Stupid Q: How does a rear derailier work!?

robthehungrymonkey

Right, I know this is a stupid question but I feel I need to sort it out!

I know how a front double/compact derailier works, limit screws stop the derailier going too far either way. Simples.

Now a rear I don't fully get.

Indexing: I *think* this is how much the derailier moves when changing between each gear. Correct? If so, when the indexing is set up correctly, in theory it shouldn't need changing.

The limit screws: do they effectively shift the whole thing either way? I.e. if you adjusted the derailier right, the every gear is moved right? Or, is it similar to the front and just limits how far it can go?

Any help in me fully understanding this would be greatly appreciated!

MountainMonster

The limit screws are exactly that, limit screws. Stops the derailleur from going too wide off the cogs.

The indexing has to do with cable tension, so that the pull is changed to the correct amount with each change of gear. Indexing and limit screws have no effect on each other, so to say. Indexing should be done often to ensure correct tension, as cables stretch over time. It should also be done when replacing cables, or removing the derailleur from the frame for cleaning, or any reason.

benpinnick

Indexing is a function of the throw of the lever and spacing on the cassette and chainrings, and nothing to do with anything else. Your indexing wont work unless you have the start point correct. Use the cable tension to adjust this. As MM says limit screws work just like the front mech, designed to prevent the chain going too far. You also need to know about the b-tension screw, which dictates how low the mech hangs. This is a factor in how well you can shift gears too.

Oh, and gear cables don't stretch, thats a myth. They do however shift and flatten in the pinch bolt, and the outers compress and bed in.

pilot_pete

Indexing is done in the shifter. The rear derailleur is simply a hinged parallelogram which moves in and out across the rear cassette. It is spring loaded to provide a positive tension on the cable. If you remove the cable you can simply push it in and out, and if you let go it will settle at the outer limit, where the outer limit screw has been set.

The indexing is in the shifter mechanism. My Campag shifters are just a ratchet mechanisn that holds the cable end and winds it around a barrell, thus pulling the cable and hence the rear much. The movement is defined by the ratchet teeth, each one moving the cable an exact amount, which will move the mech a specific distance across cassette! Clever stuff.

PP

robthehungrymonkey

Awesome. Thanks for that, for some reason i thought the limit screws and the indexing were linked and adjusting them affected the indexing.

andy_wrx

Another way of thinking about it is that the mech moves sideways a certain distance depending how much cable is pulled (or released, when it moves the other way).

The shifter doesn't 'know' whether it's 8sp, 9sp or 10sp, it just moves the distance sideways that the shifter tells it to, as 8sp, 9sp or 10sp shifters pull/release a different amount of cable for each click, which is less cable pulled/released per click than a 9sp shifter, and in turn than an 8sp shifter, as 10sp cassettes have the cogs closer together than 9sp, than 8sp

This is why Shimano, Campag and SRAM are not compatible : Shimano shifters pull & release a certain amount of cable and Shimano mechs are designed to move the correct distance for the cassette cog spacing because of the cable pulled. Campag shifters pull & release a different amount of cable, which Campag mechs are designed to use, so Shimano mechs don't move sideways the correct distance based on the cable pulled by a Campag shifter. Same with SRAM.

Yes, as explained above, the limit screws just act as end-stops, preventing the mech going too far so that the chain is pulled beyond the last cog on the cassette.
The adjustment knob on the cable tightens/loosens the cable, so that the chain is centralised over the cog on the cassette rather than slightly left or right : having done the adjustment, the shifter cable-pull I describe sorts-out changing from one cog to another

Cables do stretch, but only when new. The cable is a set of individual strands - it doesn't strech by elongating like a spring, but when it's a new cable and you start using it, the strands move slightly and bed-in against oneanother, so the cable does lengthen slightly - but only slightly, there'll be much more bedding-in of cables into their endstops or relaxing into their curves, etc. Which is why the bikeshop tells you to go back after a month or so and have your 'initial service', which is just tweaking things like this to allow for bedding-in