79pmooney

Steel (non-stainless) rusts. Leather gets soaked and dries harder. Light oils can get washed out of parts needing lubrication. Compressed air can force grit and debris into chains, bearings and other places it doesn,t belong.

That said, I've put 50,000 miles on a steel bike that has seen plenty of rain, including some rides where "downpour" would be an understatement. Now that bike 1) has had the insides of the frame treated with rust prevention, 2) had bearing greased with plenty of quality grease, 3) leather was treated with quality leather treatments (toestraps, leather covered racing seats and for around 15 years, leather wrapped handlebars). Moving parts need lubrication. Bearings need to stay lubricated and free from water. On a bike ridden regularly, heavier oils and greases are your friend. I use light Tri-Flo for the chains of my summer bikes but Finish Line MTB wet lubricant for the winter bikes because a hard rain (or ride through a very deep puddle) won't wash it out..

Now, as others have said, frames of carbon fiber, aluminum and titanium don't care about water. Aluminum doesn't either.

If you plan on doing more rain day rides, tell us what you're riding. Not everything wants the same treatment.

Edit: Fenders are your (and your bike's) best friend. Get full fenders with a deep front flap or plan on making a good front flap. (Quote me and I'll tell you what I do.) And if you ride in the rain enough, get a rain bike and save your good one for the better days.