What's all this talk of washing bikes? I ride mine on the trails so often, if I washed it 'properly' after every ride it would erode, not corrode.
When I get back, I swill the majority off with the hose, then attack the stuff that's stuck on a little better with a paintbrush, rinse, bounce the excess water off the bike, then spray the expensive stuff with WD40 to remove the water, then stick it in the garage ready to ride again the next day. 10-minutes, done.
If the weather's nice I might put it in the bike stand and do it properly, but with work, wife, kids, and lots of other stuff going on in my life that's rare. When I do I use something that makes bubbles. I sometimes wonder whether it's worth it though cos 5 mins in to the next ride it looks like sh1t again, as it's supposed to. Clean MTBs just look ghey.