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 01-28-06, 02:57 AM #1 you Senior Member Thread Starter   Join Date: Feb 2005 Location: San Francsico, CA Bikes: Schwinn Voyageur '89 Posts: 324 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) wheel building with asymmetrical rims How do I find the spoke length? should the spokes be different lengths for drive side and non-drive side? Or should the be different, thanks.
01-28-06, 06:39 AM   #2

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 Originally Posted by you How do I find the spoke length? should the spokes be different lengths for drive side and non-drive side? Or should the be different, thanks.
If you use SpoCalc, at least some of the common OC rims are already in the data base. I just looked and the Velocity Aerohead OC rim is in there. The spokes from the two different sides will not necessarily be the same length. OC(off center) rims reduce dish, they don't eliminate it.

SpoCalc is available free. You just need an app that will run Excel spreadsheets (eg. MS Excel or Star Office). You can download Spocalc from Sheldon Brown's site:

http://www.sheldonbrown.com/rinard/spocalc.htm

It's great for several reasons. First, it has a very large database of rims and hubs. It does all the math for you. Once you plug in the numbers it automatically calculates the spoke lengths for radial, 1x, 2x, 3x, and 4x lacing patterns. If you can run spreadsheets it's a great way to go. Even if your specific rim or hub is not in the database you can do all the entries manually and the calculations are still automated. There is a specific variable entry for OC rims. It is "O.S.B." This is the "lateral spoke bed offset (from wheel center). Non-zero for asymmetric rims".

If you can't get Spocalc up and running, I can run the numbers for you, but you will have to supply them for me. For the rim you need the OSB and the ERD. For the hub you need five different measurements. The best bet is to supply the specific model numbers for the rim and hub and see if they are already in the SpoCalc database.

I looked at the other two spoke calculators I have used (DT Swiss and Promechanics.com) and neither of them accounts for offset rims.

Does anyone know of some calculator (other than SpoCalc) that takes an O.S.B. variable?

Last edited by cascade168; 01-28-06 at 07:23 AM.

 01-28-06, 01:17 PM #3 you Senior Member Thread Starter   Join Date: Feb 2005 Location: San Francsico, CA Bikes: Schwinn Voyageur '89 Posts: 324 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) I used spokecalc and wasn't sure if the spokes should be the same length or not, I also noticed that the only rim dimension taken into account is the effective rim diameter not how the rim is dished or whatever. I am using the Aerohead OCs.
01-28-06, 01:39 PM   #4

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 Originally Posted by you I used spokecalc and wasn't sure if the spokes should be the same length or not, I also noticed that the only rim dimension taken into account is the effective rim diameter not how the rim is dished or whatever. I am using the Aerohead OCs.
If you look on the "rims" page, you will see that there are two columns that are tinted yellow. These are the variables. The first one is "E.R.D." (effective rim diameter) and the second is "OSB, mm". You should notice that the entry in OSB for the Aerohead OCs is 4.0mm. Look at the rest of the rims and you will see that they are almost all 0.0mm, except for some Ritchey, Bontrager, and a couple of others. Any rim that has an entry other than zero is assymmetric.

So, are they different lengths? I'd guess that they are 1mm different. Most rear wheels have spokes that are 2mm different from drive side to non-drive side.

 01-28-06, 02:20 PM #5 you Senior Member Thread Starter   Join Date: Feb 2005 Location: San Francsico, CA Bikes: Schwinn Voyageur '89 Posts: 324 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) yeah they're a mm different, thanks. should the spokes be of equal tension?
01-28-06, 02:42 PM   #6

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 Originally Posted by you yeah they're a mm different, thanks. should the spokes be of equal tension?
If you go to the "Calculator" page of that spreadsheet, you should see an entry over on the far right (in the Rear Wheel group) that is labelled "Tension L/R". It should give you a ratio (in %) of the left to right sides (or non-drive to drive sides). As I stated before, OC rims don't eliminate dish, they just reduce it. This allows for the tension on the non-drive side to be higher than a normal rim. A normal rim will usually produce a ratio of ~60%. I would speculate that the tension ratio for your OC rim will be in the 85-90% range (again, just my guess). The closer you can get to 100%, the better.

To specifically answer your question, no, I don't think they'll be the same.

So, what is that ratio?

A word of caution here... I have found that ratio number produced by Spocalc to be a nice guideline, but it's most important to tension up the drive side spokes to the rim manufactureres max specification to get the strongest wheels. Get them all tensioned up and then you can finish your truing and dishing with the non-drive side spokes. I just check them to make sure there is not a spoke that is much differnt from the rest - and, I have built wheels that this happenned to me. One spoke would be way loose and the wheel would seem to be done. In this case you tighten up that spoke and re-check everything else. Don't get hung up on the tension for the non-drive side spokes. Just make sure there are no noticeably loose ones. Concentrate on the drive side spokes. They are doing the "heavy lifting" for your rear wheel.

01-28-06, 04:52 PM   #7
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 Originally Posted by you yeah they're a mm different, thanks. should the spokes be of equal tension?
Probably not. The amount of dish required is what determines the difference in spoke tension. When you get the rim centered between the locknuts, the tension difference from side-to-side is what it is and you can't do anything about it.