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I have wonderful new manuals everywhere, of brakes, wheels... In these manuals I see "tighten up to 5Nm", "tighten up to 40Nm".

There must probably be tools with such measurements on them, but I don't have any. Is there a way, an equivalent I can use to know the approximate torque I'm putting on my tools?

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    I strongly suggest you buy/borrow/hire/steal a torque wrench. You will be surprised how little torque is required in most cases. Modern bikes are light weight and built to fine tolerences. A gorrilla with a spanner can easily do expensive damage. – mattnz Oct 24 '13 at 21:22
  • Fully agree with @mattnz. Note also that torque becomes particularly important with carbon frames. – PeteH Oct 24 '13 at 23:04
  • A note that a cheap beam-type torque wrench is only 20-40 euro-dollars. And cheap beams, while imprecise are more accurate than uncalibrated clicker types. Especially if you don't use a clicker regularly, they can seize up and overtorque. – RoboKaren Feb 8 '17 at 19:13
  • If you're on a tight budget, torque wrenches may look pricey. If so, see if your town has a bike repair co-op where you can drop in and use their tools, usually for a small fee. These are pretty common in bigger cities (I'm in Ottawa and we have three co-ops). If all you need to do is spend a couple minutes dialing in torque on a few bolts, they might not charge you anything. – SSilk Apr 30 '18 at 14:27
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    Important thing to note is to get a wrench that fits your needs. Very few bolts need torque of 50+ Nm and most of screws will fall under 10Nm. Arguably it is also easier to over torque smaller (needing less torque) screws than bigger ones.There are few sub 10Nm choices (eg: IceToolz Ocarina) and some sub 25Nm options. If you go for 2-100Nm one you are most likely to suffer from not enough resolution and precision in the low end of the scale. – Furmek May 28 '18 at 23:24
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You do this with a tool called torque (sometimes called dynamometric) wrench. Without a tool you can estimate it this way:

  • Make yourself familiar with a weight of 1 kg
  • Apply the force with your simple wrench 10 cm from the bolt in question

This will give you 1 Nm of force. To get 5 Nm, use 5 kgs of weight or increase length to 50 cm. The math is simple:

τ = r * F

Where τ is torque, r is radius and F is force (1kg has 10 Newtons of force)

If your bike is expensive racing machine, this tool is a must. By applying too much force you risk making cracks in lightweight materials.

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    Those numbers are good for quick estimation, but to be more specific, 1 kg at 10cm gives .98 Newtons. This is because of gravity. The force of gravity (F from above equation), is 9.8 N/kg. So a 1 kg mass creates as a force of 9.8 N. The number r in the force above is the distance in meters. So 10 cm is .1 meters. This gives us a torque of 9.8 * .1 = .98 newtons. Using 10 as the force generated by 1 kg of mass is sufficiently accurate for most purposes of estimating torque. – Kibbee Oct 24 '13 at 12:37
  • @Kibbee thanks, I know, but you will not notice the difference without measuring it :) – Papuass Oct 24 '13 at 13:00
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    So what if you are tightening your bike, while moving at speeds close to the speed of light? Also, even for the newtonian case, one should take in consideration the local variation of the gravitational field - on top of the mountain (right before that downhill) it would be weaker! – Vorac Oct 28 '13 at 9:12
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Even if your bike isn't expensive, you need a torque wrench, particularly in two places:

  • Cranks: If you overtorque your crank bolts, you risk damaging or deforming the cranks or bottom bracket.

  • Brakes: Brake rotors bolts in particular need to be torqued to the same value. Otherwise you can warp the brake rotor by torquing one side more than the other.

And of course if you have a carbon frame or seatpost, you MUST HAVE a torque wrench.

Since torque wrenches are so cheap (20-40 euro-dollars for a beam-type), it's worth having them in your toolset -- or borrowing them from a local bike coop, for those times they are required.

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  • Just bought a cheap torque wrench. Almost destroyed my seatpost because it simply wouldn’t click and with the long lever you have much less feel than with a hex key. – Michael May 24 '18 at 17:19
  • Cheap clickers are problematic. If you’re going to go cheap, get a cheap torsion bar type. – RoboKaren May 24 '18 at 17:20
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It is, in theory, possible to be vaguely accurate torquing bolts without an actual torque wrench.

Torque is measured in applied force multiplied times the length of the lever arm. You achieve the same torque applying a 10 pound force to the end of a 6-inch lever as you do applying a 5 pound force to the end of a 12-inch lever. In both cases the torque is pounds times inches -- 60 inch-pounds. (And you can easily do the conversion to foot-pounds or newton-meters or whatever.)

Measuring the length of the lever is fairly easy, of course (though you do need to take note of where on the lever you apply the force -- the "length" is from that point to the pivot point). Getting a good estimate of force, however, is trickier.

Skilled bike mechanics tend to develop a "calibrated arm", and can feel, within an acceptable margin, how much force they are applying. (Understand that being within a factor of 2 is generally adequate.) The Saturday afternoon mechanic, on the other hand, may not be so skilled.

One could presumably dig out a spring balance and use that to apply the force to the lever (socket wrench handle), but if you're going to that much trouble you might as well get a torque wrench. But, if you have some objects for which you know the weight, it is possible to lift the "reference" weight, getting a "feel" for it, and then apply the same force to the lever. One can even be "sorta" accurate scaling by half or by 2x (though beyond that is questionable).

Note that I'm not saying that such "shade tree" techniques are to be preferred over having the proper tools, just that there is the option, if you are caught in a bind.

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