When I would wear a backpack, I found that the straps would start to stress my chest / pecs causing me to be more tired than riding without it. But this was a general backpack... probably the higher end ones are more aero and I would hope that they would have a chest strap to alleviate strain...

I think droy45 was just talking about the comfort factor of weight on the torso (adding to pressure at the butt/seat interface) vs weight on the bike. But perhaps you meant to respond to Aravilar, who was asking about mass and speed...

Good info real useful to bicycle commuters
Obviously filling the large backpack with greased bowling balls will allow a speed obsessed commuter to shave a few micro seconds off the commute, especially if the commute is on the face of the moon.

Congratulations, you completely missed the point of my post.

All I read was some useless exposition on a scenario completely unrelated to cycling. I just made sure other people wouldn't be confused by it.

Precisely so, I must have clicked on the wrong post to quote.

I'm just letting the side conversation run, although I will say that weight matters on the coast-down due to higher density objects having a higher terminal velocity. And more momentum per drag area when the slope levels off. Not sure where a vacuum enters into it.

Thanks for doing this experiment. Do you mind uploading or sending me the source data?

That said, are you serious? There is no air in a vacuum, hence there is no drag. If there is no drag, terminal velocity is only limited by how far you have to fall until you reach relativistic speeds.

If I can dig it up I could just post it here.

Re vacuum, it's not very pertinent to coasting down a hill or biking to work. That's sufficiently handled with rho in the drag equations, which you utilize for terminal velocity. I think for our purposes we can consider the air density to be constant during a ride, and even disregard the actual value entirely if you're just looking for a ratio of drag coefficients (it divides out).

Thanks!

We're talking about two different things; for your experiment, the drag coefficient is essentially constant. I was speaking re: vacuum regarding the other conversation about falling in atmosphere and falling in vacuum.

Good experiment, especially given the budget...

If you had a barometer and a thermometer you'd have enough info to solve P=rho*R*T but over the course of your test series it seems unlikely to change much and not enough overall to change the conclusions for different weather.

If you are interested in a condition where drag doesn't matter - one of my longtime online aquaintances was a judge recently at the NASA rover challenge where high school teams build folding 4-wheel offroad HPV's and ride them around Redstone. I can't think of any way that streamlining would help those guys.