I had to make a change on the bike last week - the mini-pannier had to come off. That meant returning to riding to work with a small back pack containing clothes and things.
The pannier didn't weigh much; and neither does the back pack. However, just the small adjustment - that small shift of weight - has had some decidedly odd effects.
For starters, the first time I got out of the saddle to really hit the pedals, I sat straight back down again as the back end felt like it was about to wobble off. The bike was flipping so quickly from side to side, I thought either the back wheel had collapsed or one of the rear stays had cracked or a weld had split. Nope. It was simply the removal of the weight represented by the pannier. When you stand up, you throw the bike from side to side as you pedal. The weight of the pannier slowed that motion down quite considerably. With the weight gone, I was throwing the bike all over the place.
Then I started having trouble cornering - I almost smacked into a few kerbs because I wasn't leaning over hard enough. Previously, the weight of the pannier would pull the bike over - now, I have to work at pushing the top of the bike down when going fast into a corner.
I'm sure an engineer could explain the impact on the various forces governing the bike, but the really strange result has been what's happened around my chest. After a few days, it felt like my pecs (at least that's my description of the things under my nipples) had detached from the underlying muscle and that every intercostal muscle had been ripped. I also got a bad attack of what I thought were stomach cramps.
It turns out the "stomach cramps" was actually my abdominal muscles protesting at having to do some unusual exercises. Again, just that small shift in weight off the rear meant different stomach muscles were being used to balance the bike and counteract my leg movements.
Things seem to have settled down now - I managed to get home without scraping any concrete off kerbs with my pedals, and the "cramps" have disappeared. But how weird is that? You shift about 800 grams of weight, and all hell breaks loose.