I've somehow turned into the neighbourhood bike mechanic. I work on my bike on my driveway on a stand. My neighbours see this and come over with their bikes and ask for help. Often these bikes have been neglected for years. I can handle most fixes but sometimes, I'm perplexed by derailleur adjustment. I tweak one thing and the noise goes away but then it throws something else out. I think I need a more systematic approach. So how do the pros do it? When they get an unfamiliar bike and it's shifting rough, how do you approach it?

Let's assume it's a decent, non-rusty bike with modern drivetrain. The chain, chainring and cassette are in good shape. The front derailleur is set at the correct height and straight. The rear derailleur hanger is straight. But the bike is shifting rough, lots of rubbing going on.

Do you start first with the front derailleur?

Start with the rear derailleur (RD). Shift into high. Loosen the cable clamp and tighten all barrel adjusters all the way in. Adjust the RD high limit screw so the RD pulleys align with the small cog on the wheel. Reattach the cable, pulling hand tight. Try shifting once. Adjust cable tension with the barrel adjuster until it works cleanly. Shift into low gear, then adjust low limit screw to prevent shifting into the spokes. Fine tune barrel adjuster for clean shifting in all gears.

FD is similar, starting in lowest gear and adjust low limit to just prevent chain rub on FD cage. Tighten cable clamp, being sure cable is routed on correct side of clamp. Adjust cable tension so there's no rubbing while in middle chain ring (assuming triple) while shifting through full range on rear. Adjust high limit to prevent shifting off large chain ring.

FIne tune as needed. If this doesn't work, check that the rear wheel has the right number of gears for the shifter--I'm amazed at how often a wheel gets swapped without this simple check.

Many old shifters need a soaking of WD-40 in the indexing mechanism. Many grip shifters are just broken.

Tweaking one thing then another is the wrong approach, as the adjustments are interdependent.

First check that the derailleur hanger is not obviously misaligned, such as by dropping the bike on its drive side; kids do this a lot. If the hanger is not straight you will have problems making adjustments correctly. Fix this first.

Follow this procedure from the beginning, without skipping any steps. Do not proceed to the next step before the current one is correct, as they depend upon previous steps being right.
https://www.parktool.com/blog/repair...eur-adjustment
and the front: https://www.parktool.com/blog/repair...eur-adjustment

Make your adjustments gradually, in 1/4 turn or less increments, so that you do not miss the sweet spot. Even 1/8 turn can make the difference between so-so and excellent shifting.
Unless something has gotten replaced or broken only small adjustments should be needed; these bikes presumably were ridden previously so should not be that far out of adjustment.

NB; what many think is a derailleur issue, often is the cable/housing, shifter or a bent RD hanger.

RD has as described above , stroke limit screws and, perhaps, the B spring pre-load screw..

it reacts to the cable, pulling it inward ..

Bob's nearly hit the process on it's head, nearly. I suggest to separate the various aspects of a system when accessing it. Move the der independent of the cable or lever. Move the cable then the control independent of the rest. Set your der initial adjustments with out the lever of cable being used. Then move onto the cable being pulled mid way along it's length to shift the der. Only use the control/lever when all the rest is working well.

Other aspects of a bike benefit from this same process. Understand what is first needed for the second aspect to be able to be looked at. Example are those who true wheels but don't bother with a bearing adjustment first. Wrong order. Do the bearing work first as how the hub works can affect how the rim looks. Andy

Thanks for the great tips.

Are there some gear combinations that are going to rub no matter what? For instance, smallest cog to inner chainring and vice versa? These are gear combos that will likely never be used.

Small-small often rubs, and in addition engages very few cogs at once, so will wear the cogs and chain much quicker. A similar ratio is available combining the next larger chainwheel with a larger rear cog.

Small/small and big/big frequently rub. They typically don't on larger men's bikes, but on bikes with shorter chainstays they typically will.
Triples are especially prone to rubbing and in the outer and inner front rings, you'll typically only be able to access 2/3rds of the cassette on a small frame.

One thing not mentioned yet, set the cables/ferrules etc after tightening the cable clamp down by holding the RD while clicking off a few shifts without pedalling to prevent it from shifting. The cable will pull everything tight so you won't have to readjust tension after a week or two.

Given the situation described in the OP, the first thing I would suspect is contamination or damage to the shift cable.

When I was selling bikes one of the things I noticed was the tendency of manufacturers to spec rear derailleurs one grade up from the rest of the components on the bike. That's the first part that customer's want to look at. Conversely, my personal experience is that the rear derailleur just may be the most reliable component on a modern bicycle.

The mark of a rookie is the tendency for readjusting derailleur limit screws.

I feel your pain. We have a shop in town now, so more and more I've been pointing people toward the shop. (I highly recommend sending people to a shop).

I start from first principles by checking the hanger alignment using a gauge made for the purpose, then limit and b-screw settings, then dialing in the indexing. Very often I install new cables and housing, because any bike that someone brings me typically needs pretty much everything done to it anyway.

And clean everything. And lube the chain. And hope the shifter isn't some whack-a-mole grip-shifter from the dime store.

Not sure if that makes me a rookie, but many bikes that come in my shop have poorly set limit and B screws of the RDs, so I make sure to check those when servicing. Especially the "low" (towards the spokes) screw.

First eyeball the rear derailleur to see if it looks clearly misaligned, but otherwise I'll shift through the RD to see if any correction is needed or if it seems like an obvious, easy fix (like cable tension) and check limits and b tension. If it's being troublesome at all, I remove the derailleur, align the hanger, and reinstall the derailleur. This is a good time to just move the derailleur with your hands and see if any pivot points have either excessive play or turn roughly--potentially lubricating pivot points can be helpful. Eye rear derailleur for alignment (it may be bent even when the hanger is straight)--ideally replace if possible, otherwise try to adjust with correctly aligned hanger, and maybe if really in a pinch try to bend derailleur/hanger so that hanger operates in alignment. Then I loosen the cable anchor and assess cable.housing friction as well as I can. Replacement of cable and housing may be advisable. If replacement isn't strictly necessary or approved by customer and if it's easy to do so, I'll use this as an opportunity to lubricate the cables. Then adjust the HL with the cable uninstalled (center of guide pulley to outside edge of cog). Set barrel adjustors in and pull cable reasonably taught. Shift into the next easiest gear and roughly align guide pully under that cog. Shift into biggest cog and adjust low limit so that limit screw is as far in as possible while not adversely affecting shifting. Adjust b screw--typically very close to cog on most 2x or 3x setups, potentially farther out for wide range cassettes (15mm for SRAM 1x 11-12sp, for example). Shift through all gears and fine adjust cable tension so that shifting is balanced to be as fast as possible for all shifts.

For front shifting, again, assess performance first. Before getting too deep--look at the derailleur and check for excessive play in the pivot points and seriously bent looking cage and advise replacement if necessary. If it's not working well, first check to see if the derailleur is as close to the chainrings as is possible--ideally 1-3mm above the large ring. Sometimes on 3x systems you have to check to make sure the inner cage clears the middle ring when in the big. For most systems (SRAM Yaw is an exception) the outside plate of the cage should be parallel to the chainring. Loosen cable and set derailleur into ideal position, reattach cable. Set the inside front limit so that it just barely doesn't rub in the small chainring and big cog. Shift into the middle chainring and adjust cable tension so that there is minimal rubbing at both extremes of the cassette. Shift into big chainring and adjust limit so that it barely doesn't rub when in the small ring--increase cable tension if necessary to get it into this position. In an ideal world everything should now shift great--you may have to slightly loosen the FD limits to get reliable shifting, and you may have to alter the cable tension a bit to get the best possible shifting. You may need to adjust the angle of the FD slightly to improve shifts. To judge this, look at where the chain needs additional contact to make the shift.

If you want to help people out with this kinda garage wrenching, probably would make sense to get bulk der cables/housing. The Park der hanger tool is sanely priced. Unless you have a pile of old parts, learn to recognize when to kick it to a coop or a lbs when they really just need some new parts.

As an example of calling it, I work professionally as a wrench and had a customer come in with some 10sp Shimano drivetrain. Front shifting was atrocious. Dorked around with setup for a while but called it pretty fast when I determined it was in a pretty good adjustment and that the cables and shifter was fine. Grabbed a hold of the derailleur and found the pivots were really worn and loose. New der and shifting was amazing right off the bat with a basic, sane to spec adjustment.

So don't beat your head against something that just won't work even though it's set up seemingly correctly.

Thanks everyone. I printed some of the responses and saved them in my "mechanic's notebook".

One question - if you're assessing the cable and housing, how do you judge that it should be replaced? If the cable shows any kind of rust, should it be immediately chucked?

And are the cable caps crimped at all or do they just stay put with friction? I do have the Park cable cutter/crimper tool and it looks like there is one crimp position that's bigger than the other (which is meant for cable ends).

On most bikes, it's easy enough to free the housing from the slotted frame stops. Shift into low, stop pedaling, shift into high, and you'll have enough slack to pull one housing out. Then pull all the housings off and you can check and service the cable. If time and budget are tight, you can clean (fine steel wool) and oil surface rust. Replace anything that's frayed or deeply corroded. You can inspect the housing ends for failure and crimping. You can check the shifter too, with no load on it.

Cable ends are crimped on with the cable cutter tool. It's often easy to yank them off with pliers if a der or housing needs to be replaced and the cable is otherwise okay.

I do not know what to tell you because it's my uncle who deals with all this