How to store a bike outside and still avoid rapid decay?
A small shed seems like the best option if you have space–some insulation inside the shed would help stop condensation.



A bike kept outside will rust given enough time, its gears, etc. will also not like being wet all the time.  Just as importantly, it is more likely to get stolen if it is just left outside, and I would always rather get on a dry bike than a wet/cold bike.