Torque specs, just wondering

[Crazy440]

Just finished torquing the ball joints, on the upper and lower control arms to 100 ft. lbs. While doings so, I wondered how that number was arrived at? How is a torque spec decided? Why 100# not 90 or 110? What happens if you only go to 90? My cam bolts where 65#. Why not 70 or 80?
Inquiring minds want to know. =)
Just wondering

[ws23rt]

When a bolt is tightened it starts to get longer like a rubber band or spring. Different qualities of metal stretches differently.

As it gets longer it takes more force until its max strength is reached. Beyond that point which is called yield it will no longer return to it's original length.

As an example for a cylinder head if the bolts are not tight enough the combustion forces would do some of the bolt stretching.

A torque value is estimated considering many factors. Dry threads, lubed threads, thread pitch etc. This is why we see what seems to be conflicting torque values given in different cases.

By far the best way to get ultimate bolt strength is to measure it's stretch. (A long bolt will get longer than a short bolt and a high strength bolt will stretch less than a lower grade bolt.

On very large bolts (4"-5"- 6"dia.etc) it becomes impractical to use a torque value and the number of degrees the nut is turned will give the bolt stretch needed. Some large studs have a hole down the center for a heater to expand the stud to allow turning the nut.

There is more to this topic but this is the basic stuff.

[Crazy440]

Thanks for the info, ws23tr. 
Does the same principle go for tightening a nut, onto a stud?  If you were to stop at 90# instead of the 100#, the stud threads would not be stretched correctly? Or would the part, just not be secured tight enough?

[ws23rt]

Thanks for the info, ws23tr.  
Does the same principle go for tightening a nut, onto a stud?  If you were to stop at 90# instead of the 100#, the stud threads would not be stretched correctly? Or would the part, just not be secured tight enough?

Yes tightening a nut on a bolt is the same as tightening a bolt into a threaded hole.

The objective is proper clamping force on the parts being connected.

You mentioned thread stretching. That is off a bit. The bolt body under tension is what is going on here. The threads are a means to accomplish this.

In most applications full bolt strength is not required. In that case a generic bolt torque chart for the size and grade of bolt being used is good with a wide margin for error being no big deal. (bolts that connect panels for example are much stronger than the panels and need not be torqued to their ultimate capacity). But when a torque is called out for in an assembly like a trans. or engine it needs to be followed with the correct strength bolts/nuts.