Simple green, in my experience is a great paint remover.... if you let it sit long enough full strength on the paint. Never the less I have no reservations about using weaker solutions on painted (bike or other) surfaces, as long as it is cleaned off within a short period, which is pretty much what you do when cleaning a bike. I used it at about 10% conc to clean an industrial 1800# milling machine of 30yrs accumulated grease, grunge and coolant before repainting a few years ago and the paint didn't peel off before and the new paint is still adhering. Try it on some other painted metal first, but it is unlikely to be a problem, there is a lot of painted metal in engine compartments.