I already have panniers and have several sizes of waterproof stuff sacks for camping. I plan on some weekend and overnight bike trips this summer and was wondering if there is any advantage to a trunk bag over a stuff sack and a couple of bungie cords. BTW, my tool kit and spare tube are carried in a triangle frame bag above the seat tube water bottle, so no worries about where it will go. My wife wants to get me something for cycling for my birthday and the two things I could think of were a trunk bag or a mini-frame pump.

1. Probably sturdier
2. Easier to get in there to access stuff

On the other hand, the stuff sack approach has

1. Waterproof, assuming it's a dry sack
2. Cheaper
3. Lighter

Personally I'd just use the stuff sack and not put anything in there that I'll need to take out during the day's riding. Then you can get the mini pump - a Topeak Road Morph or Mountain Morph is perfect.

Some trunk bags are insulated and mostly waterproof, so they can be used as a makeshift cooler. Fill with ice and your choice of beverage once at camp, and maybe it'll also work for perishables overnight.

I adapted a $3.99 6-can soft cooler/lunch box for use as a handlebar bag, and it does keep the inside dry in the rain, except for some minor seepage through the zipper in a downpour. Much better than a straight nylon bag I had before, and I do use it as a cooler once in camp, too. The best method is to stop at a dollar store before camp and get a small Styrofoam cooler, but that's not going to be feasible everywhere...I tried to leave the cooler at the hike and bike site for the next day's travelers, but the rangers insisted on scouring the campsite clean every morning.

no advantage for me, but several disadvantages.

trunk bag will often extend over the rails, making it difficult or impossible to mount
your panniers. takes up the solid, flat space needed to carry your tent or six-pack.

if the trunk bag straps don't interfere with the panniers, you still may have to remove
the trunk bag each time you try to mount/remove the panniers.

not as versatile as bungeed-on stuff sacks.

i only use the trunk bag for local rides. h-bar bag with wallet and cell phone and
map. trunk bag has tools, spare, and pump. has foldout expando sides if i decide
to pick up some magazines or such. inside is large enough to hold a pineapple or
a couple coconuts. i never use the trunk bag for longer rides with panniers.

Stuff sacks are variable in size, lighter, and can be totally waterproof. I'd always favor them over a trunk for touring. The trunk's big selling point, for me, is easy access while out on the ride, but I can get that same easy access from my front panniers.

My Sleeping Bag and Tent went on top of the rear rack , no trunk bag is that size.

though I did Use a REI Rucksack as a place to stuff my stuff sacks into ,
and gain Lash Points to secure the rucksack , solidly on the top of the rack..
+ i could wear the rucksack, off the bike..

I have used both, but not for touring -- for randonneuring. The trunk bag definitely has the advantage of being able to organise stuff into discreet groupings in the pockets. I am a keen fan of Topeak's original rectangular rack backs, although less so of their newer sloped-top ones. As for waterproofing, I think Topeak offers a cover rather like the ones used for their handlebar bags.

I used Sea to Summit bags to pack away a change of bike clothes and a sleeping bag on some randonnees. The disadvantage, as stated, is that to get to the stuff at the bottom, you have to take everything out. If there is small stuff in there, that brings up the possibility of dropping it and/or losing it. Another slight disadvantage is wear from contact points with the rack and straps/bungies that secure it. The distinct advantage is waterproofness without having to worry about an extra covering.

As I note in another thread, our touring bikes are equipped with the Topeak Tourist racks that have a lower bar for attaching the panniers. This obviously was designed to overcome the issue of trying to carry the rack bag and panniers together. By offering this rack with the extra rail, I can, if I want, leave a rack bag permanent on the bike and use it for commuting, leisure rides, randonnees or whatever, and just add the panniers without hassle when I need to. Mind you, that does present an issue with finding a place for the tent.

One other out-of-the-square thought is... have you thought of a Carradice saddle bag? You don't absolutely need a Brooks saddle to use one, and as Machka has shown in a number of her pictures, the Nelson Longflap she has fits quite well over her Topeak rack with the panniers underneath. And our experience with both our Carradice bags (yes, we have several, each) is that they are waterproof, roomy and durable.

I prefer a small duffel that is essentially a large waterproof stuff sack. It gives me more space than I need, so when I leave the grocery store I have the volume that I needed to hold everything that I bought. Fully loaded without food my duffel is only about 20 percent full but it is great to have expandable space for when you need it for more food or water bottles.

In my experience, it is very nice to have a bag that is both removable and easily accessible to carry valuables and snacks in. So, that means you either dedicate the top of one of your panniers to a pocketed bag, use a handlebar bag, or get a trunk. Others may disagree, but this has been my experience.

you don't need a trunk bag,

you DO need a frame pump.

the advantages to the use of stuff sacks is myriad. you can easily strap extra things - a newspaper, takeout box, small load of firewood or a six pack - above or below stuff sacks and bungies. you can dry clothes more easily with the modular bag/bungie system.

Frame pump it is. Thanks for the advice.