Join Date: Mar 2006
Location: Vancouver, Canada
Bikes: Rodriguez Shiftless street fixie with S&S couplers, Kuwahara tandem, Trek carbon, Dolan track
You don't really need a drop bag for a 100 mile road event. A 100 mile mtb event, though, you just might. Knowing what little I do about off-road riding, it always take longer to go any distance offroad. So I'm just going to assume that you might be riding into the evening/night, and that the weather will be iffy.
So to start off, you need a waterproof bag that you can zip completely shut. Always assume worst case: the organizers will have your bag waiting for you mixed in with a hundred others in the middle of a wet, muddy field, and it will be raining. (It's happened on PBP!) How about a small orange storage bag for use on kayaks? Or a small cooler bag?
As far as contents, I like to have a fresh, dry t-shirt to change into, and for rides of more than ten hours, a fresh pair of shorts. If it's been raining, you will want a towel and fresh socks, as well as a fresh jersey and spare rain jacket. And heading into night time, you will want to have your battery lights inside the bag, or spare batteries. All of this should fit into a 2 ft x 2 ft bag. I don't think you'll need any food, since I would assume there will be food stops. If not, put food in there! But nothing that will leak & get everything else wet. Advil also helps sometimes.