Rim offset wheel build question?
 

I'm sure some of you are getting tired of seeing my incessant questions about building wheels and such. So I apologize. :) I just don't want to order the wrong size spokes or have my wheel all jankity.

I got a rim from my LBS that I don't know the brand of. I've measured all the pertinent dimensions, but I'm confused about the rim offset.

The ERD of the rim is 615mm. The rim is 26mm wide making the half-width 13mm. The distance from the side of the rim to the nearest spoke hole is 11mm. So that would make the offset(OSB) 2mm?

When I enter ERD-615mm OSB-2mm along with the standard AW hub dimensions it gives me spoke lenghts of 292.5 for the left and 292.8 for the right. Does that sound right? Is the difference in spoke lengths only because of the rim offset? And is the reason it is such a small difference because an AW hub is not offset either?

Most importantly though, as long as my calculations are correct I don't think a .3mm difference even matters. 291mm or 292mm spokes should do the trick... I think.

I've had experience with cattywhompus wheels and whopperjawed wheels but never jankity wheels, so I'm afraid I have no idea.

I would go with a 292. 291 will probably work fine. As someone pointed out that the spokes stretch about .5 mm when
you tighten them. And a single turn of the spoke is about 1 mm (forget the exact amount, but I am certain that less then
2 turns equals more then a mm).

What tool are you using to get the spoke lengths?

Can't say I've ever had a jankity wheel, but jbonamici is still way ahead of me on cattywhompus and whopperjawed wheels. :}

Offset doesn't come from the rim, only ERD. Do not try to measure the offset in the spoke holes on the rim for calculation purposes
Offset comes from the rims relative center position to the hub flanges, and this only on the rear wheel.
A front wheel will have both sides of the spokes the same length.
The rear wheel will have shorter spokes on the drive side and longer spokes on the non-drive side (in my humble experience it is usually around 2mm to 3mm of difference between the two sides on the rear)

Added: Usually on my calculations I would round down for spoke lengths.

Go with it. Plug 5mm into your calculator as the OSB and it probably wouldn't change the spoke length enough to worry about. Spoke length is most sensitive to ERD, flange (at the spoke hole) diameter and number of crosses, so get those right.

It does if you have an OCR or other rim with an offset spoke bed.

Haha, well I'll just have to wait for others that have "jankity" wheel experience.

Well, it's a Sturmey Archer AW hub that is not offset, so I think the spokes should also be the same on both sides. Except, like Bob Barker said, the rim does have an offset spoke bed, which is where I think the difference comes in.

Bob is spot on. Many of those measurements in SpoCalc don't translate to an appreciable difference in the Spoke Length it spits out. Get the main stuff right and you will be close enough.

I stressed out about this kind of stuff really hard when I was ordering spokes for my first wheel build and the most important thing is just what Bob says, get the main things right. ERD, flange diameter, cross number and wL, wR values are the most important things. If you play around with the other values in SpoCalc you'll notice that changing them doesn't translate to more than a couple tenths of a millimeter.

If spocalc is saying 292.8 and 292.5, I'd go with 292.

I've ordered the wrong length spokes before...it was obvious before I ever started tensioning the wheel and ebikestop.com took them back and exchanged them for the proper length. All I lost was a couple bux in shipping and a few days.

If you are super stressed out, just post the numbers you are feeding Spocalc and any of us can run them through ourselves and check your work.

I dunno, that doesn't sound right - even an AW should have some offset. The driver/cog is on one side, and it isn't on the other, so therefore there is an offset unless you want either a bunch of spacers on the left or your rim off to the side looking goofy and braking funny.

I've built three set of wheels in the past few months. I found wheels that looked like mine, and then measured. It worked fine. Maybe try that, just as a confirmation? I don't trust Excel. I'd like to see it right there in front of me.

I think Mark is right; the AW hub is slightly dished. I tried to check Dan Halem's spoke calculator but can't find it... is it gone? For good?:eek:

yep. It was hosted on Geocities. unfortunately many C&V resources were hosted there and it would seem a lot of folks didnt realize that Geocities was closing and they needed to act to save their data.

Hell, I wish I knew that nobody was caring for those resources, I would have gladly hosted any of that stuff....might be good to make a mirror of all the good Sheldon Brown articles just in case Harris decides to stop hosting them.

I measured the ERD and rim OSB using the methods from this page.

Rim (unknown brand)
ERD=615mm
OSB=2mm

Hub (SA AW)
dL (left flange diameter)=65mm
dR (right flange diameter)=65mm
WL (center to left flange)=27mm
WR (center to right flange)=27mm
S (spoke hole diameter)=2.4mm

Here is a pic of the hub in it's original configuration. I will need to remove some spacers because the OLD is wider than the dropouts. It appears there are enough spacers already that I could remove enough of them to reduce the OLD and keep the hub centered so it wouldn't be dished. Is that a bad idea?

http://lh5.ggpht.com/_aQvE2vU5Quo/Su...eyarcheraw.JPG

292 or 291 mm spokes will work fine. The first wheel build can be a bit worrysome since there seems to be a lot of variables and money to spend before things get started but there's more room for error than you're concerning yourself with.

More direct to your question - everything you're dealing with is simple trigonometry. You can figure out spoke lengths with a calculator and a pad of paper if one were so inclined but when you've got a problem like this one where you're not certain you've got the right answer try drawing it out in a right triangle and see how much something like 2 mm of spoke be offset will change the necessary length. The answer is really not much.

can't see the pic.

If you are adding/removing/moving spacers around you should do that before you make your measurements.

Don't forget, when you calculate wR and wL you measure from the Left Locknut to the Left Flange and from the Right Locknut to the Right Flange then subtract that distance from half of the OLD.

wL=(OLD/2)-(distance from left locknut to left flange)


in short, if you move spacers around your wL and wR values will change.

Get the spacers set so the hub sits correctly in your dropouts, then measure.

Ignore the rims offset...

There, I fixed the picture.

That's a good idea Craven. I will do that. I don't know why that never occurred to me before.

Ok, so I have the hub in the dropouts now, however, the flanges are not exactly centered but they are close. I think I could get them completely centered with the right size spacers. The cog I think needs to go a little farther to the right to match up better with the chainring. Is there a way to move the cog over with spacers without moving the hub?

Just build the wheel. Your looking to deep into this...

Don't ignore chainline issues.
If you start swapping spacers around you're messing with the factory chainline.

you're kind of going about this the wrong way.

The #1 thing is that the cog is in the optimal position for perfect chainline. Use the spacers to achieve good chainline and don't worry about where the flanges are.

Once you've got the chainline, take the hub off and do your measurements.

The flanges don't need to be centered on the frame. The Rim does.


an like miamijim said, you are kind of overthinking this. All this moving around of spacers and stuff...its not gonna change the spoke length calculations significantly. There's enough fudge factor room on a spoke. Get your spokes and get yer wrench.

Haha, ok, thanks guys. I'll do that. Wish me luck.

you'll be fine. Its easy to be overwhelmed by all the reading, when you get to doing you will see its really quite easy.

Good Luck! Just take your time and you'll be fine. I wanted to add that you will find you have a fair amount of offset adjustability once the wheel is together and round, so even if you are off by a little to start with you can easily move the rim sideways to compensate.