have thought that 1mm would make all that much difference. Perhaps shorter is preferable to longer, as the former would guarantee that the spokes would not be long enough to puncture the tube.

    - Wil
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  4. #4
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    Wow-- thank you for the extremely thoughtful and helpful reply! I'm impressed that you're building a bike for your mother; that casts both of you in a very positive light, IMHO.

    I'm using the Shimano Nexus Inter-8 "Red Band" premium hub (SG-8R25) and a Mavic MA3 rim (36 spokes). I did the calculations for 3 cross.

    I expected to get the rim Thursday, but it won't arrive until tomorrow-- that has given me too much time to overthink and worry . After reading your comments I'm confident enough with the 282 mm spokes that I'll give it a try. I'll post back when I finish the wheel.

    This should make for a very versatile bike. I'm going to mount the shifter on the very end of my drop bars in a way that it can be readily removed. I'll route the shifter cable and housing to the frame with zip ties. All I'll need to do to go from gearing to fixed will be to loosen and remove the shifter, cut the zip ties and swap out the wheel.

    Many thanks,

    -D

    Quote Originally Posted by MacG
    Interestingly enough, I built a set of wheels yesterday with an internal gear hub and used spocalc, too. The building (and the tubes and tires) are a mothers day gift towards my ma's new bike that is slowly materializing.

    I've noticed that when using spocalc, the length it comes up with seems to be a tad on the long side. One wheel I built exactly according to the specifications has the threading extending through the back of the nipples by about 1 to 2mm, which doesn't leave much breathing room on the threading. The wheels I built yesterday came out pretty close, but still wouldn't have suffered terribly if the spokes had been a mm shorter.

    As a general rule of thumb, I would say to subtract about 1mm from what spocalc gives you. Either 283 or 282 should work without too much trouble. I guess I would be inclined to go with the 282s myself based on what I have seen spocalc do.

    What pattern are you using? Judging by the spoke length, it sounds like a pretty big shell. With a big hub and deep (or just small) rim, you should avoid patterns with many crosses due to the sharper than normal angles that will be formed at the rim. I built the Nexus/CR18 wheel with a 2-cross pattern on Jim's recommendation, and it looks great. I think 3-cross would be okay, but starting to push it a bit. 4-cross is right out.

    I also laced the front wheel (another Sun CR18 but laced to a Shimano Deore low flange hub) 2-cross because the spokes worked out better that way and it doesn't need a high cross pattern like a rear wheel (or a wheel with some kind of hub brake) does. For what it's worth, 2-cross is ligher than 3-cross or 4-cross because the spokes are physically shorter.

    If you decide on a spoke length that you can't get premade, call up Jim at the Hiawatha Cyclery in Minneapolis. He has a Phil Wood spoke cutting and thread rolling machine at his shop. Basically he can cut blank spokes to any length you want down to the millimeter. When he builds wheels for people, I think he charges $0.75 for straight spokes and $1 for double-butted. I don't know if the prices would be different for just spokes alone or not. They are all DT Swiss spokes as far as I know; good stuff. Not as cheap as Nashbar, but this would get you exact lengths if you need them.

    Good luck with the build!

  5. #5
    Cornucopia of Awesomeness baxtefer's Avatar
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    i like the way you think.
    I've been contemplating building my next bike around being able to accept a Nexus, while being fixed the other 98% of the time.

    One thing though..... The shifter won't fit on your drop bars. But i'm sure you know that already and have ordered that ugly bar extension thing from Sheldon.
    {o,o**
    |)__)
    -"-"-

    O RLY?

  6. #6
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    How much leeway you have in spoke length depends mostly on the lacing pattern. Assuming you're working with 700C rims, I wouldn't change farther than the nearest calculated millimeter if I was lacing radially. For single cross, the same rule applies. For 2X, you can go a millimeter short and never notice it. For 3X or 4X, a millimeter short, and 2 millimeters if you really need to.

    I don't find with most rims that Rinard's Spocalc gives measurements that are too long. There are a number of rims that have bounced around in ERD (diameter at the ferrules, basically) by 1-3 millimeters. Spocalc tries to give you the known alternatives, but you still have to determine which one you want. It's always worth it to have two long spokes and to mount them in opposing spoke holes. Have a piece of plastic tape near the end of one spoke. Thread the nipples until they're perfectly flush, then put another piece of tape on the other spoke to mark the edge of the first piece of tape on the other spoke. Then disassemble, reassemble the spokes with the nipples in the same position, lay the spokes next to each other with the pieces of tape in the same relative position, and measure the length to the base of the nipple (the flat surface on the inner side of the nipple where it rests on the ferrule). This gives you an accurate ERD. Try the same measurement in 2-3 other pairs of holes just to be sure the rim is round and you're getting an accurate measurement.

  7. #7
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    Thanks again, everyone, for your help. I built the wheel last night, and the length of the spokes was fine. My build isn't the greatest; the spokes are uniformly tensioned, but there are some problems with radial trueness. However, it is not detectable on the bike so I'm going to leave it alone for now.

    Regarding the Nexus hub and shifter: I got the $55 adaptor to attach it to the end of my drop bars. It works well, and most importantly makes it so I can indeed switch from geared to fixed very quickly. This is an expensive solution, but if you just occasionally need some gears it is pretty cool. I've only ridden the hub for a couple of miles so it is premature to review, but so far so good. It isn't as smooth as I may have dreamt, but it works reasonably well. I'll more than likely write a long review after I get a few hundred miles on it.

    Sincerely,

    -D

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