It took me a while, but I finally figured out why the backs of my pantleg was always getting these big black streak marks. I thought it might be from oil leaking from the gearshift console, maybe grease from the track the seat slides back and forth in, the edge of the door, the moving sidewalk at the airport, every fence or doorway I brushed against.
No, it's the Volo. When you use the shoulder strap, it positions the street grime-covered rear wheels right behind my knee. And so with every step, my pants help clean the wheels a little more. Unfortunately, the Volo's center of gravity doesn't really allow you to carry it more horizontally, like a golf bag. And even if it did, that'd just put the grimy front wheels right under your arm, where your shirt, sweater or jacket could clean them.