I stopped on the way home tonight to buy these:
I've decided I want to get an upper body workout (other than lugging my 45 pound bike around).
When you strap two 8lb dumbells on said 45 pound bike, it has surprisingly little effect on how hard it is to move around the city- because it's a small fraction of the overall weight.
However, they were attached to my rack with elastic straps, and the period of their vibration was significantly out of phase with the rest of the stuff that's attached to the bike. Most stuff that I strap back there either has less mass, or it's spread out over a larger volume, so it's not as noticeable.
But every time I went over a bump, there was a distinctive clunk a half a second after everything else had finished oscillating. Kind of disconcerting, although not really that big a deal.