Bicycle Mechanics - wheel dishing

having laced my first set of wheels I've
come up with a bit of a problem on rear wheel
when I try to dish it to fit frame.
To start off, I used 3 different spoke calculators,
and the LBS used whatever magic they have and
all came up with the same spoke lengths.
I made sure when I laced the wheel that I had
the correct spokes for left/right side of hub.
My quandry is that to get the dishing correct
the non cassette side spokes are slightly
loose. If I tighten them they pull the wheel
too much and dishing is out of whack.
Any ideas as to what I might be doing
wrong?
Am I being overly obsessive (hell I'm building
my own wheels, how obsessive is that)?
thanks,
Marty

Marty,

When finished, the drive-side spokes will have more tension than the non-drive-side spokes. The difference is very noticable.

Getting a wheel built properly requires several variables to come together at the same time: trueness, roundness, spoke tension, dishing.

I go for spoke tension first. I like my drive-side spokes very tight, they're creaking (I don't use a tension gauge). With taught spokes I go for true and then round. After getting the wheel round, I usually have to retrue it some.

With the spokes taught and the wheel true and round, it's time for dishing. A dishing tool is a necessity. Tighten the spokes OPPOSITE from the side with the gap. In other words, tighten the spokes on the left side to move the axle to the right. Go in 1/4 turn increments on every spoke. Theoretically you shouldn't affect true and round, since all the spokes will be tightened the same amount. Continue until the wheel is dished. If the spokes are getting too tight, you may have to loosen the spokes on the other side - again, in 1/4-turn increments.

Now start over. True, round, dish. The adjustments should be minor a this point, but it still make take 2 trips through the process.

Woody

Woody,

Thats generally the tact that I've taken
although I went for roundness first, then dish
then true and so on and so on ad naseum. . .
I just wondered about the difference in tension.
while I'm not using a dishing tool the
Truing stand has a pretty good gauge so I can
get a good approximation of dish. I then have been
mounting the wheel on the bike and giving it the
old eyeball test. So far so good.
Geez, I can't wait for my copy of Brandts book
gets here, can't hurt.
Might have to look into dishing tool tho.

Marty

Dishing is determined by tension, not by spoke length. On a rear wheel, the cog-side is typically tighter. There is no way around it, and it is not a problem. Make sure the cog-side spokes are fully tight, and don't worry about the left side, it will be looser.

For more on truing see http://www.parktool.com/repair_help/howfix_truing.shtml

Originally posted by Calvin Jones
Dishing is determined by tension, not by spoke length.
Calvin,
I beg to differ with this opinion.
all the spoke calculators came up with 301 in non
cassette side and 302 on cassette side to make up
for difference in flange to center of hub distances.
My understanding is this allows for a more uniform
tension of the spokes on both sides.
I was just wondering about the degree of looseness
on the non cassette side, seemed a little excessive to me.

Marty

Spoke length has no effect on tension. That is, dish has nothing to do with spoke length BEFORE lacing the wheel. In other words, using longer or shorter spokes does not determine dish at all. Good spoke length is important, but it does not by itself dish the wheel.

However, if you consider spoke length to be "effective" spoke length as distance from the hub spoke hole to the rim, the right spokes will be shorter than the left.

Try a simple experiment. Take somebodies front wheel, and cut a spoke. Cut another one in another place. Now, replace one spoke with one 2-3mm shorter, and the other with one 2-3mm longer. True the wheel. The long spoke and the short spoke have the same tension as the cut ones. One may have poor thread engagement and may be barely holding on, and the other may have the spoke sticking up in out of the top of the nipple and may even poke the inner tube, but the tension is the same as all the others.

By using different spokes on left versus right side you get better thread engagement at the nipple. View a wheel from the top as a two dimensional triangle. There is a longer leg from the left side and a shorter leg from the right. So, the left side gets a longer spoke.

On multi-speed rear wheel, and also on front disc hubs, the two flanges are not centered to the middle of the hub. If the left and right side spoke tension were even, the rim would be centered between the flanges. On non-disc front wheels the left and right flange are symmetrical and centered to the middle of the hub center. The tension is the same of left and right side, and the rim is centered over the hub center, between the flanges, and between the hub locknuts. It is most critical that the rim be centered between the hub locknuts, not flanges.

However, on a rear wheel, the right flange is closer to the hub center by about 12-18mm as compared to the left flange. You want the rim centered over the hub locknuts, not between the flanges. You want the rim closer to the right flange than the left.

It is common on rear wheels to have as much as 30% less tension on the left side than the right. This is why pulling up a wheel tight is so important. If the flanges are moved together by design, you can get this figure lower, but there is always some difference.

To look at the issue from a physics standpoint, draw a vertical line through the centerline of the hub, with the rim centered on the centerline as well.

Now draw the spoke angles to the left and right flanges, as viewed from the rear. The angle between the centerline and the non-driveside spokes is greater than the angle between the centerline and the driveside spokes (this is because one flange is further from the centerline than the other).

The sine of these angles, multiplied by the respective spoke tensions, represent the force vectors pulling the rim left and right. Since the force vectors must be equal for the rim to be in the center, and since the two angles are different, there is no way the tensions can be the same. Spoke length never enters the matter.

I still haven't thought of a good way to describe, in words, the proper spoke tension for the drive side, but it should be, well, pretty tight. The other side's tension can only be the tension that results in a dished wheel, unless you're deliberately fudging the dish for a specific reason (goofed frame, etc). Anyway, I hope your wheels are a source of great pride and enjoyment. :)

Look at the bracing angles of the left and right sides. Since the horizontal components of the respective spoke tensions have to be equal, it is not unusual for the right-side spokes to be twice as tight as the left. Sheldonbrown.com can lead you to some good musical-pitch-related tensioning advice.

A typical 9-speed, 130mm wheel is RADICALLY dished and structurally less sound than it would be with either fewer cogs or a wider rear axle.

Marty,
Are you using a hub that has a larger flange on one side? I haven't built my own wheel, but I have studied it and would have thought that the NON drive side spokes would need to be longer because of the longer effective difference between the flange and the rim due the offset. Though I haven't built a wheel from scratch, I do my own truing, tensioning, spoke replacement, etc..

Actually this is just one of the reasons I buy pre-assembled wheels and just true, stress-relieve and re-tension them myself. It is actually cheaper to buy the components and let the Performance wheel shop put wheels together than to do it myself. They do the grunt work of lacing and spoke length, and I just touch them up. Saves a lot of time and aggrevation. One day when I screw up a rim or something I will try it from scratch, but for now I would rather spend my limited wrench time cleaning and repacking bearings and other things to make the bike work better.
FWIW,
Raymond

Originally posted by RainmanP
Marty,
Are you using a hub that has a larger flange on one side? I haven't built my own wheel, but I have studied it and would have thought that the NON drive side spokes would need to be longer because of the longer effective difference between the flange and the rim due the offset.
Doh!
well yeah, thats what I meant, and thats how I laced em.
I just didn't translate to print effectively.
No I'm not using a HiLow hub (haven't seen those
in a day).
Aside from the frustration of not being able to ride the
new ride, I'm really having a good time doing this.
There is a huge sense of pride in knowing I built it
all myself, and the wheels are a major factor here.
Its like I'm crossing some mythic line, or milestone.
I'll probably get a set of prebuilt wheels (clincher)
to compliment my homebuilts, and will pretension
and stress relieve em.
oh yeah, welcome back Rainman. . .

Marty

Marty,
I kind of figured you just had things reversed, but you never know. Sometimes things aren't as intuitive as they seem.

I know what you mean. I really get a lot of satisfaction from building something from scratch and from being able to fix things myself. Heck, I enjoy working on my bikes almost as much as riding them. I am always swapping things around and trying new components. I have probably built the equivqalent of 4 new bikes. One I actually assembled to check for fit, disassembled for painting, then reassembled, as well as a lot of swapping stuff around. Also old bikes I have pulled apart and put back together. One day I will build a wheel. Right now I'm just having fun with other stuff.

If I remember correctly the tension on drive-side spokes is approximately 2.5 times that on the non-drive-side when properly dished/centered.
Regards,
Raymond

Thanks guys. These were the answers I was looking for. Clear and precise.

Check out Sheldon Brown's website for wheel building tips. see shortcuts at the beginning of the Mechanics forum.