I actually found a way to keep metal from touching my rear tires... I figured out a way to mount a high power earth magnet in front of the tire on my swing arm to catch debris before it ends up in my tire, and it blends in with the framework so you can't see it... now, to buy the magnet..... inexpensive too...
I've found that simonize tire slime works great in bike tires, especially the rear tire... I caught a long screw in september, haven't lost air since, and I've ridden it at least 200 miles since then...