For my commuter bike, I rode my 1985 Stumpjumper (bought new in 1986) for a year on my 28-mile round-trip commute, and after that year I decided I needed a lighter bike for commuting, so I bought my Jamis Coda Comp in 2003. It really made my commute easier.

For my road bike, I crashed on my 1983 Vitus in 2003. I had that bike since 1987. The crash did it in, so I took that opportunity to shop around and ended up with my Luna in 2005--two years without a road bike. I don't expect I'll upgrade from that, but I have put a compact double on it to replace the regular double.

I bought my Bike Friday last year because I love to see the world by bicycle, and that was not practical with my Luna. All of my vacations involve bringing my bicycle with me and riding wherever I am.

So I guess my answer is that I upgrade out of necessity--when my bike can't do the things that I ask of it.