Rebuild 20mm Carbon Tubular Wheelset for Disc Use?

[yz_387]

I have a set of 20mm carbon tubular rims from planet X that I built up on BHS hubs a few years ago. I have used them for 1 season on the road and a couple seasons of cross. I have been really happy with them, but I have since moved to disc brake cross bikes and don't really have a use for them anymore. What I could use is another set of disc brake tubulars so I am considering rebuilding these with a pair of low cost disc hubs. They are both 24 hole but I am a little concerned that since they were not designed for disc brake use, I might run into issues at the spoke holes. Is this concern legitimate, or would they be fine? Maybe use nipple washers to distribute the load?

[bm0p700f]

Your main issue will be that disc brake hubs have worse bracing angles than road hubs (some are better than other) so lateral wheel stiffness will be reduced. Your current wheels are not overly stiff as they are so this may present a problem given the low spoke count. Your rims will survive braking they won't disintegrate. If a problem occurs it will manifest by a spoke pulling through in time.

[NovemberDave]

The front bracing angles will be worse for sure, but if your disc bike has 135mm rear dropout spacing the rear bracing angles will improve, especially if your current hubs are for 11 speed.

I think that as long as you are under about 175#, depending on how you ride, they'll do well as disc rims.

[bm0p700f]

Do the sums, NDS bracing for the rear NDS is worse o a disc brake hub compared to a rim brake hub. NDS flange to centre is normally 34mm. This is made up by some hubs putting the DS flange 20mm from centre but it does not fully make up for change from 11 speed road to 11 speed disc brake. Why do you think disc brake rim are all wide it is because they are stiffer and that solve the bracing angle problem.

[NovemberDave]

Could be my bias from building a preponderance of WI hubs, but the tension ratios on CLD are notably higher than on T11, and T11 are among the better 11 speed hubs for that. Flange spacing on a CLD is 19/33 (C to C). I haven't done an exact comparison of measuring stiffness of, say, and SL25 built into both a T11 and a CLD, but the SL25 and SL23 have very similar raw rim stiffness measures, and an SL25 built with a CLD is stiffer with better tension balance than an SL23 with a T11. And the SL23/T11 combo is quite stiff.

[yz_387]

I guess I should have included a little more information. The rear is currently built with BHS SL210 (10 speed) which has a pretty small DS spacing of 16.75. I always struggled to balance spoke tension with this wheel and played with multiple different builds 2x, 1x heads in on DS, etc. For a 2x build spoke calc comes up with as DS BA of 3.3 and a NDS BA of 7.4 for a ratio of 45%. I am thinking of using the BHS (Bitex) MTB 270 rear (135mm) which has a larger DS spacing of 19.5. I loose a little bit of NDS spacing so for a 2x build the BAs are 6.8 and 3.9 for a ratio of 57%. While far from ideal, it seems like the new build would feel stiffer than the previous one, correct? Good point about the loss of stiffness in the front wheel. I had thought through the rear, but hadn't really considered the front.

BTW, I weigh in at 150 pounds for the record.

[NovemberDave]

While I haven't built a Bitex hub in about 6 years, I recall the 10 speed version having tension balance issues (a big part of why we didn't choose them at the time). And I haven't yet built with a Bitex disc hub, but I'd say you're going to wind up with a slightly better rear wheel, and a slightly worse front wheel. You're light enough that the front wheel's decreased stiffness should be inconsequential and the rear wheel should be absolutely fine if the rim brake build worked well for you.

[yz_387]

Thanks for the feedback; very helpful.

[bm0p700f]

Wi cold hubs are good but you are comparing tension balance which is not reflective of lateral stiffness. Take a dt Swiss 240 road hub. The tension balance on these is very good but the nds bracing angle is low so the lateral wheel stiffness is lower than with almost any other hub.

With the Wi cld hubs vs the t11's the tension balance may be better on the cld hubs but it does not mean lateral stiffness is higher. Work out the bracing angles. Lateral stiffness is proportional to the sum of the squares of the cosines of the bracing angles.

FYI personally I have never had issues with hubs with a tension balance of 45% or a bit under. So long as the wheel is stiff enough it does not seem to matter. If you can maximise bracing angles and tension balance then you have a winner. Op try the novatec d772 hub the bracing angles are a bit better and tension balance is about the same. There's not a lot in it though.

[dzmlyk]

Lateral stiffness is a little more complicated than simply bracing angle. The total cross sectional area of the spokes has an effect as well. (I'm sure that you've heard of a guy by the name of Rinard.) Number and type of spokes have a huge effect.

To address the OP, I think the low spoke count will have a greater impact on lateral stiffness. But how much will you notice that on 25 psi CX tires? As far as spoke nipple pull through, I think most open mold carbon rims have pretty darn thick spoke beds. I wouldn't worry about that at all.

[yz_387]

Thanks for the suggestion of the novatec d772; it looks like a good option. Anyone know if there is a source for these in the US?

[NovemberDave]

bm0p700f - I'm totally crystal clear on bracing angle, tension balance, and stiffness. As I said, while I haven't measured the exact same aluminum rim in both disc and build (though have done so with carbons, which proved what I'm about to say), when measuring stiffness in basically the exact same way that the old Damon Rinard tests were done, an SL25 with CLDs was stiffer than SL23 with T11s. Not by very much - barely beyond margin of error - but stiffer. SL25s and SL23s, measured just as rims, are within the margin of error of each other. Carbon rims measured both in T11 and CLD builds were stiffer in CLD build.

The applicability of these tests may or may not be 100% relevant to the OP's situation, but I have every confidence that if his current Bitex road build (with hubs that were notorious both for poor tension balance and for stiffness) works for him in an application, a competent disc rear, assuming 135mm spacing, should absolutely be sufficient.

dzmlyk brings up a very good point. In designing a build, I would never ask spoke gauge to do what I'd rather have spoke count do, but if spoke count is constrained as it is in this case, spoke gauge can be modified to the build's benefit.

[bm0p700f]

I also know spoke gauge is important in making a stiff wheel. I know rin hards works I have read it it fine. My comments come from the best disc brake hubs have flange separation on the rear of 54mm at best the road hubs have a centre of flange to centre of flange difference of 57mm. There are no doubt exceptions for example the flange to flange distance of the WI CLD hubs compared the WI T11 hubs is partially the same at 53mm so it is hardly surprising that wheels built with both hubs have the same lateral stiffness. This however is not true of all hubs. If the op want to maximise lateral stiffness for a given spoke count and spoke gauge then the novatec d772 or d772 hubs could be used or a dt Swiss 240 or 350 disc brake hub. Both have the widest flange to flange distance of any disc brake hub apart from the 28h only shimano CX75 which has the widest but the drilling precludes it use.

[yz_387]

I am finally getting around to building these wheels. In the past I have always been happy with using sapim lasers, but I see that they are not recommended for disc use? Do I really need to go up to sapim race instead?

[eliflap-scalpel]

i have a 20000km rode carbon tubular rims from Nextie Bikes

23mm wide

bought in fall 2013, laced to extralite straight pull disc hubs with pillar spokes, used on road for 20.000km ... and counting ....