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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
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up vote 10 down vote accepted

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:

tau = r * F

Where tau 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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Great answer, many thanks :) –  Antoine_935 Oct 24 '13 at 12:05
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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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