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And 30 miles to go.Can get 5 to 9 working and I,m fortunate to have 3 gears up front.Tried tightening it and no joy.Guess ride with front gearing and take it easy.Thanks for any advice.
That's rather less information than is needed to help with a diagnosis.
What mech, what shifter, how many speeds?
Have you hit something/ checked the mech is straight?
Does it feel like inside of the shifter is stopping you?
A 'hard' stop from the mech limit stops?
Does the shifter work but not enough movement from the mech?
Does the mech move freely the full extent of the cassette if you move it by hand while rotating the pedals?
Does the top jockey wheel collide with biggest sprocket or is there a wee space?
(Etc)
I'm guessing Shimano road shifter with frayed cable at the lever end where it passes through the shifter
Shifters are easy to deal with. Just disassemble it by removing all the visible screws, circlips etc., then shake it until springs and other tiny pieces stop flying out, then sweep up the pieces and put them in the bin and replace it with a new shifter.
Often when it's a whole section of the cassette you can't access there's something physically blocking the movement of the derailleur. What have you changed recently? Undo that. Shifters tend to mess up gradually, unless thumped.
I’m guessing Shimano road shifter with frayed cable at the lever end where it passes through the shifter
this!
Agree with the frayed cable hypotheses. Just make sure if it is that you get every tiny piece out when you replace it.
Doubt if it's the cause this time, but it's worth mentioning anyway, i've come across quite a few of the newer 12sp Shimano mechs that have caught the loose end of the cable between the limit screws. It all works fine until you release enough cable for the end cap to stop the mech moving any further. Caused a little head scratching until I spotted it.