Short version: A little difference with panniers vs Backpack, significant improvement strapping it to the rack.
It was the same protocol as before, four tests each with panniers, backpack, and strapped to rack. I threw out the runs contaminated with passing cars and averaged the remaining runs, smoothed with a 3-point moving average. All downhill runs began from a dead stop and without pedaling. The riding position is on the hoods.
Here is the test bike, my foul weather beater:
(don't laugh at my ride. It's served me well for a lot of miles)
with panniers:
these are actually canvas bags strapped together and held over the rack with a bungee cord. The panniers are holding slacks, shirt, socks, belt, shorts, phone, pump and the backpack, the same as if I were commuting except for the extra backpack. Altogether it weighs about four pounds.
backpack:
which also contains the pannier bags. This time the backpack is full and at commute weight. The straps are short so it rides high on my back.
For the last set, the backpack was simply strapped on top of the rack.
The chart shows that there is almost no difference between wearing the pack and having panniers. Strapped on the rack however there is about a .6 to 1 mile per hour improvement at 25 mph, and that is a significant difference. Anyone care to calculate the ratio of CdA, or I will later (and then the traditional "saves x watts at 25 mph" format).
Zooming in on the highest speed range of the tests:
You can see that the panniers actually beat the backpack by up to about .3 mph, and the rack was up to 1 mph better than the backpack.