Unwanted downshifts over bumps

Duncan Smith said:

Gears on the MTB (Shimano XT) are all set up nice, instant changes, no
noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to
down- shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking the
trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

Look at the cable and cable guides first of all, or flex or anything that
could cause the cable to slacken.

Btw, even though it's physically down to a smaller sprocket, it always seems
odd to me to call shifting to a higher gear "downshifting".

~PB

"Pete Biggs" <[email hidden]> wrote in
message news:[email hidden]...

Quoted message said:

Duncan Smith said:

Gears on the MTB (Shimano XT) are all set up nice, instant changes, no
noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to
down- shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking the
trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

Look at the cable and cable guides first of all, or flex or anything that
could cause the cable to slacken.

Btw, even though it's physically down to a smaller sprocket, it always
seems odd to me to call shifting to a higher gear "downshifting".

Maybe - but it's his front shifter which is having the problems :-)

cheers,
clive

Duncan Smith said:

Gears on the MTB (Shimano XT) are all set up nice, instant changes, no
noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to down-
shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking the
trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

The mech spring will be trying to pull towards the innermost sprocket
anyway, and the only thing that restrains it is the pod. Logically,
therefore, the mech is fine since it's doing exactly what it is
supposed to do. (The only fault it could have would be that the
spring was too strong, but that seems unlikely.)

This, and the fact that the gear indicator changes as well, puts
suspicion on the pod. Sounds like some clamping / restraining
doohickey has worn out, but the innards of these things are not the
domain of mortal man, at least not this one, so I can't offer any more
insight. Shimano aren't known for making their components user-
serviceable though, so I reckon you might be needing a new pod.

Colin

Clive George said:

"Pete Biggs" <[email hidden]>
wrote in message news:[email hidden]...

Quoted message said:

Duncan Smith said:

Gears on the MTB (Shimano XT) are all set up nice, instant changes,
no noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to
down- shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking
the trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

Look at the cable and cable guides first of all, or flex or anything
that could cause the cable to slacken.

Btw, even though it's physically down to a smaller sprocket, it
always seems odd to me to call shifting to a higher gear
"downshifting".

Sorry, I'd don't know what's happening to my brain these days. I was indeed
thinking of rear shifting there.

Anyway the advice remains the same. First of all, check for anything that
could be causing the cable tension to momentarily slacken.

~PB

Colin MacDonald said:

Duncan Smith said:

Gears on the MTB (Shimano XT) are all set up nice, instant changes, no
noise - perfect on the road. But when pedalling over bumps (pot
holes, off road etc) the chain-ring deraillieur has a tendancy to down-
shift by one towards the inner-most sprocket.

This is confimed in the trigger pod window as it will either have
moved from 3 to 2, or from 2 to 1. To make sure I wasn't knocking the
trigger I fisted the left handlebar grip - still does it.

Any thoughts on where I should be looking to solve the problem - in
the pod or at the mech??

The mech spring will be trying to pull towards the innermost sprocket
anyway, and the only thing that restrains it is the pod. Logically,
therefore, the mech is fine since it's doing exactly what it is
supposed to do. (The only fault it could have would be that the
spring was too strong, but that seems unlikely.)

This, and the fact that the gear indicator changes as well, puts
suspicion on the pod. Sounds like some clamping / restraining
doohickey has worn out, but the innards of these things are not the
domain of mortal man, at least not this one, so I can't offer any more
insight. Shimano aren't known for making their components user-
serviceable though, so I reckon you might be needing a new pod.

Colin

You might be able to confirm this - while off the bike, try to re-create
the downshift by whacking the pod only.

In article
<[email hidden]>

Duncan Smith said:

MTB (Shimano XT)

I'm not familiar with your components, but I have experienced similar
symptoms, and it was down to the lever.
In my scenario it was a rear derailleur changing up by itself. It was
really annoying and caused a couple of crashes. It got so bad a small
disturbance to the cable (like turning the bars) would change gear. Took
me a while to identify the guilty component - Suntour thumb shifter - it
just wouldn't stay in the 'click' positions. A workaround is to switch to
friction mode instead of index mode.
G.

Quoted message said:

I'm not familiar with your components, but I have experienced similar
symptoms, and it was down to the lever.
In my scenario it was a rear derailleur changing up by itself. It was
really annoying and caused a couple of crashes. It got so bad a small
disturbance to the cable (like turning the bars) would change gear. Took
me a while to identify the guilty component - Suntour thumb shifter - it
just wouldn't stay in the 'click' positions. A workaround is to switch to
friction mode instead of index mode.
G.

Cheers, I don't think there's a friction switch on the XT pods any
more (if there ever were?). Last time I saw one of those was on my
hack that had a Shimano Biopace groupset - a very nice feature too,
pity they're not that common any more...

Regards,

Duncan