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Currently, I have Shimano Tiagra 4600 series brifters on my bike, and the barrel adjuster on the front derailleur cable slips when the shifter/derailleur is in the smallest chainring position, when the cable is at its slackest.

The adjusters appear to be Jagwire Rocket adjusters (see photo), and the cause of the slipping seems to be that the notches on the screw part and base are somewhat different in size and don't engage well enough to prevent the adjuster turning due to road vibration.

I could fix the problem with the existing adjusters by either filing out the notches on the fixed barrel to be slightly larger, or possibly using a mild threadlocker. However, I don't feel that these are good solutions to what appears to be a design flaw with the adjusters (which came with my bike from the factory).

I have a number of questions as to the best fix for this. Should I use the cable adjuster designed by Shimano , subassembly (4) on the ST-4600 exploded view, which contains an alternative baseplate to place on the shifter itself that provides the indexing and retention of the adjustment.

Alternatively, should I use an inline cable adjuster like the Shimano SM-CA70 (see spread 11/page 20 on the 24 MB 2013 Shimano Spares catalogue). I know that these are intended for brifters with under-tape gear cable routing, but are they higher quality than the adjuster that would be built into the Tiagra shifters?

Another option would be adjusters that fit into the downtube cable stops, but my frame (Cannondale Synapse Alloy) has somewhat unusual non-threaded ones mounted on the sides of the head tube, and I fear that the stops being mounted so far forward on the bike would pose an issue with sharp bends for the cable exiting the adjuster if the bars were turned a large angle.

The current adjusters on the bike, it's hard to see but the 'teeth' on the rotating part are too wide to snugly fit in the notches on the fixed part

Small edit: If I had to summarise the question to be a bit more objective, it is this: do all cable adjusters for the front derailleur cable have issues with slippage due to the lower cable tensions, or are the Jagwire Rocket adjusters just unusually bad at keeping their position? Do either inline or cable stop adjusters generally perform better than the other option?

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How old is the bike? –  Batman Apr 20 at 19:10
    
It's a 2013 model, bought around the start of this year. I've become quite adept at adjusting the front derailleur if I notice that the adjuster has slipped. I found a forum post that suggests the option of filing the adjuster. –  jbuk Apr 20 at 19:31
    
Is there any slack in the cable when it's in the lowest gear? –  birthofearth Apr 20 at 20:54
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You should be able to take it back to the shop and get them to fix it for you, if you're nice to them. –  Batman Apr 20 at 20:58
1  
@jbuk from that 2nd photo they look more like detents than (near-)locking stops - I'd expect the notches on the black part to match the teeth on the grey part, as my (flat-bar shimano) adjusters do - a dog clutch would be a bit too extreme. Putting something (e.g. threadlock) in the thread might work, but I'd be wary of it either going in the wrong place or locking the thread too hard as it's not designed to get a spanner on, also this isn't how it's designed to lock. Personally I'd file 1 part to match the other - but only after complaining to the store (even online), they might help. –  Chris H Apr 21 at 13:13

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