Here is the problem that this solves.
If you want to "Hack" your gear train because there is no way that your existing gear train will allow you to keep your cadence on the flat terrain AND and this is important you also want to climb steep hills without blowing out your knees then you have to get this derailleur.
Why?
A few things with this derailleur that most people will not notice wth three of them being MOST IMPORTANT.
1)The length of the long cage as it is longer than most derailleurs in existence unless you want to buy a third party $400 + long cage (Shimano only) upgrade kit accessory for your existing derailleur.
2)The size of the two jockey wheels
3)The rotation point of the cage on the derailleur chassis
These three details increase what the industry calls "cog" or chain capacity. Or to translate a mix of gears with more teeth.
Note that for Shimano Derailleurs you can buy a $400 kit to do the same thing.
Further note with Shimano Derailleurs you can't mix Shimano and SRAM unless you also want to change out your shifters as SRAM moves directly in a one to one correspondence to cable pull as Shimano does not.
And yes there are exotic parts to makeup the difference but that sort of thing is not for most non bicycle geek normal people.
Take those three differences together and you can then mount a larger gear range cassette on your bike/recumbent/trike add some links to your chain to accommodate the size of the cage jockey wheels and your largest cog and now you can shift.
Thus you can go from a typical 9 speed 11-32 to a 9 speed 11-40 cassette or a 10 speed 11-42 from an 11-34 cassette
Or if you really want to hack and that requires you to have a great deal of patience and care so you can go into tiny adjustments to take you to the very expensive cog territory of 10 speed 9-42 cassette.
Why is this important?
If you don't know it now you must discover the famous and unfortunately deceased "Sheldon Brown" and his web pages on bicycle technology.
With this derailleur You can greatly expand the "gear inches" range of your bike/recumbent/trike.
To explain with my 26 inch diameter wheel trike.
Starting with the standard gearing 52-42-30 front and 11-32 rear my highest high is 122.9 Gear Inches and my lowest low is 24.375 Gear Inches.
Now changing out the rear cassette to an 11-40 with this derailleur the highest high stays the same and the lowest low changes to 19.5 Gear Inches.
Or to put it into Cranks and moving forward with one pedal crank in my lowest gear before the change I move forward 76.53 inches and after the change 61.23 Inches. This translates into significantly less effort. AND for the record with either the original equipment or the changed out cassette at your highest gear one crank will move your forward 353.77 inches.
Now change out to a ten speed 11-42 cassette and your lowest low becomes 18.4 Gear Inches or 56.95 inches moving forward per pedal crank.
To recap with this derailleur either the 9 speed 11-40 or the ten speed 11-42 will serve you well if you need to grind up Colorado Mountain Passes (Actually the Blue Ridge Parkway in VA has shorter STEEPER grades).
But if you also need speed speed speed on the down hills like the back side of Wolf Creek Pass Colorado or flats on the Outer Banks you can go for the very expensive 9-42 rear cluster.
At that point your range is from 18.4 Gear Inches to 150.22 gear inches so on one crank at low you go 56.95 inches to one crank in high moves you 471.69 inches.
Why is all this math important?
Ultimately it is your gearing + knees + your lung power that sets your speed.
So in the highest high you can put together with this derailleur plus 9-42 cluster if you take a comfortable rate of 50 cranks per minute (the term for that is Cadence) every minute you will move 23584.5 inches. Then times 60 minutes in an hour you will move 1415070 inches or 22 miles in that hour and that is 22 MPH.
Now on the lowest low to grind up the 6-12% long mountain grades "spinning" at 50 cranks a minute it all works out to 2.69 MPH.
And no you don't do the entire 5-7 mile hill at that speed. What happens is that you start of that speed then get moving then shift though a number of gears to get to a comfort zone of roughly 50 cranks at 5-7 MPH which is adequate to get you up over the 7 mile grade in about an hour.
There is one more item that will set you back roughly $22.00 and you may have to buy it and test it and determine if it helps.
With adding this derailleur and a larger range cluster plus links in your chain you are messing with a very carefully designed relationship between all of the mechanical components and the measurements of your frame. Got to Sheldon Brown's pages to learn all about it.
You may need to make a very slight "adjustment" to these changes to make it work better and more smoothly.
You can't add 1/2 links to a chain and there are limits to derailleur adjustments.
So the solution is to add a Wolf Tooth Components...
