A dirty little secret about disc rims

Disc brakes are gaining ever more adoption through road and cross uses. There's even an emerging category of aero road disc bikes, which may seem an oxymoron to many (honestly, myself included), but it's a wide wide world and there you go.

One thing that most people have probably never thought much about is that aluminum disc rims that are actually designed as disc rims and aren't just fully anodized versions of rim brake rims, the rim sidewalls aren't machined. Machining a brake track is a lot like sending a piece of wood through the planer; you make the opposing sides darn near perfectly straight and parallel. Obviously, this is super important when you are going to use the rim as a braking surface. Any irregularities on the brake track would make braking interesting, which is precisely what braking should not be. 

When you don't machine the sidewalls, those little irregularities are left in the rim. The extrusion process isn't that precise, than then when you roll the extrusion into the rim more imprecisions are introduced. Functionally, these are completely meaningless in a disc wheel, because the role of the rim as a brake rotor os eliminated. The number one thing we're working toward in disc rim builds is making sure the tire bead hooks are as parallel and aligned as possible, and the outside of the rim gets lower priority than it does in a rim brake build.

It's not like you're going to see the wheel wobbling all over the place, or anything like it. If you put it on a stand and know what you're looking for, depending on the rim, you'll notice it. Our tolerances on rim brake builds are pretty tight - we like to refer to it as "slice of paper straight," as in you have trouble fitting the thickness of a slice of paper into any variance. On aluminum disc builds, that's not as regularly achievable, not is it something even to worry about. Everything is as straight as can be without compromising the overall integrity of the build to chase out functionally meaningless little imperfections. You still worry about round like it's nobody's business, though.

Carbon wheels aren't subject to this discussion since they're molded to high precision without need for machining. 

This is probably something that 99% of wheel owners would never notice, but since we have it in mind that each wheel we send out will be inspected with withering scrutiny, we figure it's worth mentioning. 

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Thanks, John.M. – There's a huge element to that, yup.


With all these – do you tru your wheels now from the inside? At the Bead hooks ? It makes more sense that way


That is why I love my RAIL 34s!

John C

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