I normally use a pannier on the way to and from work, but today I decided to try a small rucksack instead - same kit/load. The results were outstanding; Normally I do the journey (19miles) in anything between 1hr 11min & 1hr 17mins - dependent upon the weather/wind etc. Every journey is a race so I always push a fair bit. This year the quickest I have managed is 1hr 13min - just getting back into it and not fully fit yet. Well today, the bike seemed lighter (weather was same as last week). I was standing and pushing very hard up any slight hill (which I dont do with panniers) and the end result was 1hr 7mins. Not sure about the science, but I will now be binning the pannier and sticking to the rucksack. Maybe its a mixture of drag, wind resistance and balance? "Sweaty back" is not a problem - the amount I push/sweat means I am soaked even without the pack!! I am sure someone will provide a more scientific explanation, but panniers are now out for me.