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umop3pisdn
Howard
When the bike is fully loaded with rider and gear the fork should already be 'in' to a percentage of its travel. It's usually about 20%-25%. On hardtails this affects the head angle, bb drop, seat tube angle etc. So helpfully Cotic take their measurements using an A-C that accounts for 25% sag in a given travel length (sagged AC = fully extended AC- (120mm/4)).
FWIW 66deg HA when 25% in the fork's travel is pretty, um, 'progressive'.
Just be aware of how the 'assumed' A-C of the fork will not be the 'actual' A-C of the fork if it does not take in to account sag. So a frame design where the HA is 66 static with a fully extended A-C will in reality not ever ride with a 66 deg head angle.