Look at the picture at the bottom of this page:http://www.rodbikes.com/articles/brakes.html
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Typically it is a very bad idea to jury rig disc brake tabs onto a fork or frame that wasn't designed to handle the additional stress from disc brakes. For cross maybe it's ok because of the low speeds and flat courses, but I certainly would never do that with a road bike. For what you'd pay for a disc conversion, you'd probably come close to what you could buy a new disc-compatible cross frame.
Not so fast. Calfee adds their disc tabs to an ENVE fork and they have undergone stringent testing to make sure the fork is strong enough to handle the braking force. Per Calfee webpage, "We had our ENVE disc brake fork tested by ACT and it passed the EN 14781: 188.8.131.52 – disc brake fatigue test."
Link here - scroll to paragraph near bottom of page.:http://www.calfeedesign.com/carbon-fiber/model-calfee-adventure/