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Thread: torque wrench

  1. #1
    mnoutain bkie rdier
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    torque wrench

    I now have a torque wrench from Park tools with the bit set. Ratchet type.

    Question:
    Doesn't torque change from bolt size to bolt size? If so, how can I know if 5nm on a 4mm bolt is the same as on a 5mm bolt sorta thing?

    **Edit With regards to the stress placed on a frame. See post 3 if you wish..

    Thx.
    Last edited by rydbyk; 02-13-2013 at 02:01 PM.

  2. #2
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    5 nm is 5 nm regardless of bolt size. The wrench doesn't know what it's torquing, only applied force!

  3. #3
    mnoutain bkie rdier
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    Quote Originally Posted by J.B. Weld View Post
    5 nm is 5 nm regardless of bolt size. The wrench doesn't know what it's torquing, only applied force!
    Maybe I am way off, but imagine trying to torque a bolt that is 6 inches in diameter. Seems like it would take quite a bit more force (showing up as torque on your meter) to turn the darn thing. Correct?

    I know that is an exagerated example, but an increase in mm is an increase in torque if what I said is correct.

    Real world example: My frame recommends a 5nm torque on a seat collar. Well one seat collar I have is a 4mm bolt and the other is a larger 5mm bolt. Do I just assume that 5nm on BOTH bolts are going to put EQUAL STRESS on the seat tube?

    Maybe I am taking crazy pills here...

  4. #4
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    So a torque rating is in Newton-meters. A newton is a unit of force, and a meter a unit of distance. The combination is like saying 1 newton of force applied at 1 meter distance. Here's a graphical explanation on Wikipedia:

    Name:  Torque_force_equivalence_at_one_meter_leverage.png
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    Newton metre - Wikipedia, the free encyclopedia

    So 1Nm of torque is 1Nm of torque regardless of bolt size.

    However, you are correct that different size bolts can be subjected to different torque loads. A larger bolt can have more torque applied to it before the threads fail because it can distribute the force over a greater area. Here's a page showing some typical maximum torque values for different metric bolt specifications:

    STANDARD BOLT TIGHTENING TORQUE

    As you guessed, *maximum* torque that the bolt can be subjected to is larger for larger bolts. However, that needn't mean that you have to torque each bolt to its maximum allowed torque. If a bolt is over-spec'd for it's application, it may be fine with less torque applied.

    Here are some values typical of bicycle components:

    Park Tool Co. ParkTool Blog Torque Specifications and Concepts

    So in your real world example, with a grade 10.9 4mm bolt you may not want to exceed 4Nm of torque, while a 5mm bolt may be OK at 6Nm of torque.

    However, if you were to apply 4Nm of torque on a bolt of either 4mm or 5mm diameter on a seatpost collar, so long as both bolts had the same thread pitch, you would be applying the same force to the seat tube. You don't necessarily need to torque the 5mm bolt more just because you can.

  5. #5
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    I agree that the torque applied to the bolt will be the same.

    The clamping force may not. The pitch of the bolts may be different. A screw is essentially a ramp where a different pitch means a different gradient with more or less leverage.

    English pitch bolts are often different pitches (distance between thread crests) with different sizes; metric bolts often have the same pitch for different size bolt, but would exert a different clamping force.

    But I would feel that this is over analyzing this.There's a lot of slop - close counts.

  6. #6
    Plays with tools
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    If the part has a spec, torque it to that spec regardless of size. Somebody did the math to determine what that spec was so you didn't have to worry about it.

    Now, for the academic sake of this conversation what your thinking about bigger fasteners taking more torque to rotate is completely true. In some precision assemblies they will specify that you run the fastener down and then use a beam type torque wrench to measure the drag that assembly creates as it snugs up. Then you will add that number to the specified torque rating. But this is space shuttle kind of anal (and those things even blew up) and has zero application to anything on a bicycle.

  7. #7
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    One thing that would affect the # would be the coating applied to the treads prior to tightening.

    Every lubricant has a torq coeficient and that number is used and applied to the actual number to alter the actual required value.

    Oil and synthetic lubs for example, used on the same fastner in the same location would have different numbers..

    Just like extensions and adaptors have simular multipliers.
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  8. #8
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    Quote Originally Posted by Harvie View Post
    One thing that would affect the # would be the coating applied to the treads prior to tightening.

    Every lubricant has a torq coeficient and that number is used and applied to the actual number to alter the actual required value.

    Oil and synthetic lubs for example, used on the same fastner in the same location would have different numbers..

    Just like extensions and adaptors have simular multipliers.


    Interesting, I knew lubricant changed the torque # but I did not know there was a torque coefficient chart.

    I admit that for the majority of my bike wrenching career I never laid a hand on a torque wrench and tightened everything by feel. I was always an advocate of clean threads with a light coating of grease because bolts will tighten easier, and stay tight with less effort at the wrench handle then if left dry.

  9. #9
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    torque wrench

    Don't forget the material that the fastener is being threaded into. Soft aluminum will not stand higher torque values as, say steel. When you have light weight aluminum bike parts you shouldn't crank down on the bolts or you'll strip/stretch/gall the threads in the part before you damage the fastener. Your also stretching the fastener and that's what holds the torque. Tightening too much can also make a fastener loosen because its stretched too thin and looses purchase area on the thread causing it to loosen in a high stress/vibration application.

    IOW, tighten to recommended setting and don't lubricate the threads unless noted.


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  10. #10
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    Most pro race teams build engines using the 'STRETCH' of the bolt instead of torque. The tool that measures the stretch is a little high....for the bike crowd.
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  11. #11
    mnoutain bkie rdier
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    So, someone who has always done torque by hand could perhaps become more dangerous with a torque wrench if they do not understand all of this.

    Cool. I wish they would always just us a 4mm bolt with the same pitch everywhere that is possible. No guessing needed.

    So, if you called your frame manufacturer and asked them how much torque to use on the seat collar, do you all think they would say "Depends...what size bolt is on the seat collar?"

    Nope.

    Do they adjust suggested nm on the seat collars that have it printed on the seat collar based on 4mm vs 5mm?
    Last edited by rydbyk; 02-19-2013 at 12:55 PM.

  12. #12
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    You should be asking the manufacturer of your seatpost collar for the torque, not the frame manufacturer.

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    torque wrench

    @ Harvie, yes some engines use stretch for rod bolts in a engine as its more accurate to how much torque is on the fastener. A lot also use a set torque in Lb/Ft then use a angle torque after that setting for stuff like head bolts and crank pulley bolts. It's cool stuff!




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  14. #14
    Fat & Single
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    See the singlespeed forum....

    Does a 40t chainring on the front and a 20t cog on the back .....

    Put less tension on your chain than a....

    22t front chainring and 11t rear cog ????

    Can you ride up hills better with a 180mm crank arm than a 165mm crank arm ???
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    Quote Originally Posted by 4SEVEN3 View Post
    @ Harvie, yes some engines use stretch for rod bolts in a engine as its more accurate to how much torque is on the fastener. A lot also use a set torque in Lb/Ft then use a angle torque after that setting for stuff like head bolts and crank pulley bolts. It's cool stuff!




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    torque to yield head bolts are used by many auto manufacturers today.


    The lengths of fasteners are measured when possible, like rod bolts as mentioned.

    Not much to do with bikes though.


    A nice write up on fasteners:
    ARP-bolts.com | ARP - World-leading Fastener Technology

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