The Science of Strongman: The Curious Case of the Bumper Plate PR

By Mark Jones 

There are a number of terrible reasons to love bumper plates. They are rubbery, bouncy and don’t smell like rust. They take up tons of room on the bar, creating the illusion of lifting lots of weight. You can even slam them down from overhead if you enjoy using a bent barbell. Most importantly, you mysteriously set PRs every time you deadlift with them after training with steel plates.  I hate to be the one to burst your bumper plate bubble, but deadlifting with bumpers is significantly easier when compared to iron plates.

This doesn’t mean that your coveted PR no longer stands. It just means that you have introduced another variable to consider when measuring your strength. When you PR your deadlift using bumper plates, you obviously still lifted the weight. But were the conditions the same as the last time you hit a personal best? Did you really get stronger? Or did your equipment simply give you additional mechanical advantage? The real answer is probably some combination of the above. We can solve the mysterious case of the bumper plate PR with a little science and engineering.

 

Bumpers induce more bar bending.

To understand this concept, we first need to understand beam bending and moment arms. For the sake of simplicity, we can approximate a basic model of a barbell bending as two cantilever beams. This means that the bar is fixed at the center (holding the bar) and has forces acting near the ends (the weights) and in the middle of each bar half (the weight of the bar itself). [***] See notes at bottom of page.

The weight of the bar, as well as the plates added to each side, induces bending in the bar. The bending is a result of a “moment” and is caused by a force that acts a certain distance from where the bar is fixed. The further away the force acts, the greater the moment.

Moment = (Force) x (distance away from axis)

This means that there are two ways to bend the bar more: adding more weight, and moving the weight further away from the center of the bar. If you don’t like formulas, try holding a 25 lb. plate to your chest and then out in front of you. Boom, concept learned.

When you throw four bumper plates onto a bar, the ridiculous thickness of the plates moves the force further away from your hands. Let’s look at how this extra distance affects the bend in the bar.

δmax : how far the ends of the bar deflect

P : force generated by weight (gravity) at its mass center

L : how far away P is from your hand

E : material constant based on properties of steel

I : geometry constant based on cross section of barbell

 

As you can see from my poorly drawn diagram, the “L” term is cubed. This means that if bumper plates move the center of mass out by 5.5 inches, this will cause the bar to deflect over 68% more than steel plates. [1] Deflection of this magnitude is absolutely non-negligible.

 

Deadlift bars pretty much have the same effect as bumper plates

In case your panties weren’t already in a bunch, a little math can help explain your love affair with deadlift bars. You may remember all the way back our cantilever beam bending equation from before. The “I” term in the equation is based on the geometry of the cross section of our bar. For a barbell (cylinder), our moment of inertia term “I” is shown below where “R” represents the radius (“M” is mass in case you were wondering.)

The diameter of a standard deadlift bar is 1.06” vs. a standard power bar with a diameter of 1.14”. If we plug into our equation, the difference in diameter alone will increase the deflection in the bar by 15% (mass held constant). A standard deadlift bar is also 4” longer than a standard power bar. Lets now refer back to our 405lb deadlift example. When we compare a standard power bar with steel plates to a deadlift bar with bumper plates, the extra length combined with the smaller cross section and 2” collars causes the deflection to increase by a whopping 149%[2]. Keep in mind this math was done with 4 plates. The numbers get even more ridiculous when you add another plate to each side.

 

More bar deflection means performing less work

When you begin a deadlift, dozens of things happen before the bar leaves the ground: your shoes compress, the floor sinks, the bar moves to the top of the holes in the plates, your arms stretch, your spine compresses, the bar bends. After ALL of those (and maybe more) things happen, only then can the weight leave the ground. Before the weight actually leave the floor, you are only pulling with a fraction of the force required to actually lift the weight.

 

By the time this happens, your hands have moved up a very measurable and non-negligible distance. When you deflect the barbell even further with bumper plates, you have now moved the same weight less distance. The weights (and therefore the mass center of your system) are closer to the floor at the lockout, which means you are performing less mechanical work.

Bumper plates and deadlift bars drastically change your starting position

As in any strength sport, mechanical advantage is absolutely key, specifically during static lifts. The deadly combination of bumper plates and deadlift bars put you in an extremely advantageous position. The more deflection that the bar sees before the lift starts, the closer to lockout you are able to get before picking up any actual weight. A higher starting position effectively turns a deadlift from the floor into a block pull.

All of these factors pile onto the laundry list of reasons that lifters are in love with bumpers and deadlift bars. Both put you in a much higher starting position, and both allow you to accomplish less work while lifting the same weight. This is not to say that bumper plates and deadlift bars are useless or that you are cheating if you use them. Each has their place in both training and competition.

Obviously you can only use what you have access to, but if you have the choice, you probably want to train with what you will see in competition. Powerlifters may want to stick with steel plates on a standard power bar. A strongman may more closely represent competition settings with bumper plates on a deadlift bar. However you choose to train, remember the wise words of the late Jón Páll Sigmarsson “There is no reason to be alive, if you can’t do deadlift.”

 

[1] Calculation done for a 405lb deadlift with (4) 45lb plates on a standard Texas power bar. Compares 1” thick steel plates (York barbell steel) to 3.75” thick bumper plates (Rogue bumpers). This calculation ignores the weight of the barbell itself, and was done under cantilever beam bending conditions.

[2] Comparing the dimensions of a standard Texas power bar with a Buddy Caps Texas Deadlift bar.

[***] While a 4 point bending model more closely models a deadlift, the hand position is unknown and greatly affects the results. 3 point bending model (supported in the center) yields the same results as cantilever as far as relative bending since L^3 still exists in the numerator and I still exists in the denominator. I have chosen to use the cantilever model for simplicity and ease of understanding. I believe this to be a very close approximation and certainly useful to demonstrate the principles of beam bending.

 

By Mark Jones

 

Mark is a lightweight amateur strongman, long time powerlifter, practicing mechanical engineer and owner of your favorite stone sleeve company, 4Armour LLC. As a student at The Pennsylvania State University, Mark studied mechanical engineering, mechanics, red beard growing and was an athlete on the Penn State Powerlifting team. Since then, Mark has been working in product development, and directly applying his knowledge of physics to lifting smarter both in the gym, and wherever heavy things may be hiding from him.

Check out his hand-crafted, American made stone sleeves in the Starting Strongman store!

https://store.startingstrongman.com/product/4-armour-stone-sleeves/

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Questions or feedback about this article? Email Mark at [email protected]

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