I have read some accounts of the rear wheels collecting rocks which get caught inside the wheel and do considerable damage to the wheel. Some even say the damage is severe enough to cause a flat. I am not having much success finding a source for flaps or guards to help prevent such damage. There have been some pictures but are all those custom made parts? I ride Arizona deserts, lots of rocks, and would like to prevent a problem before it happens. Anybody here have first hand experience with this issue? How to prevent it?