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Old 12-16-09, 07:13 PM   #1
yellowsir
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Can I place an 8 speed cassette on my current 7 speed MTB wheel?

Hello everyone. I have a question. Recently acquired a Trek 800 from a local flea market for cheap ($20). The bike had a couple of problems like missing rear brake and some minor scuffs. But as I start to tune the bike up I realize that the bike is originally has an SRAM 24 speed grip shifter system. Problem that I am having is that the previous owner placed a 6 speed wheel on the back that I believe doesn't belong there. Of course left side shifter has 3 speeds, and right side has 8 speeds. So I went out to the flea market and picked up a 7 speed 26X1.5 mtb wheel. Its working out great, but I feel bad that I cannot properly shift the bike through all the gears.

Now my question is, is there a possibility I can buy an 8speed cassette for example at PERF BIkes and place it on the 7 speed hub with no problems, or do you guys think I will run into complications? Will there be a spacing issue? Do i have to buy a wheel with an 8 speed cassette already on (not easy to find)? Any input will be greatly appreciated. I just got into bike mechanics as a new hobby, and this is my first real bike that I have worked on. So far the bike is throrougly lubed, and rides great. TIA
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Old 12-16-09, 08:12 PM   #2
garage sale GT
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It could be a freewheel hub. A lot of low end MTBs come with a 7sp rear freewheel. I saw some 8sp freewheel bikes in costco but I don't know if they sell the freewheels separately, and for a cassette, there's no hope.

They had 8sp freewheels several years ago but dropped them due to axle breakage. If costco has bikes with them again, it may be a special volume buy by them and that means there's no reason to expect the parts are standard and available.

You can put a 7 speed cassette on an 8/9 hub but you need a spacer. That implies the freehub is longer. Maybe there's a workaround, I don't know.

Last edited by garage sale GT; 12-16-09 at 08:17 PM.
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Old 12-16-09, 08:23 PM   #3
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Originally Posted by yellowsir View Post
Hello everyone. I have a question. Recently acquired a Trek 800 from a local flea market for cheap ($20). The bike had a couple of problems like missing rear brake and some minor scuffs. But as I start to tune the bike up I realize that the bike is originally has an SRAM 24 speed grip shifter system. Problem that I am having is that the previous owner placed a 6 speed wheel on the back that I believe doesn't belong there. Of course left side shifter has 3 speeds, and right side has 8 speeds. So I went out to the flea market and picked up a 7 speed 26X1.5 mtb wheel. Its working out great, but I feel bad that I cannot properly shift the bike through all the gears.

Now my question is, is there a possibility I can buy an 8speed cassette for example at PERF BIkes and place it on the 7 speed hub with no problems, or do you guys think I will run into complications? Will there be a spacing issue? Do i have to buy a wheel with an 8 speed cassette already on (not easy to find)? Any input will be greatly appreciated. I just got into bike mechanics as a new hobby, and this is my first real bike that I have worked on. So far the bike is throrougly lubed, and rides great. TIA


On the difference.

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Originally Posted by garage sale GT View Post
It could be a freewheel hub. A lot of low end MTBs come with a 7sp rear freewheel. I saw some 8sp freewheel bikes in costco but I don't know if they sell the freewheels separately, and for a cassette, there's no hope.

They had 8sp freewheels several years ago but dropped them due to axle breakage. If costco has bikes with them again, it may be a special volume buy by them and that means there's no reason to expect the parts are standard and available.

You can put a 7 speed cassette on an 8/9 hub but you need a spacer. That implies the freehub is longer. Maybe there's a workaround, I don't know.
7 cass on 8/9/10 shimano freehubs require a 4.5mm spacer.
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Old 12-16-09, 08:31 PM   #4
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Either way, if it's a 7sp wheel you're going to either need to buy a new freehub body (or wheel) and cassete (if that's an option) or buy new shifters. Cheapest and simplest option is new shifters. Don't waste your time on an 8sp freewheel. I suspect it's probably shimano compatible SRAM on there (MRX) but you should post the model names of your rear derailleur and shifters too to confirm everything will work correctly together. Either that or ask a mechanic at a local shop if it is compatible.
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Old 12-16-09, 08:33 PM   #5
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why not just set the stop screws on the derailleur to keep from losing the chain when you go into first, and leave the 8sp shifters? That's an "OK" solution. More or less good enough.
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Old 12-16-09, 08:38 PM   #6
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why not just set the stop screws on the derailleur to keep from losing the chain when you go into first, and leave the 8sp shifters? That's an "OK" solution. More or less good enough.
Except for the part where 8 doesn't index 7 properly.
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Old 12-16-09, 08:55 PM   #7
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The ABSOLUTE cheapest and easiest solution would be a RH friction shifter.
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Old 12-16-09, 10:16 PM   #8
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Well the shifting system is a sram 5.0
I have a pair of 7 speed shimano shifters which I really wanted to use. But later realized that I think they are not compatible with the sram 5.0 derailleur.
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Old 12-16-09, 10:18 PM   #9
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If the wheel is a 7 sp freehub, your cheapest solution is to use a 7 sp cassette, with your 8 sp shifter and attach the shift cable to the rear der as shown in the Alt Cable Routing paragraph at the bottom of this page http://sheldonbrown.com/derailer-adjustment.html You then will not be able to use one of the positions on the shifter, 1 or 8, but it will shift smoothly over the 7 cogs. I did this when I converted a 21 sp hybrid to drop bars with 8 sp Sora brifters. It worked well for a year then old age cracks appeared in the rim and I bought an 8/9 sp rear wheel and 8 sp cassette.
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Old 12-16-09, 10:38 PM   #10
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yeah...i will probably keep the bike with the 7 speed wheel for now till I come across an 8 speed wheel.
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Old 12-17-09, 09:50 AM   #11
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7 speeds are typically 5mm between cogs, 8 speeds are typically 4.8. so as our friend has already discovered, the indexing, while off a bit, should work OK.

Last edited by garage sale GT; 12-17-09 at 02:06 PM.
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Old 12-17-09, 05:23 PM   #12
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8 of 9 on 7. you will need 9 speed spacers, chain, and eight of the cogs off a 9 speed cassette. Also in the Sheldon Brown article. PGMikeT

Last edited by PGMikeT; 12-17-09 at 05:24 PM. Reason: sp.
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