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SunTour XC Pro MD front derailleur cross compatibility

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SunTour XC Pro MD front derailleur cross compatibility

Old 09-24-18, 01:59 PM
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masi61
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SunTour XC Pro MD front derailleur cross compatibility

So I’m fixing up a 90’s era Specialized Rockhopper mountain bike with a 3x7 mountain drivetrain. The Specialized branded triple crank has a 42 tooth big chainring. The stock front derailleur is a Shimano and the body has a capital”A” on it. I’m having trouble setting up the front triple index shifting. I noticed that the arc of the cage is not too tight to the big chainring. It got me thinking if I had anything else on hand that might work properly. I remembered that I have a SunTour XC Pro Microdrive front derailleur that fits this frame (34.9/bottom pull).

I was going to give it a try and hope that the cage countours will permit better index shifting across the 42/32/22 range of this crank.

if anyone has tried this with Shimano index systems, I would love it if you could share how well it worked out (or if it does not work I guess I should know the bad news as well).

Last edited by masi61; 09-24-18 at 02:47 PM.
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Old 09-24-18, 02:38 PM
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It's a front derailleur, it doesn't care who makes the shifters or indexing, and it's too bothered about the speed either (at the back, it does at the front) + Suntour only lasted till 7 speed, so it's the same as you currently have.
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Old 09-24-18, 03:37 PM
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Originally Posted by jimc101 View Post
It's a front derailleur, it doesn't care who makes the shifters or indexing, and it's too bothered about the speed either (at the back, it does at the front) + Suntour only lasted till 7 speed, so it's the same as you currently have.
I disagree with the bolded part. Ft indexing does want specific and matching components if you want best function. Otherwise matching cage shape and ring sizes is a want too. Running a ft der made for a 52/53 ring on a 42 ring makes for a lot of open ring/cage gap where it counts. Much like having your guide pulley too far from the rear cog's underside. Andy
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Old 09-24-18, 05:26 PM
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What is missing here is information about the shifter you are using
Crankset, check. Derailleur, check. Shifter? No idea
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Old 09-24-18, 07:34 PM
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Originally Posted by alcjphil View Post
What is missing here is information about the shifter you are using
Crankset, check. Derailleur, check. Shifter? No idea

Shifters are Shimano ST-EF51 Thumbshifters.
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Old 09-24-18, 08:48 PM
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Originally Posted by masi61 View Post
I remembered that I have a SunTour XC Pro Microdrive front derailleur that fits this frame (34.9/bottom pull).

I was going to give it a try and hope that the cage countours will permit better index shifting across the 42/32/22 range of this crank.

if anyone has tried this with Shimano index systems, I would love it if you could share how well it worked out (or if it does not work I guess I should know the bad news as well).
A Suntour XC Pro MD front derailleur was designed for a 42/32/20 so it should work quite fine with this chainset. The original XC Pro front shifter was a non-indexed friction so you may have some issues getting the trim right but should work.

Originally Posted by jimc101 View Post
+ Suntour only lasted till 7 speed, so it's the same as you currently have.
Suntour XC Pro went to 8 speed the last year it was offered. The only change was the cassette and the shifter, the RD was the same. At least that's all I needed to change on my Hei Hei when I upgraded it to 8 speed.
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Old 09-24-18, 09:09 PM
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Originally Posted by masi61 View Post

Shifters are Shimano ST-EF51 Thumbshifters.
FWIW: these are "Rapidfire" shifters, where the shifting is activated by clicking on a lever or trigger under the handlebar. What is normally called a "thumbshifter" is a single lever mounted above the handlebar. It's a piece of bicycle mechanic trivia that might help, particularly with surly old farts like me.

Now, that SunTour XC Pro MD derailleur was never designed with index shifting in mind. The MD means "Microdrive" so the cage is shaped to fit a small (42-tooth) front chainring, so that might work in your favor. The sticky point is the cable-pull-to-cage-movement ratio. This affects everything about the shift and I frankly don't know if it will work. Don't get me wrong... if it were my bike I'd try it out and see what happened. I'd set it up so the cage was centered over the chain when the shifter was in the middle click. If it made the shifts to the big and small chainrings from there, it might work. Test rides and tweaks would certainly be needed- front derailleur adjustment takes a lot of patience and a chunk of understanding what's supposed to happen versus what's really happening.

Your original front derailleur was a Shimano Alivio (the big "A"). It's a sturdy, reliable unit. I'm surprised it didn't fit the chainring... the cage should fit a 42 or 44 tooth chainring well.
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Old 09-24-18, 11:04 PM
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I've often wondered if a Suntour XC front derailleur could be modded to work with Shimano thumb shifters. I think it all comes down to cable pull and attach point on the derailleur. By attach point, I mean routing the cable to the opposite side of the bolt, or even through the bolt, like the old road brakes. It might mean extending arm, which would get pretty creative. But you need to figure out the difference in cable pull between Suntour and Shimano to know if it is possible.

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Old 09-28-18, 11:10 AM
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Like Jeff Wills said, it might be worth trying, to see what happens in terms of the cage travel over the chainrings.

I had a road Cyclone front derailer that worked perfectly with an Alivio Rapidfire triple shifter and SR Apex triple crankset, sothis might also work.

I would set the cable tension with the chain on the middle ring, shifter in it's middle "2" position, with the chain on the largest rear cog, and with the chain then nearly rubbing the front derailer inner cage plate as I pedaled under power.
Then I would see how the shifting was to the other rings, and how the chain cleared the cage without rubbing in the useful gears (not including the smallest chainring and smallest cogs at the same time).

On some front and rear derailers, one can move the cable anchoring to the opposite side of the pinch bolt in order to modify the derailer's travel (actuation ratio).
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