Let us look at the pros and cons of the following sprocket:
These two sprocket are great on trails or even in races with climbs of some difficulty. The 11-26 is very versatile, has only the limitation of not being in the presence of the most difficult climbs, on the other hand is fine even on flat routes. It’s ideal of hill roads, with continuous up and downs.
The 12-27 is a more specific races in which there is a lot of uphill, not steep slopes, but always with a certain altitude to climb. Compared to other solutions to the 12-27 sprocket closest among them. Remains a product not suitable, use amateur to climb very demanding.