Hi Gazlas,

what sizes do you have? There are shims that exist for making the tube fatter to fit a bigger diameter derailler clamp band.

I haven't seen the specs but it is possible that the side plates for a "10 spd" front derailler are a bit narrower than for a "9spd" front derailler. If so you are more likely to have problems with rubbing when the chain is on a bigger angle. This might be what your mechanic is referring to. If your bike has a long chainstays this will minimise this effect.

I happen to use a 10spd front derailler with a 9spd system, however I also use a friction shifter, so I can trim to solve any problems. I can say with the deeper inner plate of the new derailler it shifts really well, but I suspect that others won't be fitting friction levers anytime soon.

Cheers,

Cameron