My road bike arbitrarily shifts the rear gears ON ITS OWN, please help?

[PistolSlap]

I have a Bianchi racing bike with seven gears in the back and two in the front. The gear switcher for the front is on the frame between my knees, and the one for the back is on the right handlebar, like this:
http://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Bar-end_shifter.JPG/690px-Bar-end_shifter.JPG

This is the one I'm having a problem with. REGARDLESS of the front gear I'm in, the back gear will arbitrarily switch from the *bigger* cogs, down to the smaller ones, *when I'm riding up hill, putting strain on the pedals*. It switches on its own, every few seconds, until it's at the smallest cog.
The bar end shifter also moves on its own back to the perpendicular position which it's at when switched to the smallest cog.
*this only happens when riding up hills with all my weight on the pedals and no momentum*

This is obviously infuriating when I'm going up a hill, to have it switch to a harder gear without my command, abruptly, which often results in my falling off the pedals and swearing a blue streak.

I've tried to adjust the screw on the bar end shifter, since I thought it was perhaps loose, but tightening it as far as it goes doesn't solve the problem. I figured also that maybe the wire is too short or too tight, and so can't handle the larger cogs because of not enough length or slack, so it pulls itself back to a more 'comfortable' position.
That is just a guess though, without knowing anything.

I've been to my bike shop multiple times and they have demonstrated their incompetence, so now I am asking anyone bike savvy to please help me in figuring out how to fix this problem?

I would be very grateful.

[xerxes]

Hi,

Does the lever have any clicks, or is it just a friction shifter?

It sounds as though there's not enough friction at the lever to hold against the spring of the rear mech. I'm not familiar with bar-end shifters, but with old down-tube mounted friction shifters there was a screw through the middle with a "D" ring through it that allowed you to tighten it and increase the friction, but it sounds like you've tried that. If they got very worn, you could put an extra washer in them to increase the friction, so you could try something along those lines.

[Joe_W]

Did you lube the shifters to make them change faster / more precise? If so, that was a bad idea (I know, I tried when I was younger...)

[DaveM]

If you tighten the screw "as far as it goes", the shifter shouldn't be able to move at all. So maybe you are tightening the screw that mounts it to the bar and not the one that gives more friction to the shifter? If you can't tighten it to the point that it won't shift at all, maybe the shifter is worn or missing a washer.

Were your "incompetent" LBS guys able to reproduce the effect, or are they politely claiming that you're "nuts"?

You're not hitting the shifter with your knee are you? Had to ask...

The only things beyond the obvious stuff like that is that under heavy pedaling load, you are flexing the bars a lot. Maybe the shifter is somehow moving around inside the bar under flex. It shouldn't, but who knows. Again, maybe a missing or worn washer in the shifter. Does it matter where you hold the bars? Does it happen the same if you are climbing in the drops or holding onto the top flats of the bar?

[Bill]

Gonna take a different approach on my theory. Has the derailler been upgraded or rather the freewheel/cassette without upgrading the entire drive train? I had this problem only a little different on one of my MTB's from a previous owner who thought they could get away with just slapping on a different freewheel making a 15 spd into a 18spd without doing any thing else. Mine jumped gears under load and pressure. This is just a hunch, but check the other guys' theories first.

[PistolSlap]

thanks guys, I'll take this information down and see what my LBS guys think of it (kind of to 'nudge' them along)

I'll respond when I've figured out the problem.

[MjrIBanks]

i have no experience with this kind of thing but just thought of this, is it possible that the teeth on the cassette are worn so much that under load the chain just slips off?

[iggypop37]

right - i am having similar problems with the friction shifters on my old peugeot, and have been told it is the spring in the derailleur which is worn. i can conceive that, as it is an old classic well-used suntour part.

how do i check for spring wear - i have to say the bike is well-modified now