My Tiagra 4500 9-speed rear dérailleur has so much play now that if I select the lowest gear, the cage hits the spokes. Obviously this is a bad thing and I want to replace it immediately. The trouble is, they're like hen's teeth. There are plenty of 10-speed dérailleurs out there, but before I order anything, I want to confirm that 10-speed is compatible with 9-speed.
I've found conflicting claims on the internet, related to pull ratios. Can anyone put my mind at rest and let me order a new 10-speed dérailleur? I'd rather not upgrade the entire drivetrain as I'll be buying a whole new bike later this year.