Unit 12 · Balance, Turning and Spin

Lay a ruler across one finger and slide it until it stays level. The spot where it balances is its center of mass, the point where the ruler’s weight acts as if all of it were gathered in one place. For a plain ruler, that point is right in the middle. Gravity pulls on every bit of the ruler, but you can treat all those pulls as one pull on that single point.

A standing person has a center of mass too. For most people standing straight, it sits roughly at hip level, a little below the belly button, in the middle of the body. It is not fixed to one spot on your skin. When you raise your arms, it moves up a little. When you bend your knees and crouch, it drops lower. When you lean forward, it moves forward.

A skateboard has its own center of mass, usually close to the middle of the deck between the bolts. When you stand on the board, you and the board act together. Because a rider weighs many times more than a board, the combined center of mass is mostly set by where the rider’s body is. Skaters control balance by moving their own center of mass with their knees, hips, arms and shoulders.

Stand with your feet together and have a friend give your shoulder a gentle nudge. You wobble. Now stand with your feet wide apart and try again. You barely move. The difference is your base of support, the area on the ground outlined by everything touching it. Wide feet make a wide base.

The rule of balance is simple to say. An object stays upright while a straight line down from its center of mass lands inside its base of support. If that line falls outside the base, gravity pulls the object over. A tall, narrow bookcase tips more easily than a short, wide one because a small lean moves its high center of mass past the edge of its narrow base.

A skateboard’s base of support is the rectangle between the four spots where the wheels touch the ground. From nose to tail it is long, the length of the wheelbase, the distance between the front and back wheels. From toe side to heel side it is narrow, only as wide as the wheels are apart. That is why falling forward or backward off a rolling board is less common than tipping toward the toes or heels.

Before rolling, practice balance standing still on grass or carpet so the board cannot roll away. Wear a helmet that fits and is buckled, and add wrist guards and pads while you learn. Put your front foot just behind the front bolts and your back foot over or just in front of the back bolts. Bend your knees as if you were about to sit on a high stool, and keep your shoulders lined up over the board.

Bent knees help in two ways. First, they lower your center of mass, so a small bump moves it a shorter distance sideways. Second, they work like springs. A straight-legged rider passes every jolt from a crack straight up into the hips. A rider with soft knees can absorb the jolt and keep the upper body still. Riders make many tiny corrections to keep the center of mass over the narrow base. Each correction is a small shift of weight through the ankles, hips or arms.

The same idea explains a manual, rolling on two wheels with the nose or tail up. In a back-wheel manual, the base of support shrinks from a rectangle to a thin line between the two back wheels. The rider must hold the center of mass right over that line. Too far back and the tail scrapes; too far forward and the nose drops. Practice manuals on carpet first, with a grown-up spotter if you are young.

Flip a skateboard over and look at one of the two trucks, the metal assemblies that hold the wheels. The flat part screwed to the deck is the baseplate. The T-shaped part that holds the axle is the hanger. The axle runs through the hanger and sticks out each side, and a wheel slides onto each end with two bearings inside it.

A thick bolt called the kingpin rises out of the baseplate and passes through the hanger. Around the kingpin, above and below the hanger, sit two rubbery rings called bushings. They are usually made of urethane, the same family of material as the wheels. A nut on top of the kingpin squeezes the bushings. The bushings hold the hanger in place but let it tilt, then push it back to center.

The hanger also has a short arm, the pivot, that pokes into a small cup in the baseplate. The pivot and the kingpin together set the line the hanger swings around. That line is tilted at an angle, not flat and not straight up. Because of that slant, tilting the deck makes the hanger turn, which is the secret of steering a skateboard by leaning.

Stand on a board on carpet and press down with your toes. The deck tilts toward the toe side. Underneath, each hanger swings on its slanted pivot line. A swing on a slanted line does two things at once: it lets the deck tilt, and it twists the axle so it no longer points straight across. The wheels now point a little to one side, the way the front wheels of a car point when the steering wheel is turned.

The front and back trucks face opposite ways, with their kingpins pointing toward each other. So when the deck tilts, the front axle and the back axle twist in opposite directions. Seen from above, the two axles are no longer parallel. Lines drawn out along the two axles would meet at a point off to one side of the board. That point is the center of the circle the board will roll around.

The result is easy to remember. Lean toward your toes and the board turns toward your toe side. Lean toward your heels and it turns toward your heel side. The harder you lean, the more the axles twist and the tighter the circle. This is called carving when you ride long, smooth arcs by leaning instead of lifting the front wheels. The turning radius is the radius of the circle the board traces.

Turning the kingpin nut with a skate tool changes how the board rides. Turning it clockwise squeezes the bushings, so the board has tight trucks: the hanger needs more lean to tilt, and the board resists turning. Turning it counterclockwise lets the bushings relax, so the board has loose trucks: a small lean makes a sharp turn.

Each choice has a cost. Tight trucks feel steady at speed and are less likely to shake, but they make sharp turns harder. Loose trucks carve tight circles and feel lively, but they react to every small wobble of the rider’s ankles. Many riders adjust a quarter turn at a time, ride for a while, and adjust again. Bushings also come in different hardness, measured on a durometer scale like wheels. Softer bushings turn easier; harder ones feel steadier.

Keep the nut threaded far enough onto the kingpin that its locking ring grips the threads. A nut backed off too far can loosen on its own while you ride. Check both trucks after adjusting one, since uneven trucks make the board pull to one side. If the board turns too sharply while you are still learning to roll, tighten the trucks a little before you change anything else.

Roll a marble across a smooth floor and it keeps going in a straight line until something stops it or pushes it aside. This is inertia, the tendency of a moving object to keep moving at the same speed in the same direction. Isaac Newton put it at the center of his first law of motion in 1687. A skater rolling across flat concrete would also go straight forever if no force acted.

To travel in a curve, an object needs a force that keeps pulling it toward the inside of the curve. This inward force is called centripetal force, which means center-seeking. Swing a ball on a string in a circle and the string is that force: it pulls the ball toward your hand. Let go of the string and the ball flies off in a straight line, not outward in a curve.

On a skateboard, nothing pulls on a string. The inward force comes from friction between the wheels and the ground. When the axles twist, the wheels try to roll in a curve, and the ground grips the urethane and pushes the wheels sideways toward the center of the circle. Even at a steady speed, a turning board keeps changing its direction of motion, so physicists say it is accelerating.

Watch a rider carve and you will see the whole body tilt toward the inside of the turn. This lean is not just style. The ground pushes up on the wheels and also sideways, toward the center of the turn. If the rider stood straight up, that sideways push at the feet would tip them toward the outside, the way a rug pulled from under your feet tips you over. Leaning lines up the ground’s total push with the rider’s center of mass.

The needed inward acceleration depends on speed and radius: a = v² ÷ r, where v is speed in meters per second and r is the turning radius in meters. Worked example: a rider rolls at 5 m/s around a curve with a 5 m radius. Then a = 5² ÷ 5 = 25 ÷ 5 = 5 m/s². If the rider and board together have a mass of 60 kg, the inward force is F = ma = 60 × 5 = 300 N.

Compare that with the rider’s weight: W = mg = 60 × 9.8 = 588 N. The sideways force is about half the weight, since 300 ÷ 588 ≈ 0.51. A rider leaning at about 27 degrees from straight up keeps the push balanced in this turn. The lean depends only on speed and radius, not on mass, so a light rider and a heavy rider carving the same line at the same speed lean the same amount.

The formula a = v² ÷ r has a surprise inside it. Because speed is squared, doubling the speed makes the needed inward force four times as large. Going three times faster needs nine times the force. Cutting the radius in half doubles the force. A turn that feels easy at a slow roll can be impossible at a fast one.

Friction has a limit. The most sideways grip the wheels can give is roughly μ × N, where N is the normal force pressing the wheels into the ground and μ (the Greek letter mu) is the coefficient of friction, a number that describes how grippy two surfaces are together. On level ground N equals the weight, so the largest possible turning acceleration is about μ × g. For example, if μ were about 0.7, the limit would be about 0.7 × 9.8 ≈ 6.9 m/s².

In that example, a rider at 8 m/s on a 5 m radius would need 8² ÷ 5 = 64 ÷ 5 = 12.8 m/s², almost twice the limit, so the wheels would slip. Skaters who want that slide do it on purpose in a powerslide, turning the board sideways to scrub off speed. Softer wheels usually grip rough ground better, and hard wheels slide more easily on smooth concrete. Sand, water and leaves lower μ a lot, which is why a wet or dusty turn can send a board out from under a rider.

A rider is rolling faster than usual down a gentle slope in an empty lot. The board starts to shimmy: the trucks twitch left and right, a little at first, then harder and faster. Riders call this speed wobble. It can grow in a second or two from a buzz in the feet to a shake strong enough to throw the rider off.

Speed wobble is an oscillation, a motion that swings back and forth over and over. On a skateboard it can feed on itself. A tiny tilt steers the trucks, the turn tips the rider the other way, the rider corrects, and the correction overshoots into a tilt the other way. At higher speeds each swing happens faster than the rider can react smoothly, so the corrections arrive late and make the next swing bigger.

Several things make wobble more likely. Loose trucks or soft bushings let a small lean turn the wheels a lot. Stiff, straight legs pass every twitch through the body instead of soaking it up. Weight drifting onto the back foot is a common trigger, because it lets the front of the board wander. Going faster than you have practiced is the biggest factor of all. Wobble is not a sign that someone is a poor skater; experienced riders feel it too.

When the board starts to shake, the first move is to get low. Bend your knees and drop your hips so your center of mass sits closer to the board. A lower body swings less, and bent legs soak up the twitching. Next, move your weight forward so you press gently on your front foot. Pressure on the front truck helps hold it steady and keeps the front of the board from wandering. Then relax your ankles and look far ahead, not down at your feet.

Fighting the board with sharp, quick corrections usually makes wobble worse, because each sudden push becomes the next swing. Smooth, small pressure works better. If the shaking keeps growing and you are moving slowly enough, step off and run it out. If not, bail low: crouch, aim for the grass if it is close, and fall onto your pads and roll instead of reaching out with straight, locked arms.

Prevention is the best fix. Learn speed a little at a time on flat ground and gentle, empty slopes. Never ride hills that have cars, and never hold onto a moving vehicle. Tighten loose trucks a quarter turn, or try harder bushings, before riding faster. Wear a helmet that fits and is buckled, plus wrist guards and pads, every time you practice speed. Fear when the board shakes is a normal, useful signal that it is time to slow down.

Spin a top on a smooth table and it keeps turning for a long time. It slows only because the table rubs its point and the air drags on it. Just as a rolling object tends to keep rolling straight, a spinning object tends to keep spinning. Physicists measure the amount of spin an object carries as its angular momentum.

Angular momentum depends on two things. One is how fast the object turns, called its angular velocity, often counted in rotations per second. The other is how its mass is spread out around the line it spins on, the axis of rotation. Written as a formula, L = Iω, where L is angular momentum, I is the moment of inertia (the spread of mass) and ω, the Greek letter omega, is the angular velocity.

The key rule is conservation of angular momentum: if no outside twist acts on an object, its angular momentum stays exactly the same. A skater in the air is a clear case. The air pushes back only a little, and nothing else can twist the skater, so the spin they carry when their wheels leave the ground is the spin they have until they land. They can change how it is shared out, but not the total.

Push on a door near its handle and it swings open easily. Push with the same force right next to the hinges and it barely moves. The difference is torque, the twisting effect of a force. Torque equals the force times the lever arm, the distance from the pivot to where the force is applied, measured straight across. The unit is the newton-meter (N·m).

Worked example: you push on a door with a force of 20 N. At the handle, 0.8 m from the hinges, the torque is 20 × 0.8 = 16 N·m. Near the hinges, 0.1 m away, the torque is 20 × 0.1 = 2 N·m. The same push gives eight times the twist at the handle, because 16 ÷ 2 = 8. A longer lever arm means more turning effect from the same force.

A skateboard is full of lever arms. In a kickturn, the back foot presses on the tail, and the back wheels are the pivot. The tail sticks out past the back wheels, so a press on its end has a lever arm that lifts the nose. To spin the whole body, a skater needs torque from the ground through their feet. Friction between the shoes, the grip tape and the wheels on the ground lets the feet twist the board and body before takeoff.

Watch a skater set up a 180 and you will see a small move before the pop. Their shoulders and arms turn a little the opposite way, like a baseball batter pulling the bat back. This is the wind-up. A skater about to spin toward the front, for example, first turns the shoulders slightly toward the back.

The wind-up gives the skater a longer time and a bigger turn to work with. As they crouch and pop, they swing the shoulders hard in the spin direction. Their feet are still pressing on the board, and the wheels are still on the ground, so the twist of the upper body pushes against the ground through the legs. The ground pushes back with friction, and that outside torque gives the whole skater angular momentum just before takeoff.

Once the skater is in the air, the wind-up rule changes. Now the total spin is fixed. If the shoulders keep turning one way, the hips and legs swing slightly the other way to balance it. That is why skaters lead with the shoulders, let the hips follow, and let the board follow the hips. This counter-rotation, twisting the upper body one way so the lower body turns the other, is how a skater lines up the board in the air without changing the total spin.

Jaylen’s stool spin is a classic physics demonstration. With his arms out, his mass is spread far from the axis of rotation. With his arms in, more of his mass sits close to the axis. Nothing outside twists him either way. Since his angular momentum L = Iω cannot change, a smaller I has to go with a larger ω.

Worked example with made-up but realistic numbers: suppose Jaylen and the stool have a moment of inertia of 3.0 kg·m² with his arms out, and he turns at 1 rotation per second. His angular momentum is 3.0 × 1 = 3.0 units. He pulls in, and his moment of inertia drops to 1.5 kg·m². Then 1.5 × ω = 3.0, so ω = 3.0 ÷ 1.5 = 2 rotations per second. Halving I doubles the spin rate.

Skaters use this every time they spin. In a 360 on a ramp or a big spin in the air, a skater who tucks the arms close and keeps the knees bent turns faster with the same takeoff. Reaching the arms out works the other way: it slows the spin. That is useful near the landing. Skaters often spot the landing, turning the head to look at it, and open the arms a little to slow down and settle before the wheels touch.

For a small heavy object turning around an axis, the moment of inertia is I = m × r², where m is the mass in kilograms and r is the distance from the axis in meters. The unit is kilogram meter squared (kg·m²). The square matters a great deal. Moving a weight twice as far out makes its moment of inertia four times as large.

Worked example: hold a 2 kg dumbbell 0.2 m from your body’s axis. Its moment of inertia is 2 × 0.2² = 2 × 0.04 = 0.08 kg·m². Stretch your arm so it is 0.8 m out. Now I = 2 × 0.8² = 2 × 0.64 = 1.28 kg·m². Moving it four times as far out made it sixteen times harder to spin, because 1.28 ÷ 0.08 = 16. A whole body is many small pieces added up the same way.

This explains how skaters shape their spins. In a tight tuck, arms, knees and head stay close to the axis, so the moment of inertia is small and the body turns quickly. A skater stretched out straight, arms wide, has a large moment of inertia and turns slowly. The board counts too. A deck held flat under the feet near the axis adds little, but a board flung out to the side adds a lot.

If angular momentum stays the same, where does the extra speed come from? The answer is rotational kinetic energy, the motion energy of a spin. It equals ½ × I × ω², with ω measured in radians per second. A radian is a way of measuring angle; one full rotation is 2π radians, about 6.28 radians.

Worked example using the stool numbers: 1 rotation per second is about 6.28 radians per second. With the arms out, the energy is ½ × 3.0 × 6.28² = 1.5 × 39.4 ≈ 59 J. With the arms in, the spin is 2 rotations per second, about 12.57 radians per second. Now the energy is ½ × 1.5 × 12.57² = 0.75 × 158 ≈ 118 J. The spin energy doubled, from about 59 J to about 118 J.

The extra energy came from Jaylen’s arms. Pulling heavy bottles inward while spinning takes effort, because the bottles tend to fly outward in a straight line. His muscles did work, which is force applied over a distance, and that work became spin energy. When he reached back out, the energy went the other way. A skater who tucks hard in the air is doing the same work with arms, legs and core.

A bowl is a skatepark pool with curved walls all around. A rider can gain speed in it without ever pushing with a foot. The trick is pumping: crouching and standing at the right moments. Before you try, you should already roll comfortably on flat ground and ride up and down a small transition, the curved part where a wall meets the floor. Wear a helmet that is buckled, plus knee pads, elbow pads and wrist guards, and start in a small, shallow bowl when it is not crowded.

First, roll across the bottom with your knees bent and your weight centered between the bolts. Next, as you ride down a wall toward the curve, sink low into a crouch. Then, as the board passes through the curve and you feel your feet pressed harder into the deck, push up through both legs and stand tall, like a jump that never leaves the board. Ride up the next wall a little, crouch again as you come back down, and push again through the curve.

It feels like a rhythm: low, push, low, push. Common problems have simple fixes. If you push too early, on the flat, you gain almost nothing, so wait until you feel the curve press into your feet. If you lean back on the way up a wall, your board can shoot out in front of you; keep your shoulders over your feet. If you do fall, try to land on your knee pads and slide down the wall instead of putting out straight arms.

In a curve, the deck must push the rider toward the center of the curve, not just hold them up. So the push on the feet is larger than the rider’s weight. Worked example: a 60 kg rider moves at 5 m/s through a curve with a 2 m radius. The extra inward acceleration is v² ÷ r = 25 ÷ 2 = 12.5 m/s². At the bottom of the curve the deck pushes with about 60 × (9.8 + 12.5) = 60 × 22.3 ≈ 1,338 N. On the flat it pushes only 60 × 9.8 = 588 N.

Standing up means pushing against that force over a distance, and work equals force times distance. Raising the body 0.3 m in the curve takes about 1,338 × 0.3 ≈ 401 J of work. Crouching again on a wall or the flat, where the push is near 588 N, gives back only about 588 × 0.3 ≈ 176 J. The difference, about 401 − 176 = 225 J, stays in the motion as extra kinetic energy.

Check the gain: at 5 m/s the rider’s kinetic energy is ½ × 60 × 5² = 750 J. Adding 225 J gives 975 J, so the new speed is √(2 × 975 ÷ 60) = √32.5 ≈ 5.7 m/s. Angular momentum gives a second view of the same idea. In the curve, the rider circles around the curve’s center, and standing up moves their mass closer to that center, like Jaylen pulling his arms in. A child on a playground swing pumps in a similar way.

Skaters describe direction by which side of the body faces something, not by left or right. When you ride up to a wall or a ledge with your chest and toes facing it, you are riding frontside. When your back and heels face it, you are riding backside. This works the same for regular riders, who stand with the left foot forward, and goofy riders, who stand with the right foot forward.

Spins use the same idea. In a frontside spin, the skater turns so the chest swings around toward the direction of travel, and during a frontside 180 the skater faces forward halfway through. In a backside spin, the back turns toward the direction of travel first, so halfway through a backside 180 the skater’s back faces where they are headed. Because the words describe the body, a regular and a goofy rider doing a frontside 180 actually turn in opposite directions as seen from above.

These words also name grinds, slides and airs. A frontside grind on a curb means you approach with the curb on your toe side. A backside grind means the curb is on your heel side. After a 180, the skater rolls fakie: in their normal stance, but moving backward with the tail in front. Learning both directions balances the body and gives a skater more ways to ride a park.

Learn this only after your ollie is steady and you can roll fakie comfortably. Practice on smooth, flat, empty ground in a helmet and pads. A good first step is the frontside kickturn 180: while rolling slowly, press the tail, lift the front wheels, and pivot all the way around on the back wheels, turning your chest toward the direction you are going. This teaches the shoulder-first turn with the wheels still near the ground.

For the ollie version, set your feet as for an ollie: the ball of your back foot on the tail and your front foot just behind the front bolts. First, crouch and wind your shoulders slightly backside. Next, pop the tail with your back foot and, at the same moment, swing your shoulders frontside and slide your front foot up toward the nose. Then let your hips follow your shoulders and guide the board around with both feet. Finally, land with both feet over the bolts, knees bent, rolling fakie.

Two problems are common. If the board turns only about 90 degrees, your shoulders probably stopped early; wind up a little more and turn your head to spot the landing over your front shoulder. If you land with one foot off the board, your front foot may have dragged too far; level the board by pressing evenly with both feet in the air. Start with a small pop. Height can come later, once the turn feels smooth.

Every big spin is built from small ones. Many skaters start with tic-tac style quarter turns, lifting the front wheels and pivoting about 90 degrees on the back wheels while standing still on carpet. Next they try a full 180 kickturn standing still, then rolling slowly. Only after that do they add a small pop. Each step should feel calm before the next one begins. Fear is a normal signal that a step is too big; break it into a smaller piece.

Spinning in both directions matters. Most skaters find one direction easier at first. Practicing backside 180s as well as frontside 180s balances the body and teaches the brain to spot the landing from both shoulders. A backside 180 uses the same physics: wind the shoulders frontside, pop, then swing the shoulders backside so your back turns toward the direction of travel.

Falling small is part of every spin practice. If a landing goes wrong, step off the board toward the side it is not rolling, bend your knees, and let your body crouch. If you are going down, aim to land on pads and roll onto the side of your body, keeping your arms bent rather than locked straight. Wrist injuries are among the most common skateboarding injuries, and wrist guards help prevent them, which is why they belong on every spin session.