Interesting; that looks like what my goal is for my next short tour. I do know that 4 hours with a 5.5# backpack is too uncomfortable for me. I'll see how 3# feels.

An ounce of experiment is worth a pound of prediction...

Have you considered loading up your bike this weekend and trying, say, a metric century with the pack and the rest of your load? Call it an investment in your knowledge base or some such.

Probably a smart thing to do but not my style. :) I pretty much never ride with a load unless actually on tour.

For me it depends on the biomechanical position my back is in and whether that tends to be a strong or weak position to support weight.On a road bike I am aften in a 45% or less riding position with my back rounded at times(particularly in the drops) so my back is often in a weak position to support added weight so I rarely wear a backpack but like some others have noted bungy it to the rear rack.On mtn bike tours where I'm usually 45 % or above I almost always wear a pack on tour or day ride and sometimes quite heavy.But I always keep my back flat/flexed even leaning foward on the flat bars.From what I can tell you are talking about a performance road bike so I would be cautious adding weight to your back .It can be done with lite weight and it is convenent like you say for off bike but on my road bikes I always eventually just strap it to the rear rack and access it off bike from there(if a day pack/lite weight pack).

By the way I haven't toured since 2007 when I spent 3weeks riding my old Fuji sports touring bike around Florida 10 days of which were spent backpacking in two different National Forests.And yes the backpack,which wasn't all that light,was bungied to the rear rack.I just got a 86 Miyata Six Ten off ebay so I am planning some trips again.

Interesting that you find it more of a problem when using lower bars. My thinking was the opposite. I find it more annoying when more upright since it hangs on my shoulders. With the amount of weight I am proposing I think it will be fine either way, but I ride with bars 4-5" below the saddle so the more upright posture doesn't factor in. I figure that what I expect to have in the backpack is less than I have at times jammed in jersey pockets (at times I have carried a couple liters of water plus some other stuff there).

I'm more concerned about my lower back which tends to get tight at times(and which is more vulnerable to strain when your back is rounded).But since you're talking about light weight and in place of a rear rack ,I say give it a try.It may help to ocassionally stand on the pedals and stretch out and flex you back if it starts to get tight.

I found a nice solution with the Inov-8 Race Pro 4 lumbar pack. It sits low enough so shifting weight is not an issue, and even the speculative reviews boasted a massive capacity. I'm planning to use it with the included reservoir so I can mount a frame bag where my bottle holders go. It's supposed to ship to me tomorrow. I can update with a review then.

Here's a link: http://www.zappos.com/inov-8-race-pr...e?zfcTest=fw:1