Hit the deck

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Date:31 December 2006 Tags:

Frontside forces and fakie flight: exploring the physics of skateboarding tricks

With nothing more than a plank on roller-skate wheels, the sidewalk surfers of the 1930s, 1940s and 1950s had a straightforward mission: start at the top of a hill and ride down.

The primary goal was to merely stay on and avoid collisions, and given the humble equipment and rough road conditions, it was no small challenge. Now, thanks in part to improvements in design and materials, skateboarders have a higher calling. In a blur of flying acrobatics, skaters leap and skid over and on to obstacles, executing flips and turns of ever increasing complexity – all at top speed.

For onlookers and beginners, it can be hard to follow the action, let alone answer the question that springs naturally to mind: how on earth do they do that? While it may seem that modern skateboarders are defying the laws of physics, the truth is that they’re just using them to their advantage. Let’s take a closer look at some fundamental skateboarding moves and the physics principles behind them.

Invented in the late 1970s by Alan “Ollie” Gelfand, the ollie has become a skateboarding fundamental, the basis for many other more complicated tricks. In its simplest form, the ollie is a jumping technique that allows skaters to hop over obstacles and on to kerbs. What’s so amazing about the ollie is the way the skateboard seems to stick to the skater’s feet in midair.

Seeing pictures of skaters performing soaring 1 m-plus ollies, many people assume that the board is somehow attached to the skater’s feet. It’s not. What’s even more amazing about the ollie is that to get the skateboard to jump up, the skater pushes down on the board! The secret to this paradoxical manoeuvre is rotation around multiple axes. Let’s take a closer look.

Just before a skater performs an ollie, there are three forces acting on the skateboard. One of these forces is the weight of the rider. Another is the force of gravity on the board itself. Finally, the force of the ground pushing up on the skateboard. These three forces balance out to zero. With no net force, the skateboard doesn’t accelerate, but rolls along at a constant speed.

Notice that the skater is crouching down. A low centre of mass will be crucial to getting a high jump. (Don’t believe it? Stand perfectly straight and try jumping without crouching… you didn’t get very high, did you?) Now let’s follow the changing forces that go into making an ollie.

The skater accelerates himself upward by explosively straightening his legs and raising his arms. During the jump, his rear foot exerts a much greater force on the tail of the board than his front foot does on the nose, causing the board to pivot counterclockwise about the rear wheel. As the tail strikes the ground, the ground exerts a large upward force on the tail. The result of this upward force is that the board bounces up and begins to pivot clockwise, this time around its centre of mass.

With the board now completely in the air, the skater slides his front foot forward, using the friction between his foot and the rough surface of the board to drag the board upward even higher. The skater begins to push his front foot down, raising the rear wheels and levelling out the board. Meanwhile, he lifts his rear leg to get it out of the way of the rising tail of the board. If he times this motion perfectly, his rear foot and the rear of the board rise in perfect unison, seemingly “stuck” together.

The board is now level at its maximum height. With both feet touching the board, the skater and board begin to fall together under the influence of gravity. Gravity eventually wins out, and the skater bends his legs to absorb the impact of the landing.

A skateboarder launches straight into the air from the top of a ramp. Seeming to hang in place for just a moment, he turns in midair and directs himself back down the ramp. Skaters call this manoeuvre a frontside 180. Physicists call it impossible. Well, they don’t really call it impossible. Just very, very sneaky.

You see, one of the fundamentals of physics is a little something called the law of conservation of angular momentum. This is what it says: If you’re rotating, you’ll keep rotating unless a twisting force, or torque, acts to stop you. Likewise, if you’re not rotating, you can’t rotate unless a torque starts you rotating.

There’s just one more important detail. If you’re in midair, the only force that can act on you is gravity. On earth, gravity can’t make you rotate; it can only make you fall. So the question is, how does a skater go from not rotating to rotating, without the help of an external force? As it turns out, skaters rotate in midair using a trick borrowed from domestic cats. This simple experiment will show you how – and why – it works.

Skateboarders in have a need for speed. The faster they go, the higher they can rise out of the pipe. Achieving greater heights is not only impressive on its own, it’s necessary for pulling off acrobatic tricks such as Caballerials and McTwists. On flatground, the conventional method for gaining speed is to push off with one foot. But half-pipes present a much more elegant option for the speed-hungry skater. It’s called pumping.

To pump in a half-pipe, a skater first drops down into a crouch while traversing the flat bottom of the U-shaped pipe. Then, as he enters the sloped part of the ramp, called the transition, he straightens his legs and rises up. By raising his centre of mass just at the beginning of the ramp’s arc, the skater gains energy and thereby increases his speed.

Pumping in a half-pipe is closely related to pumping on a swing. To get the swing to go higher, you lift your legs as you pass through the bottom of the swing’s arc, then drop them at the top of the arc. Each time you do this, you gain a little energy and swing a little higher. From a physics point of view, the extra speed that comes from both kinds of pumping is a result of the equivalence of work and energy.

As you move into the bottom of the arc, centripetal force makes it harder than normal for you to raise yourself. The net work you perform in lifting yourself is equivalent to a net energy gain. This energy gain translates into extra speed and greater height at the top of the swing or ramp.

The science and art of skateboard design
By Noel Wanner

What is a skateboard? Is it just a glorified plank with roller skate wheels on it? Or is it a highly engineered device through which kids have reclaimed the urban landscape, bringing creativity and style back to the sterile asphalt spaces of sprawl?

The basic elements of the skateboard seem pretty straightforward. A board has three parts: the board or deck, the wheels, and the trucks, which connect the wheels to the board, and allow the board to turn.

But how do you get from this relatively simple mechanism to the perfectly balanced vehicle, the tool for endless creativity on the ramps and streets?

We talked to two of the leaders in the design and production of skateboards, Tim Piumarta of NHS Inc, and Fausto Vitello of the Ermico Foundry, manufacturers of Independent Trucks, to find out about the mixture of industrial science and “feel” that goes into a great board.

Tim Piumarta has been one of the most influential skateboard gear designers over the past 20 years, as the R&D guru of NHS, creators of Santa Cruz Skateboards, Road Rider Wheels, and much more. He described to us the process of making a modern skateboard: “Modern skateboards are made traditionally from seven plies of sugar maple veneers, pressed together using polyvinyl glues in either aluminium, metal or concrete forms, generally taking around 21 bar to take up multiple skateboards in one closing of a press.

“Anything from three to five skateboards are done in one press, and after 30 minutes to an hour, the boards are removed from the press.

“At this point they have been stuck and laminated in the compound curve
or the shape, which is the concave. Then, after days of curing, the CNC routers – or hand routers, depending on the woodshop – will cut out the final shape, apply the edge trimming, paint it and send it on its way.”

Why maple wood? Because it has unique characteristics, says Piumarta. “With all the alternate materials we’ve tried, from epoxy and glass fibre to carbon- loaded thermoplastic nylon, nothing has had the combination of toughness, elasticity, feel and response of laminated sugar maple board.”

Piumarta was one of the first designers to put concave curves into boards in the early 1980s, and developed the first upturned nose. When skaters refer to “concave”, they are talking about the way that the board curves up at its edges, nose and tail. This curvature both strengthens the board and gives the rider more control.

Adds Piumarta: “There are two shapes you talk about when you look at the performance of a skateboard: number one is the concave, the 3-dimensional curves that are in the board itself, nose, tail and side to side concave. Every manufacturer has its own style or philosophy. Mine is based on actual functionality; what your foot feels like when it’s in the concave itself.

“To get there, I do a lot of prototyping in foam cutting, letting all of our pro and amateur riders have a say in what feels good and what works before we cut tooling to make skateboards. So our approach is based on a feel functionality first, and then secondly, when no one’s looking, I slip in curves and bends engineered into this 3-D curve, the concave, that makes the board stiffer, stronger, and makes it last longer.”

The other shape is called the plan form. This is the shape of the board’s outline; if you put a board flat up against the wall and traced its outline, you would be drawing the plan form. According to Piumarta, this shape is largely determined by the choices of individual riders.

“Pro riders can tell by looking and feeling with their hand… they can tell if a board is out of shape by even 0,01 mm. They can feel it, they know what they like, and what they don’t like.”

And, as Piumarta says, all the engineering in the world means nothing if it doesn’t result in a good ride.

Exploratorium. For more fun stuff, visit www.exploratorium.edu

Where the rubber meets the road
Skateboard wheels have gone through a dramatic change since the early 1900s, when kids took roller skate wheels and nailed them to a plank. Those early wheels were usually steel, which offered a rough ride, to say the least. Worse, steel wheels offered little or no traction, so riding these boards was pretty much a straight-ahead proposition.

In the late 1950s, the first commercial skateboards appeared, though most boards were still homemade. By the 1960s, some advances in roller skate design led to the appearance of clay wheels. These were an improvement over metal wheels, but not by much. As one early rider described the ride, circa 1961: “It was wobblier than hell, moved way too fast, and vibrated on the asphalt enough to jar every bone in your body and loosen every tooth. It was more like getting electrocuted than anything else.” (Bob Schmidt, quoted in , 1999). But it wasn’t until the early 1970s that a pair of wheel innovations would arrive that helped turn skateboarding from a “funky, surfing activity, what you would do when the waves were down, into a real bona fide sport”, according to Tim Piumarta. The urethane wheel and the press-in precision bearing changed skateboarding forever, and led to the next big explosion in skateboard popularity.

The first urethane wheels were the handiwork of Frank Nasworthy, who, after seeing some experimental urethane roller-skate wheels in a friend’s back yard in 1970, realised that such wheels could be used for skateboards. Nasworthy and his friends tried them, and found that the old tooth-jarring ride was gone, replaced by a ride of unprecedented smoothness and stability. Skateboarding was in a dead period in the early 1970s, but Nasworthy’s wheels, called Cadillacs, began to catch on.

Fausto Vitello explains why urethane was perfect for wheels: “Urethane has some unique properties. The first is that it has really good abrasion resistance, which means that the wheel will last a while. The second, even more important point, is that urethane gives a really good grip with the ground. It will slide if you push it hard, but it gives great traction. So that means you can control your board. And the last is really high resiliency, or rebound, which means that although the wheels have no pneumatic tube or anything (they’re solid), they’re still able to go very fast.”

How boards have changed over time
Over the past decades, skateboarding has gone through many phases and swings in popularity. Riding styles have changed many times, and are still evolving, and as Piumarta relates, skateboard designers have to change the design of boards to support and enhance those styles.

“Considering that the early 1970s were all downhill riding and slalom riding, we made boards that were almost like snow skis. Then skateboard parks and pools came along, and we had flat 25 cm-wide boards with no nose and a big kicktail, because everyone was just going forward and carving in pools and parks.

“Street skating took over, and we introduced the first street skateboard back in 1979, and of course it was called the Street Skate. As skateboard parks closed and people moved out into the street, the 25 cm boards wouldn’t work. So all the boards got narrow, and eventually I started putting upturned nose into some of our early concaves. This eventually migrated right over into narrow boards on the street with an upturned nose and an upturned tail, with concave, but all at around 21-23 cm wide.

“Vert, as pool riding was called, came back in the form of ramp riding back in the mid-1980s, and boards got wide again, up to 23 cm. Again big concaves, upturned noses, a lot more tricks being done than in the late 1970s, so the upturned nose stayed on those boards. Then with vert really dying down, and street skating going crazy, which is where we’re at right now, the boards have gotten even narrower than what was called a freestyle board back in the late 1970s.”

So the size and shape of the skateboard has been fluid, changing to fit the needs and demands of the riders, reflecting the changing interests and styles of skaters out on the street. Many former pro riders have gone into the design and production of skateboards; perhaps that is why manufacturers are so attuned to the needs of skaters.

So what the heck is a ‘truck’, anyway?
The skateboard steering devices on the bottom of the board are called the trucks. Trucks consist of a base plate (mounted to the base of the skateboard itself), an axle that pivots on two urethane cushio