John:
I'll attempt to add to the valuable information Dave and Nick have shared...
When thinking about compatibility with shifters, RDs, and cassettes; keep in
mind the RD does whatever the shifter (index, ratchet, or friction) tells it to
do, hence the "dummy" comment. So you can for the most part eliminate the RD
from your compatibility question.
Your Shimano 8 speed shifter works with your Shimano 7 speed cassette because
the "indexing" in the shifter is matched with the spacing between the cassette
cogs; hence one click, one shift. So when you click a shift the shifter pulls
the cable an amount that causes the RD to move (inboard or outboard) an amount
equivalent to the distance between the cogs of the cassette. On a Shimano 8
speed cassette the distance between cogs is 4.8 mm, so each time you shift with
a Shimano 8 speed indexed shifter the RD is going to move 4.8 mm inboard or
outboard. Shimano 9 speed cassettes have the cogs spaced 4.34 mm apart. You can
see the problem if you were to use, for example, a Shimano 9 speed indexed
shifter with an 8 speed cassette (one shift would move RD 4.34 mm, not the
needed 4.8 mm between 8 speed cassette cogs) Your 8 speed shifter works on the
7 speed cassette because a limit screw on the RD doesn't allow it to move the
full distance into the 8th position and the distance between cogs on a 7 speed
cassette are the same as 8 speed: 4.8 mm. Indexing (space between cogs) is the
same between Shimano 7 and 8 speed, so they play well together. If you were to
use a cassette from a different manufacturer in which the distance between cogs
was different from that of a Shimano cassette, then you'd likely run into a
compatibility issue with your 8 speed shifter/non-Shimano cassette (assuming
cog spacing was different from an 8 speed Shimano cassette).
One can almost look at a friction shifter as allowing infinite adjustment.
Riders that are good with them almost develop an indexing in their head/hand
feel then fine tune after a shift. No indexing to a friction shifter so you can
run a friction shifter with any make/speed cassette. Just have to find the
sweet spot yourself.
The ratchets move the shift cable in little bits (and the RD in little bits) I
believe (I've never ran them. Someone, please, correct me if I'm wrong). So
each click moves the shift cable a certain amount that in turn moves the RD a
certain amount. You'd have to do some research to find out how much the RD
moves with each click of your 1975s and then do some math. You know, at least
with your Shimano 7 speed cassette, the cogs are spaced 4.8 mm apart. How much
does the 1975 shifter move the RD each click? For perfect shifting on the 7
speed (or 8) cassette, the clicks (one, two, or three)/RD movement would have
to add up to intervals of 4.8 mm. I doubt that's going to be the case, but
fortunately there's a little wiggle room short of perfect that will obtain
satisfactory shifting. At least there is more wiggle room with 4.8 mm spacing
than there is with 4.34 mm (Shimano 9 speed casette).I guess one could look at
ratchet shifting as being somewhere between indexing (one click, one shift) and
friction (infinite adjustment) shifting.
Your research task: find out how much RD moves with each ratchet of your 1975s.
Then you'll be able to better assess compatibility with different cassettes of
any maker/speed, as long as you get the respective cog spacing. For the
compatibility issue you are inquiring about, the RD isn't really part of the
puzzle, so eliminate it.
Sheldon Brown's website has that level of trivia: cog spacing, indexing values.
And hopefully it has ratcheting values. You might find your answer there.
Good luck! And, please, report back!
Scott in Big Sky Country
On Sunday, November 27, 2022 at 05:21:38 PM MST, 'John Hawrylak' via RBW
Owners Bunch <rbw-owners-bunch@googlegroups.com> wrote:
IF I use my 1975Suntour Bar End Ratcheted shifters with a modern Shimano RD
(Deroe M531) and a Shimano7 speed HG cassette, and modern Shimano shifter
cables/housings, WILL the RD shift each gear going from thesmallest cog to the
largest cog, WITHOUT the need to ‘trim’ the RD after eachshift, especially when
moving up in the large cogs????
Secondary question: If the SunTours would work without trimming, do they
enough pull for a 7 speed cassette???
I have been using 8speed Shimano Ultegra bar ends (BS-64) with a HG-50 7 speed
cassette and likeit, 1 click, 1 shift, no need to trim. Been curious about
going back to using the SunTours rachets if themodern RD eliminates the need to
“trim”. Would like to know if others have tried it and what the results are
withrespect to trimming.
Note, NOT looking for a debate on Index vs Friction. I think index is
better, but I have troubleburying those nice SunTour bar ends & maybe I can use
them if the RD eliminates trimming. YMMV and that’s great, diversity is good.
John Hawrylak
Woodstown NJ
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