Hauling your expensive MTB around on the back of your car across the continent gets a little depressing when it starts to rain and your bike is coated in the nastiest road spray. Not only do you have to wipe the bike down just to ride it without getting filthy, but all that water and grit aren’t doing your components any favours.
So like they taught you in high school when in potentially dangerous territory protect what’s between your legs! ;)
We use whatever falls to hand readily. Usually garbage bags or plastic sheeting from Home Depot and duct tape to secure it. I only cover the drivetrain, brakes, saddle, suspension and handlebars. That leaves the wheels mostly open so cars following you can see you taillights.
One advantage to doing this beyond keeping your bike clean is it obscures your bike’s true value making it less attractive to thieves.
It takes about 10mins to wrap the bike and another 10mins to unwrap it. Not a huge deal, but I wouldn’t bother if I was just doing a day trip. I have used leftover mattress covers and bungee cord to cover bikes before. This has the advantage of being very fast. Just drop the cover over the whole bike and wrap with bungee cord. The cover can be removed and reused a few times. The downside is that the cover quickly gets dirty and obscures your taillights. I had a cop pull me over and make me remove the mattress cover for that reason. :( If you have a wide vehicle you might get away with it.