The Science of Strongman: The Curious Case of the Bumper Plate PR
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 competition 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. We can represent a deadlift, with a 4 point symmetric loading model. This means that the bar is supported at 2 symmetrical points near the center (hands) and there are 2 forces (weights) near the ends of the bar. Please understand that this model is an approximation and that I have taken a conservative approach to underestimate deflection 
The plate weights added induce 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 create a larger bending moment: 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.
Approximate 4 point loading model 
Max beam deflection
Δmax : how far the middle of the bar deflects upward
P : force generated by weight (gravity) at its mass center. Is also equal to the supporting force required by the lifter’s hands. This neglects the bar weight and is a conservative calculation. 
L : distance between mass centers of weights
a: Distance between mass center of weight and middle of 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” and “a” terms are squared. These values will grow as you increase the thickness of the plates from competition plates to bumper plates. Since a 20kg bumper is 5x thicker than a 20kg competition plate, the “L” and “a” terms grow quite significantly.
When using (5) 45lb bumper plates on a standard power bar, the deflection that the bar sees is 1.32”. On the same bar, (5) 45lb competition Ivanko plates causes a .63” deflection in the bar. This 100% increase in deflection when using bumpers may seem small, but when you add up the nearly ¾” difference over many reps, in many sets, in many training sessions, you can see how significant the difference can be. The extra deflection is the equivalent to deadlifting with competition plates placed on a thick gym mat.
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 beam deflection 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.
Moment of inertia in cylinders
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 34%. A standard deadlift bar is also 4” longer than a standard power bar. Lets now refer back to our 5 plate deadlift example. When we compare a standard power bar with competition 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 273%. The deflection rises from a mere .63” in the competition setting to 2.35” in the bumper plate/deadlift bar scenario. Considering that a conventional ROM for an average height male is about 23”, this kind of deflection becomes very relevant . The extra deflection now becomes the equivalent of deadlifting with competition plates placed on 2x4s.
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.”
 This model ignores the deflection of the bar under its own weight. If you have ever lifted an empty bar, the deflection is not discernable to the human eye. The accuracy of this model increases with the addition of more weight. To see the calculations and work, download my Deadlift Deflection Calculator.
 Measured empirically using 5’9” male subject. Your ROM may be more or less depending on stance, height, and other proportions.
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 as a mechanical engineer, 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!
Questions or feedback about this article? Email Mark at email@example.com
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