Bicycle Mechanics - A question for the engineers among us...

One thing I love about cycling message boards and websites is reading stuff by folks with backgrounds in engineering, like Jobst Brandt, and what they have to say about WHY bicycles perform as they do. I recently made an alteration on my bike that had unforeseen, baffling consequences, or I'm going mad. Check it out:
My main commuter ride is a 1987 Schwinn Le Tour, True Temper 4130 Chro-Moly, I'm guessing made in Japan. I've updated most of the components, because I absolutely love the frame and fork, which are lighter than many I see for sale nowadays,(Surly, IRO, etc.) The ride is great, but a couple weeks ago, I bought a rear rack for it. 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.

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

I am not sure if adding a mere aluminium rack on your cr-mo would affect the quality of ride by changing the natural frequency(ed note: read accoustics if you like) of the bike...

that being said, I think there maybe a possibility that what you are feeling in an increase in the rigidity of the frame as a whole, otherwise coming through as deaden feel. Since aluminium racks are attached to the seatpost, and to the rear dropouts and/or seat stays, that has the effect of putting a stabilizer bar/strut bar onto a car. The ability for the seat/chain stays to flex relative to their counterparts is prevented if the rack is "rigidly" connected to the frame.

Solution: well, if there are screws holding down the rack, loosen the screws out and run some Loctite on the threads, when you screw the rack back in, do not screw it all the way in, and make sure that there are some play in the rack attachment, that should allow the rack to stay on and not make the bike drive like a Hummer.

Tri

I think TriEngineer hit the nail on the head! The rack is acting as a stiffening truss on your seat stays. I don't, however, think that a loosened mounting bolt will work in the way he predicts. The bolts that hold the rack to the frame are about the same diameter as the holes that they're going through. It is the play between the outer diameter of the bolt and the inner diameter of the hole that you want to increase.

My suggestion, for what it's worth, is to use a smaller diameter mounting bolt at all rack attachment points, butressed on both sides of the mounting flange with small rubber grommets. The rubber should allow the play that your seat stays need to give the feel you're accustomed to. By the way, be sure that the bolt diameter you use has adequate strength to hold the rack and its load! Otherwise, your loaded rack might fall off when you hit a bump!

I don't think my suggestion will cure your problem entirely, but it should improve things somewhat. Good luck!