Simple Green is some harsh stuff, I wouldnt recommend it.
What I used is Meguirs car soap, the same stuff I used on my car.
The problem with Simple Green is that it is a degreaser, and it'll do exactly that to all the stuff you want to keep grease on, and since the dust cap on these bikes doesnt look 100% waterproof, it'll get into your bearings and act like a solvent on the grease there. But car soap is strong enough to get the grime off, but not so strong that'll dissolve everything in sight.