Written by Oz on Thursday, 16 Sep 2010.
Get your shift on!
Someone pm'd me last night about derailleurs and all the associated technical jargon that one must sort through when buying one, so i decided to put a little reference thingy together.
What the hell is low normal and high normal on a rear derailleur?
MTB rear derailleurs, for the most part, shifted in one direction when cable tension was applied; from small sprocket (hard gear) on the cassette, to the big sprocket (easy gear). This all went out the window back in the early 2k years when Shimano decided to introduce XTR rear derailluers that shifted backward.
Their justification was that rear derailleurs would fair better in shifting situations that required a downshift to a lower gear while pedaling under pressure without the use of cable tension....thus, the spring would assist the movement rather than a pull of the cable. I think this was an attempt to remedy the blown apart chain syndrome from the dreaded "i'm hammering up a climb and need to grab another gear" power shift. The idea was to let the derailleur down shift on it's own terms via a spring verses a rider forcing the shift with an eager thumb.
In my opinion, it was also a response to the 1:1 system that SRAM introduced and Shimano needed to do something gimmicky to ensure market dominance. We sold a lot of em, and pretty soon, replaced a lot of them. Most people would agree that they failed miserably and for the next 2-3 years after 2004, every bike from Specialized, Trek and the like were spec'd with them (i am sure they got them for pennies on the dollar), eventually to be replaced by top normal patterned derailleurs or the new SRAM. The issues that arose were the deathnail to the design; slow and sluggish shifting when dirty, flimsy design, the list goes on and on. They are still available today, but i rarely ever see anyone with one.....pesonally I hate them and they should be abolished (not Shimano, but rather backwards derailleurs) but to each his/her own.
So here is the run down....
Top Normal - This means the derailleur shifts up the sprockets toward the low range (1st) gear when cable tension is applied. The spring returns the derailleur toward the high gears (9th, 10th) when cable tension is released. This is the traditional format for 99.9% of the setups that people ride.
If your shit b shiftin this way under tension, u b top normal yo!
Low Normal - When the cable tension is applied at the shifter the derailleur moves toward the high gear on the cassette and the spring returns it toward the low gears when tension is released. This is the wacky way to shift.
If your shit be shiftin this way under tension your shit is buggin...that shit b low normal or Rapid Rise yo!
Yo, what length cage do i b gettin?
This is an area where people screw up all the time......reason being, you just can't drop in a short cage derailleur and let it fly. Cage length is the distance between the upper and lower pulleys. This length determines the ability for the derailleur to take up chain slack when it is in the relaxed position. What is that you may ask? This is when your bike is in the smaller chainrings up front and small sprockets out back. If your chain is to long and the derailleur can't take up the extra, it will sag like 80 year old boobies. Conversely, when you downshift to the easier gears while in bigger rings up front......not having that extra chain will cause you to rip the derailleur off the bike. You need a happy medium so to speak; the proper length derailleur cage solves all this.
Companies like Shimano and SRAM make different size cages (long, medium, short) in order to optimize shifting performance. The reason being is that shorter cages reduce drivetrain weight (less cage = less chain), the shorter length of the cage has less overall exposure and therefore has less of a chance of snagging shit, less overall slop because the short cage puts less strain on the linkages (think shorter lever therefore less mechanical advantage in regards to side to side twist) and they shift quicker.
If you run a triple up front (44-34-22) and run a standard 9 speed cassette like an 11-32 or 11-34 you have what the majority runs on their bike which is a long cage.
Short cage sounds tits...i want one!
Hold on tiger, u just can't buy and bolt. You need to figure out your derailleurs required capacity and you do that shit like this:
Take the difference between your big and small chainring....
Take the difference between the largest and smallest sprockets on your cassette.....
Add these two numbers together....
45 is the magic number that determines cage length. This states that you need a derailleur that has a chain wrap capacity of at least 45; a long cage Shimano
Lets say u got no skills and insist on running a whacky bash guard. You wanna run a 22-32 up front and be all freeride. You want your shit to be crisp, here is how u do it.
32-22=10 This b the difference out front
32-11=21 This b the difference out back
21+10=31 31 b your total chainwrap requirements.
A 2010 Shimano SLX mid cage derailleur has a max capacity of 35, your number of 31 is 4 teeth below this and thus allows you to run a crisp shifting mid on your setup.
You want to be a real fruitbasket and run a single 22 with an 11-32 cassette. You have a difference of zero out front and a difference of 21 out back. Your total max capacity is therefore 21. You can use a short cage XTR for increased crispness.