Slight differences in Young's Modulus in Steel? 2

[DKirkham]

Barbell manufacturers claim that "whippiness" of their steel bars can be changed by varying tensile and alloy content. By "whippiness" they are referring to flexing. I didn't think Young's Modulus of elasticity changed with alloy content, tensile strength, or heat treat of the steel. (I am not referring to any stainless alloys here).

A friend of mine, with over 30 years of coaching elite lifters (he was also a competitive lifter), swears different barbells exhibit different "whippness" even though they are exactly the same diameter. He has measured the bars, and they are the same diameter to withing 0.001". He is a very bright man and I do not doubt what he says.

Barbells are typically made from cold roll, 1040, 4140, or ETD 150.

Does anyone have an explanation as to why some bars could be stiffer than others? I am stumped.

Cleanliness? Varying inclusions? Perhaps a variance in the depth of the knurl?

Are there slight differences in E with different alloys and/or heat treats?

The only explanation I can come up with is elite lifters are so sensitive they can pick up a slight change in stiffness. Elite athletes are extremely sensitive to very subtle differences.

My interest in this is we are always looking for ways to increase stiffness in our cars.

Thank you all in advance.

David

http://www.kirkhammotorsports.com

 

[EdStainless]

There is probably a slight variation in diameter, just not in the grip section.
Or slight variation in the width of the bar. Spacing the weights farther apart would result in more flex.
The variations are so slight that measuring them is difficult, much less feeling them.

But it could just be the feel, the bars may actually flex the same amount but could dampen the rebound differently.

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[DKirkham]

Ed,

Thank you for your reply. I had thought about spacing of the weights. I did not think about diameter of the bar under the sleeves (outside the grip area--under the plates). Good call. Those diameters are probably not held to a strict tolerance. Also, I did not think about differences in dampening. Does alloy affect dampening? (In the normal, commercially available steel alloys we are talking about).

From what I am reading in your reply, you confirm changes in tensile and changes in alloy do not change E. Is that correct? That is what I have always been taught too.

The literature is conflicting.

In Marks' Standard Handbook I found the following: (10th Edition, page 5-4 Table 5.1.3 Elastic Constants of Metals)
Cold Rolled Steel--29,500,000 lb/in2
All other steels--28,600,000-30,000,000 lb/in2

In United Technologies Pratt & Whitney:
Aeronautical Vestpocket Handbook Twentieth Edition 1986
Properties of Materials
Metals and Alloys, Modulus of Elasticity Page 114
Low carbon steel--31,000,000
4340--31,000,000 lb/in2
9310--30,800,000 lb/in2

What I am curious about is the reference in Marks' to the range of 28,600,000-30,000,000 which is a 4-5% difference. Do you know of different alloys that exhibit different stiffness?

Thank you in advance,

David

http://www.kirkhammotorsports.com

 

[redpicker]

You are correct in that the heat treat condition has little effect on the elastic modulus. Same with alloy content on Low-alloy steels.

This does not mean that there is no measureable effect, only that the effect isn't very large.

The measurement methods will have a larger effect than composition and heat treat condition, and the information you have found in Marks and other references mainly show there is little difference.

I would suspect that the weights with less "whippness" do have a larger diameter outside the grip section than others. This may be affected by the material, but only tangentially. That is, if you are trying to make the lowest cost weight, you would probably use cold-rolled carbon steel, since it probably has the OD tolerance needed for the grip section. If you are going to "customize" this geometry, you would probably use a higher cost material, perhaps 4140, since the material cost increase is very little compared to the additional cost of the machining operations needed. This only makes sense if you can charge a higher price, which I would expect you could if this difference was noticable and beneficial.

rp

 

[Maui]

I think that a more definitive definition of "whippiness" might be needed in order to answer the original question. Are you referring to the dynamic elastic strain response of the barbell during the act of lifting, and how this response changes from one bar to the next while lifting exactly the same load distributed on the bar in exactly the same way?

Maui

http://www.EngineeringMetallurgy.com

 

[DKirkham]

Maui wrote:

"I think that a more definitive definition of "whippiness" might be needed in order to answer the original question. Are you referring to the dynamic elastic strain response of the barbell during the act of lifting, and how this response changes from one bar to the next while lifting exactly the same load distributed on the bar in exactly the same way? "

Yes. Not being an elite athlete myself, I am not sure what they can, or can not, feel--or how consistently they load the bar when lifting.

I'm looking for a materials science explanation to their observations. I do understand, their observations are inherently subjective. When I read barbell manufacturers claim that tensile and alloy changes the "whippiness" in bar I am immediately incredulous--this sounds like marketing, not science. However, I have a very intelligent friend--with years of experience--who says some bars (of the same diameter) are "whippier" than others. From my conversations with him I understand lifters think whippiness=flex.

I apologize I was not clearer in my description of "whippiness." Thank you for asking for clarification.

David

http://www.kirkhammotorsports.com

 

[Maui]

What you are describing probably correlates with the effective spring constant of the bar as it undergoes elastic deflections while under load. If you model the bar as a bending beam of uniformly round cross-sectional area A with elastic modulus E and label the distance from the center of one hand grip position to the center of the load point of the weight as L, then the effective spring constant k is approximately

k = [3*(A^2)*E]/[4*pi*(L^3)]

where pi=3.1415. The spring constant is directly proportional to the elastic modulus. It is also directly proportional to the [italic]fourth power[/italic] of the bar diameter (through the variable A) and inversely proportional to the [italic]third power[/italic] of the distance L. So small changes in either the distance from the hand grip to the weights or the diameter of the bar will have a pronounced effect on the effective spring constant. And this may translate to the perceived 'whippiness" that your friend described.

Maui

http://www.EngineeringMetallurgy.com

 

[DKirkham]

Maui and Ed,

Thank you so very, very much. This is essentially what I told him. Slight changes in diameter and hand position will have a profound effect on the perceived and real flexibility of the bar.

To add to the confusion, I told him a coach could tell an athlete one bar was "whippier" than another and the athlete would probably believe him. The power of suggestion (like the placebo effect) from a respected coach is real. For example, I likened it to a coach who has an athlete with a mental block who can not lift more than 300 pounds--but who can easily lift 295 lbs. The coach will load 305 pounds on the bar and tell his athlete he "unloaded" the bar to below 300 lbs. When the athlete tries again he lifts the 305 pounds overhead. Sports is indeed, a mental game.

Again, thank you. I think you have answered my question. I sincerely appreciate the help.

David

http://www.kirkhammotorsports.com

 

[cloa]

Its probably tube with varying thicknesses and internals- its not solid cylindrical bar to save weight so the internals cause the large variations.

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[Maui]

Cloa, the high end good quality bars for heavy lifting are typically solid cross-sections, not hollow.

Maui

http://www.EngineeringMetallurgy.com