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Can someone demystify cassette "back" spacers?

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Can someone demystify cassette "back" spacers?

Old 08-09-19, 08:23 AM
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davei1980
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Can someone demystify cassette "back" spacers?

I have an older Ultegra hub with a Deore 10 speed cassette that I'll soon be switching out.

I only recently learned that these hubs sometimes require a 1mm "back spacer" behind the cassette to take up the slack caused by the narrower 10sp cassette compared to 9 speed.

I never knew this, was I doing something wrong? I would think that if there was 1mm extra space I wouldn't be able to tighten the cassette to the freehub body with the lock ring without there being obvious slack.

When I get a new (used) cassette, will I need a spacer or will it be obvious? I would think I would notice it rattling around back there! Here are pics of my current components, I can probably get part numbers later today:


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Old 08-09-19, 08:48 AM
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Sometimes I feel like an analogue man whose been trapped in a digital universe.

If it fits, it fits.

When you are trying on shoes, if the size that you think should fit doesn't, do you buy then anyway because they are the "right" size or do you try a different size?
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Old 08-09-19, 09:11 AM
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Originally Posted by Retro Grouch View Post
Sometimes I feel like an analogue man whose been trapped in a digital universe.

If it fits, it fits.

When you are trying on shoes, if the size that you think should fit doesn't, do you buy then anyway because they are the "right" size or do you try a different size?
Let me paraphrase you:

"wait, wait - let me overthink this!"
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Old 08-09-19, 09:16 AM
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Put the cassette onto the hub and ensure that it overhangs the end of the hub by a couple of millimeters so that the lock ring can clamp it securely; if it is flush or recessed you need a spacer.
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Old 08-09-19, 09:27 AM
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That Ultegra hub has a "Shimano 9/10-speed" freehub body. So...

With a Shimano 10-speed cassette, use a 1 mm spacer.

With a SRAM 10-speed cassette, no spacer is needed.

If you're installing an 11-speed cassette, stop what you're doing and get a Shimano 11-speed freehub.

More information on freehub sizes and spacers on slowtwitch.com
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Old 08-09-19, 10:39 AM
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Great write up on hub driver spacing and how it came to be.

https://www.slowtwitch.com/Tech/Cass...rt_2_3257.html
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Old 08-09-19, 10:52 AM
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Originally Posted by terrymorse View Post

More information on freehub sizes and spacers on slowtwitch.com
...thanks for the link. It confirmed for me what I already suspected about me being prehistoric.
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Old 08-09-19, 01:19 PM
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Originally Posted by terrymorse View Post
That Ultegra hub has a "Shimano 9/10-speed" freehub body. So...

With a Shimano 10-speed cassette, use a 1 mm spacer.

With a SRAM 10-speed cassette, no spacer is needed.

If you're installing an 11-speed cassette, stop what you're doing and get a Shimano 11-speed freehub.

More information on freehub sizes and spacers on slowtwitch.com
I at least know enough to not attempt an 11sp on a 9/10 freehub body! I AM; however, new to shifters/derailleurs/cassettes.

I posted this because I realized I am NOT using a spacer but the 10 sp shimano cassette doesn't seem to rattle around back there and seems to be held tight by the lockring so who knows...
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Old 08-09-19, 01:21 PM
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Originally Posted by dsbrantjr View Post
Put the cassette onto the hub and ensure that it overhangs the end of the hub by a couple of millimeters so that the lock ring can clamp it securely; if it is flush or recessed you need a spacer.
Makes perfect sense! Just like if your QR axle sticks out to the ends of your dropouts, it's too long and not being held by the skewer!
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Old 08-09-19, 01:29 PM
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Originally Posted by 3alarmer View Post
...thanks for the link. It confirmed for me what I already suspected about me being prehistoric.
You say "prehistoric" like it's a bad thing.
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Old 08-09-19, 01:32 PM
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To remystify slightly, the Shimano HG-800 11-34 cassette requires a 1.85 mm spacer to mount on an 11 speed road freehub.

(Slowtwitch article is road-centric, and predates the HG-800.)

-mr. bill
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Old 08-09-19, 02:07 PM
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I was looking into this exact question, last night, and I'm still a bit confused (no jokes, please ). So I'm supposed to be using a spacer, with a 10-spd cassette, on my 10-spd hubs ?
I need to check to be sure, but I'm thinking I'm running without a spacer, right now.

Last edited by Brocephus; 08-09-19 at 04:51 PM.
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Old 08-09-19, 04:00 PM
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Originally Posted by mr_bill View Post
To remystify slightly, the Shimano HG-800 11-34 cassette requires a 1.85 mm spacer to mount on an 11 speed road freehub.

(Slowtwitch article is road-centric, and predates the HG-800.)

-mr. bill
That's because the HG-800 (and the new 105 variety) is basically an 11 speed MTB cassette, and there is no special 11 speed freehub for shimano MTB wheels. It all still works with the old 9/10 speed freehub. I think they're introducing something new for 12 speed though. So you need the spacer to make those cassettes work on an 11sp road hub. I bet those cassettes also won't fit on the old 7800 10sp freehub, where all this confusion stems from. Because the 7800 freehub standard was dead by the time Shimano came out with the 10 and 11 speed MTB groups, so they never bothered to support it.

Last edited by Metaluna; 08-09-19 at 04:04 PM.
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Old 08-09-19, 04:55 PM
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Originally Posted by Brocephus View Post
I was looking into this exact question, last night, and I'm still a bit confused (no jokes, please ). My bike is a 10-spd. Last year I got some aftermarket wheels, that came with a relatively thick spacer. Then I just got another set of ( used) wheels with another 10-spd hub, and that freehub had a much thinner spacer on it (that wheelset hasn't had a cassette installed yet).
So what's the deal with these two spacers ? Any idea?
Were those aftermarket wheels 11 speed compatible by any chance? If so, that spacer is probably 1.85mm thick, which is needed to adapt "normal" 8,9, and 10 speed cassettes to 11 speed freehubs.

To squeeze in the 11th cog, Shimano had to make the cassette, and therefore the 11sp freehub body, a little bit longer than the classic 8/9/10 speed freehub body. So, "normal" 8/9/10 speed cassettes need a 1.85mm spacer to push them outward a little bit so they can make solid contact with the lockring and be tightened down.

However some (maybe all?) Shimano 10 speed road cassettes aren't normal. They're actually 1mm thinner than other cassettes. To use them on a conventional 8/9/10 speed freehub you need a 1mm spacer. The reasons for this are explained in the Slowtwitch article. The spacer that came with your used 10 speed wheels is probably that 1mm "thinner" spacer. These are usually included with the cassettes that need them, but sometimes they come with the hubs too.

Other manufactures, like SRAM, make their 10 speed cassettes the same thickness as 9 speed cassettes, and so when mounting them on 9/10 speed freehub bodies they don't need the 1mm spacer.

However, when mounting a 8/9/10 speed cassette on an 11 speed road freehub, *everybody* needs the 1.85mm spacer. So those aforementioned Shimano cassettes actually need *both* spacers for a total of 2.85mm of extra extension.
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Old 08-09-19, 05:28 PM
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Originally Posted by Metaluna View Post
Were those aftermarket wheels 11 speed compatible by any chance? If so, that spacer is probably 1.85mm thick, which is needed to adapt "normal" 8,9, and 10 speed cassettes to 11 speed freehubs.
.
The wheels came from a pretty well known wheelbuilding operation, but was a closeout wheelset built on 105 hubs, and all the closeout wheels were advertised as being "8-9-10-spd"compatible, but I couldn't swear that some wires weren't crossed somewhere, and it's actually 11-spd (they sell them too, so it's conceivable).
Anyway, thanks for the outstanding (clear and detailed) explanation !

Last edited by Brocephus; 08-09-19 at 06:37 PM.
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Old 08-09-19, 11:33 PM
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Originally Posted by Metaluna View Post
That's because the HG-800 (and the new 105 variety) is basically an 11 speed MTB cassette, and there is no special 11 speed freehub for shimano MTB wheels.
And to demystify why, the inboard sprocket is cantilevered over the spider on mountain bikes because the inner sprocket is large enough to clear the spokes.

-mr. bill
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