Originally Posted by lightspree

Another way to look at inertial mass: If you were to try to move an extremely heavy object weighing sextillions of tons (a planet, say), it would be rough. Try it. You have a planet handy. Try it. It will barely budge. Try it. Seeing is believing. Just try it and see.

And don't say, "There, it moved!" or I shall be forced to play the Dead Parrot Sketch again.

Or take a somewhat less extreme example: Trying to move an aircraft carrier. It is floating in still water, next to a dock. You brace both feet against the edge of the dock, and push with all your might. You might be able to move it, but it will be slow. It resists acceleration, or change of motion, in accordance with F=ma.

Now try the same thing with a little unloaded dinghy. It will move easily and quickly, and can easily be moved back and forth.

Another experiment: Set up a test of two (full suspension) bikes, A has 50 lbs of unsprung weight attached to the rear stays, B has 50 lbs of sprung weight attached to a strong seatpost rack. Then set up a camera at night, attach a light to the side of each pannier or bag, and take a time lapse photograph of the motion of the light as each bike passes over a series of washboard bumps.

The time lapse will show that bike A's 50 lb load will have to move up and down, in a wave motion. Bike B's time lapse will be much closer to a straight line, or a much milder wave motion.

Now repeat at increasing speeds.

At high speeds, bike A will probably fail. The stresses will increase and increase as the speed is raised. At some point, the stress due to the 50 lbs resisting the required sudden acceleration (to go up and over the bumps) will cause the wheels or the mounting hardware or the bags or some other part of the system to fail. Bike B will float over the bumps, and the time lapse will look more like a straight line.

But maybe you are thinking along other lines.

Heavy loads are harder to suspend on a bike than lighter loads. Up to a point, it's pretty easy - just look at all the bikepacker setups. But after that, as you go heavier, it's harder to find bags or mounting systems that will handle the weight. One approach is to put the dense, heavy items low in a frame bag....

It's rare to see a really heavily loaded touring bike that has the entire load suspended. I've done it, but only with moderately heavy loads (full suspension bike, strong seatpost rack with topdeck and side panels with panniers, and frame bags). It wasn't that great. I preferred having a stable, solid platform in the rear, at least for heavier loads. If I were on really bumpy surfaces, it might be different. I would still want to keep the weight down. It makes sense to minimize weight and bulk. With today's wide selection of ultralight gear, there is rarely any need to have the sorts of loads that were typical in the old days, or even not so many years ago.