how many chain links?

[buddha]

And what happens if the largest sprocket is very large (not that mine is though)? Or if the small ring is very small?

With a 53/39 > 11-28 setup, the Shimano - "pulleys at 90deg to the ground" does not work when 53>28 is selected. The chain fouls the 'tabs' on the derailleur cage. (I know you shouldn't use that combination, but I like to have it available, just in case).
That's why I used the calculator, I linked to in earlier.

With a 53/39 > 12-25 cassette it was okay though.

 

[rockyraccoon]

My bike has 50/34 and 11-28 cassette

 

[e-rider]

I've just shorten my chain based on shimano tech instruction. I didn't have to count the number of links.

According to it, the the guide pulley and tension pulley should be in line with the centre of the cassette (90 degrees to the floor) when the chain is on the largest chainring and smallest sprocket.

I checked this and my chain needs to be in the middle range of the cassette to get the 90 degrees - so in the smallest sprocket its behind the line suggesting that my chain is too long. However, Shimano also say that 'behind' is ok for double chainsets but not triples! (I use a double). I'm thinking that if it remove a link so it's closer to 90 degrees. If I wanted to shift to a 25T or 27T the rear mech is going to pull very tight indeed. And, more generally , the rear mech will pull forward in every gear whilst using the large chainring if it's at 90 degrees when in the smallest sprocket - is that a good idea?

 

[slowmotion]

I used this calculator.

Thanks for that, buddha. It calculated exactly the same number that came with the bike when new.

 

[rockyraccoon]

... And, more generally , the rear mech will pull forward in every gear whilst using the large chainring if it's at 90 degrees when in the smallest sprocket - is that a good idea?

That is right! I'm using 50/34 and 11-28 cassette and it is very close to 90 degrees.. When using 50T and 28 combination (big ring/big cog), the rear mech pulls forward almost straight. However, it won't be a problem if your chain is not shorter than the number of links recommended by the calculator.

What happens is when you use the largest chainring and the largest sprocket a lot of noise will be generated due to the chain rubbing the front mech. That is "cross-chaining" (using the smallest rear cog with the smallest front ring, or the largest rear cog and the largest front ring)

Bicycle chains are quite flexible, and will work fine with any combination. Even knowing they are there available to be used and can work ok, these combination should be avoided.

As I mentioned my is very close to 90 degrees (after shortened my chain due to crankset and cassette replacements) I had to remove 3 links totalling 53 links.

I just used the calculator which recommends 53. Both methods work fine..

 

[PpPete]

The advantage of going one link "up" rather than down is that it makes thinks things a lot easier if you later change to a larger cassette..... now where was that safe place I put that "spare" link ?