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I'm building a dynamo hub wheel.

I see there's symmetrical and asymmetrical rim and not sure which one I should go with.

I'm using son dynamo 28 if that makes difference.

enter image description here

My front wheel has SRAM Level TL, 2-piston caliper, hydraulic disc.

enter image description here

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  • Just as a side note, I recommend reading your hub's manual to see what it about off-road riding. IIRC, my SON 28 was rated for "light, off-road (gravel/unpaved)" use but not "rugged mountain biking" – Paul H Dec 18 '20 at 4:34
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Asymmetrical rims raise the weaker side tension and reduce the bracing angle disparity on wheels where the hub has a different center-to-flange distance on each side, as long as the amount of offset at the rim in question brings the dish closer to center than the offset at the hub moves it away from center.

SON28 disc hubs have smaller and closer to symmetrical center to flange numbers than most disc hubs. As you can see in the numbers from Schmidt's site below, the difference in center to flange is only 2.5mm for 6-bolt and 2mm for centerlock.

SON hub dims 6b SON hub dims cl

Therefore, you have to be careful with the golf-ball type generator hubs about not running into the mostly fringe situation of an offset rim doing more harm than good. You (or at least I) really need to run the numbers to figure out if it's beneficial. We'll use a WTB 29" Asym I29 with its 4.5mm of offset to demonstrate, with the KOM Light to compare with. Being able to do this easily is one of the reasons the nerdier spoke calculators are superior. I will use SpoCalc here.

6-bolt SON28 with WTB Asym I29 29": enter image description here

6-bolt SON28 with WTB KOM Light I29 29": enter image description here

Centerlock SON28 with WTB Asym I29 29": enter image description here

Centerlock SON28 with WTB KOM Light I29 29": enter image description here

130%-ish is further away from 100% than 91%-ish, and the bracing angles are way less disparate on the KOM. So in none of these example cases would 4.5mm offset be beneficial. It would be all downside. You'll need to do the math yourself for the parts you're using, but among contemporary mountain asymmetrical rims this is probably about what you'll see with the SON28 in particular.

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  • Thanks, I can sense it has a true analytical and data driven answer. Althouth I can't decipher what that means.. I think what it comes down to is with contemporary mtb asymmetrical rims and son28 I'm probably worse off than just using symmetrical ? because son28 is almost symmetrical.. – eugene Dec 19 '20 at 12:03
  • @eugene That's right. SON28 in particular is a narrow-flanged hub, moreso than almost any other front disc hub you can name. That will cause most offset mountain type rims to over-compensate and leave you worse than if you had used a symmetrical rim. But it does depend on the amount of offset the rim has; there are road rims for example with an offset dimension where this wouldn't be the case. And because of the hi-low flanges on the 6-bolt, you can't just compare offset numbers on a hub like to see which is greater. You would need to run the numbers. – Nathan Knutson Dec 19 '20 at 18:12
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    (Or at least, again, I need to use a spreadsheet and look at the numbers before I can be confident enough about this kind of thing to order parts or start the build. Now that I'm thinking about it, it's probably obvious to some that the disc side flange being bigger is only going to make it worse if the offset comparison put you in the red already. What I'm trying to impart is that if you're uncertain, figuring it out with a tool like SpoCalc is your friend, as opposed to some advanced next level thing.) – Nathan Knutson Dec 19 '20 at 18:22
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If the hub has the mounts to install a disc, the length of the spokes each side differ from each other when using a symmetrical rim. This is one of the reasons why people use asymmetric rims with discs, and it makes it much easier to use all spokes with the same size.

Ultimately, it won't make a huge difference. If the hub has a disc, I would try for an asymmetrical rim, but if a symmetrical one is cheaper or "better" then I wouldn't think twice about using that. As I have in the past.

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  • when you say if the hub has a disc it means, hub has the mount to install the disc? So can I understand it as when hub has the mount for disk, it's preferred to use asymmetric rims. ? And it is still possible to use symetrical rims even so? – eugene Dec 18 '20 at 2:55
  • I will change the answer to make it clear. Yes that is what I meant. It is still possible. – abdnChap Dec 18 '20 at 9:40
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Is the wheel symmetrical or asymmetrical? It's usually symmetrical if it's a front wheel with rim brakes, but a brake disk on the front can make it asymmetrical. A rear wheel is usually asymmetrical due to the presence of sprockets on one side, but a brake disk on the other side can make it symmetrical.

I will add that it would be nice if rim manufacturers would specify how much offset an offset rim is designed for. One designed to allow for an 11-speed rear cog might be overdoing it for a front hub with a brake disk.

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  • I have a hydraulic disk brake on front.. In that case.. I should go with asymetrical? or it's a matter of choice and either symetrical or asymetrical is fine? – eugene Dec 18 '20 at 2:38
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    @eugene - Probably asymmetrical, but it's not all that critical. The asymmetrical rims just have one set of holes drilled slightly offset from where they would otherwise be. – Daniel R Hicks Dec 18 '20 at 2:44

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