Originally Posted by
cyccommute
And now you are missing the point. Using the same extrusion profile for both the disc and rim version says that they aren’t concerned about any supposed need for a stronger rim in the disc version. In fact, they could have saved themselves some stocking costs by just selling one version.
As for the minimal weight savings, that’s what I’ve been saying all along. The difference in strength is going to be marginal between a disc and a rim version because the difference in forces (if they even exist) are minimal. There is no need to mark a rim as “for rim brake use only” other than for legal reasons...and even those are rather ridiculous.
It's the other way around. If they decided to use a single extrusion die, then they would not design a weaker profile for rim brakes and then use the same version for disc brake. Instead they would design a stronger profile for disc brake and then use the same profile for rim brakes.
No, they would not sell one exact same version of rim for both rim brake and disc brake versions. This is because milling the sidewall for the brake track costs time and money. They would not sell a milled sidewall for disc brakes, because there is extra machine time, tooling expenses (which wear out and have to be replaced), and payment for a worker to place the rim into the milling machine and flip it onto the other side. Instead they would sell one version with a milled wall for rim brakes, and a non-processed version for the disc brake (cheaper too). That is why, if they did use a single extrusion die for both versions, the rim brake version would be lighter because material would be removed from the sidewall, while the disc brake version would not have any additional material removed (other than the spoke and valve holes). If the disc brake version was actually lighter because of thinner sidewalls than the rim brake version, then they would have used 2 different extrusion dies with different profiles.
Also 1mm extra thickness to the spoke hole bed in the rim for only 35g of extra mass is actually a big increase in strength against cracking of the rim at the holes. My cheap $15 rim is only 2mm thick. 1mm extra would increase the depth of the hole by 50%. I don't know how this would affect the increase of strength for the the nipple to bear on, but maybe if we assume that the load radiates out at a 45 degree angle from one side of the hole to the other side, that's an additional square root of 2. So that extra 1mm of thickness may increase the distance that the load travels through the material by 70%. Whether or not the extra strength is needed (or how much is needed), I don't know, because I don't know exactly how much greater is the total load on the spokes with a disc brake.
A rim might also utilize eyelets in the spoke holes to better distribute the load around the holes instead of increasing the thickness of the bed.