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I have a new trainer arriving on the 29th and obviously this will involve either using my existing cassette that’s still on the wheel or a new cassette to put on the trainer.

As my existing cassette has already run the same distance as the existing chain I’m thinking of using the existing cassette.

Now removing and installing the cassette doesn’t seem too hard and I’ve ordered the relevant tools

  • Chain whip
  • Cassette tool

How important is a torque wrench on this?

I think I remember somewhere that the R7000 cassette is 40Nm, or roughly 29 ft-lbs.

Do I need a torque wrench or is it a case of being tight is good enough?

Looking at the trainer install video the guy literally does it up with an adjustable wrench. This got me wondering how important is a torque wrench in this instance?

Install Video

Skip to around 6 minutes for the cassette installation

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    40Nm isn't that much, about 4kg at the end of a 1m long lever, no brutish force is required. The lockring and the first cog are serrated. Apply some reasonable force, they won't go loose after you've felt 4-5 clicks.
    – Carel
    Dec 27 '20 at 19:34
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How important is torque wrench on this?

It's not important at all. The cassette lockring just has to be tight enough to not come off as it's not doing anything other than keeping the cassette from sliding off - some cassette lockrings are even make of aluminum.

Use a long wrench and put a bit of force behind it and you'll be fine.

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    In general, torque wrenches are pretty important, but I’d agree they’re not necessary in this case. This has been my experience.
    – Weiwen Ng
    Dec 27 '20 at 18:36
  • Agreed This locking has a torque rating but I've found a couple of bikes with loose/finger-tight lockrings, and one completely without, which had awful shifting but the cassette didn't come off. As long as the lockring is clicked over enough nubs to stop it drifting off, you're fine.
    – Criggie
    Dec 27 '20 at 21:13
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    @Criggie Doesn’t the lock ring also clamp all the cogs together? Under tightened lockrings are a common cause of freehub bite on aluminum HG freehub bodies.
    – MaplePanda
    Dec 28 '20 at 6:45
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    @MaplePanda yes the lockring will exert some clamping force on the cassette. However the biting into the aluminium freehub bodies would happen no matter what, over time. It would also help to stop single/loose cogs from camming over.
    – Criggie
    Dec 28 '20 at 9:02
  • I agree with @MaplePanda that clamping down the individual cogs is a good reason to get the lock-ring nice and tight, reducing their movement and resulting bite into alu bodies.
    – Swifty
    Jun 4 at 16:31
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Not mentioned so far - a torque wrench has two functions. One is to ensure enough torque is used to make the nut grip as required by the application. The second is to ensure you don't overtighten/strip a thread. If you are an adult, and are using a 1 foot/12 inch/30 cm wrench, you will be able to exert much more than the 29 ft lbs and possibly damage the thread. If you were using a 2 foot/24 inch/60 cm wrench, much much more.

So I would agree with the other replies with a proviso - a torque wrench isn't required in the sense that this doesn't require exact torque. And for someone who knows roughly what 29 ft/lbs (foot-pounds, 40 Nm) is, that's fine. For someone who has no idea, and tries to tighten as much as they can, it might be necessary to avoid damage.

And as someone pointed out, these rings can be made of aluminium - therefore easy to strip/deform.

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    It is a good idea to have a torque wrench handy as I am sure you will have a need for it again. You can find bike specific ones for low torque items (e.g. seat post/rails) like this - amzn.to/2M75lHu and for greater than 20 NM you can use something like this AC delco torque meter - amzn.to/3nWT0nr . If I had to pick one I would go with the AC delco 3/8 in. adapter as it looks like it will cover all fastener torque specs on the bike (min range at 8NM). I own both and have not had any issues with either of them. For $50, I think it is a worthwhile purchase. Dec 28 '20 at 23:17
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    @TudeProductions quite right. An old mechanic told me once that "torque measuring tools are most accurate in the top half of their quoted range" and that has been pretty much right. A torque wrench rated for 0-20 Nm is fine for 10-20 but less good for 0-10 Nm and probably useless in the bottom 10%, from 0-2Nm. You can also get fixed-values like the common 6Nm etc, and I'll be buying some of them when conditions allow.
    – Criggie
    Dec 30 '20 at 8:01
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    @Criggie - Yes there is validity in the statement. If you look carefully at most certificate cards for wrenches they usually have different tolerances of error in lower vs. higher ranges. I always advocate doing a job once and while in this case it is low risk and everyone will say here is a good way to guesstimate 29 NM, it always is most reliable to use torque wrench or reference turns from snug at least to ensure both over tightening or under-tightening does not occur. I am glad the poster brought it up as I was debating whether to say it or not. Dec 30 '20 at 14:29
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Having the exact torque generally isn’t very important. I recommend finding a way to torque it properly the first time you change a cassette so you get a feel for how roughly tight it should be. It’s hard to just say “oh yea, it should be decently tight” because everybody’s opinion of “tight enough” is different. People also tend to be pretty inaccurate when it comes to estimating weights and forces. A torque wrench borrowed from a bike shop (or even automobile shop) would be best, while a luggage scale would be adequate, and even a bathroom scale might work in a pinch.

40Nm = 29ft•lb as you said, so that would be roughly 30 pounds applied to the end of a 12” wrench. You can do the math if you have a different length wrench than that.

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    Exactly. Do a back-of-the-envelope calculation to have an idea of the order of magnitude. Dec 28 '20 at 13:32

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