derailleur/shifter compatability

[ElyDave]

I have an old 10sp 105 shifter, currently mated to an even older tiagra rear mech (originally 8sp, about 15yo).  Rear mech is getting more and more difficult to dial in accurate shifting and I can't get it across the full range nowadays.

Looking on various websites, I'm concerned about current mech compatibility with the older shifter.  I cant find the model number on the shifter (tried peeling back the hood from both front and rear) so is there any easy way to tell which current derailleurs will work?  I'm needing a long cage btw - use a triple on that bike.

[Phil W]

Get an XT 9 speed rear mech. Still readily available.

[ElyDave]

Phil, forgot to mention, this is a road bike - is the XT cable pull compatible with a road casette?

[Phil W]

Phil, forgot to mention, this is a road bike - is the XT cable pull compatible with a road casette?

Yep, it's the setup I had on my road bike , now use dura ace 10 speed bar ends with an XT 9 speed rear. . The shifter will determine the pull, the rear derailleur doesn't really have a number of gears. It all works perfectly

[ElyDave]

that's kinda what I thought, but (at least some of) the newer rear mechs have a different geometry that affects movement for the same cable pull.

[perpetual dan]

This is quite a clear description that I've referred to a bit https://bike.bikegremlin.com/1278/bicycle-rear-derailleur-compatibility/
It seems that the road / MTB different cable pull (and also the distance the mech must move from low to high?) mostly starts with 10 speed.

If it used to work well and now doesn't that sounds to me more like something wearing out than a compatibility problem - though something on the edge of working might well go over that edge rather sooner.

[ElyDave]

thanks Dan I will take a look.  I don't have a current compatability problem.  I have a very old rear mech that I think is now pretty worn and causing these problems, as you speculate. 

[Phil W]

that's kinda what I thought, but (at least some of) the newer rear mechs have a different geometry that affects movement for the same cable pull.

If you get 9 speed Shimano mtn mech you'll be fine

[Brucey]

compatibility-wise you can use any shimano RD (excepting 7400 series and 4700 series) 6s to 10s or any shimano MTB RD 6s to 9s and it will shift correctly with your 10s shifters on a 10s road cassette.

An 8s tiagra RD is liable to wear out and get to the point where the thing simply won't be reliable any more.  You don't say which 10s 105 shifters you have but IME they are slightly finicky with 'washing lines' and  if the routing is under the bartape they are a very common cause of trouble. 

cheers