I run really hot so having something on my back
You convinced me right there - we're no strangers to cycling in heat. No problems.
Just asking 'coz messenger bags are less swamp-armpit and far better balanced for on-bike use.
Messenger bags just become a fight with keeping it positioned.
This is where we differ.
It's academic now - and it takes a bit of adjustment- but once you get the strap(s) right it's
far more secure (and less topheavy) than a backpack.
Somewhere on the internet is my story of delivering a ~60lb gunsmithing jig - and there is no way I could have done it with a pack, not even @telerod15's enormous duffel bag one.
A messenger bag is like a backpack with a dropper post
I realize that if I really wanted to find a solution I could. Once I start school in August I'll have a lot less time anyhow so more than a day a week isn't likely.
Again speaking academically, sign me up for
@jmeb 's n+1 camp. You want something designed to carry a load - and most current roadies aren't. Especially when people load the back rack with food+laptop+clothing and put nothing on the front. The bike becomes stiff in lean but the front wheel flops over, almost like it's not even connected to the frame. Which is
bad especially on downhills - or when coming down from a bump under the rear wheel.