mystery sluggish downshifts on front derailleur

[wle]

i have a bike with let;s just say, 'experienced components'.
they are shimano 600.
i have ridden them for over 7000 miles, not sure how much
the previous owner rode. probably not over 1000.
double chainring, 52 and 40 teeth.

here is the problem:

sometimes
[maybe one shift out of 3 or 4]
when i shift down from the big chain ring to the small
one, it takes a couple of pedal revolutions for the shift to
'complete'.

the chain will come off the big ring right away.

but then it will do one of two similar things:

a. it will go on the small ring, then sort of climb back
up on the tooth ridges for a second or 2, then go on and
fall in and connect after that. or

b. it will go from the big ring immediately to the ridges of the
small ring,
wait a second [a revolution or so of the pedals, yes
i am pedaling at the time..], then fall in.

after the shift is 'completed', there is no further problem.
i can even stand to pedal, there is no jumping or skipping after
that.

i have inspected for
improper assembly of crank / chain ring
[possibly the 2 chain rings are too far apart somehow,
making the gap harder to traverse]
worn teeth
worn chain
[ it seems to be very flexible from side to side but
the current wear is about 0.3/16" on 12 links]
derailleur alignment and adjustment.
as far as i can tell that is all ok.

i did suspect that the chain tension was low, possibly the
spring was weak, or the bearing dirty and slow.

but when i experimented with adding chain tension by hand
[bike upside down, pushing the tensioner back]
while attempting the problem shift,
that seemed to do nothing, or possibly make it worse.

i don;t know what else to do, any ideas?

it;s certainly not covered in the literature..

wle.

 

[Mike Yankee]

Here's what I'd check:

1) FD freedom of movement -- check by hand. Lube pivots if necessary.
Be sure cable isn't frayed or dirty, dragging somewhere.

2) FD cable tension. Try loosening it with the barrel adjuster. If
the cable is almost slack on the inner ring and sluggish downshifts
persist, the problem lies elsewhere.

3) FD inner stop setting. Put the chain on the small ring in front and
the largest cog in back. Adjust the stop so there is about 1/16"
clearance between the FD cage and the chain at the closest point. There
are two adjustment screws; you want the one closer to the seat tube
(turn CCW in 1/4-turn increments to loosen).

4) FD position, height and angle. This is potentially trickier if you
don't know what you're doing. Try the other things first.

I have an old Shimano 600 FD on one of my bikes, and it works great.
The cage is wider than that of current derailleurs, however. It was
probably designed originally for 1/4" chains. It works well with
modern narrow chains if the cable tension and stops (especially inner)
are set right.

Let us know what solves your problem.

 

[wle]

mike,

responding to your queries..

1) FD freedom of movement -- check by hand. Lube pivots if necessary.
Be sure cable isn't frayed or dirty, dragging somewhere.

--no, the derailleur cage itself moves immediately when the shift lever
does.

2) FD cable tension. Try loosening it with the barrel adjuster. If
the cable is almost slack on the inner ring and sluggish downshifts
persist, the problem lies elsewhere.

--yes, it;s slack, the derailleur is hitting the lower limit stop.

3) FD inner stop setting. Put the chain on the small ring in front and

the largest cog in back. Adjust the stop so there is about 1/16"
clearance between the FD cage and the chain at the closest point. There

are two adjustment screws; you want the one closer to the seat tube
(turn CCW in 1/4-turn increments to loosen).

--i will fiddle with this. but the reason i think it;s not that, is
that when it;s in low gear,
the derailleur cage is nowhere near rubbing the chain.

--wouldn;t it have to be rubbing the chain, in order to be restricted
enough to
make the shifting slow?



4) FD position, height and angle. This is potentially trickier if you
don't know what you're doing. Try the other things first.

--i did that actually.

height is about 2mm off the big chain ring.

angle is parallel to the chain rings.

not sure what 'position' means other than height and angle.

both plates look straight, other than some angles in at the
trailing edge [low gear side] and leading edge [high side]
that i assume are supposed to be there. my other bike has them
and works ok.


--wle.

 

[[email protected]]

wle wrote:
> mike,
>
> responding to your queries..
>
> 1) FD freedom of movement -- check by hand. Lube pivots if
necessary.
> Be sure cable isn't frayed or dirty, dragging somewhere.
>
> --no, the derailleur cage itself moves immediately when the shift
lever
> does.
>
I would still check the whole length of the cable and replace it if any
part is rusty or frayed. I had weird shifting problems too. I found
that the cable was frayed and rusty where it passed under the bottom
bracket. This allowed the cable to partially stretch. The cage would
move right away but not always enough to move the chain.

A new cable caused all the problems to go away.

 

[Neil Brooks]

[email protected] wrote:

>I would still check the whole length of the cable and replace it if any
>part is rusty or frayed. I had weird shifting problems too. I found
>that the cable was frayed and rusty where it passed under the bottom
>bracket. This allowed the cable to partially stretch. The cage would
>move right away but not always enough to move the chain.
>
>A new cable caused all the problems to go away.

I second that emotion. Five bucks.

Also, though you got most of this info already, check these two spots:

http://www.parktool.com/repair_help/howfix_frontderailleur.shtml

http://sheldonbrown.com/derailer-adjustment.html

Good luck!

 

[Mike Yankee]

>the derailleur cage is nowhere near rubbing the chain. --wouldn;t it
have to be rubbing the chain, in order to be restricted enough to...

This belies a misunderstanding.

The inside of the cage SHOULD be close to the chain (small ring in
front + largest cog in back). Otherwise the FD may not have enough
throw to move the chain off the big ring when you downshift.

You want about 1/16" clearance as a starting point.

 

[wle]

well.
i suck.
it;s mostly fixed but i changed so many things i am not sure what the
problem was.

i

checked the cable for fraying, stretching, seemed fine

lubed and cleaned the cable guide on the BB
[this probably did it actually]

REDUCED cage clearance on the low gear side
[actually this seemed to help]

lengthened the cable
[so that i don;t have to flip the shift lever (downtube)
so far forward for a downshift, i may have been
sometimes not completely slackening the cable..]

raised the FD slightly to about 2mm clearance, it was down
around 1mm.

now it doesn;t do that 'riding on the teeth' any more but
it does [about half the time] sort of shift in two
definite stages, each one makes a click, the 2nd one makes
the gear change.

i still want it to work better.

the bigger the back gear,
the more likely it is to shift properly.

this again points to chain tension or slack/crud in the tensioner
in the RD, which i am next going to try to take apart and clean.

though it involves a big chrome scary looking 'circlip' that
may get destroyed in the attempt to remove it..

wle.

 

[wle]

now i have finished messing with it.

final state:

it shifts beautifully BUT

in gear 6 [next to smallest] with small chain ring,
a fast shift will always end in what i call the 'trim zone'.

that is to say, the chain will be rubbing the cage and
you have to back off a hair.

or i can move the lever 'just enough' so it won;t land in the
trim zone, but then it takes a second or two to complete the
shift.

i am pretty sure i could formerly adjust it for this
gear where it would not do that.

and of course in gear 7, it rubs even more, but
i don;t use that combination, with a 7 speed rear though,
i would like to use gear 6 a lot.

i tried rotating the cage slightly either way and
nothing seems to help.

am i just stuck?

is it impossible for gear 6 to work with the small chain ring,
with fast shifts and no chain rubbing?

?
wle.

 

[Marty]

wle wrote:
> now i have finished messing with it.
>
> final state:
>
> it shifts beautifully BUT
>
> in gear 6 [next to smallest] with small chain ring,
> a fast shift will always end in what i call the 'trim zone'.
>
> that is to say, the chain will be rubbing the cage and
> you have to back off a hair.
>
> or i can move the lever 'just enough' so it won;t land in the
> trim zone, but then it takes a second or two to complete the
> shift.
>
> i am pretty sure i could formerly adjust it for this
> gear where it would not do that.
>
> and of course in gear 7, it rubs even more, but
> i don;t use that combination, with a 7 speed rear though,
> i would like to use gear 6 a lot.
>
> i tried rotating the cage slightly either way and
> nothing seems to help.
>
> am i just stuck?
>
> is it impossible for gear 6 to work with the small chain ring,
> with fast shifts and no chain rubbing?
>
> ?
> wle.
>


A worn chainring is often the cause of sluggish downshifts. Often
putting a new chainring on will make it work like err.. new!

Marty

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.

 

[[email protected]]

wle wrote:
> i have a bike with let;s just say, 'experienced components'.
> they are shimano 600.
> i have ridden them for over 7000 miles, not sure how much
> the previous owner rode. probably not over 1000.
> double chainring, 52 and 40 teeth.
>
> here is the problem:
>

spring weeds so it's time to till spring weeds.