Xt rear mech question.

'68 Fastback

Hi,

I'm going to upgrade to a XT shadow rear mech and am wondering, whats the difference between the medium cage and the long cage?

I like the idea of a shorter cage but would the medium be too short?

I'm running an XT chainset with a 42 big ring. I think Ive got a 32 on the rear.

Thanks for any advice.

zil

The more your chain length varies the longer a cage you need. If your running a triple up front with a normal mtb cassette on the back then you need a long cage. If your running a double up front then you can get away with a medium cage.

You *can* run a a triple with a medium cage but it doesn't work perfectly. You have to shorten your chain to make sure the derailleur takes up all the chain slack which means that you won't be able to shift into anything but the top 2 or 3 gears on your big ring but only the top 3 gears are actually giving you a different ratio than what you could get with your middle ring so your not actually losing any gears, just flexibility.

seamus

Seemingly what matters is the total tooth difference in the front and back.

So if your rear cassette is 11-32, then you've a difference of 21 on the back
If your front is 22-32-42, then the difference between 22 and 42 is 20 teeth.

The combination of these two figures; 21 + 20 = 41 is known as the capacity, and you need a rear mech with a capacity which is greater than or equal to this figure.

Looking at the XT mechs - you have the GS, which is long cage and the SGS which is super long cage. The GS has a maximum tooth capacity of 36 teeth and the SGS, 43 teeth.

These figures aren't necessarily hard limits. You won't kill a GS if you have a tooth difference of 37 teeth, you just need to be extra aware of the damage you will do if you shift into the largest sprocket while in the big ring.

This is why road bikes have tiny mechs compared to MTBs; A standard MTB configuration has a difference of between 38 and 40 teeth. A standard road compact setup has a 34 tooth difference.

You should try to fit the shortest cage mech that you can. In my experience, shorter cage mechs are easier to adjust and shift quicker and more solidly than longer cage mechs because there's less flex in the cage. Shorter cage mechs also look neater, IMO.

'68 Fastback

Thanks for the replies.

I'll have to double check everything again before any spending happens!

I'd like to say im diciplined about not crossing the chain but im a lazy git when it comes to the front shifter so i think the long cage might be the one.

Thanks again.

Idleater

What I did was put on a medium cage, and upgraded the back block to a road bike 12-27 and combined with (44-22) + (27-12)=37 which was exactly the max capacity of the cage.

I never really used the 22-30 bailout so 22-27 is not much different.