I started to think about what was in my payload and why it is much heavier in the summer. That is because, in the summer, I ride in bike shorts and road shoes, so I have to pack jeans/trousers, belt, wallet, change etc. on the bike. In winter, I just wear my work clothes and mountain shoes, and don't change when I get to work. So the extra items I'm wearing or transporting in the summer (workshoes + bike shorts) are probably partially offset by the extra items I'm wearing in the winter: mainly biking jacket, & other cold weather wear. So I guess I'll start weighing the rider + bike and see what the true all-up weight is in both situations. I bet the rider weighs more (stripped) in the winter, since he's getting less exercise due to the lousy weather.