Dunno. Maybe you are right in that I was working a bit harder without
realising it. The brain is a weird thing - or maybe it's just me that's
weird and not me brain

I found today's ride a touch easier than the rest of the week's because
the breeze cooled thigns down a touch. That'll help you ride home. Other
than that, momentum on the flat is increased due to increased mass, but
once you're up to speed it'll be the same effort to stay there. The extra
mass will help a bit over lumps and bumps, but uphills will hurt more.
The extra incentive to push (out of the saddle) will drop the time a
touch, so it's not that surprising.

The other way of looking at it: 12kg is going to be roughly 10% of your
total bike and body weight (assuming you're not either annoyingly light or
disturbingly heavy, and giving a reasonably leeway in "roughly). On the
flat it'll make virtually no difference, uphill it'll add 10% to your
time, downhill it'll drop about 8% from your time. The overall effect is
bugger all, but you'll certainly notice it being heavier.

Oh, and Moike - you should do OK at Physics, mail me if you ever need a
hand!

--
Dave Hughes -
"Love is a snowmobile racing across the tundra and then suddenly it flips
over, pinning you underneath. At night, the ice weasels come." - Matt
Groening