Für die Englischkenner:
Shimano uses 4.8mm spacing between cog centers for 8
speed (1.8mm cogs and 3.0mm spacers) and 5.1mm spacing for 7 speed
(1.9mm cogs and 3.2mm spacers). Suntour used uneven cog spacing for
both 7 and 8 speed. For the 8 speed they used 5.2mm between the 1st,
2nd and 3rd cogs (1.9mm cogs and 3.3mm spacers), 4.9mm between 3rd, 4th
and 5th cogs (3.0mm spacers), and 4.7mm between 5th, 6th, 7th and 8th
cogs (2.8mm spacers). The average spacing between all eight cogs is
4.9mm. The early shifting nature of the
Shimano Hyper-Glide cogs
should negate small variations between two cogs (especially if you
replace the standard Suntour top derailleur pulley with a floating
pulley), but the tolerance build-up across the entire cassette may make
the shifting poor at the extreme ends.
Here's my suggestion: If you replace the top two or three spacers
on a
shimano 8 speed cassette with 7 speed spacers, you should be able
to approximate the Suntour spacing pattern. Similarly, you could
replace the bottom 3 or 4 spacers on a
Shimano 7 speed cassette with 8
speed spacers for a similar effect, but in this case you may not be
able to tighten down the lock ring sufficiently because the cassette
will then be almost a millimeter narrower (Adding the 7 speed spacers
to the 8 speed cassette will make it less than 1mm wider, so you should
have enough threading in the lockring and cassette body to tighten it
down properly).
Just to complete the technical information, the 7/8 speed Suntour
shifters are actually just 8 speed shifters - for use on a seven speed
cassette, you just don't use the last position (spacing on the Suntour
7 and 8 speed cassettes was almost identical, having 5.2mm between 1st,
2nd & 3rd cogs, and 4.7mm on the rest). The 6/7 speed shifters
actually have two ball detent cams, because the standard six speed
spacing was closer to 5.5mm average (rather than the 4.9mm average for
7/8 speed). Of course, both the 6/7 and 7/8 speed shifters can be used
in friction mode for compatibility with any cassette or freewheel, with
the 6/7 speed shifters having a nice ratcheting action (similar to the
front shifter in both sets).
