Jumping down gears

[Polco]

I'm hoping someone on here can help! I've recently built up a 531 frame with some modern components, albeit with down tube shifters. Trouble is, when I ride on even the slightest gradient or when I push off at the lights the gears shift down the block leaving me grinding along in a big gear! 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. The hanger looks straight and the chain looks like it's in a straight line from sprocket through the jockey wheels, and I've experimented with different chain lengths with the same result, anyone got any other ideas?

 

[oonaff]

hi i would get it off the ground and hand turn the cranks,better than trying to test it by cycling and trying to watch it
as this might result in an accident. :idea:
have you routed the chain correctly through the jockey wheels?
is the chain compatible with the sprockets?
the chain length can be done by wrapping it around the big ring and largest sprocket and pulling
the deraillier untill its almost on its maximum travel.
there used to be another way with the two jockey wheels one above the other
and i beg readers pardons ive completely forgotten it
is the rear deraillear the correct cage length for your largest sprocket.
is the chain forcing the rear mech down or is it the mech ?
what running gear are you using ?are the shift levers indexed or friction.

 

[Polco]

Trouble is, that on the workstand it runs ok, but when the pressure gets put on the pedals on the road that's when it starts running down the block. It's a Shimano 9 speed, new chain, everything else cleaned thoroughly and was working on another bike prior to going on this one. All the obvious stuff (I can think off) has been checked, chain going through jockey wheels correctly, plenty spring in the mech, I thought it might be the cable run as a gentle tug on the cable will slip the gear lever but it isn't snagging and runs smoothly, currently I'm baffled!

 

[oonaff]

what about bb spindle length im clutching.you say new chain is it shimano ?[link spacing ok? was 9 speed down tube shifters ever done ?anyone.
i would change the chain give that a whirl.when it was working fine on another frame is everything exactly as it was component wise,chain everything ?sprocket teeth ok ?

 

[Polco]

Yeah, correct bottom bracket, it isn't a Shimano chain but it is a brand I've used on other bikes with no trouble at all. I am using Sun Race down tube levers, tried Shimano bar end levers too but had the same problem. Not convinced the chain is an issue as on the workstand it shifts faultlessly, and it's not as if it's jumping on a worn cassette either, it just slams down the cassette when you push hard!

 

[oonaff]

yes i understand, its when its under load. but as chains are pretty cheap its worth a try. if you had it working fine with those shifters before. sounds really odd that you had the same prob with shimano bar end shifters?
any one else got any other angles ?

 

[Robbied196]

Its always difficult to speculate on a mechanical issue without seeing it, but it sounds like you must be loosing gear cable tension. The derailleur is under tension when on the large sprocket, for it to move, the tension has to release.

Its probably worth checking the basics again, making sure the cable route is correct, the gear cable lock nut is tight, the levers are tight? Also worth resetting the derailleur stop screws and tension if there is one.

Just to clarify, are you saying when you are on the large sprocket, if you tug the gear cable at the downtube it drops down a gear?

I think oonaff has covered most other possibilities

 

[Polco]

Re:

Yes, when I give a gentle flick on the cable it will change down gears,tried everything suggested and still the same. I have noted today however that after lubricating the cable where it passes through the guide under the bottom bracket it is staying in gear unless I push really hard, so that would appear to be the biggest problem. The cable run is clear though so I will triple check it isn't snagging on something else as well.

 

[Robbied196]

It all seems to point to a cable issue. After its jumped down to the low sprocket, does the gear cable remain tight or go loose?

 

[rrarider]

Is the skewer holding the wheel firmly in the dropout? If the drive side was moving forward when your maximum force was on the crank, then it might skew the cogs a fraction, whilst under hand pressure they might remain parallel to the direction of motion?