Calling All Suntour XC Pro Experts

[Kirke Campbell]

Hello,

I'd like to do something a little different on a Susie build I am working on and am considering going with a Suntour XC Pro rear derailleur. Specifically, it's the XP00-GXB model, which is the long cage version. I'd be running this with Silver friction shifters and a Shimano 9 speed cassette. I'm curious what the biggest rear cog I can use is? This resource claims it kinda depends on the length of the hanger:

https://www.disraeligears.co.uk/site/suntour_xc_pro_gx_derailleur_xp00-gxb.html

How does one measure the length of the derailleur hanger? From the center of the dropout to the center of the derailleur hanger hole?

Any real use experience running an XC Pro derailleur with a 34t cog in the rear?

Also, stated capacity above is 40t - anyone out there pushing that? 

Thanks!

-k.   

[iamkeith]

I'm still using a couple of those derailleurs, but only with 32 t cogs.  There's a ton of room to the pully though, so I don't doubt that 34 t would work.  These are early 90s mountain bikes, but I don't know if that suggests long or short hanger.  I can measure if you need.

I'd say just try it.  You can always get an extender if needed.  Like this one:

https://www.wolftoothcomponents.com/products/roadlink?variant=3919254981&country=US&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&glCountry=US&gad_source=1&gclid=CjwKCAjwktO_BhBrEiwAV70jXoSX3MljQVBuJU-LwZiVgNC99JdWB2Ksfb_moIZOJ3VwbUw-LEqejBoCnq4QAvD_BwE

[Garth]

Yes, the hanger is measured c-c. A velobase comment here states they've used w/34t cog no problem. 

https://velobase.com/ViewComponent.aspx?ID=5446F34B-FD3D-40A2-8C95-1E6AE5B573A1

As for capacity, pushing limits on many bike things is okay, but not in a RD for total capacity. The way to work with it is if and only if you're aware enough to never use a big/big combo. While I would say that's common sense, not everyone understands that. The mfr capacities assume a user has no such discernment.. So if your big ring is say 46 teeth and the cassette ends with say 28-34 teeth, you use the 28 as your capacities largest cog measuring point. Just don't forget that or you're SOL ! Even then, it likely wouldn't go into that cog easily, which is a warning to heed. On say a nine speed cassette, I never use the largest two cogs anyways as the friction from the angle is not something I can live with but for a brief time. On my seven speed FW's I can use the six of the cogs in the big ring of a double.

[Eric Daume]

If I ever have capacity concerns, I always err so I can still run big/big. A sloppy chain in small/small(er) isn’t a catastrophic failure mode. 

But I’m mostly 1x these days, so it’s a moot point for me.

Eric

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[Ben Miller]

Not to steal Kirke's thread, but I'm also thinking about a vintage Suntour RD. So... while I've got you all here: Thoughts on the Suntour XCD 4050 Accushift vs the XC Pro RD? By my eye they look pretty similar, but with RD's looks can be deceiving. Is there a significant difference between the two?

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[Patrick Moore]

Or you can simply add just enough chain for the big/big and let the chain droop on the small/small or even the small ring and 2-3-4 smaller cogs, depending on your setup. I’ve done that many times without problem since I am even less likely to shift to the small/small with a granny ring (triple rf subcompact double) than to the big/big.

On Tue, Apr 8, 2025 at 6:08 PM Garth <gart...@gmail.com> wrote:

… As for capacity, pushing limits on many bike things is okay, but not in a RD for total capacity. The way to work with it is if and only if you’re aware enough to never use a big/big combo. 

[Kirke Campbell]

Ok, great insight here so far. What I was going to run is 26-38-46 up front and an 11-34 in the rear, which is a 43t capacity - pushing the stated 40t. Sounds like I can probably do it but just avoid the big/big combo.

I was also considering removing the 46 altogether and putting on a bash guard - not sure I'd ever really use the gears that big ring affords. I ride slow! That would solve the capacity issue there. 

@Ben Miller - Can't speak to the difference between these derailleurs but I feel like usually it's a matter of fit and finish. I'm sure the XCD 4050 will function perfectly, but maybe just a touch heavier, not quite as nicely polished, etc. 

-k. 

[ascpgh]

Not an expert (Paul Brodek rest in peace) but I have a soft spot for the sunTour derailleurs especially the XC pro. 

The hanger specs got tight with indexing (volumes to read there amongst all the players) and the rear axles no longer had slots.

Here you can see that the axle position of these Suntour dropouts are nearly vertical, creating a fixed dimension from axle center to the center of the threaded derailleur mounting hole. That's the specification and Suntour even produced dropouts in several axle-dropout dimensions, all being within their derailleurs' ability to adjust to their satisfaction. (Johnny Coast built my rando frame with the dropout on the right with two mounting points). With a 30t-46t and 34t cassette I'm nowhere near your 40t range question though.

I have used a 34t 8-spd cassette with XC-Pro long cage on my Rambouillet without difficulty butI had the TA Zephyr crank configured as a triple 26t-36t-46t, then later went wide double 34t-44t. The B-adjustment and chain length will help as will your commitment not to use big-big combination. 

Your chainring count and sizes additionally affect this. Makers were (still are) utterly dictatorial in the gearing combination specs so their indexing works at peak design potential. I can cope with microseconds slower response in shifting in exchange for gearing that better suits my riding needs and haven't had a problem.

I added an extension tab when I wanted to run another derailleur on my Rambouillet that had 28t max and truly wouldn't clear the 34t cog. It's a friction shift drivetrain so, again, I got gearing I wanted using the french Mavic derailleur I wanted so I was OK if some shifting wasn't spec sheet precise. 

Andy. Cheatham

Pittsburgh

On Tuesday, April 8, 2025 at 7:01:54 PM UTC-4 kirkebc...@gmail.com wrote:

[Kirke Campbell]

Andy - I love those Suntour dropouts and have often thought about sourcing a pair to use if I ever get a custom frame built. And if you were rocking a 34t cassette and a 26-36-46t in the front with a long cage XC Pro, then I feel like I can probably pull off what I'm planning as well. 

-k.

[Kirke Campbell]

For the record, I'm measuring a 30mm derailleur hanger length on the Susie Longbolts. My 1994 Bridgestone MB3 has a 28mm derailleur hanger. 

-k. 

[ascpgh]

Hanger specs were so short, comparatively,  BITD. The XC-Pro's specs were 24mm for 28t, 26mm for 30t and  28+mm for 32t. but no hanger spec guidance for a 34t? Disraeli Gears listed Sutherland's 6th Ed. as the source for the XC-Pro hanger specs. That big green ring binder was a bike shop standard but even it failed to extend the list to include what would be necessary to work with a 34t cassette and that volume was the problem solver of the day.

Reduced diameter drivetrains were about to come at the peak XC-Pro era and 34t cassettes were probably out of fashion with Suntour's MicroDrive and Shimano's HyperDrive C about to release. 

Big headaches ensued as shops began realizing the cost of the inventory to have spare chainrings and cassettes for all the permutations of OE spec and local preferences on hand when this new paradigm landed. The internet wasn't kicking yet and most options had to be special ordered given the lack of confidence regarding which items to physically stock with no use history. 

Piecing together our bicycle rat rod drivetrains is a bit of try it and see. Sort of the fun of it. 

Andy Cheatham

Pittsburgh

[Joe Bernard]

You are correct, Andy, 34t cassettes were a rare commodity late-'80s/early-'90s, especially during the MicroDrive/Compact-C era. It's hard to find examples of XC Pro running that cog cuz nobody was doing it. 

Joe Bernard 

Driving a bus somewhere in Lake County CA