Originally Posted by krispistoferson
This aluminum Delta rack seemed to "deaden" the ride and make the bike feel much heavier, even with nothing on it. Could the "acoustics" of my frame have been changed somehow? I took it on and off several times to make sure and I really don't think it's my imagination. Thanks in advance for your replies.
I'm an engineer, but only in electronics/systems. That said, I worked in under water acoustics/sonar systems for about three decades. Acoustics are vibrations in a media or material like the metal of your bike frame and the air in your tires.
It's certainly logical to me that the bike frame will vibrate differently when it's got an additional amount of mass clamped to it at several points on the frame. Both the mass of the rack and the rigidity or stiffness (or lack of) of the rack will cause the bike to vibrate differently; either in amplitude or frequency or both.
On your bike, the rack apparently dampens or decreases the vibrations, and/or moves the frequency range that your bike likes to vibrate in, to a different frequency range which is less stimulated by the road surface. A different bike or even a different rack, could in theory at least (but not likely), make the bike more lively than the no-rack case. A different rack could have less damping, but it would take experimentation to determine that.
Vibrations in any "system" is a function of the materials, size and shape (even the frame angles and shape of the tubes will have some affect) of that system. The system here is all the parts of the bike (including tires/tubes) and the rider to some degree.
I periodically use a rack (hollow steel tubes) on my Titanium road bike, but I've never noticed a difference. I also never noticed it on my previous steel bike with a different rack. I may not be as perceptive or sensitive to such things as you, so that's another variable. I doubt it's your imagination, but that's a possibility too.
Al