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I have the opportunity to buy 2 front Shimano Dura Ace C40 latest generation wheels at a very cheap price. The hole count for the wheelset is 16/21. Could I build / have built a rear wheel from one of the front rims, for example using a 32 hole rear hub or is 16 hole too fragile for a carbon rear rim?

I don't have the hub yet, but from research I assume this is the most equilibrated option, correct? With these, do you think I would need new spokes or can I reuse the ones which come with the 16 hole hub? One of the patterns I saw could work, it doesn't require any crossing. Maybe the rear hub could interfere a bit though? The video corresponding to the screenshots supposedly is a professional wheel-builder as he was a making a project for a 16 hole rear wheel, tough steel / aluminium one.

There is also a review from CyclingTips comparing the new generation wheelsets with the old ones. There is a part where they say only the rear tubular version rim is asymmetric.

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    I've had a 16 spoke wheel and it simply wasn't enough spokes. One broke on a 50+ km/h descent, wheel instantly went so untrue that it bound up on the frame, and the whole wheel was skidding not rolling. Had that happened on the front wheel it would have been an endo followed by mudsucking. 32 spoke minimum for me now.
    – Criggie ♦
    Mar 8, 2020 at 5:33
  • "endo followed by mudcucking" very accurate description. Quite likely to be followed by a couple of months of blendered food sucking, too.
    – FreeMan
    Mar 9, 2020 at 20:23

3 Answers 3

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Sixteen spoke rear wheels do exist, but there aren't very many of them, and to function they need to have extremely stiff and strong rims. In this case, being a Dura-Ace product, it's reasonable to assume that the everything has already been pushed to around the limit in the design. In particular, making one of these a rear would require the compromise of either pushing the drive side spoke holes way beyond what they were expected to endure in terms of resisting cracking, or making the whole thing have way less tension than intended, resulting in it being weak. Just winging it and assuming you'll be fine is not a good idea here.

These limitations don't have to do with the rim being carbon per se.

Worth mentioning but pretty far down the list of issues with this plan is that the sort of rims these are (deep carbon rims in an ultra-premium prefab set) can have directional drilling that will contort the nipples around if a pattern other than intended is used.

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  • I can't even remember very many 16-spoke rears. The older Campagnolo Shamal wheels, which had heavy 40mm aluminum rims, do come to mind. You correctly point out that Shimano didn't explicitly design that rim to withstand the tension required for 16 rear spokes - the OP should keep in mind that the drive-side spoke tension can be nearly double the NDS tension. As to the spoke hole angling, I suspect this might not be an issue, because that front rim is designed for regular 1:1 alternating lacing. The spoke holes would go left, right, left, right.
    – Weiwen Ng
    Mar 9, 2020 at 13:09
  • Most extremely-lows-spoke-count rear wheels are something like Mavic's old Cosmic Carbone SLRs (road.cc/content/review/94524-mavic-cosmic-slr-wheels). 20 carbon fiber spokes (actually 10 that run rim-to-rim) that can't really be compared to "normal" wheels with off-the-shelf style spokes.
    – Andrew Henle
    Mar 9, 2020 at 14:44
  • @WeiwenNg The reason it can come up is that rims for system wheels are free to have the cross section be as thick as they want at the drilling, and it doesn't matter because the hole can be aimed in the direction needed. Aftermarket rims have to be concerned with allowing the nipple to swivel as needed. I'm not saying it's a huge or insurmountable problem, but it's something that comes up when doing hacky things with carbon rims off system wheelsets.
    – Nathan Knutson
    Mar 9, 2020 at 16:15
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The asymmetric design is intended to be used as a rear wheel to correct the offset caused by the right hub flange being closer to the center than the left. I would keep that rim in the rear position, if these are the tubular version. You could reuse the spokes, but if you are lacing them in a different pattern, and/or if you are swapping hubs and the flanges on the new hubs are a different size, the spokes may not be the correct length anymore.

I would not feel very confident riding on a rear wheel with 16 spokes. It's just my opinion, the wheel could be built up if done properly and provide you years of fun riding, but rear wheels are subject to stresses that the front is not, such as fact that more of a rider's weight is over the rear wheel, and torque from being the drive wheel.

This is also why you do not see rear wheels with radial spoke patterns. The crossing of spokes allows the hub and rim to be braced firmly in the plane of rotation because of the opposing positions of the spokes when crossed over each other.

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  • I know rims with assymetric geometry are supposed to give some kind of balance because of the assymetric hub, but does have such an impact? I see people building wheels and you usually try to center the hub with the rim, ignoring the assymetry. Besides there are a lot of wheelsets with both symetric rims, namely the clincher version of these model.
    – Joao Silva
    Mar 7, 2020 at 16:50
  • And are 20/21 hole that much different that 16h in terms of force balance? And are you sure the rear being the drive one implies there are no radial spoke patterns? Please explain. Campagnolo wheels have a radial pattern.
    – Joao Silva
    Mar 7, 2020 at 17:03
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    Yes, most rims are symmetrical, and it's still my preferred type. Theoretically asymmetrical rims allow to build a rear wheel with slightly better spoke structure by compensating for the asymmetry of the rear hub flanges. Wheel makers sometimes use a radial pattern on the non drive flange, but if you look at those Campy wheels, the drive side still has a cross pattern. this is essential due to the force exerted on the wheel when you pedal. This is called half radial lacing. Wheels should only be laced full radial on front, non disk brake wheel builds.
    – bradly
    Mar 7, 2020 at 17:57
  • Many rear road wheels are built half-radial.
    – Nathan Knutson
    Mar 8, 2020 at 18:27
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I observe that this wheelset does not come with a 16 spoke rear wheel, because it uses half-radial and 2:1 spoking

The OPTBAL (spoke lacing design) rear wheel features a unique 2:1 spoke lacing pattern with half the number of spokes on the non-drive side,...
from https://www.duraace.com/global/en/wheels-intro-deep

I can't get an exact count on the spokes, but their page suggests its 7 NDS spokes plus 14 drive side, giving a 21 spoke rear wheel.

From linked web page
This is all I could find about the number of spokes on the rear wheel.

In the endless quest for lighter-better-faster-stronger I'm positive Shimano would have tried to reduce the spoke count. The fact they don't have a 16 spoke rear C40 wheel suggests that 16 spokes is simply insufficient.

I would not go through with your plan.

On the flip side, I'd have no problem making a rear wheel using a front rim with the correct number of spokes, which would be 32 or ideally 36. Some people ride rear wheels with 24 or 28 spokes, but that's not enough for me.

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    a unique 2:1 spoke lacing pattern with half the number of spokes on the non-drive side Unique? Campy's been doing it for decades...
    – Andrew Henle
    Mar 9, 2020 at 9:08
  • @AndrewHenle good point, that's Shimano's marketting lies/text.
    – Criggie ♦
    Mar 9, 2020 at 10:30
  • @AndrewHenle "unique" to Shimano. We'll give 'em the benefit of the doubt...
    – FreeMan
    Mar 9, 2020 at 20:26
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    Campagnolo has this pattern with groups of three parallel spokes, 2 on the drive side and 1 on the non-drive side, 7 (Zonda) or 6 (Bora) groups. They're applying the same lacing pattern to disk-brake front wheels as well.
    – Carel
    Dec 22, 2020 at 10:56

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