The front derailer keeps downshifting. I have to hold the gear shifter in place with my thumb in order to stay in high gear; as soon as I let go, it shifts itself back to the lowest.
I’ve cleaned and oiled the cable and the derailer, and tightened and loosened all the relevant screws. Yet the thing still has a mind of its own.
The exact same thing happened last year at around this time and ultimately I had to take her in to the shop. This time I resolve to fix it myself, partly because I’m cheap, and partly because I want to know how to fix it in case it happens again.
The park tools website is really good for bike repair information and step by step guides. From what you’re saying here, it sounds like its more a problem with the gear indexing than the adjustment of the front derailleur, re the let it go and it shifts back down situation. Take a look at the shifters.
Seriously, if you want to work on your bike get the appropriate “Zinn and the Art of _____ Bike Maintenace” book. They are great books and will probably address any problems or any component that you have. They are readilly available.
I used the Road Bike book to completely rebuild and upgrade the components on a road bike and it rides like a dream. The book covered every aspect.
No, no index shifters, just the old-fashioned kind.
I have looked at the Park Tools website and can’t find anything relating to my particular problem. I tried doing the other adjustments they mention, but no dice.
This isn’t just an adjustment issue. If your shifter can’t maintain tension on the cable, there’s something wrong with the shifter’s mechanism. If it were me, I’d just replace the shifter.
What kind of shifter is it? To me “the old-fashione kind” means a downtube shifter, but I suspect that’s not what you meant.
I seems to me that the tension screw on the lever centre is not holding the washer plates on the lever itself tightly enough.
Those washer plates work something like a clutch, they allow low resistance to motion in one direction and much higher resistance in the other.The higher resistance holds the derailler against the spring return action, and the bolt on the side of the lever increases or decreases that resistance.
The ‘clutch’ consists of fixed and moving parts, the moving parts operate by taking the same movement as the lever rotation. The fixed parts generally fit onto the shaft of the lever boss, but the lever boss usually has some form of keyway in it, and those fixed plates slide onto the shaft and the keyway.
This means that the fixed plates can slide up and down the shaft, but cannot rotate.
By using a a bolt to compress the fixed plates onto the moving or free plates, you can vary the friction and hence the force needed to operate the gear lever.
This is true wether the gears are indexed or not.
Sometimes the keyway part of the fixed plate becomes damaged, or does not engage into the keyway, or does is damaged such that it cannot slide down the shaft, or even the compression screw is so slack it allows the fixed plates to lock on top of the keyway.
Anything that prevents the fixed plates sliding up and down the shaft to make friction contact with the moving plate will cause the fault you describe.
It is quite common for someone to dissasemble a gear lever and put plates on in the wrong order, locate plates wrongly, lose a spacer, or put the plate on the worng side of the lever arm.
Rectification really depends upon what has failed, its usually an easy fix.
If you don’t have index shifters, I’m thinking the bike is at least a decade old, which probably means your lever has worn out and can no longer maintain the right friction.
But, since this happened last year, environmental conditions may be to blame-- and by that I mean you’re getting more mud under your bottom bracket, which ends up holding the cable in place despite your shifting. Take a look at this [url=http://sheldonbrown.com/autoshift.html]Sheldon Brown** article and see if it applies.
Having had this happen with some old bikes of mine, I think the overall easiest long term solution is to upgrade to indexed shifters. They are much more reliable than they used to be, and overall much easier to maintain.
I think we have a winner. It was working just fine, then my husband rode it in the rain (and he always uses a really low gear so he probably didn’t notice), then when I got back on it, it was all bunged. I did clean the cable at the bottom but I guess I will have to have another look.
I’ll also have a look at the washer plates, you never know what you might find!
The goal is to get her through the winter, she has been a fine and loyal mount for a long time and in the spring, I will put her out to pasture. sniff
Often there is a screw or D-ring on the top of these older shifters that you can tighten as the mechanism inevitably wears and develops slop. Try a bit of a turn on this screw (if this is what you have on your bike). I suspect this, since you said that it happened in the past and it was a quick fix at the bike shop. If you overtighten, then it will be too hard to shift either direction; just loosen the screw a bit.
Another thing that could happen - on some of the older thumb shifters there is a spring that counteracts the tension from the derailleur’s return spring - on a front shifter this spring acts to shift the chain to the larger chainring, counteracting the return spring in the derailleur trying to carry the chain to the smallest chainring. If this spring breaks (unlikely), then there may not be enough friction to hold the derailleur in a higher gear, leading to symptoms such as the OP describes. You can band-aid this by, again, tightening the friction screw on the top of the mechanism.
Hope this helps, and that I’ve not grossly mis-guessed the shifters that you have on your bike.
