Question about Discussion of Chain Lines

android said:

I see a lot of questions about chain line. But I don't understand how it
can be a problem and if it is, what you can do about it.

For example, an English BB is supposed to be 68mm wide. If the
framebuilder puts it in the exact centerline of the bike (as he should)
then the sides have to be exactly 32mm from the centerline. Whoever
makes bottom bracket axles and cranks know this, so they should make the
chainrings go to the right offset from the centerline just by correct
design of the BB + crankset.

The same applies to the rear hub. The dropouts are 130mm apart and once
again the mfg of the cassette knows this and knows where the cassette
must sit in order to align exactly with the aforementioned crankset.

Yes, I understand the chainstay length will change the angle off center
of the chainring to cassette depending what gear you're in, but that's
always the case.

Why is chainline a problem on any well made bike with standard
components from the major manufacturers?

If everything is standard (i.e., road hub with road crank or mountain hub
with mountain crank) and of roughly the same age, then chainline is
usually a no-brainer. But there are a lot of us who don't like to do
things the standard way. What if you want to use a mountain hub on a road
bike? (For instance, because they make for a stronger wheel, I use Shimano
mountain hubs on a touring bike that was originally spaced at 130mm; that
required moving the road crank outward). What if you want to convert a
derailleur bike to a fixed-gear or a gear-hub bike? What if you want to
convert a 7-speed bike to 9-speed?

On Wed, 26 Sep 2007 16:26:51 -0500, in rec.bicycles.tech android

._c_o_m_ said:

Why is chainline a problem on any well made bike with standard
components from the major manufacturers?

OK, as a builder of contraptions, I'll taks a stab: because many of us
kludge nonstandard components into the most gwadawful messes you've
ever seen... and we love every minute of it. In a "standard" world,
this group would be moot.

Jones

android said:

For example, an English BB is supposed to be 68mm wide. If the
framebuilder puts it in the exact centerline of the bike (as he
should) then the sides have to be exactly 32mm from the centerline.

Umm, 34, but go on.

Quoted message said:

Whoever makes bottom bracket axles and cranks know this, so they
should make the chainrings go to the right offset from the centerline
just by correct design of the BB + crankset.

Make which, the crank or the bottom bracket, or the cassette? They do
try, you know. Each crank has a specified bb width, which will center
the chainline if you use the cassette and rear hub spread that they
expect you to use.

Quoted message said:

The same applies to the rear hub. The dropouts are 130mm apart and
once again the mfg of the cassette knows this

They also know that some are 126, some 120, or 135, or...

Quoted message said:

and knows where the
cassette must sit in order to align exactly with the aforementioned
crankset.

How many sprockets? 6, 10?

Quoted message said:

Why is chainline a problem on any well made bike with standard
components from the major manufacturers?

Because there is no one set of standard components that applies for all
situations. But they are pretty close.

My first good bike had a bottom bracket (Campy) labeled "120", not
meaning that the width of the bb was 120mm, but that it was designed for
a 120mm rear axle, and, presumably, a 5-speed freewheel.

But most of us who fuss about chainlines are talking about a fixed gear
bike. A track bike with its original components should have a perfect
chainline, but most of the bikes we have are either converted road
frames, or have had every part except the seatpost replaced over the
years, or both.

--

David L. Johnson

The lottery is a tax on those who fail to understand mathematics.

android said:

I see a lot of questions about chain line. But I don't understand how
it can be a problem and if it is, what you can do about it.

For example, an English BB is supposed to be 68mm wide. If the
framebuilder puts it in the exact centerline of the bike (as he
should) then the sides have to be exactly 32mm from the centerline.
Whoever makes bottom bracket axles and cranks know this, so they
should make the chainrings go to the right offset from the centerline
just by correct design of the BB + crankset.

The same applies to the rear hub. The dropouts are 130mm apart and
once again the mfg of the cassette knows this and knows where the
cassette must sit in order to align exactly with the aforementioned
crankset.

Yes, I understand the chainstay length will change the angle off
center of the chainring to cassette depending what gear you're in, but
that's always the case.

Why is chainline a problem on any well made bike with standard
components from the major manufacturers?

So far so good, all you wrote makes sense. If you build on a straight
frame with a complete Campagnolo setup your chainline will be correct
and never be an issue.
But try a dozen mix-n-match parts on a single speed conversion by a home
mechanic and all bets are off!
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

android ._c_o_m_ said:

I see a lot of questions about chain line. But I don't understand how
it can be a problem and if it is, what you can do about it.

For example, an English BB is supposed to be 68mm wide. If the
framebuilder puts it in the exact centerline of the bike (as he
should) then the sides have to be exactly 32mm from the centerline.
Whoever makes bottom bracket axles and cranks know this, so they
should make the chainrings go to the right offset from the centerline
just by correct design of the BB + crankset.

The same applies to the rear hub. The dropouts are 130mm apart and
once again the mfg of the cassette knows this and knows where the
cassette must sit in order to align exactly with the aforementioned
crankset.

Yes, I understand the chainstay length will change the angle off
center of the chainring to cassette depending what gear you're in, but
that's always the case.

Why is chainline a problem on any well made bike with standard
components from the major manufacturers?

In addition to the responses you've gotten so far, there's also issues
surrounding clearance problems encountered on various types of fat-
tube bikes. For example, it's common for a crank mfg to spec a
recommended spindle length a bit longer than optimal, in order to pre-
emptively avoid clearance issues. And then lots of bikes can end up
coming with or being built with that length, even if they could have
used a shorter spindle for better chainline.

Also, for those of us brave enough to attempt to care about q-factor
in the current world of bike components, there can be situations where
q and chainline must be weighed against each other.

"David L. Johnson" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

android said:

For example, an English BB is supposed to be 68mm wide. If the
framebuilder puts it in the exact centerline of the bike (as he
should) then the sides have to be exactly 32mm from the centerline.

Umm, 34, but go on.

Quoted message said:

Whoever makes bottom bracket axles and cranks know this, so they
should make the chainrings go to the right offset from the centerline
just by correct design of the BB + crankset.

Make which, the crank or the bottom bracket, or the cassette? They do
try, you know. Each crank has a specified bb width, which will center
the chainline if you use the cassette and rear hub spread that they
expect you to use.

Quoted message said:

The same applies to the rear hub. The dropouts are 130mm apart and
once again the mfg of the cassette knows this

They also know that some are 126, some 120, or 135, or...

<snip>

How about 120, 125, 126, 127, 128, 129, 130 and 135 wide rear hubs? There
didn't appear to be any real standard until Shimano STI came along.

Chas.