How much extra energy is needed to push a Running Stroller?
(Dedicated to all the Helluva Engineers)
Last weekend, I had 14 miles scheduled in my training program for the upcoming California International Marathon. And then, I overslept. September may be officially Fall, but in Atlanta, it is HOT. So, I still needed to get the run done before the heat of the afternoon.
If you have little kids, I recommend a BOB for your life. He's always available, carries more than his weight, and is "wheely" cool. BOB is my running stroller (www.bobgear.com/stroller). Hard to believe now, but it was the first baby-thing my wife and I bought, even before a crib.
BOB lives in my garage. Of course, when I got there BOB was already dressed and ready-to-go. BOB never oversleeps. The only solution this morning was to take one of the kids with me. I filled my iPhone with Octonaut episodes, strapped in my 4-year old, and departed.
Now, running with a stroller is a completely different experience. For one, you are pushing something in front of you. One or both of your arms are holding the stroller at all times, eliminating the usual arm swing. And, downhills, unlike normal running, are not fun. You are constantly fighting for control afraid your kid will go barreling down the hill. But, it's better than not running.
Ten miles later, I was exhausted. (BOB was fine. Of course, he never gets tired.) I dropped off my 35 pound passenger and pushed through the final four miles alone. Later that evening, I got to wondering: How much extra energy is needed to push a Running Stroller?
To answer, I dug-up my old Physics I notes (Go Jackets!). A couple of free-body diagrams later, I got an answer.
Key Assumptions for Running Stroller Calculations
I ran on level ground 80% of the time and hills 20% of the time with an average 5 degree incline. (I love my neighborhood, but, in hindsight, I should have considered the running terrain not-so-subtly implied by Sagamore Hills).
Constant velocity (zero acceleration). My calculations start when I reach my goal pace, which I maintain throughout the run. (Therefore, I ignore the force to break the initial static friction holding the stroller stationary, and the force to accelerate to my goal pace. This short time is negligible over 10 miles).
The coefficient of kinetic friction is 0.67 (rubber on asphault). I Googled this.
I graduated Engineering School 13 years ago, so....if you send feedback, be nice :)
+415 Calories over 10 miles or +41.5 calories per mile. Typically, I burn 100 calories per mile, so this a 40% increase. No wonder I was tired!
As I walked BOB home after all that extra effort, I asked my son if he was ready for a nap. "No Dad, I'm not tired." So much for sleeping-in.