Old Suntour 6 speed - could it be indexed ?

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evansthecrank
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Joined: 19 Mar 2009, 6:08pm

Old Suntour 6 speed - could it be indexed ?

Postby evansthecrank » 13 Jul 2018, 10:11pm

I’m helping a friend sort out a bike to change from drop bars to flats.
The rear dropout spacing is 130mm, rear hub is Campag Record with lube clip, and the freewheel is a 6 speed (13 to 30) with Suntour AP engraved on the face and 4 gap spline. The rear changer is Suntour XCE.
Are there any modern flat bar mount thumb-style shifters that will index / work with this setup? If we have to go to friction, any alternatives?

cheers, EtC

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gaz
Posts: 12837
Joined: 9 Mar 2007, 12:09pm
Location: Kent

Re: Old Suntour 6 speed - could it be indexed ?

Postby gaz » 13 Jul 2018, 11:24pm

You are quite likely to find some Suntour Accushift thumbshifters on ebay.

IME matching with Shimano 8 speed shifters gives acceptable ather than smooth results.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Old Suntour 6 speed - could it be indexed ?

Postby Brucey » 14 Jul 2018, 8:10am

everything you ever wanted to know about Sun Tour indexing (almost...) to be found here

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

IME the shift ratio used by suntour accushift is almost identical to that used by shimano for their 7400 series Dura-Ace models (about 1.8:1, vs the nominal 1.7:1 (actually nearer 1.67:1) of other shimano systems). I have used suntour superbe pro levers (in 7s mode) and XC9000 thumbies with 7400 RDs on several different 7s systems (both with suntour freewheels and shimano cassettes). For these 7s systems the cable pull per click is almost identical to shimano 9s. I think this may work better than the reverse combination (shimano shifters and suntour RD) because

a) the shimano RDs have float in the guide pulley and suntour ones pretty much don't, and thus expose any error in RD positioning rather more. and
b) the suntour shifters still have some degree of 'overshift' built in so that a slightly baulky downshift can be made to go in.

IIRC the combination of suntour 7s shifters, superbe pro RD, and shimano 7s cassette worked less well than with a 7400 RD and the float (vs spacing details) issue is probably at the heart of it.

Like Gaz says, if you use a (non DA) shimano 8s (or 7s) shifter with an accushift RD, it will index on 6s reasonably well.

AIUI the shift ratio of accushift RDs is the same regardless of whether they were originally part of a 6s 7s or 8s system, thus three-mode shifters (such as the Superbe Pro) pull different amounts of cable per click depending on whether they are set to 6s or 7s shifting. Note also that because the shift ratio of the mech varies through the stroke, there is a tolerance on the offset of the RD, i.e. if there is a big gap between the smallest sprocket and the frame, and/or a thicker dropout than normal, the system won't work properly.

In theory the sprocket spacing in suntour index systems varies slightly across the cluster, (unlike/differently to shimano 6,7,8s systems) but I have yet to find a document that explains exactly how. I think the smaller sprockets are spaced slightly further apart. I would expect that a shimano cassette could be respaced to exactly match the suntour spacing, if required.

[edit; Sheldon says 3x 2.8 mm spacers for the low gear (large) sprockets, and 3x 3.0mm spacers for the high gear (small) sprockets, in a Suntour 7s setup, suggesting 4.8mm pitch for the low gears and 5.0mm for the high gears, since the sprockets are 2.0mm thickness.]

There is also this list of suntour accushift parts

http://velobase.com/ViewShiftSystem.aspx?ID=28186739-7c70-4804-99ec-37c8f8e48704

which includes a lot of later models (such as XCE) that are not included in the pdf above. Note that (as indicated in the pdf) alpha-3000 parts are not considered compatible with others in the accushift line-up; there may be later models that share the same characteristic too.

Note also that the shift ratio of other (even older) suntour RDs varied with model (estimated 1.7 to about 2.0 by some) and is not meant to be compatible with any index system. Nonetheless folk have occasionally made mongrel systems work well enough.

cheers
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