Chaps

If I've got Race Face Evolve XC X-type crank set( which I think, is external ISIS ) on my carbon framed Rocky Mountain Vertex 09, and fancy upgrading.

What measurement do I need to make, to make sure I don't buy the wrong size/type…. ?

Thank you very much in advance

X-Type is not ISIS i believe, it's the same as Shimano HT11 and FSA etc.

The main 2 sizes are 68mm and 73mm which you can find by measuring the BB shell of your bike.

I think all the new type external systems come with spacers to fit either size so you shouldn't have any problems with whatever you buy.

Bottom brakes? Is that so you can pull skids?

For the RF crank arm length I believe the actual measurement is stamped on the inside of the crank arm. Most common measurement is 175mm.

In regards to bottom brackets I would have thought your RF X-Type BB is 68/73mm.

Check the manufacturers website to confirm.