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O/C rim for FRONT road disc wheel build? Worthwhile or not?

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O/C rim for FRONT road disc wheel build? Worthwhile or not?

Old 11-21-13, 12:10 PM
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tcpasley
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O/C rim for FRONT road disc wheel build? Worthwhile or not?

I'm planning to build a 700c disc wheelset for a cyclocross frame, using 32h SRAM X9 hubs and Velocity A23 rims. I want the wheels to be laterally stiff with DS/NDS (disc side, non-disc side) spoke tension as even as possible, so I was thinking about using the O/C version of the A23 rims.

I know that O/C rims have typically been used improve spoke tension balance for rim brake rear wheels to better handle the effects of drive torque. Would a similar approach be worthwhile to deal with disc wheel braking torque, especially for front disc wheels?

The SRAM X9 hubs already address this issue in part by having 58mm dia. left flanges and 45mm dia. right flanges. Do y'all think that provides enough balance in spoke tension?

As always, any suggestions are appreciated, especially if they're based on actual experience. I have a set of WTB Dual Duty FR wheels (28mm wide rims = heavy), but I'm running them with cantilevers, so I don't yet have any experience running disc brakes.
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Old 11-21-13, 12:48 PM
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All things being equal, and with spoke holes in line down the center of the rim, the R/L spoke tensions will be proportional to the R/L center to flanges. Note that it's reversed, so if the right CTF is half the left, the right tension will be double.

Unmatched flanges changes this somewhat, but the effect depends on the number of crosses, with the effect freatest on radial, and zero on a full tangent (ie. 36h/4x or 28h/3x) build.

IMO tension differences of 10-25% are very manageable, and no special compensation is needed, but as it reaches 40% it becomes difficult to have the loose side tight enough without the tight side being too tight. This is where I look to things like offset rims, or mixing spoke gauges to maintain good elongation on the slack side.

I don't have the details on your hubs, so won't advise directly, but hope that this gives you a sense of how to decide for yourself.
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Old 11-21-13, 01:40 PM
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Originally Posted by FBinNY View Post
All things being equal, and with spoke holes in line down the center of the rim, the R/L spoke tensions will be proportional to the R/L center to flanges. Note that it's reversed, so if the right CTF is half the left, the right tension will be double.

Unmatched flanges changes this somewhat, but the effect depends on the number of crosses, with the effect freatest on radial, and zero on a full tangent (ie. 36h/4x or 28h/3x) build.

IMO tension differences of 10-25% are very manageable, and no special compensation is needed, but as it reaches 40% it becomes difficult to have the loose side tight enough without the tight side being too tight. This is where I look to things like offset rims, or mixing spoke gauges to maintain good elongation on the slack side.

I don't have the details on your hubs, so won't advise directly, but hope that this gives you a sense of how to decide for yourself.
Thanks a million. The advice on manageable tension differences is really what I need. Maybe later I can post some screenshots from spocalc.xls to show the tension ratios with and without O/C.
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Old 11-21-13, 02:04 PM
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Schmidt disc Dyno-hub a 1mm length difference was their data, (via SJS)

easily done with a bit more spoke tension on Rt side.
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Old 11-21-13, 03:16 PM
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Originally Posted by tcpasley View Post
I'm planning to build a 700c disc wheelset for a cyclocross frame, using 32h SRAM X9 hubs and Velocity A23 rims. I want the wheels to be laterally stiff with DS/NDS (disc side, non-disc side) spoke tension as even as possible, so I was thinking about using the O/C version of the A23 rims.

I know that O/C rims have typically been used improve spoke tension balance for rim brake rear wheels to better handle the effects of drive torque. Would a similar approach be worthwhile to deal with disc wheel braking torque, especially for front disc wheels?

The SRAM X9 hubs already address this issue in part by having 58mm dia. left flanges and 45mm dia. right flanges. Do y'all think that provides enough balance in spoke tension?

As always, any suggestions are appreciated, especially if they're based on actual experience. I have a set of WTB Dual Duty FR wheels (28mm wide rims = heavy), but I'm running them with cantilevers, so I don't yet have any experience running disc brakes.

We've built a number of O/C F&R A23 disc wheelsets and had great results. I'd recommend it.
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Old 11-22-13, 06:35 AM
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Originally Posted by tcpasley View Post
I'm planning to build a 700c disc wheelset.... I want the wheels to be laterally stiff ...
Basic wheel lore, abbreviated:spokes act as linear springs, meaning that for x amount of tension they'll elongate y mm. For 2x tension they'll elongate 2y. Hence, unless spokes go slack, lateral stiffness is independent of spoke tension, but hugely dependent on spoke gauge. Then things like choice of rim, lacing pattern, flange height etc.

Originally Posted by tcpasley View Post
.... I want the ... DS/NDS (disc side, non-disc side) spoke tension as even as possible...
For wheel/spoke longevity this is always a good idea, in theory. Whether you'd be able to ride enough to actually merit from it or not is another issue entirely.

IRL, it's been reported that one/some OCR rims were prone to cracking around the nipples. Whether due to excessive anodizing turning the rim brittle - in which case the cracking would likely have happened on a symmetric rim too, or if the fault lay in the material around the nipple flexing in another way due to the rim profile/hole location has to my knowledge never been established.

Originally Posted by tcpasley View Post
I know that O/C rims have typically been used improve spoke tension balance for rim brake rear wheels to better handle the effects of drive torque.
Kinda-sorta torque. Short to mid-term, torque as such is a non issue for a wheel. The long-term potential, and the stock explanation for spoke failure while JRA is fatigue. The NDS spokes, due to their lower tension, sees a bigger reduction in tension when the spokes pass through the load-affected zone, between the hub and the ground. And this is what's usually held as the main cause for spoke fatigue. Drive torque will add another, probably tiny, fraction of tension loss to the leading spokes, so ti certainly won't help. But if it's enough to be an important contributor, I don't know.

Originally Posted by tcpasley View Post
Would a similar approach be worthwhile to deal with disc wheel braking torque, especially for front disc wheels?
Worthwhile is a null word in these circumstances. Will it build a laterally stiffer wheel? Theoretically, yes. But it'll pretty much only do you any good when you've already pushed the wheel to the point where spokes are about to go slack.
Apart from that there's no advantage to better-than-needed.
Will it build a more long-lived wheel? Theoretically yes. But fronts have a fairly cushy life anyhow. A drive wheel will see a lot more applications of drive torque than a front will see applications of brake torque. A front carry less weight than a a rear, and a front is less asymmetric than a rear.
And again, unless you reach that amazingly high mileage, there's no advantage to better-than-needed.

So, If you really want lateral stiffness, use the O/C rim to run a heavier gauge. Maybe straight gauge both sides, or 2.0 / 1.8 instead of 1.8 / 1.5

Or use O/C rims for the warm feeling that comes from knowing that you've done that little extra,which is nice. I do it frequently.
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Old 11-22-13, 07:11 AM
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Originally Posted by FBinNY View Post
...the R/L spoke tensions will be proportional to the R/L center to flanges...
Actually inversely proportional to the distance. As the distance on one side goes to zero, the tension would go to infinity.
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Old 11-27-13, 12:26 AM
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Found this on YouTube => http://www.youtube.com/watch?v=22B579FrfjQ

Includes a mathematical analysis of spoke angles and tensions and measurements of lateral wheel deflection.
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