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!

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.

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: http://www.ctc.org.uk/desktopdefault.aspx?tabid=3946

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.

"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).

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.

I realize this is an old thread. I’m resurrecting it after Google searching because I’ve had some mix-n-match derailleur issues lately a couple of my old bikes. Until recently I was like the people going into the Co-op mentioned above and trying to mix-n-match shifting components and then wondering why they didn’t work together. I’m gaining a better understanding of how the geometry of the derailleur, and actuation of an indexed shifter work in conjunction. Regarding the actuation ratio that “dddd” mentions above...what exactly does that ratio mean with the 1.7:1, for example? Is that saying that for every 1.7mm of cable pull the derailleur moves 1mm? Or is it vice versa? And WRT the math ‘fietsbob’ suggests...what is the formula? In other words...how do you figure out the distance a certain derailleur will move when used with a certain shifter (of different brand and/or model)? Does spacing of freewheel/cassette gears play into this? Lastly, is there an outlet that might have a listing of various derailleur and shifter ratios where you can find two varying brands/models that might work together?

Dan

This lists pull ratios. Bicycle Cassettes & Drivetrains - Google Sheets

What..?

Never heard of different types of shifter cables.

http://www.yellowjersey.org/Suntour21.pdf

Science Behind the Magic | Drivetrain Compatibility ? Art's SLO Cyclery

https://www.bikeforums.net/bicycle-m...-shifters.html

Shimano rear derailleurs (most models in the 7 to 10 speed era) had an actuation ratio of 1.67 to 1.00. That is on average across the cassette. When SIS was introduced in 1984 with Dura-Ace, Shimano started with an actuation ratio of about 1.8 to 1.00. Suntour followed with Accushift a couple of years later with the same actuation ratio. A useful kludge is for those poor folks that have Dura-Ace 8-speed STI brifters, and want lower gearing than the 7400 rear derailleur can handle, install an Accushift mountain derailleur such as XC Pro or XC LTD etc. I've done it; works great.

And then with Shimano 600 and Light Action in 1986, Shimano changed their derailleur actuation ratio to the more familiar 1.67 to 1.00. This would have made the shifting less finicky to set up, and more resistant to mis-adjustment and friction in the system. It might have also served to pull the rug from underneath Suntour, causing the smaller company all forms of compatibility and customer support misery for years until their untimely demise in the early 90's. But no comment here....

So the answer to you question is that Shimano and Suntour indexing systems are not compatible. They may sort of work for 5 cogs on the cassette, until the cumulative error causes a bunch of mis-shifting and rattling.

Be assured, that as sure as the earth rotates around the sun, someone will come in later indicating that they've got these systems to work great together. However, most of the cycling public is blissfully ignorant of the mechanical aspects of their bikes, and they assume that constant random drivetrain clattering and ghost shifting is 'normal'. But I am gratified so far that nobody (yet) has offered the useless pedantic observation that everything works in friction mode.

I'm reasonably certain there are many things you've never heard of...not just in the bike world.

Not the cables themselves but the end fittings. Those differ and even today Shimano and Campy compatible cables have different size end barrels.

Back in the day new aftermarket derailleur cables came with a different barrel on each end.

If anyone is interested enough to try it my math says...

The new Shimano stuff is really close to the Ergo stuff. The charts show them one point off but I think it’s a rounding error.

That also means the Shimergo trick should work with new ratio Shimano shifters and old ratio Shimano derailleurs.

The vintage Campy has a gain of like 2.2 and should index a 5 speed with older Shimano 10 speed which had the shortest pull

Still mostly do but don't cover everything - like Huret flat ends.

https://www.bikeforums.net/classic-v...le-source.html

Hmm, what I member from 1980ish was that one cable end was standard, the other was for Huret. And I thot the Huret end worked on Sun Tour.

Great minds think alike..... So when Shimano 11-speed (and Tiagra 10-speed) came out with the modified derailleur geometries, I saw that too. Naturally, I was interested in the prospect of running a Shimano rear derailleur with Campy shifters, to have the best of both worlds. This was a couple of years back.

But alas, I've tried it, and it did not work; it overshifted. That is, the new Shimano derailleur actuation ratio is somewhere between the old standard of 1.67 and what Campagnolo requires, at around 1.50. I tried this with a 105 11-speed RD, and a new Tiagra 10-speed unit.

But then giving my head a shake, I realized that what engineer at Shimano (or with any component manufacturer) would have designed a bike component that would be cross-compatible with the 'standard' of one of its competitors? Seriously, that would be a unrecoverable 'career-limiting-move' to be avoided at all costs. So in retrospect, my little experiment was naïve from the start.

The spreadsheets out there all seem to be copies of one another but they all list the new Shimano at 1.4. For a Shimergo it might be close enough, the lower third of the shifts aren't even used and top gear is sloppy, so you can set it perfect in the middle of the cassette and it won't accumulate enough error to matter at 1st and next-to-top

It's beside the point for road bikes but nearly all the mountain bike derailleurs are roughly the same ratio. The really big 1x rear derailleurs are more set apart by their cog capacity than their left-right travel.