If you ever have the desire to haul a dumbell in a pannier, don't.
That 20lbs threw off my balance quite a bit and my commute took a bit of extra time. If I ever do this again I am strapping it to the top of the rack.
At least this morning I had the following thought after being left in the dust by a roadie "HA! You think you are so freaking fast! I'm carrying a pannier that weighs more than your bike and only going a few miles per hr slower."
Then I passed him later after he flatted out. Go figure