im sure it has been touched on, but think about the percentage that you are adding in weight.

500lb bike (which is probably a little heavier then most of the bikes we ride), say you are 180,, and have a 130lb passenger. thats adding another 20%.

basically like putting 800lbs in the back seat of your sedan... lol or 1000lbs in the bed of your truck.... you are going to notice. hahaha