Just fitted a different brake rotor. Should I go for new pads too, or try and re-bed-in the old ones, which have a reasonable amount of life left in them?
Are the old pads in any way contaminated?
I don't see a reason to change..
I don't think so. But they're certainly not as grabby as they were before. I assume its because the pads and rotor don't quite match up on a microscopic level.
If they aren't contaminated there's no real need to change, but sanding them down a little and burning them in again should help put the grabbiness back into them.
"they're certainly not as grabby as they were before"
The rotor wont be bedded into the pads yet
Rough em up with some sandpaper, bung em back on and go donuts everything back in again.
Rotors take longer than new pads to bed in. Go and find a big hill to drag your brakes down a few times is the absolute ideal. Failing that, ride round the block for a while dragging your brakes slightly. With a series of fast hard stops inbetween too. It'll get you fit too, as well as bed your brakes in nicely!
A pre-emptive word to the "just go ride it" crew... Taking the time to bed your rotors and pads in properly not only results in superior performance, but also avoids the "oh crap, am I gonna stop" moments when you're hammering down a trail and forget you've got new brakes. As a word of caution, I know someone who changed his brake pads just before setting off down a fast trail, went to grab handfuls of brake for a corner and almost nothing happened... A trip to A&E later and he had a full arm cast for the next 8 weeks! This is of course an extreme word of caution, but be careful if you're riding with new pads/rotors...