cynergy

Hi Folks,

I have a Trek 520 touring bike that I've had for many years. When I first got the bike (new) the local shop swapped out the chainrings/crankset that came with the bike and put (at my request) a crankset that has smaller chain rings. This arrangement has worked well over the years - I like the shorter gears so i can pedal comfortably up the hills in my area.

However, this setup has one issue. If the crankset is under a little bit of load (i.e., if I'm pedaling up a hill) and if I try and shift from the middle chain ring to the smallest chain ring, the front derailleur will fail to push the chain off of the middle ring onto the smallest ring.

I've tried adjusting the derailleur myself and I also recently took the bike to a large bike shop in southern NH and had it serviced. they claimed the problem should be addressed, but I tested the bike this weekend and it still has the same problem. if i'm pedaling up a moderate hill and try to downshift, the chain does not move and it stays on the middle ring.

Any suggestions on how to troubleshoot this problem?
Thanks!

Litespud

try easing up on the pedals for the split second you’re shifting, otherwise the little spring in the FD is trying to shift a taut chain off the ring. Might be too much for the FD

alcjphil

When you are pedalling hard, the top run of your chain is under very high tension. The front derailleur pushes against the top run of the chain when it attempts to shift the chain onto a smaller chainring. All that is attempting to make that shift is the spring on the front derailleur. If you are pedalling hard, that spring may not be able to push the chain onto a smaller chainring. Cable friction can make things worse, but if you want to shift to a smaller chairing you have to ease up on your pedals

Andrew R Stewart 

With the swap to smaller rings there's a chance that the OEM ft der now doesn't match up with the different ring curvature and/or the size of jump between the rings. When stuff is new and less worn fudging compatibilities has less problems. But as components wear the less then best matches can be the first to become problematic.

However what the others said is the golden rule for nice ft shifting. Ease off the pedal force. BTW the faster the cadence typically the quicker the shift is also. So bogging down and waiting to shift till you can't soft pedal a stroke or two means that you waited too long to shift. Andy

berner

I once had a similar problem after installing compact chain rings. as Andrew in post #4 has said, the curve of the cage is a larger diameter than ideal. I addressed my problem by grinding down the chain ring arc slightly with a dremel tool, maybe 1/16 inch or a bit more. Then the cage can be lowered closer to the chain rings to within a dime of touching, as recommended. I also squeezed the cage spacing a bit tighter by a tiny bit by tapping with a hammer after removing from the bike. Just a tiny bit though. This was about 20,000 miles ago with good shifting the entire time.

cynergy

Thanks all. I do try and ease off the pedal when downshifting, but the problem seems worse with this bike than with my other bikes. I suspect it does have to do with the diameter of the chain rings vs the derailleur. For now I'll keep it as is, but might eventually try replacing the derailleur once the shift cables need to be replaced.

Thanks again for the replies!

Retro Grouch 

I'm assuming you don't have any additional derailleur inward adjustability.

I used to have some 2mm bottom bracket spacers to move the BB that far toward the drive side. That's a cheap hack that would give you the adjustability that you need.

Bill Kapaun

Sometimes to can make the mismatch work by trying various tweaks.
Try mounting 2-3mm higher than "perfect".
Sometimes a tiny bit of "twist", nose in or out.
I usually finish up with a new FDER cable because I need it.

cynergy

Hi Retro Grouch

Yes, there is no more inward movement of the front derailleur. I looked at it again last night. The inner chainring is pretty tiny (i like my short gears!) and when the front derailleur shifts inward, it does OK if there's no pressure, but if there's a little pressure on the chain, the derailleur rubs against the chain but doesn't cause the shift. It seems like the front derailleur is very long relative to the diameter of the smallest chain ring and the derailleur pushes against the chain at a point farther away from the chain ring so that it is less effective at shifting to the smallest ring.

Maybe I'll try what Bill Kapaun suggested and try adjusting the orientation of the front derailleur, but it seems like it's too long/large for the size of the chain ring.

andrewclaus

Ditto what Retro Grouch says. The chain line may have been moved in a touch with the crankset replacement. Another solution would be to install a slightly longer BB spindle, or replace the cartridge BB with a longer one.

cynergy

I wonder if it's the geometry/length of the front derailleur. Before work this AM I compared the front derailleur on my hybrid commuter bike (which also has a 3-ring crankset, with small chain rings up front) and the front derailleur on my 520. The 520 derailleur seems significantly longer (maybe an inch or more) than the derailleur on my hybrid. i do not experience the same issue with my hybrid bicycle.

I might try and swap out the front derailleur with something similar to what my commuter bike has and see if it helps.

100bikes

The longer cage of the 520 derailleur should accommodate a greater differential in the size of the chainrings

While I assume that the front derailleur is at both the proper height above and alignment to the chainrings, without
photos it is tough to determine

Classic setups, expecially Suntour derailleurs were pretty fool proof it set up correctly.

Check to see if there is equal spacing between the chainrings.
It is possible that the middle ring is mounted backwards.
Another question would is if the rear derailleur is positioned to keep the proper tension of the chain when it is
on the small chainring. Suntour rear derailleurs are meant to be parallel to the chainstay(B adjustment screw)
To little tension will make the transition to the small ring a bit more hesitant as well.

One way to check is to get the chain onto the small front ring with the chain on a cog in the rear you generally ride in.
Notice if the chain has any sag to it.

BTW, if you are on the small cog in the rear, it is tougher for the the front derailleur to shift because:
1. The angle of the chain 2. The tension on the system.(cross chaining)

One other hack( have done it many times for similar situations) is to bend the leading edge of the front derailleur in a couple of degrees.
Usually on Huret and Simplex, but have done it with both Suntour and Shimano as well.

rusty

cynergy

I did some more Googling. The bike came from the factory with a Tiagra front derailleur. I believe that this front derailleur has a 20 tooth capacity with a max chain ring size of 50 teeth. so the smallest recommended chain ring is a 30 tooth ring. i don't have the bike with me, but i believe the number of teeth on the smallest ring is a lot less than 30 (I think 22 teeth, if I remember correctly). I am going to look for a used 9 speed Shimano XT front derailleur and see if that helps the problem. The replacement crank set with the smaller chain rings is a Deore XT 9 speed MTB crankset. I am pretty sure this is the reason I've had shifting issues all of these years.

Bill Kapaun

IF you look over the FDER carefully, you should see a name/Model#.
Sometimes you have to look from the NDS of the bike to see it on the inside of the cage.

deacon mark

If it was me I and you like the bike I would upgrade to compact crank and get a bigger rear cassette. Will do the same thing and be easier and lighter.