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Rear Derailer Alignment



 
 
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  #1  
Old December 26th 16, 04:10 AM posted to rec.bicycles.tech
John B.[_6_]
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Posts: 2,202
Default Rear Derailer Alignment


On a modern road frame, with vertical dropouts, the real derailer
mount is such as the centerline of the top derailer guide pulley is
located slightly forward of the cassette center line and the Shimano
instructions say to adjust the derailer so that the guide pulley is as
close as possible, but not touching, the cassette cogs.

On an older frame, with forward facing dropouts, the derailer mount is
actually forward of the axle by a substantial amount (assuming that
the wheel is mounted at the rear of the "dropout" slot) so that if the
guide pulley is moved as closely as possible to the pulley will be in
front of the cassette cog to the extent that the chain, when exiting
the cod will actually be angled upward over the guide pulley which
will be slightly forward of a 25 tooth cog.

At present I have my rear derailer (Shimano 105), on the older bike,
set so that the derailer body is horizontal and disregarded the
vertical alignment of the guide pulley with the cassette.

Should I attempt to follow the modern Shimano instructions to get the
upper guide pulley as close as possible to the cassette cog and
disregard the angle of the chain/cassette vertical alignment?
--
cheers,

John B.

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  #2  
Old December 26th 16, 03:40 PM posted to rec.bicycles.tech
AMuzi
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Posts: 13,447
Default Rear Derailer Alignment

On 12/25/2016 9:10 PM, John B. wrote:

On a modern road frame, with vertical dropouts, the real derailer
mount is such as the centerline of the top derailer guide pulley is
located slightly forward of the cassette center line and the Shimano
instructions say to adjust the derailer so that the guide pulley is as
close as possible, but not touching, the cassette cogs.

On an older frame, with forward facing dropouts, the derailer mount is
actually forward of the axle by a substantial amount (assuming that
the wheel is mounted at the rear of the "dropout" slot) so that if the
guide pulley is moved as closely as possible to the pulley will be in
front of the cassette cog to the extent that the chain, when exiting
the cod will actually be angled upward over the guide pulley which
will be slightly forward of a 25 tooth cog.

At present I have my rear derailer (Shimano 105), on the older bike,
set so that the derailer body is horizontal and disregarded the
vertical alignment of the guide pulley with the cassette.

Should I attempt to follow the modern Shimano instructions to get the
upper guide pulley as close as possible to the cassette cog and
disregard the angle of the chain/cassette vertical alignment?


There's a continuum of performance (not yes/no) and you're
right to check that. Try to get as close to these suggested
dimensions as possible for optimum shift response:

http://www.yellowjersey.org/sisend.html

Even moving the axle in relation to the changer pivot can
make a big difference.


--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971


  #3  
Old December 27th 16, 02:50 AM posted to rec.bicycles.tech
John B.[_6_]
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Posts: 2,202
Default Rear Derailer Alignment

On Mon, 26 Dec 2016 08:40:12 -0600, AMuzi wrote:

On 12/25/2016 9:10 PM, John B. wrote:

On a modern road frame, with vertical dropouts, the real derailer
mount is such as the centerline of the top derailer guide pulley is
located slightly forward of the cassette center line and the Shimano
instructions say to adjust the derailer so that the guide pulley is as
close as possible, but not touching, the cassette cogs.

On an older frame, with forward facing dropouts, the derailer mount is
actually forward of the axle by a substantial amount (assuming that
the wheel is mounted at the rear of the "dropout" slot) so that if the
guide pulley is moved as closely as possible to the pulley will be in
front of the cassette cog to the extent that the chain, when exiting
the cod will actually be angled upward over the guide pulley which
will be slightly forward of a 25 tooth cog.

At present I have my rear derailer (Shimano 105), on the older bike,
set so that the derailer body is horizontal and disregarded the
vertical alignment of the guide pulley with the cassette.

Should I attempt to follow the modern Shimano instructions to get the
upper guide pulley as close as possible to the cassette cog and
disregard the angle of the chain/cassette vertical alignment?


There's a continuum of performance (not yes/no) and you're
right to check that. Try to get as close to these suggested
dimensions as possible for optimum shift response:

http://www.yellowjersey.org/sisend.html

Even moving the axle in relation to the changer pivot can
make a big difference.


Thanks for the answer.

I had originally thought of removing the existing rear drop outs and
replacing them with the more modern vertical drop outs but the bike
went together and worked with the originals. Now, of course, with the
bike finished I have the time to sit here and speculate on what might
have been :-)

As this is a "Bangkok Bike" and is ridden in flat terrain with minimal
shifting I shall probably end up leaving it as original built :-)
--
cheers,

John B.

  #4  
Old December 27th 16, 05:15 PM posted to rec.bicycles.tech
DATAKOLL MARINE RESEARCH
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Posts: 2,011
Default Rear Derailer Alignment

JB was wondering abt getting chin on cogs ..... if not using small cog move pulley inboard for a better chainline run.

eyeball the system with string n shims on tubing ..... like blockwork
 




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