Do I need a new front mech?

[Proud2Push]

Hi

I bought a Shimano M660 SLX 9 speed front derailleur for my 8 speed bike, without realising that it was 9sp.

It is proving impossible to adjust without rubbing. Is the 8-speed chain too wide for the mech or is it just I'm not very good at getting the adjustment right.

If I need a new mech, can anyone recommend a good 8-speed one? Chainreaction Cycles have some offers on LX at the moment but they don't say whether they are 8 or 9 etc speed.

Thanks

 

[RecordAceFromNew]

The problem you have is unlikely because the new mech being a 9 speed model. The reason being the difference in width between an 8 and 9 speed chain is merely ~0.5mm.

However front mechs are designed/optimised not just for "speed" but also for road vs mtb, largest chain ring size, tooth range between largest and smallest chain ring, and also chain line. Could you tell us what make/model (if at all possible the actual model number) is the one the M660 is replacing? It might also help if you could describe the front shifter and the chainset spec.

Assuming the M660 is an acceptable replacement, imho this is the best guide on how to set up a front mech, it can be important to follow every step.

 

[Proud2Push]

Thanks, RAFN, for the helpful reply about the size difference.

I have a 1998 GT Tequesta MTB. The front changers are Shimano STX-RC RapidFire SL and the old FD was an Alivio topswing. The chainrings are 22.32.42.

Since the M660 is 44/48 does that mean it is incompatible with the 42t ring?

 

[RecordAceFromNew]

There is one thing you need to do differently to the Park Guide - since the new mech requires a 12T minimum difference between the large and middle ring, and yours has a 10T difference, you need to set the mech height by allowing up to 6mm between the outer cage and large ring (as opposed to 2mm as shown by a coin in the guide). Otherwise the cage inner plate is likely to clash with the middle ring.

Aside from that, I expect your new mech to work fine on your bike as long as you follow the guide.

 

[Proud2Push]

In my ignorance I had no idea that matching chainring sizes and front mechs was such a precise matter. I've learned a lot! Anyway, thanks to RAFN and some persistent twiddling, the changer now works fine. It doesn't make a horrible rattle any more and I haven't had to buy another part.

Just out of interest, on my 8-speed bike would it be OK in the future to replace the 42t outer ring with a 44t to give the required 12t minimum difference? Or would that break the law of unforeseen consequences?

 

[RecordAceFromNew]

In my ignorance I had no idea that matching chainring sizes and front mechs was such a precise matter. I've learned a lot! Anyway, thanks to RAFN and some persistent twiddling, the changer now works fine. It doesn't make a horrible rattle any more and I haven't had to buy another part.

Just out of interest, on my 8-speed bike would it be OK in the future to replace the 42t outer ring with a 44t to give the required 12t minimum difference? Or would that break the law of unforeseen consequences?

Great to know it is working well. Your M660 is designed to work with a large minus small max range of 22T, and a large minus middle minimum difference of 12T, so a 44T outer ring should work fine and should improve the shifting experience because it will then match the 44/48T large rings the mech is designed for.