Bike parts I've replaced (as of 2013)
For no particular reason I feel like running down all of the bits of my bike that I've had to replace due to wear since 2006. Please note that I bike a lot. People who bike less will replace less stuff.
- brake pads: repeatedly. They wear out.
- the entire drivetrain (with the exception of the front derailer, it's
still original): repeatedly. Chains (and gears) wear too.
- tires, both front and rear (repeatedly). See the sidebar.
- wheels, both front and rear. Although I don't do any extreme biking
I seem to be very harsh on rear wheels; I've gone through at least five.
(The death toll: two broken axles, two used enough that the rims started cracking around spokes, and at least one that just started breaking spokes too often.)
- pedals (repeatedly): either the physical pedals got damaged (when I was
using plastic pedals) or the bearings inside the pedals got too
worn. I've had two pedals actually break, with the pedal falling off;
fortunately both times happened on commute rides with bike stores open
- brake and shifter cables. The most memorable time was when my rear
shifter's cable snapped, immediately dumping me into an inconvenient
gear. Fortunately it was on my commute ride.
- pedal crank arms (I forget why).
- the actual brake arms (sometimes called calipers): these have springs
inside (to force them open when you release the brake levers). Mine
seize up sooner or later.
- the bottom bracket. This was replaced as part of chasing something
else but really, it was time; it had apparently basically rusted
- the headset. The bike came with an adjustable headset (that I never
adjusted the angle on); eventually the adjustable joint basically
broke. It was replaced by a perfectly good non-adjustable headset
which I expect to last forever.
- the (add-on) rear rack. If I remember right, a welded joint eventually separated.
Surprisingly I haven't replaced the bike seat. It's still the original, although it's definitely getting a bit worn by now.
(I think that's everything. If I remember something else I'll update this entry.)
Sidebar: my experience with bike tires
For most of the time since 2006 I've replaced tires when they started to get too many flats. A few years ago I wound up with midrange Continental tires (I believe one step below Touring Plus's), which I've now replaced merely because they looked like they were getting too worn; my rear tire actually wore the tread pattern completely away without, I believe, basically any flats. I currently have Touring Plus's on both front and rear so I'll have some opinions on them in a few years.
In general, either I've had very good luck with my tires or I've got much more relaxed standards of when to replace them because my tires seem to last much longer than most people's. For the front and rear Continentals I replaced this year, the rear lasted over 10,000 kms (carrying a relatively heavy load, since I keep lots of stuff in my panniers) and the front likely ran over 15,000 km. This seems to be well over the usual distance ratings.