It’s been asked a few times, suffice to say in many cases you need to select suitable Chain ring sizes and Cassette spread to accommodate your intended use.
For instance if you plan to spin your way up the odd climb then keeping a 22t granny and having an 11-32t or 11-34t cassette can be advantageous but you are then rather more limited on the size your other chain ring can go up to (36t).
If you’re a bit more of a Grauncher you can go for a higher tooth count granny and thus a bigger “big” ring (I believe the biggest allowable difference between chain rings is 14t using an SLX or similar Front mech) so a 26/40 would give you a taller top gear (40-11) but obviously you loose out on ultimate spinney gears, quite good for Proper XC racing snakes, less fun for those with a finite level of fitness/stamina
Obviously the wider the spread of your cassette the bigger the jumps between certain gears, so while an 11-34: 9speed will give you the option of having that ultimate spinney gear you will potentially have at least one uncomfortable jump between 2 gears (often near the middle).Of course an 11-28: 9speed Cassette will give more comfortable smaller steps between gears but doesn’t provide that ultimate spinney top gear should you require it….
Basically you will be narrowing the range of gears you have; where a “typical” triple setup (22/32/42 with an 11-32t cassette) gives you what could be considered a full spread, very broad choice of ratios (albeit with some replication, and possibly some minor jumps) you have to select which particular range of gearing you want with a double ring setup, either a bit tall or a bit spinney….
Did that maske any sense?
