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change 8spd cassette to 7?

Old 09-03-10, 10:15 AM
  #1  
Bianchigirll 
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change 8spd cassette to 7?

I am looking for a new 8spd cassette but can't find the one I want. it seems I can use an 8spd but with the old 7spd spacers and one less cog right?

so I could change an 8spd 12,13,15,16,18,20,23,25 to a 7spd 12-23 with my old 7spd spacers right?
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Old 09-03-10, 10:27 AM
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Maybe, maybe not. Shimano 7-speed cogs and spacers came in two thicknesses. The HG cassettes had cogs 1.85 mm thick (8-speed cogs are 1.80 mm) and spacers 3.15 mm thick. The IG 7-speed cassettes have cogs 2.35 mm thick and spacers 2.65 mm thick so it depends which 7-speed cassette you are going to use.

You could just install any complete 7-speed cassette on an 8/9/10-speed hub with the 4.5 mm spacer sold just for this purpose.

However, since you seem to be going to all of this trouble just to eliminate the 25T cog while leaving the first 7 cogs identical why not just use the 8-speed cassette and either ignore the 8th cog or lock it out with the rear derailleur's low limit screw?
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Old 09-03-10, 10:42 AM
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OH I looked but I did not see a difference in cog thickness. the 8spd cassette will not fit on a 7spd body and also it won't shift

installing a 7spd cassette on a 8spd hubs defeats the purpose since I have several 7spd wheels

I should have noted that these wheels are for 7spd bikes with DT (index and possbibly friction), barend, and rapid fire and possibly even thumb shifters.

{edit** please accept my apology for not not providing the information that I am refering to shimano 7spd HG hubs and using shimano 8 spd cassettes with the 7spd spcaers and one less cog
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Last edited by Bianchigirll; 09-03-10 at 12:54 PM.
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Old 09-03-10, 10:58 AM
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I'm confused.......
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Old 09-03-10, 11:03 AM
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Originally Posted by Bianchigirll View Post
I should have noted that these wheels are for 7spd bikes with DT (index and possbibly friction), barend, and rapid fire and possibly even thumb shifters.
OK, it wasn't clear that the freehubs are 7-speed. If you have or can get an HG 8-speed cassette and use the 7-speed spacers with 7 of the 8 cogs, it will probably work as the cog thickness difference is very small and will total only 0.35 mm over the entire cassette.
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Old 09-03-10, 11:03 AM
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Lose the Index shifter and a lot of weird things work together fine..
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Old 09-03-10, 11:06 AM
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Originally Posted by fietsbob View Post
Lose the Index shifter and a lot of weird things work together fine..
It's more than indexing. The 7 of 8 cogs and spacers have to fit on the freehub body properly and the lockring has to tighten them securely. Using HG 7-speed spacers and 8-speed cogs, it should fit and would indeed index as a 7-speed cassette.
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Old 09-03-10, 11:24 AM
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I have no problem swapping a 6 and a 7 speed freewheel , so I watch all the Cassette problems
and am glad I didnt take that Bait.

So You throw a spacer in , It aint rocket science ., Oh thats right America doesn't make anything,
so the computer has displaced mechanical skills ..
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Old 09-03-10, 12:51 PM
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Originally Posted by fietsbob View Post
Lose the Index shifter and a lot of weird things work together fine..
very true and with the exception of my Project 3 all the bikes with and to be equipped with these 7 spd wheels can be swtiched to index.

the idea was to use some nice NOS 7spd HG hubs I found on ebay but now I am having trouble finding cassettes.

again I aplologize for breaking the cardinal rule and not provide sufficient information.
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Old 09-03-10, 02:18 PM
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Originally Posted by Bianchigirll View Post
the idea was to use some nice NOS 7spd HG hubs I found on ebay but now I am having trouble finding cassettes
I've done some mixing of 7- and 8-spd HG cassette cogs with good results. I once even converted a 7-spd HG cassette to Suntour-like spacing by subbing in 8-spd spacers between the largest 4 cogs and that worked like a charm. But I've never "converted" an 8-spd cassette into a 7-speed.

One potential issue is that the smallest 1 or 2 cogs have built-in spacers and you won't be able to change them to 7-spd. So in addition to having 0.35mm less total cog width as HillRider points out, you will have 0.15mm or 0.30mm less spacer width. This gives a total width of 0.50mm or 0.65mm less than a 7-speed cassette. But as long as the cassette lock-ring still locks it all down, it should work OK. If there is play after installing and tightening the lock-ring, a very thin 1mm cassette spacer behind the cassette should work.

Last edited by Gonzo Bob; 09-03-10 at 02:27 PM. Reason: added some calculations
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Old 09-03-10, 04:31 PM
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Case #1- 7 speed freehub. You should be able to use a 7-speed cassette or 8-speed cassette after removing the second smallest sprocket. Good example is SRAM PG730 and PG830/850.

Case #2- 8 speed freehub. Use a 7-speed cassette with 4 mm spacer, or 8-speed cassette.
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