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I have an unneeded disk wheel. The rim is "disk-only", but it has sidewalls just as though it was for rim brakes. The sidewalls are painted though and not machined. (I even suspect it's some older model re-labeled with "disk-only".)

What can go wrong putting it in a rim-brake bike? After a few first slowdowns, while I take some care, the paint wears off.

Can sidewalls of "disk-only" rims, while looking just like their old rim-brake-compatible counterparties, be significantly thinner or otherwise unsuitable for using with rim brakes?

I'm looking for facts, not guesses or hypotheses (which I have more than enough myself :-) )

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    One hard and solid fact: If you do use a disk-only rim with rim brakes, and the thing blows up in your face, it'll be completely your own fault, and you cannot expect to be able to sue anyone for the damages. This holds even when the rim is actually identical to a rim with a brake track under the paint. You use some part out-of-spec, your fault when it fails. – cmaster - reinstate monica Dec 8 '19 at 10:01
  • Also, the paint may not be a very good braking surface in some or all conditions, but stripping it well isn't trivial (build it up, and spin it against fine sandpaper?) – Chris H Dec 8 '19 at 13:23
  • I would note that often the brake surfaces on a rim rim are machined slightly to make them smoother. The disk rim may not have been so machined. – Daniel R Hicks Dec 9 '19 at 23:53
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If you own the wheel and have no use for it and cannot sell it for a decent price, I would use it. Most likely the material is the same as for the rims made for rim brakes. Can you find a specification of the weigth of the rim proper and compare it to the weight of the rim-brake version of the same type/brand? That would give some evidence about wall thickness.

However, I would not trust that the paint wears off evenly by a few slowdowns or even many slowdowns. My guess is that the brake pads at first will skid a lot and make occasional rough patches where the paint gets removed. These will then become favored areas where the brake action is stronger. Your braking will get very patchy and uneven. Your brake pads may get degraded for a long time by the paint muck. Instead I recommend to use paint stripper, e.g. metal polish and steel wool, to remove the paint until you see the clean metal everywhere. Then clean the metal surface from organic residues.

Whether the wheel is worth this time and effort, is for you to decide. A new rim and respoking the wheel is not costing a fortune.

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    From my personal experience on using wheels not designated for any brakes with rim calipers. The paint, which was maybe ano, did not want to come off quickly. It took a couple of months of daily all-weather use to wear it down "naturally". Now it is really hard to tell it was there in the first place. I did not notice any degradation/improvement in braking performance early, in between and after the "stripping" process: braking at both wheels has always been adequate for my needs. Still no guarantee that the thing won't blow up under me at any moment, so I am watching rims closely for wear. – Grigory Rechistov Dec 9 '19 at 23:45
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Facts are few and far between on this topic. It's best to assume that a disc brake rim won't have sufficient sidewall material for braking, and this is true the majority of the time.

There are some notable examples of rims where by all appearances, the extrusion sure looks exactly the same between the disc and rim brake versions. DT TK540s and TK540 Discs are one such.

tk540 disctk540

However, there aren't very many current examples of rims like that, but there are a good deal with parallel looking sidewalls that are or should be assumed to be too thin to be a brake track.

The structural efficiency and weight savings of foregoing the brake track are very real, and now that disc brakes are the norm there's not a lot of impetus for manufacturers to eke more value out of an extrusion by using it for both disc and rim designs.

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    Most rims designed for braking have a wear indicating groove. If that's present on a "disc only" rim it might be a good sign – Chris H Dec 8 '19 at 13:15
  • Still not a reason for not using the proper stuff. Be it only for liability reasons.If you cause an accident and somebody else dies from it you'll face a tough case. – Carel Dec 8 '19 at 16:20
  • Even in the case of the TK540, it's obvious that the braking surface on the rim-brake model has been treated completely differently than the disk-brake version. I suspect the disk-brake version has a "braking" surface that is considerably softer and will wear much faster if used with rim brakes. A less substantial disk-only rim that doesn't share its heritage with a heavy rim-brake trekking rim might actually fail rapidly. – Andrew Henle Dec 9 '19 at 11:08
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    @AndrewHenle It could have some kind of hardening/material difference, but I suspect the only difference is the sidewall is left unground. I'd be interested if I was wrong though. – Nathan Knutson Dec 9 '19 at 18:05

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