Help with Mysterious Grinding noise?

[Bill Henry]

I have a Gary Fisher Marlin mountain bike.

When I'm in my two highest gears, there is a strange grinding/rattling
noise coming from what seems to be the front derailleur.

It only happens when the righ pedal comes around from the 12 o'clock
position of the crank to the 6 o'clock position, and on top of that,
only when I am putting a pretty large amount of pressure on the pedal.
Otherwise, it seems to be fine. Even if I spin lightly in the high
gear, it doesn't make a sound. It's only when I'm mashing the pedals.

It sounds like the chain rattling against a piece of metal. But what
could it be rubbing against? And how do I fix it?

I've already tried adjusting the high gear setting for the front
derailleur, and that didn't seem to have any effect. However I might
have just done it incorrectly. Also, the front derailler could be
slightly crooked, but it doesn't look like it.

Finally, I don't really want to take it in to a shop because I think
it's probably a small thing that can be easily fixed. I just haven't
figured it out yet!

Thanks for any suggestions.

 

[Leo Lichtman]

"Bill Henry" wrote: (clip) I've already tried adjusting the high gear
setting for the front derailleur, and that didn't seem to have any effect.
(clip) Thanks for any suggestions.
^^^^^^^^^^^^^^^^
I'll bet that when you pedal hard, you are getting some distortion in the
frame/bottom bracket/big ring, causing the chain to hit the side of the
deraileur channel. Figure out which way things move when you pedal hard
(making the noise,) and then maladjust the deraileur so it CAN'T rub in that
situation. If the noise is gone, at least then you have isolated the cause.
Fixing it may be tricky, though.

 

[The Wogster]

Bill Henry wrote:
> I have a Gary Fisher Marlin mountain bike.
>
> When I'm in my two highest gears, there is a strange grinding/rattling
> noise coming from what seems to be the front derailleur.
>
> It only happens when the righ pedal comes around from the 12 o'clock
> position of the crank to the 6 o'clock position, and on top of that,
> only when I am putting a pretty large amount of pressure on the pedal.
> Otherwise, it seems to be fine. Even if I spin lightly in the high
> gear, it doesn't make a sound. It's only when I'm mashing the pedals.
>
> It sounds like the chain rattling against a piece of metal. But what
> could it be rubbing against? And how do I fix it?
>
> I've already tried adjusting the high gear setting for the front
> derailleur, and that didn't seem to have any effect. However I might
> have just done it incorrectly. Also, the front derailler could be
> slightly crooked, but it doesn't look like it.
>
> Finally, I don't really want to take it in to a shop because I think
> it's probably a small thing that can be easily fixed. I just haven't
> figured it out yet!
>

Could be several things, too severe a chainline, the front deraileur is
out of alignment or bent, one of the chain-rings is bent, too much play
in the bottom bracket, bend in the frame.

Does it happen in the lower chainring(s) as well, or just the upper one?
If it's just the upper one, that would eliminate a bottom bracket issue.

Does it happen with any gear on the back, or just certain ones, if it's
just certain ones, like the larger cogs, then it's probably a chainline
issue, not much you can do about those.....

You may want to get a bike shop to look at it, give you an idea of the
problem, and an estimate on a repair. If it's a simple adjustment, they
might not charge much to fix it, some might not charge at all.....

W

 

[Michael K]

Bill Henry <[email protected]> wrote:

>I have a Gary Fisher Marlin mountain bike.
>
>When I'm in my two highest gears, there is a strange grinding/rattling
>noise coming from what seems to be the front derailleur.
>
>It only happens when the righ pedal comes around from the 12 o'clock
>position of the crank to the 6 o'clock position, and on top of that,
>only when I am putting a pretty large amount of pressure on the pedal.
> Otherwise, it seems to be fine. Even if I spin lightly in the high
>gear, it doesn't make a sound. It's only when I'm mashing the pedals.
>
>It sounds like the chain rattling against a piece of metal. But what
>could it be rubbing against? And how do I fix it?
>
>I've already tried adjusting the high gear setting for the front
>derailleur, and that didn't seem to have any effect. However I might
>have just done it incorrectly. Also, the front derailler could be
>slightly crooked, but it doesn't look like it.
>
>Finally, I don't really want to take it in to a shop because I think
>it's probably a small thing that can be easily fixed. I just haven't
>figured it out yet!
>
>Thanks for any suggestions.

Everytime that has happened to me, it has been dirt/water/debris in
the bottom bracket.

 

[Steve McDonald]

When you go into the higher gears on your rear
cluster, the chain angles onto the front sprocket or chainring from
farther to the right. It is scraping against the outside of your front
deraileur guide. Your chainring may be bent, causing the rattling to
occur only during part of the crank stroke. It may be simple to fix, by
just bending the derailleur guide a bit to the right to allow the chain
to have enough clearance. It could also be that a front derailleur
adjusting screw needs to be opened a bit, to allow the guide to move
farther to the right. Open it too far and the chain may fall off. The
problem could be a combination of all these things. If you get it so
that it doesn't scrape in the high gears, then it may start doing it
when you're in the low ones. Start by straightening out the chainring,
if it needs it. You might need to replace it or some other parts if
they're worn out or bent beyond recovery. I once had to install a
different crank spindle, that had a farther reach out to the right.
This moved the chainring out to a more compatible alignment with the
rear gears, to allow the chain to clear the guide in all gears. If you
play around with all of these things, you'll hopefully find how to get
it all to work without scraping at any point.

I've had old beater bikes with which I just couldn't eliminate the
scraping at some point without replacing or reshaping more parts than I
was willing to do. I just set them so that the scraping took place only
in the lowest gear, which is used the least amount of time.

Steve McDonald