Chainring Orientation

After soaking my chain in White Spirit and getting it squeaky clean I got to
work on cleaning the grimy sprockets and chainrings. Only after I had
removed the chainring did I realise that these days chainrings are not
rotationally symmetrical -- I assume the two adjacent teeth that appear as
if they have been cut short are there to allow the chain to move in and
engage the chainring more easily.

There are two pairs of these teeth, diametrically opposed, but they only
appear on the big chainring.

Why only on the big chainring? Is it because that is where the derailer is
going to be at it's limit and therefore less effective at forcing it onto
the ring?

What orientation do I want these special teeth in, in relation to the
cranks? 0 and 180 degrees offset from the cranks so that they appear at the
top of the chainring when the cranks are at 6/12 o'clock with minimal force
being applied?

Another chainring question: There appears to be at least a few mm of lateral
wobble in the big and middle chainrings, visible when I spin the cranks
quickly and focus on the chainrings against the background. Is this normal?
Can anything be done to corret it? I believe it was like this when I bought
the bike and the LBS didn't seem to think too much of it when I pointed it
out.

Dani

On Wed, 30 Jul 2003 12:34:42 +0000 (UTC), asqui
wrote:
There are two pairs of these teeth, diametrically opposed, but they only
appear on the big chainring.

Why only on the big chainring? Is it because that is where the derailer
going to be at it's limit and therefore less effective at forcing it onto
the ring?

Imagine shifting from big ring to small: Chain falls off big
ring, lands on smal..

Now imagine shifting from small to big: Chain must climb to big ring.

The chain could use a little help, so the gaps created by the
smaller teeth result in some grabby action on the chain.

Disclaimer: I am not an engineer, mechanic, lawyer, or police
officer. I could be wrong.

What orientation do I want these special teeth in, in relation to the
cranks? 0 and 180 degrees offset from the cranks so that they appear at
top of the chainring when the cranks are at 6/12 o'clock with minimal
being applied?

Probably not a concern; there may be some offset related to
something else that matters, though.

Another chainring question: There appears to be at least a few mm of
wobble in the big and middle chainrings, visible when I spin the cranks
quickly and focus on the chainrings against the background. Is this
normal?

Yes, it's normal in so far as lots of people probably never
even notice, let alone repair it.

Can anything be done to corret it? I believe it was like this when I
bought
the bike and the LBS didn't seem to think too much of it when I pointed
it out.

Read this:
http://www.sheldonbrown.com/straighten_chw.html

Dani
--
Rick Onanian

Rick Onanian wrote:
On Wed, 30 Jul 2003 12:34:42 +0000 (UTC), asqui
wrote:
There are two pairs of these teeth, diametrically opposed, but they
only appear on the big chainring.

Why only on the big chainring? Is it because that is where the
derailer going to be at it's limit and therefore less effective at
forcing it onto the ring?

Imagine shifting from big ring to small: Chain falls off big
ring, lands on smal..

Now imagine shifting from small to big: Chain must climb to big ring.

The chain could use a little help, so the gaps created by the
smaller teeth result in some grabby action on the chain.

Disclaimer: I am not an engineer, mechanic, lawyer, or police
officer. I could be wrong.

Sorry, I forgot to make it clear that I have a skanky touring bike with a
triple chainring, so really my question is doesn't the same thing apply for
when you are changing from granny to middle.

What orientation do I want these special teeth in, in relation to the
cranks? 0 and 180 degrees offset from the cranks so that they appear
at top of the chainring when the cranks are at 6/12 o'clock with
minimal being applied?

Probably not a concern; there may be some offset related to
something else that matters, though.

What do you mean?

Another chainring question: There appears to be at least a few mm of
wobble in the big and middle chainrings, visible when I spin the
cranks quickly and focus on the chainrings against the background.
Is this normal?

Yes, it's normal in so far as lots of people probably never
even notice, let alone repair it.

Oh, that's comforting.

Can anything be done to corret it? I believe it was like this when I
bought
the bike and the LBS didn't seem to think too much of it when I
pointed it out.

Read this:
http://www.sheldonbrown.com/straighten_chw.html

Damn! And I thought I had read everything there was to read on that site!

Thanks for your help.


Dani

On Wed, 30 Jul 2003 12:34:42 +0000 (UTC), "asqui"
may have said:

There are two pairs of these teeth, diametrically opposed, but they only
appear on the big chainring.

If you had three chain rings, there would be some on the middle one as
well.

Why only on the big chainring? Is it because that is where the derailer is
going to be at it's limit and therefore less effective at forcing it onto
the ring?

As the der shoves the chain over, those clipped teeth (and, on some
rings, the little nubs on the side of the ring) help to get the chain
started on to the ring when shifting. As another poster pointed out,
going down to the small ring, it simply drops in place.

What orientation do I want these special teeth in, in relation to the
cranks? 0 and 180 degrees offset from the cranks so that they appear at the
top of the chainring when the cranks are at 6/12 o'clock with minimal force
being applied?

This would be right on target. That's where Shimano put them on the
cranksets of theirs in my possession.

Another chainring question: There appears to be at least a few mm of lateral
wobble in the big and middle chainrings, visible when I spin the cranks
quickly and focus on the chainrings against the background. Is this normal?
Can anything be done to corret it? I believe it was like this when I bought
the bike and the LBS didn't seem to think too much of it when I pointed it
out.

The importance of this may be minimal; if you have no problem with the
chain doing unwanted things under hard pedalling, it's likely that it
can be safely ignored. If you have a torque wrench, you might check
the tightness of the crank arm retaining bolt or nut.

--
My email address is antispammed;
pull WEEDS if replying via e-mail.
Yes, I have a killfile. If I don't respond to something,
it's also possible that I'm busy.

On Wed, 30 Jul 2003 13:36:51 +0000 (UTC), asqui
wrote:
Sorry, I forgot to make it clear that I have a skanky touring bike with a
triple chainring, so really my question is doesn't the same thing apply
for when you are changing from granny to middle.

Maybe, but there's less tension on the chain then, so maybe
that's why it's not as important. Now I'm just grasping at
straws!

Probably not a concern; there may be some offset related to
something else that matters, though.

What do you mean?

I just mean that I don't see that it would matter much how they
line up with the crank arms, but maybe they have to line up with
something else (I'd bet there's a mark on the middle ring or
some such).

Read this:
http://www.sheldonbrown.com/straighten_chw.html

Damn! And I thought I had read everything there was to read on that site!

That link was really the only very useful information _I_
had for you in the reply.

Thanks for your help.


Dani
--
Rick Onanian

asqui wrote:
others have answered most of the rest of your questions...
....
What orientation do I want these special teeth in, in relation to the
cranks? 0 and 180 degrees offset from the cranks so that they appear at the
top of the chainring when the cranks are at 6/12 o'clock with minimal force
being applied?

The cogs should have a tab sticking down in one of the cut-outs. This
tab should be aligned with the crank arm (Shimano). Others may be the
same or may be marked in another way. It _does_ make a difference in
shifting.

Another chainring question: There appears to be at least a few mm of lateral
wobble in the big and middle chainrings, visible when I spin the cranks
quickly and focus on the chainrings against the background. Is this normal?
Can anything be done to corret it? I believe it was like this when I bought
the bike and the LBS didn't seem to think too much of it when I pointed it
out.

Mine don't have this. I'd take a look at Sheldon's site (as others have
suggested). Bottom line is: do they shift reliably or ghost shift under
power. If they shift reliably, I'd leave it be for now .

David

if that's not up to spec, you could trash the crank arm. On a couple
of occasions, I've run across setups where I was never able to get the
damthing to stop rubbing in some particular gear combination or
condition, but most of the bikes I deal with are, for lack of a less
diplomatic description, crummy. Yours should be able to work without
rubbing.

It's almost always a problem with the FD forward-aft alignment.

Also, use your barrel adjusters on your shifters while riding (if MTB).

--
Phil, Squid-in-Training