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ATB 8 speed cassette on a cheapo 7 speed wheel?

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ATB 8 speed cassette on a cheapo 7 speed wheel?

Old 06-04-17, 09:48 PM
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dglevy
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ATB 8 speed cassette on a cheapo 7 speed wheel?

Hi all,

I found two abandoned mountain bikes. One was higher quality, an 8 speed, but missing its rear wheel. The other had all its parts but was a low quality bike and only 7 speed. So my question is, can I remove the 7 speed cassette from the low quality "parts bike" and replace it with an 8 speed cassette to put it on the higher quality bike? The parts bike has a cheapo Shimano 'SIS' derailleur. The higher quality bike has an Acera derailleur.

Thanks in advance for thoughtful, well-informed replies.

--David
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Old 06-05-17, 12:24 AM
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Originally Posted by dglevy View Post
Hi all,

I found two abandoned mountain bikes. One was higher quality, an 8 speed, but missing its rear wheel. The other had all its parts but was a low quality bike and only 7 speed. So my question is, can I remove the 7 speed cassette from the low quality "parts bike" and replace it with an 8 speed cassette to put it on the higher quality bike? The parts bike has a cheapo Shimano 'SIS' derailleur. The higher quality bike has an Acera derailleur.

Thanks in advance for thoughtful, well-informed replies.

--David
Freewheel or cassette (freehub)?

If it's a cassette, 7 speed freehub is about 4.5 mm shorter - so if trying to place a (wider) 8 speed cassette, one sprocket will have to go (probably the largest, smallest one being a lockring as well, while middle ones being made with sprocket pattern to aid shifting, so removing one from the middle will disrupt that pattern). Also, one spacer will need to be thinned down. 8 (and 7) speed sprockets are 1.8 mm wide, while 8 speed cassette spacers are 3 mm wide. Removing one whole spacer and sprocket will make the cassette 4.8 mm narrower, while you need 4.5. It might work, just make sure the lockring engages properly, against the smallest sprocket, and the cassette is tightly held in place (if not, the lockring hasn't engaged properly). If it doesn't work, you'd need an about 0.3 mm wide spacer.

Another thing to mind is the rear dropout space - does the other wheel match it, or is it narrower? For alu frames this can be a problem.

If the better bike has friction shifter for the rear, you could just use a 7 speed cassette shfting in friction mode.

Lots of other options, depending on how you plan to mix and match. Getting a decent quality freehub rear wheel is also a good option IMO.
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Old 06-05-17, 07:43 AM
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Thanks so much for your very well informed reply, these forums are awesome!

That was what I was afraid of: far too much work. First thing I'm going to try is throw the low quality wheel on the bike and see how whether the 8 speed shifter will work with it and, if not, whether I can reset the shifter from index to friction shifting. Will post again after I do that.
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Old 06-05-17, 10:08 PM
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I installed the 7 speed wheel today. As you predicted, it didn't work too well. The width of the hub was fine. It shifted okay, just okay. Amazingly, the index shifting actually worked--no need to switch to friction shifting, even if the option was available. It just would get mixed up somewhere in the middle of the sprocket cluster, would jump a sprocket. No biggie. It's good enough for what the bike cost--NOTHING!...

But I'm having some trouble with the SRAM grip shift now. It's not holding fast in the lower gears, keeps slipping back into the higher gears. I'll post a new thread on it if and when I get a chance and will post the link to the new thread here.

Cheers, and thanks again for the very helpful advice!
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Old 06-05-17, 10:15 PM
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Unless the cassette is spidered. Just take it apart and sand the spacers down to 3.0mm. You will need a extra spacer(s) as noted above so the cogs are tight together.

I've done this a number of times to run a 7 speed cassette with better 8 speed shifters. It is easy and works great.

John
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Old 06-06-17, 06:11 AM
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Thanks, John, for your comment. I don't think this bike is worth putting a lot of effort into. I'm giving it to my girlfriend and I doubt she will use it much. And it's got a lot of issues besides the shifting problem. (Brakes, tires, saddle--and it weighs a ton!) Chances are, I will come across more abandoned ATBs or ATB wheels and may just happen to find an 8 speed when I do. (And there's a bike shop a few blocks from me where people leave abandoned stuff outside fairly often.) But thanks again for taking the time to post. It's knowledgeable people like you and Slaninar who are godsends!
--David
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Old 06-06-17, 10:20 AM
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I'm all for cheap on a free bike. You might be able to just put a extra cog behind the cassette and see how it works. And if you already have an old 8 speed cassette, you can just use the spacers from that one. For that matter, just use an 8 speed cassette and drop off a cog.

John
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Old 06-07-17, 02:45 PM
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'k. Thanks!
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Old 06-08-17, 08:27 AM
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Rather than the effort to sand down spacers, seems easier to check with a bike shop for a used up cassette and recycle
spacers.
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Old 06-12-17, 10:14 PM
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Thanks for the suggestion. Yeah, that might be easier but, as I mentioned in an earlier post, for several reasons I don't think it's worth putting any effort into this bike. At this juncture, I'm either going to give it to her as is--after I've made sure the brakes are working sufficiently well, OR, I'm gonna sell it and use the money to put towards a half-decent used hybrid.
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