lmk5

I've got a Schwinn hybrid bike with 7 gears on the rear and 3 chainrings up front. It uses Shimano SIS shifters, a Shimano rear derailleur, and an SR Suntour front derailleur. With the bike on level ground and normal pedaling all is well. But when under some load, the front derailleur has trouble shifting from the middle chainring to the smallest chainring. Part of the chain does transfer onto the smallest chainring but it also hangs on to the middle chainring and won't complete the shift. I have gone through all the front derailleur adjustments and while doing that I did notice that even with the "L" screw all the way out and the front derailleur cable slack, the chain will never "overshift" off the smallest front chainring. Is that an issue?

Does anyone have any guesses as to why I can easily shift from the front middle chainring to the smallest chainring while under low load, but the chain is reluctant to give up the middle gear when trying to shift under moderate loads? BTW this issue doesn't seem to be affected by which rear gear is chosen.

Bill Kapaun

Do you have an indexed or friction front shifter?
Suntour may have indexing issues with a Shimano shifter????

Is anything else different that you didn't tell us like you changed the crank or BB?

Does the cable have ANY slack when fully shifted to the small? You might just not have quite enough slack to allow the DER to move that far??

tyrion

This might be normal. You should be able to shift the rear derailleur under some load, but not so much the front one. You have to learn to shift the front derailleur before you hit the uphill.

shelbyfv

Yes, we've become spoiled by the forgiving nature of modern drive trains. Don't shift under load or move to 11 speed where almost anything goes.

lmk5

It is an indexed shifter. Nothing else was changed on the bike. It was idle for a couple years, has very low miles, and I was able to bring the rear derailleur back to life. The only problem is the front derailleur shifting under load. I did notice that the front chainrings do have a very slight wobble. Not sure if that's part of the cause.

lmk5

That's what I keep telling my wife LOL. I followed her on the trail and noticed that she always seems to shift in the middle of the incline. I know how to slightly back off the pedals to facilitate the shift, but she hasn't mastered it. It would just be nice to know how much load the front derailleur should be able to handle and whether it's a malfunction or operator error.

tyrion

Higher level components will allow shifting under greater load.

One thing you might try is see if you can lower the front derailleur on the seat tube. The min height is determined by the biggest chainring, but you might see if you can snug the FD a little tighter (lower).

Shimagnolo

When shifting the rear, you are derailing the unloaded portion of the chain, which is easy.
When shifting the front, you are derailing the loaded portion of the chain which is very difficult.
I never try to shift the front under load.

lmk5

Thanks I'll give that a shot.

lmk5

Good point. However, my wife dozes off when explaining this loading stuff.

Litespud

FDs don’t shift inward well under load - it’s trying to move a taut chain with only the force of the FD spring, which isn’t a particularly powerful spring. You have to ease up slightly so the chain isn’t so taut, but if you time it right it’s momentary. Contrast this with the RD, which is shifting “slack” chain, and the RD spring is adequate. Shifting to the big ring in front is easier under some load because you can exert more force through the FD cable. As for why you don’t need the inner limit screw, the FD has a finite range of motion, the limit screws just adjust this range where necessary for a specific drivetrain. It’s likely that your FD hits its “natural” limit at just the right spot, and the inner limit screw isn’t adding anything to the equation in this instance. I would still leave the screw in there, though, so you don’t lose it.

lmk5

That’s a great explanation and makes total sense to me. One other issue: When I’m on the middle chainring on the FD and on either of the largest 2 gears on the RD, I do get some chain rub on the inner plate of the FD. Is the barrel adjuster the only adjustment I have for this issue or is there a better way?

Andrew R Stewart 

"Part of the chain does transfer onto the smallest chainring but it also hangs on to the middle chainring and won't complete the shift." Imk5

This is what we call "chain suck", as though the chain is being sucked up and onto the rings when it usually releases at the bottom of the rings 9and thus travels to the rear). This is not a der issue but a combination of ring tooth profile (wear), ring to ring tooth placement (and why modern rings have a mark to rotationally line them up WRT each other and the crank arm) and as mentioned too much pedaling pressure.The issue is that the chain is trying to mesh with the two rings at the same time and can become "wedged" between them if the chain was too tightly tensioned during the shift (which remember starts on the top side of the rings).

The industry became aware of this way back in the late 1980s as mountain bikes began taking over the sales (from road bikes) and many of the new to MtBing riders didn't have the experience in how to better foresee a front shift and give themselves the time to soft pedal the couple of strokes to allow the chain to be relaxed during that shift. Since then some improvements with tooth profiling and better ring to ring rotational tooth placement have helped reduce this. But stuff wears and chain suck gets more likely as chains and teeth wear.

I explain this every week to customers and tell them I can make their bike work as well as it's condition, design and grade allows but I can't make the rider become a more skillful gear changer. Andy

cyccommute 

Your cable may be too tight. But the problem can also be due to the front derailer geometry. Kind of hard to tell over the InterTubz. You should not make any adjustments to the limit screws! They have no influence other than how for in- or outboard the derailer travels.

As to your shift problems, as others have said this is common (less common than 35 years ago, however). Mountain bikers learn a technique where you ease up slightly on the pressure on the pedals when the front won’t shift. It’s just a slight reduction in pressure and becomes barely noticeable after a while. Modern teeth profiles help a lot too.

Gonna rant a little here: Front derailers are stupidly designed. Shimano had a chance to make them much better when they “invented” Rapid Fail but they chose the wrong derailer to work on. Shimano’s Rapid Fail was supposed to make the front and rear lever movement in the same direction. People often get confused about up and down shifts on front derailers (and some rear) because of the reverse action of the derailers. The front derailer relies on the spring to move the chain under heavy load and it isn’t up to the job.

Suntour make high normal front derailers…most rear derailers are high normal. In a high normal derailer, the cable is used to drag the chain to lower gears under a load…think of how well your rear derailer downshifts. Suntour’s high normal front dragged the chain off the chainwheels and shifts were quick, crisp, and, most importantly, it worked under load. If Shimano had made their fronts work that way instead of giving us Rapid Fail, they would have really revolutionized front shifting.

Instead, they fell flat on their faces. Nothing since U-brakes have been such a mistake.

ShannonM

Since you've already got a Suntour front derailleur indexing with Shimano shifters, which I wouldn't have thought would work at all, maybe try a high-normal Suntour FD like the Spirt? (Not a typo, that's what they were called.) Unless Suntour changed the pull ratio on their front derailleurs somewhere along the line, it should work as well as the one on the bike.

Or just put a 9 speed Shimano Deore LX on it... If that doesn't solve the problem, there is no solution. (Other than technique, which is the right answer but seems to be off the table.)

--Shannon