How tight is 50nm?!

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  • How tight is 50nm?!
  • jayx2a
    Member

    Changed my cranks and realised my tools only go up to 16nm!

    I know 50nm is quite tight and I do really need to take it to my LBS to check, but as a general rule, how tight is 50nm apart from being Bastard tight?! Any general rule of thumb?

    I know I take the risk if I ride without getting it right but my LBS is closed until Tuesday and want to ride this weekend!

    Alloy frame, GXP BB, Eagle Crank – recommended torque 48-54nm

    Try the hope crank – 70nM they say – my LBS’ shop wrench onyl goes to 60

    dannyh
    Member

    Is just tight enough that about 50nm the other way would be required to undo it. And yes, I do realise that a proper smartypants person who actually knows their stuff about this will refute that by saying it require more to overcome the initial friction or somesuch, but I just wanted to be flippant.

    I don’t think anything on a bike should be done up so tight that the veins are popping out of your forehead using about nine inches of leverage if that helps.

    There are only three real measurements anyway:

    1. Veins just about to pop out of forehead.

    2. Snug.

    3. Soft or delicate thread – needs Loctite.

    strike
    Member

    Pretty much as tight as you can go on the cranks with a standard-sized socket driver i.e. approx. 30cm in length.  The good thing on the GXP BB is that you can not damage the BB bearings through over-tightening.

    Premier Icon jimdubleyou
    Subscriber

    Just get a metre long extension to your spanner and hang 50kg off the end.

    Edit: Hang on a sec, it might be 5kg… let me get a calculator…

    trail_rat
    Member

    τ = r * F

    t = torque

    r = radius

    F = force.(NM being KG * 9.8m/s^2)

    5kg on a 1m lever or 1kg on a 5m lever. BOSH

    dannyh
    Member

    I have managed to over-tighten pedals so much in the past that I have had to use a lump hammer on the tool to shock them undone.

    Strangely enough the crankset axle failed in a spectacular spiral fracture a couple of months later……..

    stumpy01
    Member

    It’s approximately the equivalent of holding 10 pints (in plastic glasses) on the end of a pole 1m long.

    I’d probably go for “Hnnnnnnnnnnnt”, rather than “GrrrrrrrrrrrrrrrrrrRrrrrrrrrp” It’ll be ‘reet….

    dannyh
    Member

    5kg on a 1m lever or 1kg on a 5m lever. BOSH

    How much does the lever itself weigh, though?

    pretty sure 50 is “hnnnnng” (on a standard ratchet), I may be one or two n’s out.

    stumpy01
    Member

    Just get a metre long extension to your spanner and hang 50kg off the end.

    Erm. Don’t do that. That’s 500Nm

    trail_rat
    Member

    5kg on a 1m lever or 1kg on a 5m lever. BOSH

    How much does the lever itself weigh, though?

    just enoguh to take up the slack between the 49nm that 5kg at 1m would give you due to the acceleration due to gravity being 9.8m/s and not a nice round 10.

    daimo
    Member

    Bloody tight! I had to borrow an automotive torque wrench to torque a crank arm power meter to 40 or 50nm. Seems over the top!

    Onzadog
    Member

    50Nm on a crank bolt can be felt on most cranks as a sudden change in resistance as the crank bottoms out on the splines.

    Premier Icon porter_jamie
    Subscriber

    5kg 1m

    10kg 500mm

    12.5kg 400mm

    16.6kg 300mm

    25kg 200mm

    50kg 100mm

    yay! new tool time doesn’t matter if you can’t get to the LBS, 50nm is in the range of normal torque wrenches that can be purchased from any tool shop.

    dannyh
    Member

    Erm. Don’t do that. That’s 500Nm

    Otherwise known as ‘FT’.

    PJay
    Member

    When I’ve ask about torque settings on here over the years I tended to get directed here.

    https://www.sheldonbrown.com/tork-grip.html

    Premier Icon chipster
    Subscriber

    5kg at a metre, so, about 16kg at a foot, call it 20kg on a 9″ ratchet handle. Depending how strong you are, well nipped, but not grunty. 💪

    HTH.

    Edit : what porterjamie said ^

    dannyh
    Member

    pretty sure 50 is “hnnnnng” (on a standard ratchet), I may be one or two n’s out.

    The “hnnnnnng” system of measurement is also my personal choice. If you want to be more exact though as “hnnnnng” can be influenced by the shape and padding of the ratchet handle. You need a combination of the “hnnnnnnng” and the “aaaaaaahhhhh” scale of palm-pain, really.

    bigyan
    Member

    About half as tight as a “normal” car wheel bolt (normally torqued 90-110NM).

    You can get a torque wrench from screwfix for £25, it will be a bit rough but functional enough for that.

    jayx2a
    Member

    Thanks for the responses! lol!

    dannyh
    Member

    pretty sure 50 is “hnnnnng” (on a standard ratchet), I may be one or two n’s out.

    No worries – you have to bear in mind that my responses come from a man who owns a torque wrench but still relies on “hnnnnnnng” as I can either never find it or can’t be bothered to use it………..

    daern
    Member

    Can I mention at this point that SRAM’s “tighten crank fasteners up to FT torque” design is really crap? If you read the instructions, if you have play after fitting, you tighten it up even more. If you still have play, dismantle, grease and repeat. But it’s OK, because you’ll get really good at doing it because the GXP BB will probably need replacing before the winter is out. That will give you the opportunity to clear out the crap that has accumulated in the little gap that’s left between the crankset and BB too.

    Shimano HT2 has a precise bearing preload using the “knurled finger tool” thingy and then a pair of low-torque pinch bolts to hold everything together. And they never, ever come lose when riding (unless you’re a muppet and didn’t put them together right!)

    I’m not saying that everything that Shimano do is good, but a little part of me dies every time I fit a GXP crankset.

    PJay
    Member

    yay! new tool time doesn’t matter if you can’t get to the LBS, 50nm is in the range of normal torque wrenches that can be purchased from any tool shop.

    Made me smile; when I started maintaining my bike many years ago the general consensus seemed to be that ‘proper’ mechanics didn’t need a torque wrench and relied on feel (being a newbie and cackhanded I was after torque wrench recommendations); I think that carbon components changed things though.

    I’ve always used a torque wrench but you do start to develop a feel over time. A word of advice though (from experience) – use common sense when tightening with a torque wrench and don’t keep cranking blindly – it’s easy to miss the click or mis-set the wrench!

    Premier Icon nickc
    Subscriber

    Question I’ve always wondered about. Does the value of a given hnnnng vary proportionally to the proximity to exposed chainring teeth?

    daern
    Member

    A word of advice though (from experience) – use common sense when tightening with a torque wrench and don’t keep cranking blindly – it’s easy to miss the click or mis-set the wrench!

    Good call and if you’re in any doubt at all, start with a lower torque setting and then increase. Don’t just blindly follow what it says in the instructions – use your own head too!

    Premier Icon nwmlarge
    Subscriber

    have you tried a local garage? their torque wrenches will be in the range you want but their drive will be 1/2″ you may need adaptors.

    daern
    Member

    FWIW, I have three torque wrenches in the garage – a little one for doing normal fasteners, a medium one for bigger bike jobs and then a big one for car / caravan wheel nuts. The GXP cranks are, I think, the only thing on any bike that I maintain that needs the big one!

    Premier Icon neil the wheel
    Subscriber

    Shimano HT2 has a precise bearing preload using the “knurled finger tool” thingy and then a pair of low-torque pinch bolts to hold everything together.

    Except that thy insist on using a 5mm allen bolt which has to be torqued to 13nm, which is more than enough to round out the bolt head once it’s been done a few times.

    jayx2a
    Member

    Pretty much as tight as you can go on the cranks with a standard-sized socket driver i.e. approx. 30cm in length. The good thing on the GXP BB is that you can not damage the BB bearings through over-tightening.

    Sounds like a plan!!!

    trail_rat
    Member

    “Except that thy insist on using a 5mm allen bolt which has to be torqued to 13nm, which is more than enough to round out the bolt head once it’s been done a few times.”

    I believe sir is doing it wrong.

    I have ht2 cranks dating back to the ark and have had them on and off plenty with this issue happening precisely zero times .

    I have had to sort out the aftermath when people have had ago them selves with Xmas cracker Allen keys though

    Question I’ve always wondered about. Does the value of a given hnnnng vary proportionally to the proximity to exposed chainring teeth?

    No – that’s where the aaaaahhhh scale come in.

    dannyh
    Member

    Can I mention at this point that SRAM’s “tighten crank fasteners up to FT torque” design is really crap? If you read the instructions, if you have play after fitting, you tighten it up even more. If you still have play, dismantle, grease and repeat. But it’s OK, because you’ll get really good at doing it because the GXP BB will probably need replacing before the winter is out. That will give you the opportunity to clear out the crap that has accumulated in the little gap that’s left between the crankset and BB too.

    Eliminate the little gap with some washing machine rubber seals that are slightly too small for the axle. They sit at an angle and ‘spring/squidge’ to fill the gap. It also stops the stupid little insert they had to introduce from wandering out of the BB bearing along the axle and causing massive and immediately noticeable play. Quite why I should have to do this (or SRAM would introduce a wavy washer bodge) for a product costing a reasonable amount of money is beyond me, however. But that is the SRAM way: How many parts does this need? Take that number, double it and add two and that is how many parts it will eventually consist of.

    Shimano HT2 has a precise bearing preload using the “knurled finger tool” thingy and then a pair of low-torque pinch bolts to hold everything together. And they never, ever come lose when riding (unless you’re a muppet and didn’t put them together right!)

    I had a HT2 crank come off and leave the pedal and crank dangling from my foot on a reasonably high speed descent once. But I’m a muppet. Incidentally, I reckon my buttocks clenched to about 50nM at the time, though.

    daern
    Member

    Except that thy insist on using a 5mm allen bolt which has to be torqued to 13nm, which is more than enough to round out the bolt head once it’s been done a few times.

    Only if your allen keys are knackered, I’m afraid. If your worried, swap them for stainless ones – I keep a bag in the garage for just this purpose.

    willard
    Member

    I thought it was measured in knuckles? 50Nm should be about three, possibly four, knuckles when using a standard 1/2″ ratchet driver.

    On a serious note, I am loving the conversions and now planning on how I can convert these into the various lengths of bar needed to support me for given torque settings, although this may mean either short drivers, or me losing quite a lot of weight.

    50Nm is a couple of notches back from pulling out your tripe with a std socket driver.  so tighten it until it goes slack, then back it off a 1/4.

    Otherwise known as ‘farm boy tight’

    joshvegas
    Member

    Torque is inversely proportional to proximity and in the direction of various stabby stabby cutty cutty things.

    fossy
    Member

    GXP is tight, very.  I’d greased my bearings and did it up tight(ish) so no play, but the BB developed a creak. Re-greased then saw the 50nm. Doh.  Stems are usually 5nm so 50 is very tight – probably as tight as you can go on an 8mm allen key.

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