When do bars start to bend?

[RH709]

It honestly depends on the age, the bar, and who manufactures it. That's the basic answer, here's the scientific one.

When a bar bends, it's basically going through something known as deflection. That's when a member (a pole, plank, or bar) begins to bend because there's enough force added onto it somewhere that causes it to permanently bend. The formula that determines how much it bends (or deflection) is dependent on the material being used, the member's length, and its thickness.
While steel is a relatively strong substance, it is not perfect, and there are different types of steel as well. That's why several people are giving different answers, but generally staying around 300-400's. It's because different types of steel play a part in the amount of weight needed to bend a bar, as well as how long the bar is, how thick it is (even though that generally stays around the same), and how old it is (because the older the bar the more it's used and thus the more likely it is to bend).

Hope that makes some sense!

The bar's elastic modulus, diameter (use the bar portion not the collar section), and length from fulcrum to applied force are needed to calculate deflection.

Young's (elastic) modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain.

Find the Area moment of inertia for a solid rod.
https://www.engineersedge.com/calcul...re_case_11.htm

Find deflection based on specified load at specified distance.
https://www.engineersedge.com/beam_b...flection_9.htm

Now you are an engineer. You're welcome.

For the Rogue Ohio bar with a 28.5 mm diameter and a 190,000 PSI tensile strength and assuming the human eye will not recognize the deflection unless it is at least one inch, that means during a squat of 380 lbs you would see one inch of deflection. Deadlifting the same weight would change the fulcrum point and the deflection would change.

This will change if you have a bar with a higher tensile strength that will result in less deflection. During OLY lifts they discuss bars that whip. Bars with higher whip have less tensile strength and experience more deflection.

[RH709]

Once it starts bending, then it's just a short time before it's totally hosed.

From last year:

The bar I've been using since then is starting to show signs of slight curvature too, but I'm laying off the really heavy training to try and save it until I buy a Texas Power Bar : http://www.troybarbell.com/store.php...on=show_detail

Not true. The bar will only be structurally unsound if it is loaded past the elastic region of the material. This is called the plastic zone and it now has permanent deformation.

The bar can bend in the elastic region countless times and be 100% ok.

https://www.youtube.com/watch?v=t9eB0PKYAt8