OK, I know that a 45 pound bumper plates weighs the same as a 45 pound iron plate. My experience, however, is that bumpers seem to go up easier. True or myth?
If true, wondering if people have explored or exploited this idea.
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Is this a trick? – Dave Newton Jun 14 '12 at 13:49
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No, it's a serious question. Consider, as a thought experiment, the difference between lifting plates of the same weight made out of iron, out of lead, and out of gold (if that was affordable). – portabella Jun 14 '12 at 14:10
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2Density? You're suggesting it's easier to lift less-dense objects of the same weight, all other things being equal? And when you say, "within limits", you're suggesting humanly-detectable limits? Myth. – Dave Newton Jun 14 '12 at 14:37
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1If this is about pinning the purported difference on physical constants, this would be better suited for physics.SE (where it will get summarily dismantled, unless I'm egregiously wrong) or skeptics.SE (where it will get closed because there's no notable claim). – Dave Liepmann Jun 14 '12 at 14:50
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1Is this happening on the first lift, or could this be due to bouncing on the second and successive reps? – Dave Liepmann Jun 14 '12 at 15:27
I haven't experienced bumper plates feeling any lighter than iron or iron-coated-in-rubber plates. Mass is mass.
Psychological tricks
Most probably you're experiencing a psychosomatic aversion to the iron plates because you're envisioning the bill from your landlord about the floor if you have to dump the bar loaded with iron plates.
Like entrepreneurship increasing alongside universal health care, bumpers (and a proper platform) make going for tough lifts a little psychologically easier because you've got a safety net for the rare occasion when things go wrong.
Barbell rotation
The other difference I've noted is that I lift iron with my crappy old bar that doesn't rotate so well, whereas I lift bumper plates with my less-crappy, less-old bar that rotates pretty well. How smoothly the bar rotates makes for a stunning difference in the difficulty of the fast lifts.
Similarly, the size of the plate might also affect the difficulty of the lift, due to rotational inertia. If you're comparing 25-pound iron plates to 25-pound bumper plates, the shorter 25-pound iron would seem easier.
Test Your Hypothesis
Maybe your iron 45s are actually 50s. Maybe your bumpers that say 25 are actually 23.5. Maybe the bar you load with bumpers is 40 instead of 45--that happened to me. Take your plates and bar to the scale and be sure that they're labelled properly.
answered Jun 14 '12 at 14:33
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Aren't they the same size as the 45lb plates, diameter-wise, with the weight differences reflected in width? Rotational inertia would depend on weight being further from the axis, IIRC. If you had a mixed bar (either bumper plates + < 45 iron, or 45 + < 45 iron) I could envision a difference if you needed to rotate the plates, maybe? – Dave Newton Jun 14 '12 at 15:09
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It's definitely true that I haven't controlled for all the factors that could be relevant; that's why I'm throwing this out as speculation only. I do think "psychological tricks" is a bit of a dodge as an answer; from a strictly pragmatic point of view, anything that lets you lift a heavier weight is "legal". In this case, I own my house and lift outside, so it's not a psychosomatic aversion to dumping the bar; it really does feel lighter. – portabella Jun 14 '12 at 15:09
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@DaveNewton Yes, you're correct--I meant 25 pound bumpers (same height as 45s) compared to 25 pound iron (usually significantly shorter, except for cases of competition powerlifting). – Dave Liepmann Jun 14 '12 at 15:10
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@portabella I don't mean to connote anything negative by using the word "tricks". I mean like magic-eye pictures, or thinking "up! up! up!" on the down phase of a press. Tricks that work are good! – Dave Liepmann Jun 14 '12 at 15:25
In the interests of clutching at straws: the only physical differences between plates of different density could be:
The moment of inertia of the plate. This has no effect unless you are rotating the bar, whence the energy required to produce the rotation goes like the angular velocity squared (wrist curls). Assuming uniform density of plates of the same thickness then the moment of inertia of when rotating would be greater. Plates of the same diameter would have identical moment of inertia (where the necessarily greater volume of the plate would be accounted for by additional thickness). Not only is this totally negligible, any effect is probably contrary to what you seem to experience.
The air resistance acting on the plate. A lower density weight will have a larger volume, and if it is to remain a plate must have a slightly large cross-sectional area to pull through the air. Again totally negligible and and contrary to the hypothetical effect.
The young's modulus of the plate. It is conceivable that the elastic response of the plate could have an effect on how easy the bar is to lift. This is really stretching credibility, but in the extreme case one can imagine lifting a rigid inelastic weight through direct contact and lifting the same weight attached to elastic straps. In the latter case the force required to initiate the lift would grow gradually through the push. The energy required to lift would be same (actually it would be slightly more since the elastic would dissipate some work as heat), but it is possible to imagine that it might be physiologically favourable for the maximal force of the lift to come later in the movement. With essentially rigid plates this is also totally negligible.
So yeah, mass is mass is a pretty adequate summary. Your plates may well actually be slightly lighter. Have you weighed them?
Assuming you have weighed the bumpers and the metal plates and found them to be the same (or at least close enough), the difference is one of perception. One way to dispel that in your mind is to hold the lockout on a deadlift as long as you can. After a few seconds or so, those bumpers will feel just as heavy as the metal plates.
Now, some people have taken to cheating with their lifts when using bumpers. This cheat is more effective with high-bounce bumpers than the more firm competition bumpers. Essentially, they start off the deadlift by pushing down on the bar first to use the elastic energy from the bumpers to help them break the bar from the ground. That is not doing anyone any favors, least of all you.
Depending on the gym you go to, not all bars will weigh the same. You may have some 35lb bars and 45lb bars. That will definitely lead to the perception that 45lbs in bumper form is not the same as 45lbs in iron. But at the end of the day a pound is a pound is a pound. Even if you had a 45lb plate made from feathers (assuming you could get the density right), it's still a 45lb plate.
answered Jun 14 '12 at 16:33
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I'd be interested in seeing some numbers about how much of a "boost" this actually provides--I'm skeptical it's statistically meaningful. (What would "getting the density wrong" imply?) – Dave Newton Jun 14 '12 at 16:50
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Regarding the feathers? If density were wrong, you wouldn't be able to make a plate that would fit on the bar. Feathers are not dense at all. Even rubber needs metal inserts to help increase the weight. – Berin Loritsch Jun 14 '12 at 17:41
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1Regarding the boost of the cheat, if you used something like the Rogue Hi-Temp bumpers which are essentially high bounce tires, the boost is significant. The less bounce in the bumper (meaning closer to competition quality), the boost becomes less significant. However, it is still more than what you would get if you had metal plates. – Berin Loritsch Jun 14 '12 at 17:43
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"More" yes, but I'd be interested in something actually empirical. – Dave Newton Jun 14 '12 at 18:43
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@DaveNewton Try bounce-deadlifting 1.5xBW or bounce-power-cleaning 0.75BW for reps with iron on dirt versus bumpers on a platform. That's pretty empirical. Do you mean measured? – Dave Liepmann Jun 14 '12 at 21:29
