Hi there,

New to this but hoping I can get some help. I'm a novice biker who
Recently purchased a used hardrock specialised mountain bike which is
A few years old. On my first go of the bike the dérailleur seemed to just come
Away from the bike and seems to be bent. I want to replace this. I'm aware that there is GS and ss and different makes etc.

As far as I can tell a GS Shimano RD 2400 Claris 8 Speed Rear Dérailleur should
Be compatible. As I don't want to spend a lot and can pick this up for about Ł18.
Is there any reason this would not be compatible with the bike? If so what would be a better option? All help greatly appreciated.

Thanks in advance
Paul.

Claris is a "road" RDER and won't have enough chain wrap capacity. It possily wouldn't handle your largest cog either, depending on how large it is.
Get a "Mountain" RDER, such as Altus, Acera, Alivio, Deore etc.

Chain wrap =
(Largest-smallest chain ring) + (largest-smallest cog)
Example- if you had a 22-32-44 crank & 11-34 cassette.
(44-22) + (34-11)
=22+23
=45T chain wrap capacity needed.

It appears a Claris GS has a max of either 37 or 41 according to a quick internet search.
Looked it up-
2400 SS has 37T
2400 GS has 41T
Largest cog 32T

Thanks for your advice bill.
Forgive for being thick but how do I know the size of crank cassette etc. do I literally count
The teeth on the cogs etc.

Yes

Ok. After what you said I am thinking a [h=3]Shimano RD-M410 Alivio 7/8 Speed MTB Rear Mech[/h]

IF you have a 7 or 8 speed cassette.
That RDER will "work" with 9 speed, but an RDER designed for 9 speed will work better.
You haven't told us how many cogs your cassette has.

BTW, the brand is Specialized and the model is Hardrock.

Ok thanks.
I'll need to count the cogs tomorrow.
So even though I know the bike is 8 speed
A mech designed for 9 speed will work better?

If you KNOW the bike is an 8 speed, why would you count the cogs.
The 8 speed RDER will work fine.

Maybe first check if the derailler hanger is bent.