Marathon rear derailleur upgrade

[eightkrate]

Hi everyone,
I have an old raleigh marathon mid eighty's manufacture I think.
Long story short my son borrowed the bike, and broke the rear derailleur.
The front derailleur is marked suntour 7 the rear only says suntour and really looks cheap, not to mention it is broken into two pieces. It is a 12 speed bike and I really want to get it back on the road. My question are do I need a long cage derailleur and how do I determine what will fit.I guess I want to know what my options are.
Thanks In Advance
Brett

[JohnV]

I would first start by checking SunTour's web site to see if that model is still being made. Here is the link to their site: SR Suntour. If they don't show the dérailleur group on their site, do a Google search to see if there are any available and also to see if there are any specs on them so that you can match it with a different dérailleur.

[eightkrate]

I called Suntour first. I think you people know more about the products they make than they do. They can't do any thing with out a part number I will try and post a couple of pictures later. Hopefully you can tell me if it is a long cage derailleur.
Thanks For the Response
Brett

[DaveM]

You may not need a Suntour derailleur. If the shifters are indexed (click into gear shifting) then you need a Suntour or compatible derailleur. (I think you could probably get away with a Shimano, but am not sure on 6 speed). If it's friction shifting, it doesn't matter what brand derailleur you get.

You should probably get a mid to long cage derailleur. You don't want one of the really tight short cage high end racing bike ones. They are made for very tight gear ranges.

You also need to check if your derailleur has a hook that clamps under the axle nut or if it screws directly into a built in hanger on the dropout.

[cyclerUK]

Long cage derailleurs are meant for use with a triple front crankset.
( the long cage is to take up the chain slack between the large and small "granny" ring).
As you have a 2 x 6 setup you only need a short/medium derailleur unless you expect to change to a triple crankset in the near future.
Being a mid eighties bike I would think you have a friction shifter changer so any make ( Shimano or other) will suffice.
Get one that will deal with 8 speed or less to accommodate the chain width that six speed uses.

What DaveM is asking "does your old derailleur have a hanger with a "hook" fitting as shown on this one"? :-
http://www.sjscycles.co.uk/sunrace-m2t-6-7-speed-rear-derailleur-with-hanger-long-cage-prod23545/

If so you can get a hanger that will adapt a standard screw type derailleur to fit your frame.:-
http://www.sjscycles.co.uk/shimano-rear-gear-hanger-bracket-fixing-bolt-prod406/

A last requirement - how many teeth are on the largest sprocket at the back? If above 28 teeth then go for an MTB derailleur if below then a "road" version will be O.K.

Have a trip down to your LBS and see what they have to offer