Originally Posted by gregjones

Yeah, that came out muddled. The simple non-techFred explanation:

Weight only makes a significant difference in pulling away from a stop or climbing a hill. Once rolling, the biggest thing working against bikers is air, having to push it aside to move through it.

A set of wide panniers on a bike is kinda like strapping an open umbrella to the rack, or a parachute on a skydiver.......it catches wind an will slow you down increasing pedal effort to remain at a set speed.

My panniers only weigh about 5.5lb but I have to be 2 gears lower on the trip to the store that I am without them. I can load them up and on the return trip I am still 2 gears lower than usual even though they now weigh 15 to 40 pounds. The weight isn't slowing me down---it's pushing the panniers though the air that increases the pedal effort causing me to gear down.

A solid bag will always hold you back, a basket would be almost nothing except for the contents.

These highly scientific findings are the result of seconds of worth of study to determine effort gauged by the effort expended by my calibrated 58 year old butt moving my bike at a speed determined my ultimately accurate bike computer and tires inflated to whatever they are at the time.

Balance isn't a problem. If you are carrying your feather collection and your rock collection to a friends house, put half of the feathers in each bag and leave the rocks at home. You will be fine.