Spontaneous chain ring shifts??

TomYoung said:

... now I'm getting occasional spontaneous shifts from the
middle chain ring to the small chain ring when the chain's on the
large rear cog. It's happened a few times while I've been soft
peddling along and a couple of times when starting out from a standing
stop. It's certainly not an issue with the front derailler; it's
not touching the chain when the chain's on the middle ring and
pressing the lever to move the chain back up to the middle ring puts
the chain right up on the big ring.

Given that the rear der has no effect on the approach angle of the
chain to the front sprockets on top, which is where any shift must
begin, then the rear der can't have any bearing on this problem unless
you're back-pedalling at the time...and if you're doing that, well,
don't.

Quoted message said:

I'm a bit mystified that a small adjustment to the rear derailler is
apparently causing this problem up front but since this phenomenon
didn't show up until I made the rear adjustment I'm relating one to
the other. Any thoughts on what's the problem here?

In my opinion, it's unrelated. You made two changes; one of them was
a chain swap. Retrim the front der; the chains are probably just
slightly different in width.
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TomYoung said:

Hi all:

About 3 weeks ago I converted my road bike to a triple-ring setup,
using all new stuff. It seemed that my initial adjustment was pretty
good as things were working fine except for one little issue: I was
getting some sound and vibration from the rear derailler if I moved

the

Quoted message said:

pedal backwards when the chain was on the large rear cog and middle
chain ring.

Three or four days ago I installed a new chain (I swap chains for
cleaning) and decided that the rear derailler issue was due to the
derailler swinging in toward the wheel a bit too much when I moved

the

Quoted message said:

chain to the large rear cog. So I screwed in the lower stop a bit at

a

Quoted message said:

time until the chain would no longer shift up to the large cog and

then

Quoted message said:

backed the screw out a quarter turn. Problem solved: no sound from

the

Quoted message said:

rear derailler when turning the pedal backwards.

However, now I'm getting occasional spontaneous shifts from the
middle chain ring to the small chain ring when the chain's on the
large rear cog. It's happened a few times while I've been soft
peddling along and a couple of times when starting out from a

standing

Quoted message said:

stop. It's certainly not an issue with the front derailler; it's
not touching the chain when the chain's on the middle ring and
pressing the lever to move the chain back up to the middle ring puts
the chain right up on the big ring.

I'm a bit mystified that a small adjustment to the rear derailler is
apparently causing this problem up front but since this phenomenon
didn't show up until I made the rear adjustment I'm relating one to
the other. Any thoughts on what's the problem here?

2 thoughts: are you using the correct length BB? Correct length chain?

I agree with werehatrack... it could also be that the new chain is
stiffer laterally than than old one was. Same solution as above.. maybe
adjust the cable tension just a bit as well

The BB is Ultegra's "triple" BB so it's at least proper in that sense.

I set the chain length according to the instructions that came with the
rear derailler and both chains are the same length so I wouldn't think
that could be an issue as I didn't have the problem with the first
chain.

Thanks.

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In article <[email hidden]>,

TomYoung said:

Hi all:

[snip]

Quoted message said:

However, now I'm getting occasional spontaneous shifts from the
middle chain ring to the small chain ring when the chain's on the
large rear cog. It's happened a few times while I've been soft
peddling along and a couple of times when starting out from a standing
stop. It's certainly not an issue with the front derailler; it's
not touching the chain when the chain's on the middle ring and
pressing the lever to move the chain back up to the middle ring puts
the chain right up on the big ring.

I'm a bit mystified that a small adjustment to the rear derailler is
apparently causing this problem up front but since this phenomenon
didn't show up until I made the rear adjustment I'm relating one to
the other. Any thoughts on what's the problem here?

_ I don't think they are related at all. I assume that your
chainrings have "shift assist" ramps and teeth. If this is
so and you have a sufficently large rear cog, then no derailler
adjustment will solve this problem. The relatively large
horizontal angle and shallow vertical angle is causing the
shift assist teeth to engage the chain and shift it downwards.

There are some things to try

1. Try a different brand of chain.

2. Move the chainline inward by switching to a different BB width,
however this may cause more problems than it solves.

3. Use a smaller large cog on the rear.

4. Buy a cheap Salsa middle chainring with no shift assist
ramps.

_ Lastly, you could just live with it and not use the
middle/large combination. With a triple and anything but a very
closely spaced time trial rear cogset, the middle gears are
largely duplicates of gears available on the small or large
chainrings. Even though you nonimally have 27 different gears, if
you look at the ratios, you only really have about 14-15
significantly different gears.

_ Booker C. Bense

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