Chain won't drop into highest gear...

[Mazz]

Shimano Tiagra rear derailleur.

Chain won't drop into highest gear when commanded from shifter (btw, all other gears are selectable).

How can I correct this?

Do I turn the H screw clockwise or anticlockwise?
Or
Do I turn the L screw clockwise or anticlockwise?

I also tried the barrel adjuster on the rear derailleur but I can only turn that anticlockwise and that pushes the chain upwards to the lower gear.

Thanks

 

[richardfm]

If it is the limit screw stopping it then try turning the 'H' screw anti clockwise

 

[Paulus]

Don't touch the barrel adjuster, it'll be the rear mech not going far enough over.
Unscrew the H screw a quarter turn at a time and try it.
Has the mech been knocked, or has it always been like this?

 

[Dogtrousers]

Has it worked OK in the past? If so I'd be tempted to leave the H screw where it is for now (and definitely leave the L screw alone).

If it has worked OK before then you have to ask yourself what has changed. I can't imagine that the H screw has sneakily tightened itself or the barrel adjuster has tightened itself to take in cable slack. So I'd ask -has the bike taken a knock? I'd give it a really good clean to make sure it's not being stopped from moving by gunge. I'd see if I could force it (with the bike on a stand) to go into gear by pulling on the mech. Is the gear cable still tight when you get to the top gear?

Only if everything seems hunky dory would I start fiddling with the barrel adjuster to let in a bit more slack into the cable maybe. Normally the H & L screws are set and forget, they rarely need tweaking once it works. In my experience anyway. The problem with fiddling with the barrel adjuster is that you're likely to balls up your indexing.

 

[Mazz]

Update... turning H anticlockwise has no effect (did 7 full revolution anticlockwise...done 7 clockwise now to get it back to where I was originally).

 

[Ajax Bay]

My bet: the hanger has been bent in and needs to be (gently but firmly) bent back to vertical.
Disconnect the cable. Does it drop to the smallest sprocket? No? Unscrew H half a turn and repeat.

 

[Mazz]

Disconnect the cable. Does it drop to the smallest sprocket? No? Unscrew H half a turn and repeat.

Yes mate, when cable's disconnected, I turn the crank and chain drops into highest gear.

 

[presta]

Update... turning H anticlockwise has no effect (did 7 full revolution anticlockwise...done 7 clockwise now to get it back to where I was originally).

Did you check that the cable was slack when you were doing this? If not and there's not enough adjustment on one barrel adjuster, try the other, else let some cable out through the clamp.

 

[Mazz]

Did you check that the cable was slack when you were doing this? If not and there's not enough adjustment on one barrel adjuster, try the other, else let some cable out through the clamp.

Both barrel adjusters bottomed-out and can only turn anticlockwise which means they only adjust to the next lower gear.

 

[Ajax Bay]

Ensure the STI is clicked to the very end. Screw the barrel adjusters in and then connect the cable 'just loose'. And then adjust (anti-clockwise, which will slowly tighten the cable) so that one click gets you to the second smallest sprocket. Ride on.

 

[roubaixtuesday]

Lube inside the outer cable and/or replace the inner, its possible fraying or friction is preventing full release of the cable.

Then follow the Park Tool guide to set it up.

https://www.parktool.com/en-int/blog/repair-help/rear-derailleur-adjustment

 

[Mazz]

Lube inside the outer cable and/or replace the inner, its possible fraying or friction is preventing full release of the cable.

Then follow the Park Tool guide to set it up.

https://www.parktool.com/en-int/blog/repair-help/rear-derailleur-adjustment

I don't know the recommended frequency of replacing gear cables but mine's not been changed for maybe 6 years (bike used for daily commuting). So I'll do the above.

 

[Mazz]

Ensure the STI is clicked to the very end. Screw the barrel adjusters in and then connect the cable 'just loose'. And then adjust (anti-clockwise, which will slowly tighten the cable) so that one click gets you to the second smallest sprocket. Ride on.

This worked and solved the problem.
Cheers 👍