At long last, I’m finally getting around to writing ‘The One About The Grind Box.’ You may or may not have any idea what I’m talking about, but this post has actually been a very long time coming. In fact, there’s so much to say about it that I’m breaking it up into two separate posts.
In this first post, I’ll deal with the straightforward stuff — designing the box, building it, funding it, etc. In the second post, I’ll deal with the rhetorical complexity of both the box and the academic issues tied up in it.
So let’s get to it. Here’s a picture of the finished box:
It looks nice, right? Well, let me tell you, it IS nice. It cost about $500 to make. And while it’s only three separate pieces, a huge amount of work went into creating it in exactly the form it has.
Let me explain.
Originally the box was built only as the larger, grindable section on the top. However, it was designed to accommodate—some day—the two removable (and optional) risers that can raise the coping/deck from about-knee-high to about-hip-high.
The grind box itself is 16 inches tall (to top of coping) and it’s 24 inches wide (from outside of coping to outside of coping). The entire box is exactly 16 feet long. The risers are 13 inches tall, exactly 37 ½ inches wide, and 4 feet long. With the box on top of the risers, the whole thing is exactly 29 inches tall.
The grind box portion itself is incredibly sturdy. It’s covered with 3/4″ plywood on the sides, ends, and top. The sides and ends were sheeted with CDX grade plywood, and the top was sheeted with some really fancy cabinet grade plywood—smooth side up. In the interior, the skeleton is framed out of 16 foot-long pressure treated lineal 2x4s, and it has double reinforced cross members every 16 inches (along the long axis of the box). The entire thing (except for the coping) is painted with scratch-resistant latex deck paint.
As for the coping, it’s made of 16 gauge 2-inch diameter steel tubing and it’s secured every four feet with 3-inch deck screws. It has a generous ½-inch lip on both the top and the sides, and is painted with some VERY fancy acrylic modified alkyd enamel paint (about $150 per gallon retail).
The whole business about the grind box portion is pretty straightforward. It’s more or less a standard skatepark-quality grindbox (read as “beefcake”), but the risers represent a much larger problem. In fact, the problem with the risers represents such a large problem, that I felt the need to go full on “academic” about it, and brought in a Phd in physics to figure out the math.
But here’s the catch. I didn’t want the box to be merely “my box.” Even though I’m its main builder, “owner,” and caretaker, I think of myself more as the box’s steward. It will be here in Austin much longer than I will, and it belongs to the guys who skate it every week, who helped build it, who help maintain it, and who helped pay for it.
So when I brought in Dmitry Meyerson — a rollerblader and Phd student from the University of Texas Department of Physics –I wanted everybody to be there. Too often, in my view, academics are brought in to some situation to solve a problem, but again, too often the end users understand little or nothing about the nature of the solution itself.
So we had “Physics Day” in the garage of my old house.
My old garage was really something of a rat hole. I mean, a literal rat hole. It was dusty and dingy and smelled of stale cigarette smoke, old coffee, and spilled beer. And there were actual rats — not the nasty New York City subway rats though. A rather strange family of small white rats had taken up residence in my backyard. Essentially, this made my garage a perfect rollerblader hangout — replete with homies from several different species.
Anyway, with the dry erase boards that I used to plan classes, design articles, and sort out any number of other tasks and the large picnic table I built to provide seating for the garage, it was the perfect spot for an ad hoc physics class. I got to work explaining the risers I wanted to build, and Dmitry got to work sorting out and explaining the physics.
But let me not get ahead of myself. One of the first questions I wanted answered was the weight of the box. We knew it was heavy because we’d been transporting it every Sunday for about six months or so at the time Physics Day was held. The task of moving it was made substantially easier by using a small cart I made that lets it roll at one end and be carried at the other end using two Arc-of-the-Covenant-style lifting pipes. But the box itself was still really heavy, and therefore potentially very dangerous.
If we were going to raise the box off the ground, we had to know how much weight was going to be potentially threatening us, and, because of that, we needed to make sure the width of the base of the risers would be wide enough to ensure that the box would never fall over as a result of skating it.
An additional problem with finding out the weight of the box is that I only own a small, glass bathroom scale — suitable for my wife and me to use, but not nearly robust enough to weigh something as giant and heavy as the grind box. So Dmitry came up with an ingenious solution: put a fulcrum one foot away from the center axis and have someone lift the box a tiny amount while standing on the bathroom scale.
Here’s how that worked: putting a fulcrum a foot away from the center of the box and lifting it a tiny bit will tell you what a volume of the box weighs per one foot of length. Multiply that by 16 (the total length of the box) and you get a good estimate for the box’s total weight. After doing this procedure (and subtracting Jarrod’s weight), we estimate that the box weighs approximately 538 pounds.
Now here’s the thing, the weight of the box doesn’t actually matter to the equations that Dmitry brought to bear on the problem, but it does emphasize the need to get our numbers right. A 500+ pound box simply CANNOT fall on someone. Such an accident could break an arm or a leg, or worse. The fact that less than half of Austin’s rollerbladers have health insurance only underscores this problem.
So, having a good estimate for the weight (and potential threat of what we were undertaking) in hand, we got to work solving the actual problem of the day, which is this: How wide does the base of the risers need to be so that it absolutely will not fall over during the regular course of skating it?
That’s really not a simple question. What exactly is normal skating? What’s the weight of a NORMAL skater? What speed does he or she skate at? At what angle does the weight/mass/energy transfer occur? Is friction involved? Is rotational torque involved? How much energy does a skater transfer and how much does he absorb himself? These aren’t easy questions to answer.
So we did what everyone (always) does: we created a representation of our problem. (I won’t elaborate much on this here because it will be the focus of the next post).
Basically, our representation took the following form. A 200-pound skater is willing to jump up and grind the box at 10 mph. We know this because we did experiments in the road in front of my house. We towed people (on their skates) with a car at 10 mph and asked them whether they’d jump onto the box and grind it at that speed. Most people said yes, but qualified that such a speed was significantly faster than they would normally skate the box. So, content with the 10 mph number, we took the high estimate for weight and figured for a 200-pound dude.
But the direction of impact also matters a lot to our calculation. For the sake of safety, we used the 200 pound/ 10 mph numbers and applied the force transfer at an angle perpendicular to the normal direction one might skate the box. We also assumed perfect energy transfer.
In effect, our representation was that a 200-pound rollerblader could do a Zangief-style (jump attack) stall at 10mph on the coping and have the box not fall over.
So, given that representation, which in our view was meant to be overkill, Dmitry eventually gave us our number for the riser width. Since he was calculating in metric, we originally got a solution of 1 meter (or about 39 inches), but several of us wondered if we couldn’t shorten that up a bit so that there would be less angle to have to jump over (You don’t want to clip your toe every time you jump on the box, after all). So we crunched the numbers again and came up with a smaller number: 37.5 inches.
Satisfied with this number, it was now only a matter of building the risers.
So several months later, Mick Casals finally pushed for a “build day” to finally get the whole project finished. Several Austin bladers came over to help out, including Mick, Heath Burley, Jay Geurink, Jarrod McBay, and myself.
Heath and Mick got to work building the frame:
And Jarrod and I got to work building the base and angled portions of the risers:
The final product included our calculated width of exactly 37.5 inches, and also included a handy collar design that allows the grind box to sit snugly on top of the two risers.
While several of us fronted the money for the initial build, the rest of the dudes in Austin took on the brunt of the cost. Over the next few weeks following the introduction of the new risers, pretty much everybody in our entire scene chipped in five or ten bucks, spreading the cost of the box across several dozen of us.
The whole mission has, for me, really been an instructive lesson in community. I know it sounds cliché, but we really can accomplish a lot if everybody contributes what he can. Austin now has a permanent fixture in the form of this grind box, and it’s one of the most rollerblader-friendly objects I’ve ever skated.
Since we’ve been taking this box to the park every week for the last 9 months or so, the whole scene has gotten closer. Everybody has had a good opportunity to learn new tricks, tweak and improve the old ones, and everybody has had a ton of fun doing it.
In the next post, I’ll examine the problems and issues involved with this box through the lens of something called Epistemic Rhetoric. We’ll also deal directly with the physics and math Dmitry used in the calculations.
Until then, thanks for reading.
Post Script: Many thanks to all the Austin dudes who helped out on this project, especially Dmitry Meyerson, Jarrod McBay, Mick Casals, Heath Burley, Scott Wells, and many thanks to Jay Geurink who also photographed the whole process.
Here are some additional (and some redundant) pictures of the process. NOTE: Click on the first image to view the gallery pictures in a separate view. Once in that view, click each image to see the next image in the sequence.