Now, I’ve had this explained to me a million times but I keep forgetting
I’m putting together my new 29er and trying to figure it out again

42-32-24 up front and 11-13-15-17-19-21-24-28-32-36 rear

Can I get away with the shorter GS mech?

In a word, no. But it will work if you know that you never use cross over gears. Have a look at the shimano tech documents page. It will tell you the maximum sprocket size and the maximum capacity of the mech which will be (42-24)+(36-11) in your case.

Cheers, I’ll take my socks and shoes off and get calculating 🙂

But it will work if you know that you never use cross over gears.

You’re prepared to run the risk of an accidental shift, or someone else taking your bike for a spin round the carpark, resulting in total destruction of the rear mech and/or worse, the mech hanger on the frame? 😕

Right tools for the right job methinks is in order…

Can use a shorter cage mech if you’re only running 1 or 2 chainrings, if you’re running a triple (especially with a wide ratio cassette such as the 11-36) you need a long cage mech to counter for the massive differences in chain tension experienced.