When does the flexibility of the bar for the deadlift even start to matter?

[sunetdanne]

Hope that someone with some mechanical engineering background could help to clarify this.

I notice many powerlifters tend to think that lifting with wl-bars is a big no no, reasons beeing

2) The flex of the bar, the weightlifting bar will flex so much so that if they pull 600lbs with it they then cant pull 600lbs with an pl bar in competition, due to the fact that they were uses to pulling with an flexible bar.

Metal plates,

Most of them also think that they can only pull with metal plates since the thicker bumper plats will give an longer leverage for the bar, hence more flexible bar,

IPF allows rubber plates in competition, if the same bar is used and it’s not flexible, it does not matter! A typical 25kg plate is 27mm the 25kg wl plates are 2,5” so the wl plates are around 2,3 times wider.


So my questions are; does it matter if one use an weightlifting bar instead of an powerlifting bar when pulling heavy deadlifts, will the weightlifting bar flex so much that it will make a difference(easier pulling position) If so at which load?

Will using metal plates instead of rubberplates change the pull? Would rubberplates make the bar flex more due to the longer distance(distance from com of weig

[Michael Wolf]

Those things each make a difference at sufficiently heavy weights, and together make more of a difference than each individually. At what weight does that difference start? I don't know. At what weight does that difference become significant to make a difference for a competitive lifter? I don't know. But my guess is that it has to be a truly significant amount of weight to matter to the extent that it would screw with someone's meet performance, and if a lifter isn't very close to being nationally competitive, it probably doesn't matter.

I think someone tried to calculate the bar deflection from bumpers vs metal plates a while back down in one of the other sections. Don't know how that turned out. Maybe someone can find or bring up that old thread.

[Chris Lippke]

Deadlifting with powerlifting bar and metal plates vs weightlifting bar and bumpers

Rip answers this question on his board where you asked the same question yesterday.

[Michael Wolf]

Babu Bhatt!!! Good work, Chris.

Dan, you've been on this board for over 6 years. You should know better.

[Crom]

With very approximate numbers: if you pull the center of a 28mm bar that is loaded with 600 lbs up from the very center, it will deflect about 0.8" before leaving the ground.
Spread your hands out, and it is a little less. Move the weights out (bumper plates) and it is a little more. Use a thicker bar, it is a little less.
Does .8" to 1" make a difference ?
Not to me ( I am not even close), but I am sure that to the top competitors it does.

[Michael Wolf]

Where do you get your approximations from, Crom? Are these based on calculations, empirical testing, from the deep dark recesses of your asshole?

The two things depend on one another, right? A 29mm stiff bar with metal plates vs a 28mm whippy bar with bumpers. The particular thickness of the bumpers and metal plates (regular steel, competition discs, competition bumpers, cheapo thick bumpers). How much does the tensile strength of the steel in the bar play a role, and then that also interacts with the thickness (29mm with higher PSI/29mm lower PSI/28mm higher PSI/28mm lower PSI)? Grip width, as you alluded to. Etc etc... that's why I ask where you get 0.8-1" from in such a multi-factorial situation. Are you assuming a 28mm bar is whippy and loaded with thick bumpers?

Generally, I'd say if you're competitive in the sense that you actually go to win meets, not to challenge yourself and set PRs, it matters and you should train as close as possible to meet conditions equipment. Not everyone can, but this would be better. If not, then meh.

[manveer]

The two things depend on one another, right? A 29mm stiff bar with metal plates vs a 28mm whippy bar with bumpers. The particular thickness of the bumpers and metal plates (regular steel, competition discs, competition bumpers, cheapo thick bumpers). How much does the tensile strength of the steel in the bar play a role, and then that also interacts with the thickness (29mm with higher PSI/29mm lower PSI/28mm higher PSI/28mm lower PSI)? Grip width, as you alluded to. Etc etc... that's why I ask where you get 0.8-1" from in such a multi-factorial situation. Are you assuming a 28mm bar is whippy and loaded with thick bumpers?

Bar diameter, distance between collars, collar width, plate width per unit mass and grip width are all factors. Knurl depth too. Tensile strength is independent of Young's Modulus, which is what you care about in this case. So, ignore tensile strength. Young's Modulus is pretty close to the same across different steels, especially across steel alloys that are going to be used to make barbells. The steel used isn't going to impact deflection to any meaningful degree - it all comes down to the geometry of the bar and plates (as well as grip used).

This was all discussed in great detail, but I think the thread was deleted from E&P. I still have the spreadsheet I created back then to calculate deflection based on 4-point bending.

All that said, I think Rip's response was a good one.

[Crom]

Where do you get your approximations from, Crom? Are these based on calculations, empirical testing, from the deep dark recesses of your asshole?.

I get them from the deep dark recesses of beam bending theory, as expounded in Roark's Formulas for Stress and Strain. 6th Edition ISBN 0-07-072541-1

[Michael Wolf]

I get them from the deep dark recesses of beam bending theory, as expounded in Roark's Formulas for Stress and Strain. 6th Edition ISBN 0-07-072541-1

Ha! An engineer of any type or sort I am not, but this is related to Young's Modulus mentioned by Manveer, no? Thinking about it a bit, I do question his assumption that the steel used doesn't matter. Sort of. I've seen cheap globo gym barbells start bending at 315 when held in the rack, even with the two spaced out points of contact. But if we're talking about a bar that any serious lifter would be using then ya, probably it doesn't matter.

[manveer]

I get them from the deep dark recesses of beam bending theory, as expounded in Roark's Formulas for Stress and Strain. 6th Edition ISBN 0-07-072541-1

I think you'd be better off with this: Bending, Deflection and Stress Equations Calculator for Beam Supported on Both Ends Loaded Two equal Loads | Engineers Edge | www.engineersedge.com

Ha! An engineer of any type or sort I am not, but this is related to Young's Modulus mentioned by Manveer, no? Thinking about it a bit, I do question his assumption that the steel used doesn't matter. Sort of. I've seen cheap globo gym barbells start bending at 315 when held in the rack, even with the two spaced out points of contact. But if we're talking about a bar that any serious lifter would be using then ya, probably it doesn't matter.

It's not an assumption, it's what the math bears out. In your example of a bar in the rack loaded to 315, even taking the min and max room temperature Young's Moduli for different steels from here, you're talking about .12" deflection at the center versus .11" deflection at the center given the same nominal bar and plate dimensions. Must've been something else different about the bar geometry.