Suntour GPX
 

I have an English bike (NIgel Dean Senator) with a full GPX set, but the rear hub dissolved (left in a leaky shed). It has a seven speed freewheel gear set, and I managed to find a
NOS rear hub - but it's a freehub taking a cassette, so not compatible with the OE gears, and it's also a '6s', at 126mm.

First question is whether that Freehub will take a 7s Suntour Accushift Cassette? I understand the seventh gear is attached to the sixth rather than the splines, is that correct - and therefore it would be ok on a 126mm hub?

Second question is whether there is an alternative to the Suntour 7s cassette that would still work in index format - I read a lot about GPX incompatibility issues?

One post mentioned [MENTION=108582]RobbieTunes[/MENTION] was keen on GPX - is he around at the moment?

You don't state the brand of the new freehub. The SunTour cassette spline pattern is incompatible with Shimano. IIRC, on a 7 speed SunTour cassette, the larger 6 cogs are splined and the outer cog threads onto the freehub body. 7 speed, roads hubs are typically 126mm.

I would have if it wasn’t Suntour. What IS compatible - other Suntour cassettes? Thank u for cog 7 confirmation. Suntour had 130 as well as 126, hoping 126 is enough?

!26mm is enough for any proper 6s cassette hub.

124mm is normally fine as well (if a couple of 1mm-thick washers are removed from the axle assembly), and the clearance will only be missed if some axle-stop or claw-hanger hardware happens to be present on the driveside dropout.

Suntour cassette hubs should not be built into wheels imo, since the cassettes are so hard to find at decent prices. Shimano cassette hubs can be harvested from bent rear wheels and the Shimano cassettes can even be re-spaced using sandpaper on a few of the plastic spacers.

In general Suntour indexing is more finicky than Shimano, so having the shifter, rear derailleur and freewheel/cassette all matching/compatible Suntour is you best bet for acceptable indexing performance. Plenty of folks have successfully mixed/matched Suntour/Shimano, and plenty have come to tears. Matchy-matchy is safest.

I'd say your most trouble-free route would be to use another 126mm freewheel-compatible hub, either re-rimming it with your rim, or replacing the wheel, then using your original 7speed Suntour freewheel.

7 speed AP freewheels work fine but I think the later cassette models worked even nicer until the company went down.
It'll never be crisp-n-quick like Shimano, it's got slow mojo instead.

As you said, Robbie's built at least a couple GPX bikes that he liked but he has a knack for it. :thumb:

Hi - I rather agree about the prices, likely to cost more than the whole bike did. The Shimano idea appeals, although I read a lot about the dangers of incompatability with Shimano - I assume you mean reducing some of the spacers with sandpaper? I mustn't have made it clear the original freewheel set up was 7s, and I was asking whether a 7s freehub cassette would fit on a 126mm '6s' hub.

Hi - That was my first thought, but haven't managed to find a replacement 7s freewheel (Suntour GPX) hub - do you think another make, or another Suntour would work? Having found a GPX freehub I was also thinking that a cassette would in effect be an upgrade due to the better geometry of freehubs meaning less wear over time?

You're right but as mentioned earlier, you need to find the Suntour cassettes and they are rare and very expensive. I held on to an NOS XCPro freehub for years and never saw a cassette for it so I sold it cheap.
edit. Sorry, I'm having a hard time following the combinations you've mentioned. Sure you can put a Shimano freewheel on a Suntour hub but it won't shift properly. You'd probably need mad skills to adjust the spacing to match the Suntour freewheel. Other anecdotes suggest some 8 speed chains help a lot.

I might have found a cassette but even the cheapest are expensive! The freewheel I haven't even found one - except for full kits that are even more expensive again...

The AP freewheels are far more common than the cassettes. They show up once or twice a year on the Sales thread. Take the membership and make a WTB post?

If I'm understanding your question correctly, any freewheel hub spaced to 126mm will work, either with your original freehweel, if it's still functional, or any other _Suntour_ 7spd fw. The indexing performance is all about the shifter/der/fw-cass/chain, it is agnostic about the hub. Suntour hub, Shimano hub, Campy hub, Mavic, Miche, Maillard, etc, etc, as long as the hub spacing is correct and the freewheel threads match, doesn't matter which you use.

If you want to rebuild using your old rim, you'll have to pay attention to spoke length. If you want to re-use your old spokes, you'll need a hub with the near-identical flange/spoke-spacing dimensions.

If you're replacing the entire wheel, the world's yer oyster!

I would be looking for a 7s Suntour Alpha freewheel (has the greek letter alpha on the lock ring).

I find that modern 9s chain is the best choice for Suntour 7s indexed shifting, due to it's narrow outside width helping to prevent the bit of overshift movement at the shift lever from causing the chain to contact the next-larger cog than you are shifting to. Shifting performance with this chain and freewheel can be first-rate even by modern standards, much better than it was back in the day.

I'd go with a standard freewheel hub, SunRace chromed 7-speed freewheel (MFR30 in 13-25, or MFM30 in 13-28, both are equally good), and KMC Z72 or equivalent chain. That's what I've used for a couple of years on my Ironman with GPX group, no problems.

I have Suntour Alpha freewheels -- the original 13-24 and a 13-26 that I hoped might shift better and run more quietly -- but the SunRace work shift more crisply and run more quietly. The original Suntour Alpha freewheels were persnickety with every chain and setup I tried, never completely quiet, never shifting trouble free across the entire range of cogs, in index mode. I could only get consistently quiet running and smooth shifting in friction mode. The SunRace freewheels are compatible with both Suntour Accushift and Shimano STI index shifters, due to the cleverly shaped teeth.

Swapping in freehub wheels with appropriate cassettes works fine too. I've never seen the Accushift freehubs/cassettes and probably wouldn't look for them unless I had the budget, space and time for that setup as a historical oddity.

I worried a bit about longevity with SunRace freewheels. Some of the cheaper carbon steel freewheels are just so-so. But the chromed 7-speed MFR30 ("road") and MFM30 ("mountain bike") have been reliable and excellent values. Ditto the equivalent SunRace chromed cassettes.

BTW, I've been able to squeeze 130 OLD wheels with freehubs/cassettes into the Ironman too, and continued using the GPX Accushift downtube shifters in index mode. It needed little or no tweaking with SunRace cassettes for reliable operation. I didn't cold set the frame, just gently pried it apart to accept the wheel. No problems. I did that for awhile this summer after both of my original freewheel rear rims (Araya CTL-370 and Wolber Alpine Super Champion) cracked. Possibly my fault for homebrewed quickie tuning on the bike to reduce wobbles, without checking spoke tension. I'm going to have a friend who builds wheels make a rear wheel out of one of the remaining front rim. I always liked the looks of those classic box section low profile rims. Moderately aero profile rims are stiffer and seem to hold true longer. I can't get accustomed to the look of true aero rims.

This.
I ran the same freewheel on my GPX Ironman with no problems. A Sach/Malliard freewheel will also run better than the Alpha. There's one in my Accushift Schwinn 594 that works smooth as grease on glass.

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If that rear hub is toast, and it's an actual Suntour freehub/cassette system, well.... punt.

If you've found a GPX rear hub that takes a freewheel spaced at 126, you can put a 6s or 7s freewheel on it, no sweat.
Threads are more universal than slots/splines, and a heck of a lot easier to find mates for.

If you've gone the freewheel route, by all means use a Sunrace or Shimano or Sachs/Maillard.
Anything not "straight-cut" will shift better.

With Suntour indexed, when that chain settles on a cog, you know it.
It's not a mystery. It's not a maybe. It's a fact...."thunk."

I'm running a set of GPX hubs with a Shimano 600-level 6-sp freewheel.
The rest of the bike is Suntour Superbe Pro and it's money.

I have another set of the hubs, but I'm quite sure I promised them to a guy for a decent lunch.

Only the 7s Accushift has it's own proprietary cog spacing, and a 7s Sunrace freewheel might work well enough if a floating top pulley is incorporated.

The spacers between the bigger cogs of a Shimano cassette can be narrowed the necessary .2mm with good accuracy using a sheet of sandpaper with the uniformity of thickness monitored using a digital or dial caliper.

I wouldn't expect the 7s Sachs/Maillard freewheel to work great with 7s Accushift but I haven't actually tried it. The Alpha freewheels famously can transmit pedaling force during shifts if a modern chain is used, their predescessor NW freewheels not so much and I've modified teeth on the latter to eliminate chain "skating" or slippage during shifting.

The Sachs/Maillard freewheel works MUCH better than the Alpha on my Accushift '90 C'dale Black Lightning. I believe the chain is the original SunTour item. Works great.
On my GPX '89 Ironman, I ran the SunRace 13-25 with an original, unmodified RD and shifters and that worked great as well. That rig had a KMC X-series 8 speed chain. Both indexed perfectly under load.
[MENTION=108582]RobbieTunes[/MENTION] understates the sound signature of GPX/Accushift. It's more like 'click...clack...THUNK!".

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On a carbon bike, it would be a sonic boom.

The GPX Accushift is somewhat more subdued with SunRace freewheels. Less klunk!!twang!! from the rear derailleur, compared with the original Suntour Alpha freewheels. It felt and sounded like the squared off teeth hung onto the chain for a split second, enough to stretch the RD spring a bit, then release it like a slingshot. With the SunRace freewheels you only hear the shifter click, while the RD is almost silent.

DA indexed downtube shifters sound like a percussion instrument on the early '90s Trek 5900 OCLV. klack!!bong!! Those fat carbon fiber tubes resonate nicely. So I named it the Noisy Cricket.

Hey OP, there's a very nice 7 speed AP freewheel on the sales thread at a very fair price if you're interested.

Thank you but the GPX 7s freewheel I have - it's the hub I don't have. On the other hand I have a cassette hub (NOS) but not the cassette!

I had a GPX freewheel hub, accushift 7s freewheel, GPX rear derailleur and GPX shifters.

The gpx shifters set to the other indexing mode didnt index nicely with a regular freewheel. Only the accushift mode worked properly. This is one of the reasons I got rid of that bike. I am tired of proprietary systems that restrict gearing and consumable component replacements.

FWIW, my Suntour GPX derailleurs played nicely with a set of Shimano L-422 Light Action friction shifters. I tried that setup for a month this summer, just for giggles, since I had the shifters in a box but couldn't remember why I bought them. I'm guessing my rationale was they'd make a good backup shifter in case the GPX shifters ever broke. Sure nuff, they worked very nicely. And the freewheel was less critical. Didn't matter whether I used the Suntour Alpha or SunRace freewheels. Even a too-wide 130 OLD rear wheel from a newer bike with cassette worked fine.

The GPX group has a reputation for being a bit finicky, which Suntour itself alluded to in its shop guide for setting up the group. But after trial and error I found the biggest improvement was simply switching to the SunRace freewheels I mentioned above, and KMC Z72 or comparable chain. After that all the persnickety minor adjustment problems I used to gripe about vanished.

Suntour itself could have preserved the reputation of that system by supplying freewheels with better shaped teeth, and better chains -- the original Suntour chain was awful, one of the most primitive, poorly made chains I've ever seen. I have to wonder how many people were put off the entire Accushift groups concept when the main problem was probably that terrible chain, and the Alpha freewheels (which could have been improved with a hand file or moto tool, for the daring home craftsman).