advice on derailleur capacity needed

how to count if short cage derailleur capacity (30T for sram) will be ok with a single ring set up?

i found this


capacity required (T) = (BIG ring - small ring) + (BIG cog - small cog)

i have single 36T ring in the front amd 12-27 cassette.

so capacity required (T) = (36 - 0??) + (27 - 12)=36T+15T=42T??

It doesn't make sense.

Also found that for up 27T cassettes short cage is ok and for more you need medium.

It's all kind of confusing and costs me hours of net surfing to find out.
A quick expert's advice would be higlhy appreciated.

Thanks
Luka

You've outdone yourself this time Luka! But I will need another diagram to make an accurate misinformed opinion.

Yes, i know. i'm pretty good at it. You have to excuse me though - I was gonna be a Pope.

The answer is 15 teeth difference.27 -12 =15.You have a single ring up front,so nothing to worry about there regarding teeth difference.So a short cage should work fine.

Cheers.
Snapped mine last saturday so just wanted to make sure i'm getting a proper one and it wont cost me 70 quid again.
Just didn't know if you include the front ring when there's only one.

Thanks.

sweet Mother of Satan !! only i've actually met you I would say you were a madey up Forumgoober!!

deffo i'm not. i'm not ashamed of asking stupid questions and let's put it this way - here on this forum you need a lot of courage to do so. you're the boss here so here i kneel before you and leave my naked back ready to whip

**** off you pervert !!!

no need to count it just is ok simple as that. The length of the cage has more to do with the amount of rings your running up front. As there is alot of slack in the chain to be taken up by changing from the big ring to the granny ring on a triple ring up front so you would need a long cage. With a single ring you can run a short cage mech. All sram mtb mechs can handle the same sizes of cassettes. On the other hand if your running shimano road rear mechs they can only handle up to a 28th cassette on the rear. Think the sram road ones can handle bigger cassettes.

jesus Byrner - look what you reduce people to, you should go to confession and stay there for a month!!

No way it would combust around me !! I have to say , I'd love to have someone do the whipping for my amusement but its not something I'd like to do myself !
That has to be the most bizarre post on this or any other MTB Forum ever.

Luka, in fairness you really do come up with some gems... the likes that have never been seen or heard before.. the diagrams are the best though.. keep them coming..

In regard to your derailler issue.. what Al said is bang on... its all about your spread of gears or gear range.. as in 3x9, 2x9 etc.. in general you'd use a long cage mech on a 3x9 set up... you can get away with using a medium cage but you need to be aware of what gear your in at the back when changing between middle and big at the front..

At the moment I'm running a 2x9 on the Rize, 34t at the back and 38t at the front.. and I'm using a super short cage X0 mech.. according to your formula that shouldn't work.. but it does with absolutely no problems whatsoever..

Forget about formulas and calculations, they are great for discussions on interweb bike nerd sites for fussing and faffing over gear ratios, chain lengths and the likes.. but in the real world they don't mean jack..

OH MY GOD Gerry, your drive train isn't optimised, NOT OPTIMISED !!!

Something might happen, and you don't want that, when you're out, on your own, in the DARK, down a hole, WITH AN OWL !!!

An owl in a hole ..wheres the diagram, Luka !!