Originally Posted by
Dave Mayer
The Emonda SLR RSL rim frameset weighs 640g. Specs here:
Émonda SLR RSL Frameset - Trek Bikes (CA)
The disc equivalent is 665 g, but the disc fork, because it has to overcome forces at the ends of the blades instead of at the rim brake crown, it has to be considerably bulked up. The relative fork weights are harder to find on the Trek website, for a reason.
Of course, this bulking-up renders the disc brake fork less flexible and less compliant. Perhaps one reason for the irrationally fat tires (>28) being spec'd for road bikes. Because the disc-specific fork absorbs less road buzz, the tires are forced into this role. Plus we all getting fatter.
According to
Damon Rinard's fork deflection test results, the (rim) brake fork with the greatest front-to-back deflection (labeled "Trek Classic Carbon") deflected just under two tenths of an inch. Someone with trigonometry skills would be able to work out how much vertical deflection that represents, but it would be less than two tenths of an inch.
Even if the difference between the disc and rim brake versions of the Emonda fork in front-to-back stiffness were as much as two-tenths of an inch (i.e., if the disc fork were perfectly stiff, which is obviously impossible), there is no way that would be perceptible to the rider, because the difference would be swamped by the much greater compliance of the tire.
Which is one of the major points Rinard makes regarding fork stiffness versus comfort in the text at the link.
Of course, confirmation bias can be worth at least two-tenths of an inch of measured deflection. Personally, I've never felt any difference in the "comfort" of any fork on any road bike I've owned or ridden that couldn't be accounted for by the tire's deflection or, at least as significantly, the bike's geometry, but maybe that's confirmation bias, too.