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Old 07-11-12, 01:47 PM   #1
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Derailleur Geometry

I'm a novice mechanic (been doing it for about a year now at a privately owned shop in Seal Beach, CA) and have some questions about derailleur geometry. Tried doing most of the research myself before posting here but still have a couple questions.

We received a Trek Multitrack 750 in for a tuneup. Looks like it's from the late 80's or early 90's, with SunTour components on it. The cable for the rear derailleur was rusted and corroded which made the shifting do funky things. Without thinking twice about it, I snipped the cable at the derailleur and removed it. Only to figure out that it's a cable that is no longer made. I replaced the X-Press SunTour shifter with a 7 speed Shimano trigger shifter/brake lever combo because I felt that was my only option. After everything was installed, the cable tension was super funky when trying to adjust the shifting. Another mechanic suggested that I replace the derailleur and see what happens. Sure enough, I replaced it with a Shimano Altus derailleur and it worked (shifted correctly).

To my dismay I realized I had done this once before. The SunTour derailleur didn't work with a Shimano shifter. After I screwed up this 2nd time, I did some research on rear derailleur geometry and actuation ratios. What I concluded is that Shimano derailleurs move 2mm for every 1mm of cable movement and SRAM derailleurs move 1mm for every 1mm of cable pull. So my question is, are SunTour derailleurs supposed to work with new Shimano shifters? If not, then what actuation ratio do the old SunTour components have? If they are supposed to work, and I'm just a horrible mechanic, what am I doing wrong? I saw on Sheldon Browns website that there are different ways to route the cable underneath the bolt on the rear derailleur. Could I have been doing that wrong? I thought SunTour/Shimano had some sort of compatibility? But maybe that's with the freewheels?

Anyway, any information on derailleur geometry, actuation ratios, and SunTour/Shimano compatability would be GREAT. Thanks!
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Old 07-11-12, 01:53 PM   #2
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Measure and do the calculations, It will be entertaining to do.

see classical mechanics of leverage.. pivots are fulcrums.
the lengths involved are where the differences show up.
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Old 07-11-12, 02:49 PM   #3
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Current Shimano rear actuation ratio is 1.7:1, and the pre-1997 Dura Ace (and also Suntour, iir) was 1.9:1.

Refer to this data page:

Perhaps the "hubbub" modification will rectify your Suntour derailer?

Also, what was special about a Suntour shifter cable? (??????) You gotta be kidding!

Suntour had unique, asymmetrical sprocket spacing for their 7-speed freewheels, which was later changed to a newer "plug-and-play" (Shimano) standard during Suntour's last couple of years in the early 1990's.
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Old 05-05-14, 01:28 AM   #4
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My small run of experiments leads me to believe Suntour and Shimano actuation ratios must be very close or the same.

I've run a Suntour Superbe Pro rear derailleur (84ish) with indexed Shimano shifters and 7-speed Shimano cassettes for 8 years with no issue. I've used indexed XT m730 thumbies and XT m739 rapid fires.

I would be very interested to know more about Suntour's actuation ratio as I have a lot of old Suntour stuff and I love it - except for the shifters and cassettes.

Last edited by philg; 05-05-14 at 01:36 AM.
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Old 05-05-14, 04:23 AM   #5
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"Only to figure out that it's a cable that is no longer made. "-aaronmichael

Just an FYI. You can get replacement cables for Suntour Express shifters (the end looks like a smaller version of a mountain brake cable). Jagwire makes them and are available to most shops through QBP (Quality Bike Parts-a big national parts supplier).

Last edited by bikejunkie; 05-05-14 at 04:24 AM. Reason: additional text
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Old 05-05-14, 11:20 AM   #6
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I hate mixing old Suntour with modern Shimano 7-8 speed indexing. It sometimes can be "ok" but usually just ends up being an exercise in frustration. To me the hassles are not worth the experimentation unless you are doing it for a personal bike and don't mind taking the time to figure out a mounting method for the cable that allows the suntour derailleur to actuate properly with the Shimano index shifter.

People come into our bike co-op and try to use nice old Suntour derailleurs like Arx ones with their modern Shimano shifters and complain that they can get it to shift nicely at one end of the cassette but not the other. Easier just to explain to them that old pre-index Suntour stuff isn't compatible with modern Shimano index shifting.

I keep nice Suntour derailleurs for use with friction shifting bikes. THey are very nice derailleurs after all.
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