Biker2022

My early-part-of-this-century Mongoose Switchback "has trouble" shifting to the lowest gear. It works pretty well, shifting between the highest gear and back to the middle gear. It also, in the past, has shifted pretty well between the middle gear and on to the lowest gear.

The H screw (the one on the right) seems to work fine, and I can see it move the "cage" from left to right, but the L screw seems to do nothing to the "cage". (The shifting cable was replaced recently.)

If anyone has a suggestion about getting the lowest gear to function, I'd appreciate it.

I'm not an amateur bicyclist and I'm certainly not a professional -- I just say this so you have a general idea of what I know about bikes.

Kontact 

It sounds like you are asking about a front derailleur. If the derailleur won't move far enough to the left to allow the chain to go to the smallest chainring, either the L screw is too tight, or the shift cable is too tight. Given that you tested L and have a new cable, it sounds like the cable is too tight. Check for a barrel adjuster on the shift lever and screw that in (clockwise). If not, the cable anchor bolt on the derailleur will have to be loosened to create a little slack.

This is assuming that nothing else was changed with the height, angle or any other part of the derailleur set up.

79pmooney

Very first thing to look at is the hanger for the derailleur. The hanger is the tab extending down from the dropout that the derailleur is mounted on. It can be part of the dropout itself in the case of typical older steel frame or a separate piece that is secured to the dropout, usually with two screws. Hangers get bent. Bikes fall over, hitting the derailleur and that push bends the hanger in slightly. Once this happens, the derailleur is no longer in line with the cassette cogs and drivetrail. Shifting suffers.

Put the bike on a stand if you have one. Or lie down on the garage floor just behind the bike and look forward past the cassette to the crankset. Align your eye with the middle cog (7 or 9 speed) or gap (8 or 10 speed) and the same on the chainrings. Now look at the derailleur cage and the vertical line of the chain. Is it parallel to the cogs? Does it skew in or out? NOw, looking from above but still in line with the cassette and crankset middle, is the derailleur cage parallel to the cogs and bike?

If no - it is bent. If steel, there is a special tool all bike shops have that both measures the error and serves as the tool to bend the hanger to alignment. Unless you are serious about doing your own work, this is not a tool you need. Probably a $25 15 minute visit to a bike shop. ($100 tool that takes up a fair amount of space.)

If this is a separate piece, it is likely aluminum. Aluminum doesn't like second bends at all so replacement is the only good option. You or a bike shop needs to find that hanger.

Take that look and report back. If the hanger looks OK, on to the next step.

Edit: I was assuming rear derailleur. If front, see kontact above. (Never hurts to occasionally look at that hanger.)

FBinNY 

Front or rear, it's the cable and cable alone that controls shifting. The aptly named LIMIT screws do exactly what the name implies, specifically determine the inner and outer limits of the working range.

Imagine an elevator. The motor pulls the cables to raise or lower the car to each floor. The system has limit switches top and bottom to prevent the car from crashing into thd roof or basement.

As Kontact said, your cable is probably too tight, not giving enough slack for low gear (assuming FD). Once you address that, you'll need to reset both limits so the chain won't fall off either end.

wheelreason

Assuming it worked properly before, cable is too tight. Remove cable, adjust L screw, then reattach cable so it's snug without moving the dlr on low gear.

Bill Kapaun

You didn't mention what kind of shifter, but there may be a barrel adjuster on it that you can screw in. Try a couple clicks at first and add as needed.

Kontact 

Or maybe the cable is too tight and he can check the shifter for a barrel adjuster.

Biker2022

Thank you, guys. I read your remarks about the cable being too tight. (The barrel adjuster can't be turned anymore in a clockwise direction.)

I've taken several pics, one of which shows the slack in the cable – taken when the shifter (Shimano) was in "low gear."

(The chain was replaced this past fall. The freewheel was also replaced this past fall.)


This was taken, looking down at the derailleur.

This shows the cable (was taken with the shifter in low gear).

The derailleur taken from the side of the bike.

Andrew R Stewart 

Do I see a front der with frozen pivots? The der should be able to move in and out with only fingers proding it (and the cable not attached to or influencing it). Does it? If so this wouldn't be the first time I've seen this. The ft der is pretty much in the direct line of the road spray off the rear tire. Add the well crafted nooks and crannies that water seeps into and not evaporate out of well and no wonder the thing might rust. Especially on the more economically materials used on lower cost stuff.

If it is frozen ft ders can sometimes be freed up enough to work. Brute force to move the der through its range, back and forth, many times and adding a penetrant then oil often loosens the pivots. If it is rusted hopefully the limit screws are not yet frozen in place too... Andy (who has ridden too many miles on a bike with a broken ft der before)

FBinNY 

As long as you're going to disconnect the cable anyway, might as well check the FDs motion.

Oil the pivots, then shift the FD manually by pressing on the lever arm. It should have no problem shifting to any chainring, and you can now adjust the limits so it trims correctly either by return spring on the inner, or hand pressure on the outer. Then connect the cable, based on based trim for the middle ring. Usually thats barely perceptible slack on the inner.

ScottCommutes

Oddly, I dealt with this exact problem on my son's Mongoose of similar vintage. The front derailleur is spring loaded and should automatically get itself to the lowest gear, unless it is held up by the limit screw or the cable. Fortunately, you can check all this easily. My problem was like this - I could shift into low gear. I could see that there was some slack in the cable, so that wasn't the problem. I could also see visually that the tip of the "L" limit screws wasn't touching anything at all, so that wasn't the problem. I could also move the derailleur freely with my hand, so nothing was frozen.

At this point, I determined that the derailleur was shot. Most likely the spring was just too tired to pull the derailleur all the way over, but the reason didn't matter. I went to the local bike shop, had them order me a new derailleur, put it on the bike, and I was back in business. Total cost was $15.

Edit - I also needed a quick link to put the chain back together after I split it and snaked it through the new derailleur.

ScottCommutes

Also, front derailleurs are in many ways optional. Many new bikes today omit them altogether in favor of more gears on the back. As long as you have the middle ring on the front and a good rear derailleur, you have at least 85% of your gearing.

alcjphil

Not helpful

ScottCommutes

Sorry. I was just pointing out to the OP that he was chasing a relatively minor improvement. For all we know, there are bigger issues with the bike that he might decide to focus on if this one is kicking his a$$.

Mr. 66

That all looks fine, crusty but fine. I would oil the derailer pivots and spring. Work that by hand. The chain definitely needs lubricant. WD40 the cable as well, your shifter may need some as well.

I think the rest your bike probably needs service.

Biker2022

When I think about biking on grassy areas with hollows, I realize my dependence/reliance on the lowest gear in trying to stay upright.

stratman

I'm not sure why you think that the H screw "seems to work fine", because you can see it moving the cage. I would actually worry that you're putting strain on the indexing in the shifters by having the H screw too far in.

Let's lay out how the components of the system work together.

1. The two limit screws are exactly that (as has been said earlier). They are limit screws. Their purpose is to stop the chain going past the inner or outer chainring and falling to the outside or inside. In a good setup I wouldn't expect them to be in contact. Have a look at the other end of the screw (the one without the slot) to see how they work. I'd like to see a slight gap there when in either the largest or smallest chainring.

2. The tension in the cable is what moves the cage so that it pulls the chain on to a larger chainring.

3. The tension in the derailleur spring is what pulls the cage, and hence the chain, onto a smaller chainring.

Understanding how those elements work together is what you need. Without it any amount of fiddling is just random.

I would back out the limit screws so that aren't having any effect. Unclamp the cable. Does the derailleur drop to the inside? It should. If it doesn't either the pivots are seized or corroded, or the spring is done for. Maybe the spring is just tired and weak. Some very thin lubricant (triflow?) on the pivots is a good idea. And looking at the bike in general, there's a lack of lubrication going on I would say. But if the derailleur moves in and out freely (you pull it outwards against the spring) the problem is elsewhere.

2. and 3. above have to be balanced. The tension in the cable is acting against the tension in the spring. Before you reclamp the cable turn the adjuster so it's in the middle, so you can adjust in or out. Then clamp the cable. You'll now be able to adjust cable tension up or down. A new cable probably would help.

On my nine speed triple, when I'm on the inner ring there is no tension in the cable, I can move the exposed inner very slightly.

Duragrouch

Another possibility: The front derailleur spring is broken in some way, such as the end of the coil normally has a bend in it to catch somewhere in the linkage; If that end has broken off, or slipped off where it's supposed to catch, the derailleur won't push to the inside gears; The cable will pull it to the outside gears, but as they say in engineering school, "you can't push on a rope". The checks described above, such as manually manipulating the derailleur cage inward, should give a clue on this.

mpetry912 

the other thing I noticed from the "very helpful" pic was that the FD cage is dragging against / being restricted by the pants guard ?

am I seeing that right ?

/markp

oldbobcat

I can try to guess what your problem is and give you advice that might address the problem. Or I can show you where you can learn to really fix it yourself. https://www.parktool.com/en-us/blog/...&area%5B%5D=52

Kontact 

Can you grab the derailleur cage and push it to the left? If not, the pivots are frozen - oil them. Can you push it, but it moves back and forth with little effort? Broken spring.

You can replace this derailleur with a used one from a coop for $5.

Duragrouch

(above) +1. Perfectly concise.

EDIT: Make sure you push on the derailleur as described, with shifter positioned for the low ring. If shifter on high, it will be pulling the derailleur outward and you can't push inward, even with broken spring or froze pivots.