When adjusting both front and rear derailleurs, which should be done first?
There are three main steps to adjust both derailleurs. The first is usually not necessary unless the rear is very far out of adjustment.
- Adjust the rear derailleur enough that it will stay in the lowest and highest gears, even if you have to help it a little with finger pressure on the derailleur arm. This may require adjusting the high and low limit screws.
- Adjust the front derailleur: Start by putting the rear in one of your middle gears. Then run the front through each chain-ring and make sure that it will smoothly move from ring to ring. Adjust as necessary. Set the rear to the highest (smallest) gear. Adjust the front so that it does not rub in any of your 2 or 3 chain-rings. Set the rear to the largest (biggest) gear and repeat the tuning process. In a 2 chain-ring setup, you have to make sure that the cross chain combinations (small-small and large-large) don't rub either. In a 3 ring setup, it's not always possible or necessary. If you have 1/2 clicks on the front shifter (most often in 2 ring setups), make sure to include them in the process. If you can't seem to get the front to adjust without rubbing, it's possible the height and/or twist of the derailleur clamp has changed. Check manufacturer's recommendations for position, if so.
- Adjust the rear derailleur: Put the front in smallest ring in front and tune the rear derailleur until you are satisfied with the speed and quietness of the shifting. If it's noisy or grinds, it's not right yet. Don't forget about the B-screw when tuning. If it's out of whack, then your shifting will be difficult or inconsistent, depending on which way it's out of adjustment. Next, move to your next biggest chain-ring and repeat the adjustment process of the rear derailleur. If you have 1/2 click front derailleur adjustments, you will need to include them in your tuning process.
In a nutshell, adjust the front derailleur first, then the rear.
If the derailers just need minor tweaking (they are basically in adjustment but are "not quite right") then you adjust the one that is obviously wrong, check the overall adjustment again, then again adjust what needs adjusting. Often only one derailer needs adjusting and often the adjustment is just a half-turn of the barrel adjuster.
If they are completely mucked up (but nothing visibly bent/broken), I usually first check the front for parallel and sprocket clearance (they often get knocked out of alignment), get it on the center sprocket, then address the rear. Once the rear is basically correct, get the front to where it will shift to all rings with modest accuracy, then, trying all combos, adjust the rear some more. Finally, fine-tune the front, then fine-tune the rear.
But it varies with the bike, how recalcitrant the derailers are, and specifically what is out of adjustment most.
Every part of the system affects every other part of the system. So you could spend ages making the rear perfect, then go and tweak the front which puts the rear out again.
Like truing wheels, back and forth is your best answer to focus down on the best position for both.
Don't forget to clean everything before you do adjustments. Makes life easier.
It does not matter. The derailleurs are located on opposite sides of the cogs, so the the relative location of derailleur and chain depends only on shifter position, derailleur adjustment and which cogs the chain is on. The derailleur on opposite side can not change chain position, except by moving the chain to different cog.
It is far easier to adjust the derailleurs by sight, so that rear one lines up with cogs and front one has correct limits, than try iteratively adjusting until the shifting is smooth. If you want to do the latter, you may need to switch between front and rear.