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Rear cog has 7 gears, shifter has 8 settings

Old 06-23-24, 08:56 AM
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Rear cog has 7 gears, shifter has 8 settings

This is a used bike purchased at auction. It seems to be a police bike. Shifting is difficult to impossible. I don't know if the components are mismatched or something else is wrong.The rear derailleur has 7 gears and the shifter has 8 settings: is that workable? Are the parts mismatched? Thanks for any help!

Well, I can't post photos because I have not yet posted 10 messages to the site. That stinks. Anyway, the rear derailleur is a Deore XT. The shifter is STX RC underbar with 8 numerical settings. I'd love to show you the pics, but I can't.

I can email the photos if anyone is interested.






Last edited by nicomp; 06-23-24 at 09:04 AM.
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Old 06-23-24, 09:05 AM
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That can work just fine. Shimano 8 speed spacing is just a 0.2mm smaller than 7 speed, so if the adjustment is dead on for gear 5, it will only be off a maximum of .6mm at gear 7, which is not enough to matter. I've used 7 speed shifters with 8 speed cogs and it worked.

You'll want to use gears 8-2 on the shifter if it isn't already.
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Old 06-23-24, 09:08 AM
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When you come in possession of bikes that have had previous owners. You should expect that at times one of the previous owners may have done something strange to it. Perhaps it worked well enough for that previous owner, or perhaps that's why you now have the bike.


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Old 06-23-24, 09:14 AM
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The most common reason is that the cassette (or complete rear wheel) was replaced with what was "available" (be it by the rider or a shop). The cog center to center dimension is just different enough (between 7 and 8 cog cassettes) so that after a couple/three shifts the differences add up enough to show. If any other issues are present (worn chain, sticky cable movement, poor cable casing seating, bent parts,..) the tolerances go down and shifting issues happen sooner.

But taking the OP's description as "fact" the solution seems easy enough. Replace the cassette with the proper 8 cog one (and likely the chain at the same time due to miss matched wear). However, stated simple assumed solutions without the ability to have a hands on assessment can often be found to be lacking.

We are the service shop for a couple of police/security forces and some of the bikes we see have had well intended improper work done by the departments/riders (in the goal to save money usually). Some of their bikes have been recovered property that was never claimed. Some of the earlier "police specific" bikes have used parts that lost out in the greater marketplace (like rapid rise ders and silent clutch freehubs) and with the near abusive use that some police bikes get it's no surprise that some of them don't work too well at all. Andy
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Old 06-23-24, 11:56 AM
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derailleurs don't have "gears"... Cassettes and Freewheels have gears...

and there may be other problems that are causing the bike to not shift well, such as a bent derailleur hanger/bracket, a poorly adjusted shift cabl.

best guess is that someone stuffed a 7 speed wheel onto the bike, and didn't realize that the 7 and 8 speed parts don't work well with each other, and no amount of fiddling around will make them work well together.

is the rear hub a freewheel type, or a cassette type?
how to tell the difference, if you aren't sure...
https://www.parktool.com/en-us/blog/...freewheel-type
i changed the link..

Last edited by maddog34; 06-23-24 at 12:20 PM.
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Old 06-23-24, 01:15 PM
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Originally Posted by nicomp
This is a used bike purchased at auction. It seems to be a police bike. Shifting is difficult to impossible. I don't know if the components are mismatched or something else is wrong.The rear derailleur has 7 gears and the shifter has 8 settings: is that workable? Are the parts mismatched? Thanks for any help!
No need for pictures. Yes they are mismatched. Some people report that with careful adjustment they will work reasonably well, but that requires smooth running cable, good chain and a pinch of luck. Assuming the derailleur hanger isn't bent, you can try adjusting the cable so the middle few gears shift smoothly, so the outer ones aren't terrible, but it's likely you need to replace parts to make it work. If the rear hub has a freehub/cassette you'll need to know if it's a 7 speed, or an 8 speed hub with a 7 speed cassette on it - 8 speed is a bit wider, so if you have a 7 speed cassette there will be a spacer on the back, if it's 7 speed you can't just fit 8 speed, you'll need to use 7 of 8 sprockets and a spacer. If it's a freewheel you'll need to check there's room to replace it with an 8 speed freewheel. Or if the 7 speed freewheel/cassette isn't worn out you could just fit a 7 speed shifter. After that you may still need a new chain, derailleur alignment, jockey wheels ... OK maybe pictures would help.
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Old 06-23-24, 03:14 PM
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The rear derailleur does not have "gears" that is controlled by the shifter. If there are 7 cogs on your rear wheel and your shifter has 8 choices, they are not compatible
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Old 06-23-24, 04:01 PM
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One sign is, are the wheels mismatched? If assembled from parts, someone could have simply found a 7 speed rear wheel and stuffed it in for whatever reason.
If you could simply find a used wheel with an 8 speed cassette, swap that in.
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Old 06-23-24, 04:13 PM
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If you have a cassette based wheel get a new chain and cassette in 8 speed and you will be fine. I have all my XT 7-8 speed derailleurs running 9 speed flawlessly but that requires a new shifter and I lucked out on one of them and got a lovely XTR 9 speed shifter. You might not have such a rich parts bin.
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Old 06-25-24, 11:30 AM
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You can run 8sp shifters on the 7sp cogset, you can get it adjusted to work well enough to run 7 of the 8 clicks.

You may be tempted to replace the 7sp cogset with an 8 speed cogset - whether this will work depends on what rear wheel you have (freewheel or freehub, and if freehub, the freehub width). It could be that your current wheel will support a 8sp cassette, or it could be that you'll need a new wheel.
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Old 06-25-24, 11:48 AM
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What model shifter, what rear derailleur?
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Old 06-25-24, 12:00 PM
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Most likely the entire rear wheel was replaced because the original was stolen, and the replacement wheel will have a 1970's-80's style freewheel, and not a 1990's-2000's freehub with cassette like the lost original. This might have been done by a prior owner or the auction may have done it to make a complete bike.

You can probably get it to work. As earlier respondents said, the shift spacing is close enough. Shimano RD's of that age have some slop in the upper pulley ("Centeron") to tolerate some error. Just follow the normal rules for adjusting an RD and you will wind up with the lowest gear shift on the shifter blocked from happening by the inner limit screw on the derailleur.
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Old 06-26-24, 10:36 PM
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It is tough to make assumptions, but an XT RD and STX RC shifter, if in good functional condition, is probably not a problem.

If it were me, I would first try and determine if you have a freehub & cassette or freewheel. If it is a freewheel, you’ll need to see the mfg. hopefully it is Shimano and you can fudge the setup to be good enough.

If it is Suntour, you’ll need to replace the freewheel with a Shimano compatible.

If it is a 7speed cassette, there is a good chance you can’t install an 8 speed cassette. Unless there is a spacer behind the 7 speed cassette you are stuck with running only a 7 speed cassette.

I have taken 7 speed cassettes apart and sanded the spacers to match 8 speed shifters, because good enough has not been for me.

As others have noted there could also be a bent RD hanger problem and that needs to be fixed regardless of what you do.

John

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Old 06-27-24, 06:36 AM
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Suntour index freewheels work fine with Shimano.
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Old 06-28-24, 01:15 PM
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Originally Posted by Kontact
Suntour index freewheels work fine with Shimano.
yep.. however, the 7 speed cogs still won't play well with 8 speed shifters.
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Old 06-28-24, 04:29 PM
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Originally Posted by maddog34
yep.. however, the 7 speed cogs still won't play well with 8 speed shifters.
Disagree. They aren't different enough for the centeron pulley to not make up the difference.
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Old 06-28-24, 07:14 PM
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Originally Posted by Kontact
Disagree. They aren't different enough for the centeron pulley to not make up the difference.
maybe you need to look at the thread-opening post again...

"Rear cog has 7 gears, shifter has 8 settingsThis is a used bike purchased at auction. It seems to be a police bike. Shifting is difficult to impossible. I don't know if the components are mismatched or something else is wrong.The rear derailleur has 7 gears and the shifter has 8 settings: is that workable? Are the parts mismatched? Thanks for any help!"

i corrected this same issue on a bike last week... the lady just couldn't understand that the gears are spaced differently, and figured she could just shift one less time. i had to measure the gaps in front of her to demonstrate the differences...
this will be my last response on this thread to you since you know that the spacing is different, admitted such, and now insist on being stubborn and acting superior to reality, just to prolong contact with the outside world.

bye bye.
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Old 06-28-24, 08:20 PM
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Originally Posted by maddog34
maybe you need to look at the thread-opening post again...

"Rear cog has 7 gears, shifter has 8 settingsThis is a used bike purchased at auction. It seems to be a police bike. Shifting is difficult to impossible. I don't know if the components are mismatched or something else is wrong.The rear derailleur has 7 gears and the shifter has 8 settings: is that workable? Are the parts mismatched? Thanks for any help!"

i corrected this same issue on a bike last week... the lady just couldn't understand that the gears are spaced differently, and figured she could just shift one less time. i had to measure the gaps in front of her to demonstrate the differences...
this will be my last response on this thread to you since you know that the spacing is different, admitted such, and now insist on being stubborn and acting superior to reality, just to prolong contact with the outside world.

bye bye.
We don't know why the OP's bike isn't working. As I have bikes blending 7 and 8 speed successfully, jumping to the conclusion that .2mm out of 5 is the one and only issue is dumb.
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