So today at the gym a guy told me that lifting a barbell loaded with, let's say, 200 kg of iron plates is actually harder than lifting a barbell loaded with 200 kg 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.
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.
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.
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?
Measure it. The bar diameter is a far bigger factor than metallurgy, given that bars are not made of pot metal.
The tensile strength is not relevant in terms of stiffness. It's usually a good correlate, but the elastic modulus and cross sectional areas are the factors you need. Bar manufacturers do not publicize the elastic modulus.
Well, three plates of any kind .... is not "heavy." Five? yes, the COMs are quite a bit farther apart.
And no, tensile strength doesn't go hand in hand with modulus of elasticity.
As Mark said bar diameter has more to do with it really. Type of knurling cut too....cut away the very edge of the bar, you are effectively narrowing the bar.
Not always the case, but when talking about bumper plates, you are usually talking about a bar used for Olympic lifting (thinner). Those two things go together quite a bit (not always).
A thicker powerbar? no so much whip.
Then consider those thin comp 25 kg plates they use at hard core PL gyms....more mass even closer in.
Don't know if this will clarify or muddy, but the amount by which steel deflects in response to load (young's modulus or modulus of elasticity) at ambient temperature is basically the same for plain carbon steel as it is for fancy high strength alloy steel. What then is the practical difference between fancy and plain steel? The fancy alloy will deflect (bend or stretch) farther without yielding (permanently deforming, indicated by change in length or shape after load is removed).
So, regardless of alloy, skinny steel bars are floppier than fatter steel ones. No getting around that. Bar stiffness is proportional to the diameter raised to the 4th power...a 1.19" diameter bar is twice as stiff as a 1" diameter bar.
The elastic modulus of different steels do vary practically significantly – especially once you start to add in heat treatments. Why do you believe otherwise?
Are you sure you are not confusing yield strength and elastic modulus?
In my almost 15 year career, I have never used an E value for steel that wasn’t 29,000,000 psi. I’ve used several different yield strengths depending on the ASTM designation.
We need a definitive article on the technical aspects of bar metallurgy and mechanical characteristics. You guys want to collaborate on this?
At 91, Mrs. Rizan does a sit-up accidentally. We have not been doing any ab work, just deadlifts, leg presses, benches, lat pulls, and some getting-up-off-the-floor work. Carmen Phillips is her trainer, and I'm very proud of both of them.
Impressive. Why does Carmen keep her hands on the bar though? If the weight is too heavy for Mrs. Rizan, why not take off the plates?
She's lifting the weight herself, but her control of the bar path is not good. She's almost blind.
She’s been training about 3 months. I neglected to mention that, sorry. She started with us about 2 weeks before her 91st birthday. She has stopped using her walker and her cane.
What's that, 55 lb on the bar? The bar speed is real good, looks like that weight's well within her capabilities.
Is she doing full range deadlifts as well? I hope I'm that badass if I ever make it to 91.
It's about 35. But she does full deadlifts, up to about 50 now.
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