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COG CONFUSION? 7/8/9 speeds - By Gavin Donahue
Bike Friday® has used a variety of industry cassette standards.
First there was 7speed. This standard was expanded to
8 speeds by using a wider cassette drive body and adding
the 8th cog. The chain width and shifter indexing spacing
stayed the same, but one more shift point was added to
the shifters. The component makers then developed narrower
cogs and chains so that 9 cogs could fit on the 8-speed
drive body. This is termed 8/9 speed spacing. The narrower
9 speed chain works well on both, but the 7/8 speed chain
will not work on the 9 speed cogs, as it is too wide.
Additionally, the 9 speed cassette spacing requires the
narrower 9 speed shifter indexing spacing. A persistent
point of confusion stems from our practice of using the
popular 9 speed components on the Sachs/SRAM 3x7 hub.
This hub has a 7speed drive body. We have produced many
bikes with standard 7 speed components on the 3x7 hub.
The market trend to 9 speed components has, however, created
a better selection of high quality 9 speed shifters. By
removing one of the cogs from a 9-speed cassette, an 8-speed
cassette is created with 9speed spacing that allows the
use of the desirable 9 speed components. This modified
cassette is equivalent to the width of the older standard
7 speed cassettes, and therefore, fits the drive body
of the 3x7 hub. While having 8 cogs, this is not traditional
7/8 spacing and will not index correctly unless 9 speed
shifters are used. This creates an extra 'ghost shift'
at the 9th shift point that is absorbed by the limit stop
in the derailleur. This practice has changed with the
availability of the new SRAM 3x9 hubs. These hubs have
the wider 8/9 speed drive body. Matching 8 or 9 speed
shifters are needed, depending on which spacing standard
is used. It should be possible to install a 7-speed cassette
by using a spacer to replace the missing 8th cog. This
requires the 7/8 indexing standard. The older 3x7 hubs
are no longer available.
Additional confusion stems from the shifter and derailleur
and cog compatibility issues between different manufacturers:
Campagnolo has a different cog spline pattern and different
cog spacing than Shimano and SRAM, which are the same.
This prevents Campy cogs from installing on the more common
Shimano/ SRAM pattern drive bodies. There are after-market
Campy cog spacers available, that will space Shimano cogs
to the Campy indexing standard. In this case, Campy shifters
and derailleurs must be used. SRAM makes a series of components
that are Shimano compatible. These have matched cable
pull geometry to the Shimano standards and allow mixing
and matching of shifters and derailleurs. SRAM also makes
shifters and derailleurs to their own standard, which
will not index properly if intermixed with the Shimano
standard components.
I hope this clarifies some of the issues with the ever-changing
world of bicycle chain drives. For the technically minded,
the following is a more detailed explanation that expands
on the preceding general information:
7 & 8-speed spacing are close but not the same. 7 speed
spacing is 5mm, 8 is 4.8mm. (Shim 9 is 4.34 and Campy
10 is 4.12. But Campy 8 is 5.0mm.) With the full shift
range, some combinations will shift either 7 or 8 speed
spacing for 7 cogs, where the derailleur is accommodating
a potential difference of 1.2mm over the 7-speed cassette
width. When set up correctly, about half of the distance
(0.6mm) is accommodated at each end; and since this results
in an over shift, it is limited at each end by the derailleur
limit screws.
Gavin Donahue.
Bike Friday® Customer Service Manager,
emergency phone (541) 521-2213
[email protected]
http://support.bikefriday.com |
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