Rear derailleur cable tension adjustment?

[LeadFarmer]

Looking for some advice on understanding the tension adjustment for my rear derailleur please.

My Shimano Deore XT rear derailleur is damaged and I've bought an exact like for like replacement that I need to fit. I'm ok with the actual removal & refitting, and adjusting the stop sets and the B adjustment screw, it's mainly the tension adjustment thumbturns at the handlebar end of the cable that I don't understand.

I know this applies/reduces tension on the cable, but I'm not sure exactly how it does this, and what adjustment it will need to do.

Should I loosen the tension before removing my current derailleur, do I leave it untouched, do I adjust it after fitting the new derailleur???

What happens if the tension isn't correct? If the tension is too loose will the derailleur not move? If too tight, will it move too far with each gear change?

Edited by LeadFarmer on Sunday 11th April 10:43

[benny.c]

For all things mechanical I follow the advice of the Park Tools guy with the cool ‘tache....

https://youtu.be/UkZxPIZ1ngY

[LeadFarmer]

Thats a very useful video, thanks.
It shows what the cable tension adjuster does, which is the bit I wasn't sure of. Seems that the spring on the derailleur is tensioned to always be pulling itself in a certain direction, and the cable tension adjuster limits this movement, allowing the chain to line up with each cog.

Edited by LeadFarmer on Sunday 11th April 14:34

[mikey P 500]

In simple terms anti clockwise screws it outwards making the outer cabling longer or the inner cable tighter this will help adjust to reaching the largest cogs at the back, but screwing jn (clockwise) will make cable outer shorter or cable less tight allowing the spring of deralier to pull easier onto small cogs. It basically allows for fine adjustment after fitting cable at rear(so therefore should be in middle or mostly in, when fitting. Then un screw for fine adjustment, and to allow for cable stretch.

[LeadFarmer]

All now fitted, but got a little problem...

It won't go up into the largest gear. I thought at first that the stop screw needed backing off, but the screw isn't even touching so its not the screw that's stopping it. Clicking up through the gears it gets up into the second largest gear, but then there's no more clicks left in the gear lever for it to go up any further.

Working down the gears it goes into every gear fine, and into the smallest gear, but it feels like there's an extra click left in the gear lever. The chain doesn't move any further, but again it's not the stop screw as that one isn't touching either.

So it's clearly not set correctly, despite working fine in all the other gears!! Could the cable be too tight?

You can see in this photo that the stop screw for restricting it moving beyond the largest gear still has a gap...

Edited by LeadFarmer on Sunday 11th April 17:39

[IrateNinja]

Not enough tension, which based on that photo is not surprising at all. That cable should have gone in the bin when changing the mech.

[mikey P 500]

Cable is not tight enough, either pull cable about 2mm tighter or use mentioned adjuster at shifter to turn anti clockwise about 4 full turns if has that much adjustment left.

[Mastodon2]

Is that the old cable? Changing the rear derailleur would have been an ideal time to stick a fresh cable in. Also, you'd want to pull some excess through and crimp an end cap onto the cable, to stop it fraying and to account for some stretching over the life of the cable.

[LeadFarmer]

mikey P 500 said:

Cable is not tight enough, either pull cable about 2mm tighter or use mentioned adjuster at shifter to turn anti clockwise about 4 full turns if has that much adjustment left.

This solved it. I loosened the cable clamp and pulled it through a little further, now it all works fine, thanks.

Next step is to change the cable, but it feeds through the frame. Whats the normal method for feeding a new cable through? I'm sure I once saw a video where they used some narrower tubing as a guide somehow?