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Are disc specific rims a requirement?

Old 12-11-09, 12:07 PM
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daven1986
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Are disc specific rims a requirement?

Hi all,

I am looking to build a front wheel and have disc brakes. Are disc specific rims a requirement? If they aren't, do they provide any benefit over normal rims?

Thanks

Daven
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Old 12-11-09, 12:17 PM
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frankenmike 
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Definitely not a requirement, but due to the lack of a need for a braking surface are often a bit lighter.
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Old 12-11-09, 01:03 PM
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Originally Posted by frankenmike View Post
Definitely not a requirement, but due to the lack of a need for a braking surface are often a bit lighter.
Not necessarily. Mavic XM-717 discs are listed as weight 460g while the XM-717 weighs 475g. However the XM-317 disc weighs 445g while the XM-317 weighs 440g. The XM-117 disc weighs 457g while the XM-117 weighs 440g. I don't see why the nondisc rims weigh less, unless it's due to the machined brake track. Overall, the weight differences wouldn't be noticable.
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Old 12-11-09, 01:37 PM
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some disk rims look really cool. And of course, you can use those 'pretty' coloured deep v's without needing the machined brake track.
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Old 12-11-09, 02:17 PM
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If you build disc wheels using non-disc rims, they can be used on milpitple bikes if needed too.
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Old 12-11-09, 03:08 PM
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well the wheel will only be used on 2 bikes, both with disc brakes. However I read somewhere that suggested that the dishing of the wheel is different due to the disc brake rotor, and that disc specific rims are built for this dishing so you don't need extra tension in 1/2 the spokes.

Thanks
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Old 12-11-09, 04:22 PM
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Originally Posted by daven1986 View Post
..I read somewhere that suggested that the dishing of the wheel is different due to the disc brake rotor,
So it is.
Originally Posted by daven1986 View Post
.... and that disc specific rims are built for this dishing
They could be, but I haven't seen any with that feature.

Originally Posted by daven1986 View Post
....you don't need extra tension in 1/2 the spokes.
Well, the dishing offset you need to make place for the rotor in a front wheel is nothing compared to the dish most rear wheels have to be built with b/c of the cassette, and they do fairly well despite that. For a rear it's even less of an issue.
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Old 12-11-09, 04:40 PM
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fair enough, good points there dabac thanks
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Old 12-11-09, 04:59 PM
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Originally Posted by daven1986 View Post
well the wheel will only be used on 2 bikes, both with disc brakes. However I read somewhere that suggested that the dishing of the wheel is different due to the disc brake rotor, and that disc specific rims are built for this dishing so you don't need extra tension in 1/2 the spokes.

Thanks
There are off-center rims but those are used for compensating for dish on the rear wheel. The off-center drilling goes in the wrong direction for compensating for the disc. You might be able to turn them over but that usually leads to the spokes leaving the rim at a weird angle.
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Old 12-11-09, 05:29 PM
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well as it doesn't seem to be much of an issue for rear rims - as dabac says - then it should be all good with discs on normal rims. If it isn't though, I'm only buying a cheap rim so it can wait until I need a rear wheel on a non-disc bike!

They do look nice though...but cost 10 euros more!
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Old 12-11-09, 07:08 PM
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Originally Posted by cyccommute View Post
There are off-center rims but those are used for compensating for dish on the rear wheel. The off-center drilling goes in the wrong direction for compensating for the disc. You might be able to turn them over but that usually leads to the spokes leaving the rim at a weird angle.
I didn't notice any weird angles when I built my front disc wheel using an IRD Cadence VSR rim (700c). Got almost even tension left and right though. Maybe I'm not paying enough attention but it seems to me that spoke holes are drilled towards the centerline of the rim, not angled to account for a crossed-spoked wheel (I realize they are offset for left/right sides). If they were angled as you say, a radially spoked wheel would be subject to some of that angular weirdness too (though only half as much as with the flipped rim situation). However, I don't think rims are drilled that way. All rims would need to be marked for left/right if they were, unless I'm missing something.
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Old 12-14-09, 05:29 AM
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Originally Posted by joejack951 View Post
.. it seems to me that spoke holes are drilled towards the centerline of the rim, not angled to account for a crossed-spoked wheel (I realize they are offset for left/right sides). If they were angled as you say, a radially spoked wheel would be subject to some of that angular weirdness too (though only half as much as with the flipped rim situation). However, I don't think rims are drilled that way. All rims would need to be marked for left/right if they were, unless I'm missing something.
Rims are indeed drilled with alternate angles for left/right flange, but whether there's any alignment intended to deal with the flange height (and numbers of crosses) or not I can't tell.
But I can tell that you need to be pushing the envelope a fair bit before it becomes a problem.

Rims are drilled for the key spoke going to a specific flange. If you're building with all new bits this isn't a problem, there's always a doable pattern available.
(warning: some speculation ahead) If you have a used hub and a rim that's entirely symmetrical this isn't a problem either. You can always flip the rim over(have it rotating "backwards) and it should sort the pattern out.
If you have a used hub and a side specific rim (either through decals or through offset drilling) you might have an issue with getting the key spoke and the wear notches in the hub flanges to line up.
But low profile rims can usually be built lateral cross with what appears like zero consequences.
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Old 12-14-09, 05:48 AM
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The rim and hub are both brand new

Have ordered the Mavic A119 rim - not disc specific. When they come will get the LBS to build up the wheel thanks for the help.
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