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8 speed with 7 speed index

Old 06-11-07, 05:15 PM
  #1  
bfloyd
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8 speed with 7 speed index

I found a great blowout deal at Nashbar on a set of 7 speed downtube shifters so I thought ok, go a little retro. I'm using these on one of my bikes that has an 8 speed rear cluster. I am setting the rear shifter to work on gears 2 through 8, but they don't seem to line up with all the gears quite correctly. It seems to line up well in the third cog and the fifth gog but then the fourth cog seems to skip just a bit. Is there a spacing difference between 7 and 8 speed index shifters? Can I not run the 7 speed with the 8 rear cog or should it be able to be adjusted to work? Thanks.
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Old 06-11-07, 05:55 PM
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chunts
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EDIT: scratch that link, its the other way around. but yeah, they way I understand it for indexing, you have a given length of cable pull, and your shifter has 7 stops and your cog has 8 gears. think about two distances of the same length, one split 7 times, one split 8. they dont match up. your shifter is clicking slightly more than the distance between each cog each time you shift.

Last edited by chunts; 06-11-07 at 06:04 PM.
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Old 06-11-07, 06:50 PM
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you may be able to convert them to friction, though, eliminating the problem altogether even if you want to go to 6 cogs in back.

My Exage downtube shifters have a washer in them, flip it one way, it's indexed 6-speed, the other way, it's friction for however many speeds you have in the span of the derailer's throw.
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Old 06-11-07, 07:20 PM
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Funny, 8 speed shifters work great on 7 speed cassettes. Sheldon Brown says so and so do I because that's how I have this bike set up:

Why wouldn't the opposite work, too? Don't I recall that their was a ghost 8th speed setting on some 7 speed shifters? Maybe adjustment is your problem.
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Old 06-11-07, 07:38 PM
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It's more finicky, but it can definitely be made to work. I know because a customer brought in a bike with shifting problems (among other things). He couldn't get into one of his gears at the top end of the cluster. I adjusted the gears to index very cleanly before noticing that he had 8 cogs in back! So it can be done. The obvious problem, of course, is that you can't use one of your gears.

You could also use the shifters in friction mode. All indexed Shimano DT shifters that I know of have this option. You can also get a 7-speed cassette - they're cheap, and if you're dead set on using indexing, you might as well have the full range of gears available. All you need to do is slide a 4.5mm spacer onto the cassette body before installing the cassette to take up the extra space left over because of the shorter cassette.
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Old 06-11-07, 07:48 PM
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There is a ghost click on the seven speed DT shifters. And seven and eight spacing are the same so it will work either way.
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Old 06-11-07, 07:58 PM
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Originally Posted by Rev.Chuck
There is a ghost click on the seven speed DT shifters. And seven and eight spacing are the same so it will work either way.
7s spacing = 5.0mm
8s spacing = 4.8mm
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Old 06-12-07, 03:18 AM
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Originally Posted by vpiuva
7s spacing = 5.0mm
8s spacing = 4.8mm
+1

Multiply that .2mm over the 7 times you shift in an 8 speed cassette and that's 1.4mm out of adjustment. That's more than you think.


Tim
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Old 06-12-07, 04:37 AM
  #9  
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Originally Posted by vpiuva
7s spacing = 5.0mm
8s spacing = 4.8mm
+2.

I've worked with various combinations of 7-speed and 8-speed shifters and cassettes and I've never been able to get them to index to my satisfaction. Sometimes it takes a double shift in one direction and a single shift back. I've got them to work fine but, every time that I changed direction shifting up or down, I had to shift twice. I've sometimes been able to get them to work on the workstand but not consistantly on the road or trail.

I know there are lots of guys who swear they have gotten these combinations to work. Either they are better tuners than I am or they are less picky than I am. Right now I have a bike that uses 7-speed barcons and an 9-speed cassette. I use it on the friction setting.
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Old 06-12-07, 02:42 PM
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Thanks everyone. I understand about the 7 and 8 gear spreads over the overall throw. However, I have 3rd gear dialed in perfect and 5th gear dialed in perfect the 4th gear still skips. I would think that if 3rd and 5th are good 4th should automatically line up.

If I can't get this then I'll go 7 speed (I think I have one laying around somewhere). What size spacer is required for the 8 gear hub?
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Old 06-12-07, 07:33 PM
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This is how I decided, after noticing the ghost click, that you could use an eight speed cassette with the seven speed down tube shifter.

I compared the two:



Looked good enough to me. I decided not to consult Shimano, and have done it more than a hundred times with no complaints.
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Old 06-13-07, 10:18 AM
  #12  
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Originally Posted by Rev.Chuck
This is how I decided, after noticing the ghost click, that you could use an eight speed cassette with the seven speed down tube shifter.

I compared the two:


Looked good enough to me. I decided not to consult Shimano, and have done it more than a hundred times with no complaints.
They look almost the same. How does it work?

Tim
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Old 06-13-07, 11:57 AM
  #13  
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Originally Posted by cs1
They look almost the same. How does it work?

Tim
You can't be serious!
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Old 06-13-07, 09:20 PM
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Originally Posted by cs1
They look almost the same. How does it work?

Tim
Very well. I have done this so many times, based on looking at the two cassettes side by side, and never had a problem. Looking at my photo it looks like the gears offset towards the table, this was an optical illusion because of the Macro closeup. I lined up the two lower gears with a piece of paper folded until the cogs were even. So, tho they look a little misaligned, they are even, as much as the two top cogs. I almost believe Shimano is/was passing out propaganda, just to get people to upgrade to STI(This was in the early 90's )
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