ljg

I've got a Surly Long Haul trucker that I just tried to put a new Front Derailleur on.
The old Deore XT front derailleur shifted my 44 tooth tripple just fine, but it need replacing.

The Xt FD-M770 Front Derailleur that I just tried to put on wouldn't go inboard enough.
I did a bit of internet research, and measuring, and it turns out when my bike was built for me, it was set up with a 45 mm chainline. (the new derailleur is meant for a 50mm chainline, and can't move inboard enough for me)

Are there any Front Derailleurs out there that would be able to handle a 44 tooth max chainring at 45mm chainline?

Thanks,
Leslie

fietsbob

chainline refers to the crank/BB spindle length.. in reference to the back
the middle cog on the back and the middle chainring on the crank are in a line parallel
to the center line of the bike.

hit the bike shops .. ask .. bring the bike

FD wont go in far enough?, you need to move the crankset chainline further outboard.

Henry III

Maybe toss a spacer between the drive side cup and the bb shell?

ljg

These are great suggestions. I have a Phil Woods BB right now, that I am loath to change.

My first choice would be to keep my current chainline (and thus keep my current BB) and find a derailleur that works with it.

I have read that road derailleurs are meant to work with a 45mm chainline, Does anyone know if I can run a road deraileur with a 44 tooth triple crank? (and a 22 tooth difference in the front)?
I've never used a road derailleur, so I don't know.

JiveTurkey

There are a few potential issues:
- Different cable-pull ratio between road and MTB front shifters/derailers. This can be remedied by using a Shimano "flat-bar road" FD, which is a road FD in most respects, but uses MTB cable-pull ratio.
- Chainring sizes, as you mentioned. The curvature will be different. IME, this issue has more to do with shifting performance over compatibility per se.
- Chainstay angle. Road FDs typically call for a 63-66 angle--and MTB 66-69 angle--between the chainstay and seat tube. (MTB bottom brackets are higher, widening the angle). Operating outside this range can affect things, though I don't know exactly how.

Currently, I'm using a road front derailleur on 22/38/bashguard chainrings with 47.5 mm chainline on my MTB turned touring bike with front road shifter (105 STI) and 8-speed cassette. I tried using a large chainring (can't remember now if it was 48, 44, or 42) but I just couldn't set it up so that I could use each chainring how I wanted to. I can't remember exactly, but I think to use the highest gear (big chainring/smallest cog) I had to increase the cable tension to the point that I couldn't use the middle chainring with several of the largest cogs. I tried several heights and angles of the FD. I don't know if it was the chainring sizes, chainstay angle, both, or neither.

Ultimately, I decided I didn't need the largest chainring and replaced it with a bashguard and it works very well now: in the 38T, I can use all cogs (though avoid the largest), and the 22T can be used with as many as I want it to (at least half..I couldn't tell you the limit unless I test it).

(By the way, the above issues nearly become non-issues with a friction front shifter.)

peterw_diy

What shift levers do you use?

ljg

I've got Shimano 9 speed bar end shifters (dura-ace I think).
The front shifter is not indexed at all, so I think I should be able to run road or mountain.

ljg

Thanks for the detailed info!

How much of a shifting performance loss is there due to the difference in curvature of a road front derailleur vs my 44 tooth chainring?

Does the seat tube angle make much difference at all in front derailluer function?
My Surly is listed as having a 72 degree seat tube, but my old Deore XT front derailleur seemed to work fine.

Thanks again, this is all really helpful.

tastest

A trekking series will work, FD-t671. It is specifically designed for road bikes with flat handlebars. It is 66-69 degree and has related 44 and 48 max chainring cranks. Works with related flat bar shifters. I have not tried in real life, as not available in north America, you need to order from Europe. Shimano only want us NA's to be racing and has no recognition of other uses for bikes. I am now looking at SRAM.

JiveTurkey

Front shifting has been just fine for me and I'm jumping 16 teeth, albeit only between two chainrings.

It's not the seat tube angle per se, it's the angle between the seat tube and chainstay (as it affects the angle of the FD relative to the line between the chainrings and cog).

HOWEVER, since you have friction front shifting, the issues I brought up probably won't even apply anymore. It's indexed front shifting that's finicky.

I have limited personal experience with friction front shifting, but from what I've read, people have used pretty much any friction front shifter with any FD and it's worked. The only possible issue is whether the total amount of cable the shifter can move is enough to allow the FD to move across all chainrings. But, I just haven't seen this actually be a problem.

You may need to play around with the vertical position for the FD. I'd make sure that I could find a spot for the shifter so that the chain on the middle chainring doesn't rub on the cage no matter which cog was used in back (save one or both extremes).

Otherwise, set up the FD normally. For friction, you really just need to set the Limit screws so you don't overshift in either direction.

peterw_diy

There's also the matter of the lateral range of motion of the derailleur (as OP discovered with the FD-M770) -- my Tiagra FD-4500 double couldn't go outboard quite enough for my 26/36/46 crank but a FD-2303 triple does just fine. This is with the same shifters as the OP, on a Surly Pacer. The picture below was taken right after install. It's close, but I think with a 44t outer it would still clear the chainstay.

onespeedbiker

The proper fix is a wider BB; your Phil Woods BB is adjustable, so you may be able to simply move it over a few millimeters.

Otherwise, which older XT derailleur worked for you? It may be as simple as finding one in better condition ..; otherwise you will need to jump through some hoops, hoping a derailleur not designed for your setup will work. The old XT derailleur also fits that bill, you just know it works.

ljg

Thanks I will check into that one!

ljg

I'm really hoping I can find a fix without changing chainline (I like 45mm it really is the "ideal" straight line from the middle chainring to the middle cassette sprocket on a 9 speed cassette - and I use all chainrings with all gears)

However, finding another identical XT derailleur in good condition that would work is a great idea. The problem is I don't know how to find the part number. The bike was built in 2008. The derailleur says "Deore XT" on it. It has a 28.6mm clamp (bottom clamp traditional style derailleur).
But I can't even find a Shimano lookup table that would tell me 2007/2008 parts lists.

I'll try to take a photo of it when I get home and post it. But without knowing the part number, I don't know how to even search for a used one.

Good suggestion though.

onespeedbiker

Since it's an XT, the number should be "FD-M7##" The model number is stamped or a sticker on the back of the inside fence.


or the back of the plastic piece where the adjustment screws are..

hamster

45 mm is the standard road triple chainline. If your bike worked correctly for you with a Deore XT FD and a crank with a 44T big ring (presumably a mountain bike crank) and they were both set at 45 mm, getting that to work involved some black magic and possibly some custom parts.

The easiest solution would be to live the crank alone and to get a new road FD. I think that a current Shimano FD should work correctly, especially since you have a friction front shifter.

onespeedbiker

The problem with your idea is the OP has a MTB crank with a 44T large chainring. A road front derailleur is made to shift a 52-48 tooth large chairing.

hamster

I have a road front derailleur and a 42T large chain ring on my bike and I haven't observed any issues so far. (Maybe I'm not trying hard enough?) There's nothing in the specs for my FD about the size of the large chain ring. Unlike, say, the gap between small & large.

shelbyfv

Are you sure you don't have something keeping the new XT from retracting all the way? Limit screw, cable, spacer of some sort? For the time period, 9 speed, seems like your new FD should not be so different.

onespeedbiker

The issues of using the wrong front derailleur for the max size chainring I'm sure are mitigated with individual applications and the quality of shifitng one expects; from Sheldon Brown;
[h=3]Large-Chainring Size[/h] Front derailers shift best when they are mounted as low down as is possible without having the cage foul the teeth of the chainrings. In particular, the bottom edge of the outer cage plate should be closely matched to the curvature of the largest chainring.

If you use a front derailer with a larger ring than it is meant for, the rear of the cage will get too close to the chainring before the front of the cage is close enough to provide crisp shifting.

If you use a front derailer with a smaller ring than it is meant for, the front of the cage will get close to the chainring while the rear of the cage is still far away from it. This won't hurt shifting, but will require excessive "trimming " of the front derailer as you change gears at the back.

Mismatched derailers and chainrings may be satisfactory with friction shifting, but will likely cause problems when used with indexed shifters, such as Shimano STI units.
If you have unusual gearing requirements, and are handy, you can reshape the outer cage plate with a grinding wheel to match a non-standard large chainring. https://sheldonbrown.com/front-derailers.html#chainring

As far as not showing up up in the tech docs, Shimano gives you a list of the cranksets that are appropriate for the front derailleur; these cranks fall within the range the front derailleur is designed for. Further, if you check the Shimano Flat Bar gruppo, you will find MTB derailleurs that have a cage rating for larger chainrings. Even then, Shimano has one front derailleur for a 50T chainring crankset and another for a 52T chainring crankset https://techdocs.shimano.com/techdocs...&bmUID=jWLY1Rz