Jumping down gears

[mattr]

Do you have too much paint on the gear lever boss, or is it too long?
This might mean that the screw holding the shifter together is not applying enough load to the friction washer in the lever, and it slips.
Or have you doused it in oil, and its slipping?

 

[Polco]

Wish I knew what was going on, visually everything looks fine! The bosses were taped prior to painting, and the threads are the only parts that have been lubricated, good suggestion though!

 

[Polco]

Wondering if fitting this might solve the problem?
http://www.ebay.co.uk/itm/221046455961? ... 1423.l2649

 

[Woz]

Wondering if fitting this might solve the problem?
http://www.ebay.co.uk/itm/221046455961? ... 1423.l2649

I think we really need pictures of your full set-up.

From what you have said "I've noticed even a slight tug on the cable at the downtube will shift the lever into the next sprocket, the derailleur is screwed in tight, I've tried it with two different levers with the same result."

In my opinion can be these things - one or a combination of all:

- the cable routing on the rear mech is wrong; I mean it is not set properly around / near the pinch bolt and it
results in being overly sensitive to any cable movement. It's an easy mistake to make.

- rear mech is worn and has sloppy pivots, but this I'm doubtful of as it would need to be really really worn. May
be an idea to make sure your jockey wheels are correct, the top one needs to be a "floating" type - on Shimano
it is marked Centurion - G.

- the rear cable is simply crap and stretches. Try a new inner (standard Shimano) and a compressionless
outer that connects to the rear mech.

- the lever is simply not tight enough or damaged, but since you tried indexed bar ends this does not sound that likely but
on the other hand in friction mode could explain the magnitude of the shift going down the whole block - a bit perplexing.

- frame flex may compound the issue and be the trigger. Yes it exists but there are 1000s of steel frames that
don't have this problem. A little pulley wheel will only help to reduce cable friction which I don't think is the
issue at hand. You can reduce friction by putting a teflon tube under the BB too - but I'm
not convinced this is the root of the problem unless there are obvious signs of frame damage and alignment issues.
While the bike is stationary, you can put your foot and weight on each pedal to see what is happening and just how
much things flex. Typically frame flex symptons are more likely seen with the chain rubbing on the front derailleur cage
when in the big ring.

Hope it helps.

 

[Polco]

The first two points have already been checked, I stripped the mech to clean the jockey wheels and checked they were the right way round. Interesting point about frame flex, I've found going up a steep hill, standing on pedal, which because of the gear jumping issue is a huge gear, that the front mech moves in and rubs against the chain. Not nearly as much or as easy a slackening of cable as the rear mech but I suppose it demonstrates the frame is flexing and at least contributing to the problem!

 

[Robbied196]

Re:

Photo time

 

[Woz]

The first two points have already been checked, I stripped the mech to clean the jockey wheels and checked they were the right way round. Interesting point about frame flex, I've found going up a steep hill, standing on pedal, which because of the gear jumping issue is a huge gear, that the front mech moves in and rubs against the chain. Not nearly as much or as easy a slackening of cable as the rear mech but I suppose it demonstrates the frame is flexing and at least contributing to the problem!

It's actually not the front mech moving in, but rather the top of the chainring moving outward relative
to the seat-tube. Caused by:
- very slight flex in the seat-tube - BB junction
- possible play in the bottom bracket bearing
- cranks not sufficiently tight on BB spindle.
- chain too wide for front derailleur cage (eg. 7 speed chain with 10 speed front mech)
- chainring bolts loose, chainrings buckled, chainrings and spider flexing under load with the crank arm.

It's somewhat "amplified" with bigger chain rings, longer bottom bracket spindles and skinny tubes.

 

[Polco]

Re:

Under these circumstances I would suggest it is the front mech moving in,it isn't a case of the chain intermittently rubbing but rather permanently moving in even when less pressure is applied requiring the lever to be readjusted much like the issue at the rear.

 

[oonaff]

Re:

im going on hunger strike untill this problem is resolved :|

 

[gavr]

Re:

im going on hunger strike untill this problem is resolved :|

Don't give up it will be something really daft. The last time I had that problem, intermittent and painfully random down shifting, it was just a frayed cable. A frayed edge catcing inside the housing...