Fixie chainline discrepancy?

I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?

Cheers,
Bennett Fischer
Brooklyn, NY

On Nov 18, 6:00*pm, Bennett Fischer wrote:
I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? *Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?


So long as your additional spacers are the same width, the
Surly should give a 43 mm chain line. Are you sure some of
the discrepancy is not from measuring to the inside of the
front sprocket and the outside of the rear?

Th' General method that I use is:
1) Double check the rear spacing to 0.1mm
2) Know or measure the down-tube diameter.
3) Measure the front sprocket to downtube from the center of
the sprocket & calculate to the center of the downtube.
4) Using the number from item (1) above, center the measuring
device on the rear hub center @ some value & find the center of
the rear sprocket.
5) Subtractively arrive at rear chainline & compare.

You can treble check by laying a know good straight edge against
the front sprocket & seeing how close it lines up with the rear. Do
use more than one rotational position for the front sprocket, though.

Notes: the first sprocket I measured (some old suntour 16
tooth for 1/8" chain) looks to be ~2.8mm wide. 4mm of variance
in chainline doesn't seem important, at least anecdotally (NB all
evidence is anecdotal). Bicycles can have rather poor alignment,
use the Sheldon brown method to check that if your numbers are
not coincident with your observations.

On Nov 18, 6:00*pm, Bennett Fischer wrote:

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly?

I just recalled another bit from an old thread on
fixedgeargallery.com:
some leaky-headed assemblers have been known to sneak spacers
under the sprocket.

On Nov 18, 4:00*pm, Bennett Fischer wrote:
I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? *Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?

Cheers,
Bennett Fischer
Brooklyn, NY

What does it look like? Is the chainline parallel to the top tube?
Have a look and go from there, instead of relying only on the
equation.

On 19 nov, 14:26, Qui si parla Campagnolo wrote:
On Nov 18, 4:00*pm, Bennett Fischer wrote:





I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? *Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?

Cheers,
Bennett Fischer
Brooklyn, NY

What does it look like? Is the chainline parallel to the top tube?
Have a look and go from there, instead of relying only on the
equation.- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Excactly. Or better hold a straight ruler with sufficient lenght
against the outside of the chainring towards your sprocket.

Lou

On 19 Nov, 15:04, Lou Holtman wrote:
On 19 nov, 14:26, Qui si parla Campagnolo wrote:



On Nov 18, 4:00*pm, Bennett Fischer wrote:

I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm)..
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? *Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?

Cheers,
Bennett Fischer
Brooklyn, NY

What does it look like? Is the chainline parallel to the top tube?
Have a look and go from there, instead of relying only on the
equation.- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Excactly. Or better hold a straight ruler with sufficient lenght
against the outside of the chainring towards your sprocket.

Lou

What about a bent ruler, or a taut string?

String is simple. Trap it under the chain at the front srocket and
turn the wheel, holding the string against the chainstay until the
string bridges the front sprocket. Keep the string taut and move it
in and out to check the natural chainline for the front sprocket. If
it is within 1/8" its perfect. 1/4" error is acceptable, 1/2" NEEDS
correcting.

On 19 nov, 16:21, thirty-six wrote:
On 19 Nov, 15:04, Lou Holtman wrote:





On 19 nov, 14:26, Qui si parla Campagnolo wrote:

On Nov 18, 4:00*pm, Bennett Fischer wrote:

I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

http://sheldonbrown.com/chainline.html

Any clues as to why the discrepancy between my measurement - 49mm - and
the 42mm spec from Surly? *Am I doing something completely wrong? I
notice there's an approximately 5mm spacer built into the rear sprocket
- is that a common feature to all fixed gear sprockets? Before I change
the bb, should I just flip the rear sprocket to place the spacer on the
outside (and the sprocket teeth closer to the spokes)?

Cheers,
Bennett Fischer
Brooklyn, NY

What does it look like? Is the chainline parallel to the top tube?
Have a look and go from there, instead of relying only on the
equation.- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Excactly. Or better hold a straight ruler with sufficient lenght
against the outside of the chainring towards your sprocket.

Lou

What about a bent ruler,

I said a straight ruler. Geezz.

String is simple.

It is also a good method, but it needs meer eyeballing/
fingerspitzengefuhl.


Lou

On 19 Nov, 15:44, Lou Holtman wrote:
On 19 nov, 16:21, thirty-six wrote:



On 19 Nov, 15:04, Lou Holtman wrote:

On 19 nov, 14:26, Qui si parla Campagnolo wrote:

On Nov 18, 4:00*pm, Bennett Fischer wrote:

I'm a fixed gear newbie, and noticed a curious thing about the chainline
of a fixed gear bike I recently bought, that I hope someone in this
forum can explain to me.

The bike came with a Surly fixed gear hub (120mm spaced out to 130mm).
Now, according to the Surly speccs, this hub gives you a 42mm chainline
in the rear, which should match up pretty well with the 103mm bottom
bracket the bike came with. (Sugino 103mm bb, with a Sugino RD crank.)

However, using Sheldon Brown's chainline formula - front chainline =
distance from center of downtube to front sprocket (44mm); rear
chainline = 1/2 frame spacing, minus the distance from the inner edge of
the fork end to the rear sprocket (49mm) - *I get a difference of 5mm,
which indicates that I'd be better off with a 113mm bottom bracket.

On Nov 19, 9:21*am, thirty-six wrote:

String is simple. *Trap it under the chain at the front srocket and
turn the wheel, holding the string against the chainstay until the
string bridges the front sprocket. *Keep the string taut and move it
in and out to check the natural chainline for the front sprocket. * If
it is within 1/8" its perfect. *1/4" error is acceptable, 1/2" NEEDS
correcting.


"Perfect" is "where you can't measure an error", not 1/8" off.

Quickly adding: I've ridden FG bikes with at least 1/2" "off"
chainlines, and gotten away with it-- low gears down steep hills and
all.

Ignorance is bliss, as long as the chain stays on. I might have gotten
away with that because I was riding a "converted" touring frame with
17-1/4" nominal chainstays, where the middle of the long DO actually
gives 17-1/2", axle to BB centers.

Since those days, having seen a tossed chain or two at the track,
where pista-specific equipment is used and one might think that
chainline disparities would be largely, if not entirely, thus avoided,
I've converted to "perfect" as in "best I can measure", backed up by
sighting along the chain from cog to chainring (nod of agreement with
Lou H), both of which I freely admit worked a lot better before I
started having to wear reading glasses g.

Straightedge, string, laser device-- whatever. Get the thing measured,
and WTH, make it right and then you don't have to worry about it.

Also: straight means your chain and cog/chainwheel teeth will last
longer, and the chain will run quieter, too.

John Dacey's Business Cycles chainline page:

http://www.businesscycles.com/tr-refspec.htm#chainline

Sheldon "The-Good-That-Men-Do-Sometimes-Outlives-Them" Brown's
reference:

http://www.sheldonbrown.com/chainline.html

My current fixed, a Hans Schneider "sprinter" pattern, old school pro
frame, OS steel and steep and tight, has a chainline that was checked
by me and then checked again by a real pro mechanic with many years'
experience. It's quiet, smooth, and has been real fun to ride the last
couple of days since I declared Fixed Gear Season was open at this
address.
--D-y

On 19 Nov, 23:00, " wrote:

"Perfect" is "where you can't measure an error", not 1/8" off.

When the sun's shining 1/8" off is perfect, see you at dusk.

Quickly adding: I've ridden FG bikes with at least 1/2" "off"
chainlines, and gotten away with it-- low gears down steep hills and
all.

But they wear unecessarily.

Ignorance is bliss, as long as the chain stays on. I might have gotten
away with that because I was riding a "converted" touring frame with
17-1/4" nominal chainstays, where the middle of the long DO actually
gives 17-1/2", axle to BB centers.

Test that the chain stays on by spinning the wheel and pressing a cone
spanner against the side of the chain in an attempt to derail it.