Inline cable adjuster

if you have put on a new mech best bet for optimum shifting would be to replace the inner and outer too if you can, using the old outer (assuming it is right length) as a guide for overall length of new setup, incl adjuster.

As above, may as well change the whole inner and outer cable while you're doing it unless it's very new. It's one of those jobs that you realize once you've done it that you've put off far too long.

For your original question, depends on the bike.

MTB's tend to have adjusters on the shifters, so no point adding it in the loop from the handlebars, may as well put it next to the rear mech.

Roads bikes don't have adjusters on the shifters so makes more sense to put the adjuster either directly on the lever (older shifters where the cable exists outside the tape) or under the stem (newer ones with hidden cables).  I've got some nice neat Ritchey ones that plug directly into the shifter which is great as I can trim the mech without even taking my hands off the levers.

What combination of shifters and mech are you running  that has no adjusters?

will I need to fit a new cable inner as surly by adding the adjuster it’s making the outta longer ? Or do I cut out roughly the same as the adjuster if that makes senses ?

A little bit of both. Unless the cable is very straight you'll probably find that the curve needs to be a few mm more or less to get a nice bend.  Road bikes are generally way more picky about this though with short cable runs and tight bends.  MTB's have enough length that a few cm of extra cable doesn't matter.

Put the adjuster either directly on the lever (older shifters where the cable exists outside the tape) or under the stem (newer ones with hidden cables).

This. I have some nice jag wire adjusters that mount directly to the shifters. The external cables shift better then internal too!. Normally only a FD needs an inline adjuster as there's none on the derailleur. In line RD is for on the road adjustment, normally needed only when a wheel change is done in a race.

Some of the inline adjusters are quite long. Others mount onto the frame if your frame has external routing. Shorten the cable outers accordingly and use new inners.

Hey

so it’s a hybrid bike with shimano Altus shifters which are abit cheap and there is an adjuster on the handlebar but I can’t quite perfect the adjustment hence the wanting to use an inline one. In regards to the cable inner and outta both are fairly new replaced 4 months ago irrc.

If there are already adjusters on the levers then I'd learn to use those rather than introduce another element into the cable run. They're doing the same job.

The rear mech is a shimano Deore.

Is it just a case of there not being enough adjustments on the shifter cable adjuster? If so wind it all the way in, loosen the cable at the mech end, pull it tight, tighten then adjust away.

Sorry if I'm teaching you to suck eggs.

No not at all. Always learning. It’s an odd one it just feels very vague as an adjuster. Abit all or nothing.

I’ve had inline before on an old bike and always found them more precise than the one on the handle bars that was all.

In line aren't limited to quarter turns. Shifter and derailleur adjusters are. That might make a little difference to really fine tuning. Or it could be some friction in the cables, or slight misalignment in the hanger. I just switched from a cable to eTap on a gravel bike. I thought it was the cable and could never really get sharp adjustment despite changing and aligning the hanger. The switch to electronic (and perfect) shifting tells me it was not hanger alignment, but was play in shifter and cables along the line. That was with an inline adjuster and a RD adjuster too!

Sounds more likely the hanger (or mech) is bent slightly.  Did you replace the mech because it was damaged?

Usually the symptom is that you can only get it indexed well at one end of the cassette or the other and it drifts out of alignment at the far end.   So if it's proving difficult to index I'd consider that.

Usually on the old 9s drivetrains there was at least a few clicks of the adjuster in the middle where it was fine.

The rear mech is a shimano Deore.

Just to be certain, have you checked that the mech you've bought is compatible with your altus shifter? They don't all move the same distance for a given amount of cable pulled by the shifter. If that's wrong it'll never index correctly.

Hey

so I replaced the mech because the old was one had so much play in it. It was just cheap. Ha you know I never thought about it been compatible.  I thought been deore it was an upgrade from the old. It’s 9 speed and the correct length. How do I know if it will work with my shifter ? I never knew this was a thing. Everyday is a school day.

I’ve mixed and matched loads of parts over the years. Never cared about weather it’s xt, deore surly a shifter is a shifter. It’s 9 speed?