Caad8 replacement cassette advice

Think my cassette has had it and looking at a replacement. Would be grateful for some advice please from the resident mechanics!

Is this a like for like replacement? Could it be anything other than 9 speed?
There’s two chain rings on the front (compact?).
Is 11-30 the norm?

9 speed tiagra
Any reason I couldn’t put a 105 on and save a bit of weight:

If it is older 9 speed Tiagra, then it should be 50/36 and have a 12-28 on the back for compact gearing. You’ll need another 9 speed cassette because 9 and 10 speed are not compatible. If it is a newer CAAD8 and the Tiagra shifters have windows with indicators for gears, then it is 10 speed and any 105 cassette will be just fine, including 105.

I would suggest a 12-25 10 speed cassette unless you ride some serious hills on long rides. 36/25 will get you up most hills in the UK as a one off. And you will need a new chain.

Might it be easier/more accurate to count the number of sprockets, and what size they are, than ask online without even saying what model/year the bike is?

If it’s much worn then you’ll need a new chain too.

Thanks both!
It’s a 2011 CAAD8, R501 Shimano Wheel.
(The problem started when it fell apart on a ride yesterday, I thought originally it’s worn, but I think now I just fitted it really badly…)

I counted nine chain rings, with 27 teeth on the big one and 12 on the small one.

Is it right that the lock ring screws into the hub? If so, I think the problem is that the cassette extends further than the hub, so it can’t screw in properly. I don’t know what was going through my head when I put it on, I’ve done a fair few miles on it before it exploded!

Am I putting a square peg in a round hole, or should this cassette go on? If so how?!

Here’s some pictures, hopefully it makes more clear:
https://picasaweb.google.com/104061892447017957979/Cassette?authkey=Gv1sRgCJCm1qOF_a3idA&noredirect=1#

Could you explain “exploded”? And I could be wrong but is the 12 tooth on the pile the wrong way round? So the spacing is wrong?

I was standing up on the pedals going up a steep hill and it came undone!

I could well believe the spacing is wrong…I’ll see if I can find a guide to indicate the right spacing and see if that helps…

It says CS-HG50-9 on the lock ring. So perhaps this is the diagram?

http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/CS/EV-CS-HG50-9-1880C_v1_m56577569830799948.pdf

Ok, I’ve got it on but when I turn it to tighten up it feels like it’s grinding. Ie, I can turn to tighten and feel a grinding of notches.

Any advice welcome!

That’s exactly how it should feel. AS long as the cogs are evenly spaced and you don’t apply stupid amounts of torque, you’ll be fine. For a replacement, you will need a 9 speed cassette. That rules out 105.

The lockring and last sprocket are serrated so they lock together to keep it done up without using massive amounts of torque.