90s MTB Front Derailleur Problem

[ruffstuffbadger]

For some time I haven't been able to shift from the middle to small chain ring when in the 3rd/4th through to 8th sprockets at the rear. I have no previous experience with front derailleurs but have followed the instructions that came with the shifter and when that didn't work some online tutorials, which also failed to fix the issue.

If I do set it to shift down from the middle to small chain rings when in the higher sprockets it won't shift back up from the small to middle chain ring. I took it to Halfords today and the mechanic couldn't do it either. He said the mech was old and it would be best to replace it.

I am happy to buy a new derailleur but wanted to first make sure that it really is faulty and, if so, to find out which derailleur to replace it with. So. . .

1. Could there be another cause for this problem?

2. Is there a more definitive way of testing a derailleur to establish whether it is faulty or not?

3. I measured the seat tube as best I could with a piece of string at the point where the current mech is sited. The length was 95 / 96 mm which gives a diameter of 30.2 / 30.5 mm. Measurements for seat tubes are 28.6, 31.8 and 35.0 mm. Do I therefore choose a 31.8 mm derailleur?

4. Does top swing or bottom swing matter?

5. Do I have to get an 8 speed front mech or could I use a 9 or 10 speed?

6. Can anyone recommend a serviceable bottom pull front derailleur that will fit my bike which is a 1990s Carrera Flex MTB with a 30.2 / 30.5 mm seat tube, a Shimano SIS chainset (48/36/26) and Alivio SL-M410 8 speed shifters?

[Brucey]

my suggestion; try a different chain/chainring.

Chains vary enormously in how they shift (both in design and wear), and the first shift that goes belly up is the one that is giving you gyp. Chainrings tend to 'hang on' to the chain if they have burrs that make each tooth slightly wider than normal so they can cause trouble too.

Another suggestion; use a strip of paper to measure the seat tube circumference more accurately.

BTW front mechs don't mysteriously 'go wrong', they get bent, they get worn pivots, they seize up, in all kinds of obvious ways.

cheers

[ruffstuffbadger]

Thanks Brucey.