Join Date: Aug 2006
Location: Victoria BC Canada & La Quinta CA USA
Bikes: Birdy Red 8 speed, Birdy Blue 21 Speed, Birdy Monocoque 24 Speed, 2002 Devinci Desperado, 1996 Rocky Mountain Hammer Race
In principle both front and rear suspension probably do take away some power as do a lot of other things such as non-slick tires, puncture resistant tires, sprung seat or suspension seatpost. In practice it varies a lot depending upon suspension travel, geometry and pedaling technique.
It seems very noticeable with my Cannondale full suspension mountain bike, it seems noticeable but less so with my KHS Westwood folder (Front suspension + soft tail) and I don't notice it at all with my full suspension Birdy. I think that my Rocky Mountain hardtail mountain bike was about the same as the KHS. This is however all subjective and none of them had the same tires.
I think that with suspension that is well designed for road use (eg the Birdy), any loss of power is probably insignificant except for racing.