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Old SIS vs. New SIS - 6/7sp Compatibility Issues

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Old SIS vs. New SIS - 6/7sp Compatibility Issues

Old 08-25-10, 10:59 AM
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mixtemaniac
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Old SIS vs. New SIS - 6/7sp Compatibility Issues

Hi all,

I'm just about finished with a "modern commuter conversion" based around a '74 (could be '75) Schwinn Le Tour step-through. Rooting around in the parts bins of a local bike co-op, I found a nice pair of top-mount (thumb) SIS shifters. Based on my research, it seems they are 6sp low-end Tourney (with friction/sis switcher) shifters from the early 90's. Model: SL-MY20

From my parts bin, I have added a modern rear derailler (Shimano Acera RD-M340 - mega range compatible) and tried two different 7sp freewheels. A Shimano Mega Range with bail-out granny gear (14-34T MF-TZ07) and a Shimano 14-28T 7sp. Both are virtually identical except for the bail-out 34T cog on the mega range. These parts are in nearly brand-new condition. The chain is a new SRAM PC-830.

I had thought that these older 6sp SIS thumb shifter would shift a 7sp freewheel just fine, but it seems otherwise. I can't get either freewheel to shift properly in SIS mode. I spent about 2hrs so far trying to fine tune the rear derailler cable tension to no avail. At best I can get about 90% accuracy.

The bike will be used for a daily commute of roughly 8-10miles roundtrip with one largish hill (hence the granny gear).

I think I've narrowed it down to the following options:
-keep the shifters and simply replace the freewheel with a 6sp 14-28T (which I already have). I'm disappointed to lose the granny gear though.
-keep granny gear mega range freewheel and replace shifters with some modern SRAM 7sp twist-grip shifters.

Have you priced 7sp thumb or bar end shifters on ebay lately!? Highway robbery!

Any input would be greatly appreciated.

Obligatory photos:






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Old 08-25-10, 11:24 AM
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I like what you're doing with the bike; looks like a nice commuter. You didn't mention friction as a possible option. One flick of a switch and your problems are solved, unless I missed something.
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Old 08-25-10, 11:27 AM
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Thanks! Yes, friction does work fairly well. However, the preference is for index. The rider using this bike is currently using friction and isn't too keen on it
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Old 08-25-10, 12:07 PM
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No way a 6 speed shifter will index with a 7 speed freewheel. You need a 7 speed shifter, or 6 speed freewheel.

You may be able to lock out the small cog with the limit screw, and run the 7 speed freewheel as a 6, though you'd lose the 14t.
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Old 08-25-10, 12:18 PM
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+1 You are trying to use a six speed index shifter for seven speed? Huh? Spacing is different, etc., etc. Either install a six speed freewheel, or buy nice seven speed Shimano Tourney indexed shifter. Niagara sells a set for $12, including cables and housings. So for about the cost of a six speed freewheel, you could have nice seven speed shifters instead. I personally would go seven speed.

Last edited by wrk101; 08-25-10 at 12:37 PM.
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Old 08-25-10, 12:49 PM
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What they said.
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I have unfinished business.

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Old 08-25-10, 01:36 PM
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Even if you are counting on the 'phantom shift' to get from 6 to 7 speeds the cog spacing of the freewheels isn't the same. You should be able to see the difference by looking at the freewheels side by side. You could try to rig something up using 6 of the 7 speed sprockets with the 6 speed spacers on the 6 speed body but no guarantees.
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Old 08-25-10, 03:36 PM
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Originally Posted by Old Fat Guy View Post
No way a 6 speed shifter will index with a 7 speed freewheel. You need a 7 speed shifter, or 6 speed freewheel.

You may be able to lock out the small cog with the limit screw, and run the 7 speed freewheel as a 6, though you'd lose the 14t.
I tried this actually, but couldn't get it accurate enough for my taste.
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Old 08-25-10, 03:41 PM
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Originally Posted by wrk101 View Post
+1 You are trying to use a six speed index shifter for seven speed? Huh? Spacing is different, etc., etc. Either install a six speed freewheel, or buy nice seven speed Shimano Tourney indexed shifter. Niagara sells a set for $12, including cables and housings. So for about the cost of a six speed freewheel, you could have nice seven speed shifters instead. I personally would go seven speed.
Are you referring to these: http://www.niagaracycle.com/product_...ducts_id=15889 ?
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Old 08-25-10, 03:57 PM
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Originally Posted by mixtemaniac View Post
I tried this actually, but couldn't get it accurate enough for my taste.
Which is why I used the qualifier 'may'.
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