I've used an old cheapo plastic bike cover to protect bike and contents from dew and rain at night and it is rapidly deteriorating. I would appreciate any suggestions from anyone who uses such an item on a tour. I've seen the Topeak cover but doubt it would fit over panniers, etc. I haven't seen anything similar to what I have but it was inexpensive, relatively light and big enough.
I just put something over the saddle, my cork handlebar tape does not seem affected. Other than that covering the bike at night seems like a waste of time. It needs to be functional in a day long downpours so what is a little dew.
You can get light tyvek from kite stores. Silnylon might be appropriate.
The most I've ever used was a light, old groundsheet. Most of the time though, I just put a plastic grocery bag over my saddle ... and the panniers come in the tent with me.
You could also use those large black plastic garbage bags. I line my panniers with those, but always have a few left over in the package which I fold and carry in the bottom of my panniers. They'd probably work as a bicycle cover.
Raleigh Aspen touring/off-road hybrid, and a Bob Yak trailer. Yak very useful for us car-free types that like to buy lots of beer.
I cover my bike at night because I stealth camp. Any bike shop around here has lots of these bike covers that fold into themselves in a tiny little ball when not in use. I always have one in my panniers.