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Refining a prior posted question. 15 speed 1986 Specialized Stumpjumper-Sport has original 5 cog freewheel, 14-30. Original Analog shifters. Biggest cog needs replacement. Individual cogs are no longer available. 5 cogs is plenty for my needs, but 5 cog freewheels seem pretty rare these days. Shimano makes two versions of a 6 cog freewheel that are sold through Walmart.com for $10 - $15. 14-28 (MF-TZ20), or 14-34 (MF-ZH30). Prices seems pretty good. What are the issues involved in transitioning from a 5 cog to one of these 6 coggers?

  • Would it fit in the same axel space?
  • 6 cog chain compatibility spec states 3/32". Does that imply a Shimano Hyperglide chain?
  • If so, would SH chain be compatible with existing front chainwheel cogs?
  • Any derailleur/cog clearance issues switching from 30 to 34 cog?
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I'll add something about my riding style. Dirt trails only, no paved road riding. Favorite ride is a 4 mile out and back, 8 miles total. All uphill, some parts very steep. Elevation gain is 1000 feet. 90% of time in lowest gear (30T rear cog). Seldom use any front chain ring cog other than the smallest. On the return downhill segment seldom do I pedal, too steep, I just brake and coast. –  George_San_Jose Jun 17 at 3:30
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Why do you not just edit the information from your comment into your question? When doing this, it would also be great if you could expand the information into whole sentences which would reading your question a lot easier. –  Benedikt Bauer Jun 17 at 10:14

2 Answers 2

  1. (axle width) Maybe, there is a pretty good chance your bike is already at 130mm O.L.D. spacing. Road bikes would be at 120 or 126mm from that era, but a mid 80's MTB has a reasonable chance of being at 130mm.

http://sheldonbrown.com/gloss_n-o.html#old

2-3. All chains for geared bikes are 3/32 interior dimension, the difference between 7-11 speed is the outside width. You should buy a new chain if you buy a new freewheel, any quality 8spd chain is fine and should work on your current chainwheels.

  1. For an MTB derailler 30 -> 34 should not be a problem. You will likely need to make adjustments to the B screw to make sure the cage clears the largest gear.

http://sheldonbrown.com/gloss_ba-n.html#btension

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Due to (1), I'd probably stick with 5 speeds.

0,4) Presumably by analog, you mean friction shifters. Friction shifters don't care about how many gears you have in the back. You just need a derailleur with sufficient chain wrap to regulate the chain tension appropriately (a wide range derailleur in this case, which most mountain derailleurs are).

1) You need to do some work. To quote Sheldon Brown: "Hubs originally intended for 5-speed freewheels can be converted to use 6- or 7-speed freewheels. You will generally need to add a few spacer washers to the right side of the axle, building it up far enought that the cone locknut can reach the frame dropout through the wider freewheel body. You will need to re-center the axle but probably won't need to replace it with a longer one. You'll also need to re-dish your wheel , and probably re-space your frame. If you're fairly handy, this is not an unreasonable thing to do."

2,3) Just get a modern Shimano-type 7 speed chain, like the SRAM PowerChain 850.

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Given the age of the bike there is a fairly good chance it's already at 130mm hub spacing and can take a 6 speed freewheel without any change. –  Fred the Magic Wonder Dog Jun 17 at 15:11

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