Bumper plates vs iron plates

[Mirat]

So today at the gym a guy told me that lifting a barbell loaded with, let's say, 200kg of iron plates is actually harder than lifting a barbell loaded with 200kg of bumper plates.
I immediately thought it was bullshit, but then another guy who had overheard the conversation stepped in and confirmed what was being said. The reasoning behind this is that bumper plates are bigger, so the weight on each side of the barbell is applying a larger lever arm on the bar which will make it bend more than that same weight of iron plates. I studied a fair amount of physics and that made sense. So with bumpers, the bar will bend more easily, but why would it be easier to lift a bar that bends more ?. For the deadlift, I think that the bar will have bended a fair amount before it leaves the floor, so one might actually start the pull from a more mechanically advantageous position. For the other lifts, I can't think of a single reason why it would be easier to lift a bar that is bending than a bar that is straight.

[Mark Rippetoe]

The longer the moment of inertia, the more whip the bar shows. Whip is not good on your back in a squat. And 200 pounds of feathers and 200 pounds of iron both weigh 200 pounds. So, the guy is wrong.

[heavyiron8]

it won't matter for the squat but for the deadlift, more whip does indeed make the initial off the ground pull a little bit easier, as the outer plates are still in contact with the ground while the inner plates are already mid-air.

[Mark Rippetoe]

It does matter for the squat, but not until the weight gets heavy.

[Mirat]

It does matter for the squat, but not until the weight gets heavy.

Approximately how much is heavy ?

[Leanne M. Lloyd]

More whip in your squats make them more difficult, not easier.

[Mark Rippetoe]

Approximately how much is heavy ?

Depends on bar diameter, but up in the 400s. Certainly in the mid-500s.

[AndrewLewis]

I didn't notice it in the squat until the 405lb. If I pay really close attention, I can feel it at lower weights with shitty bars, but I can't even feel the whip on the B&R bar at 405lb.

[Eric Schexnayder]

Wouldn't the tensile strength of the bar determine this more than just the moment of inertia? I mean we're not talking that much difference. The center of mass of three bumper plates versus three iron plates cannot be that far apart, an inch or two?

[Mark Rippetoe]

Measure it. The bar diameter is a far bigger factor than metallurgy, given that bars are not made of pot metal.