Hi Steve!
Here is a story and photos of my largest underseat bag, my Ortlieb Medium saddlebag mounted on my former Sherpa:
http://www.thorncycles.co.uk/forums/index.php?topic=3809.0More pics here:
http://www.thorncycles.co.uk/forums/index.php?topic=3896.0...And on my Nomad, throughout the posts listed here:
http://www.thorncycles.co.uk/forums/index.php?topic=4523.0This is the largest of my underseat bags, it is completely waterproof and the smallest I could get that would hold the items mentioned earlier, including a Schwalbe SV13 presta valve tube. It does also hold a spare (mini) pump, a Crank Brothers model. Where I go, a pump failure could really cause problems. The Crank Brothers model is small, light, and switchable from high volume to high pressure and works great for emergency use. I also carry a Zefal Pressograph manometer (air pressure gauge), a small sample bottle of Purple Extreme chain oil, a small toothbrush with cut-off handle, and the core from one of my sister's old mascara brushes; the latter two are for brushing the desert playa dust off the chain before I re-lube it (the lot carried in a small zip-top sack, as is the spare tube, so it will stay fresh. A photograph of the bag and contents appears below.
Nearly all my other underseat bags are a fraction of this size (one-half to one-third, in one case one-quarter, by volume), since they only need to carry a spare road-bike tube, a couple patch kits, and the tire lever/jack and some coins. Photo below. I generally don't take the multi-tool for those bikes, since I have a small holder that fits beneath one one of my bottle cages that holds two double-ended allen wrenches, which takes care of nearly anything that might loosen on the road.
Sadly, my favorite multi-tool is no longer made; I would have recommended it without hesitation for the USD$15 I paid on sale. It was marketed about 15 years ago under the PerformanceBike SpinDoctor label here in the States, but there are similar ones available on the market today. I've attached a photo of it below so you can see what it includes. It weighs 260g/9.2oz, but I like it because it is sturdy and has the following features:
= Frame-mountable case
= Two glass-filled nylon tire levers that won't break or mar alu rims.
= Chrome-vanadium tools for 2.5,3,4,5,6mm allen wrenches, Phillips #2 screwdriver.
= Locking(!) stainless tools for 8,9,10mm 12-point hex wrenches, 14g and 15g spoke wrenches, 5mm slotted screwdriver, bottle opener, serrated knife/spatula, 15mm pedal wrench (used with 6mm hex wrench on back side of pedal to share force).
= Stainless (non-locking) tools for 2mm, 8mm allen, chain-rivet tool.
Chain-connector links for each bike's chain live in the patch kit box. The usual USD$20 bill is very tear-resistant thanks to high linen content and it makes a dandy tire boot in the event of sidewall damage.
I decided I preferred an underseat bag hidden under the saddle so my rear rack would be free to carry my rack-top pack or a touring load (typically my sleeping bag, pad, silk liner and air pillow in a dry sack and also my tent in its own sack). As mentioned, the underseat bag keeps the most-needed repair kit handy so I don't have to dig for it; this makes flat repairs much quicker and easier, as the bags all stay on the bike even when touring with a full load.
Hope this helps.
Best,
Dan. (Everything I need, nothin' I don't, and every bit of it handy when needed...)