Confused over M739 Front Derailleur

[Cloverleaf]

Not that I recall, other than I remember the original (M735 IIRC) top swing, top pull one really didn't work well on my 14.5" Trek back in the day because of the angle the cable came in at. Is the BB length definitely right for the crank? I do remember they were quite fussy because of the arc the mech ran through.

 

[hamster]

I'm happily running 44/32/22, with indexed front shifting and Shimano front mechs. I've run 44/29 and currently even 48/32 using a road mech. The only reason I went with road was that it had to be braze-on.

 

[bikeworkshop]

The other thing that interferes with front mech operation is too much play in the pivots, as well as a distorted cage.
Basically if the rings and chainline are correct, then there must be something wrong with your derailleur. (shifters either work or don't, and sticky cable is obvious)
There's only ever a difference in derailleur cage shape between 42/44 and 48 in the mtb ranges, and even then its rare.
Poor chainline, too long BB axle or unusual seattube angle are other reasons.
Oh, and incorrect cable routing.

 

[hamster]

Recently I sold an M750 XT one (low band) which was fine for a 50T chainring in terms of the cage curve. Shimano had to build in loads of freedom for seat tube angles. 2T difference is nothing. There is something else wrong.

 

[Lou Carpenter]

Well I'll throw another one on and see if that works. To be honest everything is correct as far as I can see, and I'm pretty experienced. Maybe it is sloppy joints, but the bit of stamped out metal that knocks the chain off the middle ring onto the granny just doesn't look low enough to me. Shimano usually makes a lot of their stuff essentially the same and better models will have bits of aluminium instead of steel etc. So with this style of FD all the lower spec ones are for 42-32-22 combos and it is only the XT versions that offer the 44-32-22 option. I don't know about XTR. Maybe Shimano, like me, just thought that'll work, and perhaps it usually does. All else fails I have a M750 in the post.

 

[Lou Carpenter]

Thanks for your help, appreciated.

 

[Lou Carpenter]

Not that I recall, other than I remember the original (M735 IIRC) top swing, top pull one really didn't work well on my 14.5" Trek back in the day because of the angle the cable came in at. Is the BB length definitely right for the crank? I do remember they were quite fussy because of the arc the mech ran through.

Yes after thinking some more you could be right. The BB is the correct length but maybe the chainline is quite far out and the FD's arc just doesn't like it. Like it moves too much sideways instead of sideways and down. I'll have to measure it. Thanks again.

 

[My_Teenage_Self]

Without pics this is tricky, but have you tried rotating it around the seat tube?

 

[Lou Carpenter]

Without pics this is tricky, but have you tried rotating it around the seat tube?

I admit that's a new one on me. I've always aligned the FD parallel because I thought that's what you were supposed to do. I could try it when messing about tomorrow and see what happens, thanks.

 

[bikeworkshop]

Kicking in the tail (by rotating), or even putting a little extra bend (using an adjustable Spanner) in the outer cage plate where its just not pushing hard enough to derail can be enough to extend the life.
I think those topswing style derailleurs wear out fast because the pivots are so far apart and that low down they get covered in cr4p