Need a new rear mech...
 

I have a tatty old rear mech on my 8 speed bike - What is a decent Shimano rear mech to buy? I also don't know weather I need long or short cage.

A lx is a good derailleur. If you want to go even cheaper go with a Deore, I wouldn't go any lower though. For the minimum price difference it just isn't worth it. To be safe go with a long cage derailleur, not many bikes come with short cage.

I had been thinking along the lines of LX but for some reason I thought I worked in reverse or something. What are the benefit of long and short cages?

Short cages shift a little quicker, weigh less. However a short cage may not work with your current setup.

How do I know if short cage will work for me - I am running an 8 speed 11T-28T cassette on a 16" Giant Trance.... any ideas?

a short cage should work with the 28t rear cog. Also the "working in reverse" you spoke of is called rapid rise. Don't get that crap.

rapid rise is far superior for xc riding

sure it is :roflmao:

Keep telling yourself that:rolleyes:

lowcel, you better get rid of that XO setup of yours, it being inferior and all..

Sure, I'll get right on that. ;)