Unit 11 · Grinds and Slides

Skaters use two words for riding along an edge. In a grind, the metal trucks scrape along the edge. The axle and the hanger, the T-shaped metal part that holds the axle, take the contact. In a slide, the wooden underside of the deck rubs along the edge instead. Both tricks use a ledge, a long edge like a curb or a concrete block, or a rail, a metal bar or pipe.

Start low. A painted curb at a skatepark or a flat bar a few inches off the ground is the right first obstacle. A flat bar is a straight metal rail that sits level, close to the ground, on short legs. If you fall from a low curb, you only drop a short way. Stairs, handrails and tall ledges come much later, at a skatepark, when a skater has years of control.

Before you try, put on a helmet that fits snugly and is buckled, plus wrist guards and knee pads. Wear flat-soled shoes that grip the grip tape. Pick a dry, smooth place away from cars and away from people walking. Most grinds and slides need a steady ollie while rolling, so practice that first. If your ollie gets you a few inches off flat ground every time, you are ready to begin.

New concrete and fresh paint can grab a board and stop it short. Skaters fix this with wax. Skate wax comes in small blocks sold for skating. Some skaters use plain paraffin, the kind of wax used in candles. Rub the block along the top corner of the ledge in short strokes, then rub it in with the side of the block until the edge looks shiny.

A thin coat works best. Too little and the trucks still catch. Too much and the edge gets so slick the board can shoot out from under you, or it builds up in sticky lumps. Try one run, then add a little more if the board stops short. Wax wears off as people skate, so a ledge may need a fresh coat after a session.

Wax only edges where skating is allowed, like the curbs and ledges at a skatepark. Wax leaves marks and can make a public step slippery for people walking. A good habit is to bring your own small block, wax just the edge you are using, and leave the area as clean as you found it. Wax also works on a flat bar, though metal rails are already smoother than concrete.

Before rolling, learn what a grind feels like with the board still. Set your board on the curb so both trucks sit on the top corner and the wheels hang over the side you will ride from. Hold a friend’s shoulder or a fence. Step your back foot over the back bolts, then your front foot over the front bolts. The bolts are the screw heads that hold the trucks to the deck.

Now feel where your weight must go. Lean too far toward the ground side and the board tips off. Lean toward the top of the curb and the wheels drop onto it. Somewhere in the middle, with your knees bent and your shoulders level, the board balances on the edge. That middle spot is the feeling you are hunting for when you are moving.

Next, try a rolling drill that is not a grind at all. Ride slowly next to the curb, almost parallel, and ollie up onto the flat top of the curb. Roll along the top and off the end. This teaches you to judge the distance and the timing. When that feels easy ten times in a row, start aiming a little lower, so your trucks meet the edge itself.

The 50-50 is the first grind most skaters learn. Both trucks grind the edge at the same time, so the weight is shared fifty-fifty. It came from pool skating in the late 1970s, when skaters ground the coping, the rounded edge at the top of a pool wall. On flat ground it is done on curbs, ledges and flat bars.

Here are the steps. First, roll toward the curb at a slow, steady speed, nearly parallel to it, with your feet in ollie position: back foot on the tail, front foot just behind the front bolts. Next, when your front wheels are close to the edge, ollie up and a little toward it. Then guide the board so both trucks land on the corner at once. Bend your knees as they land and keep your weight centered between the trucks.

Ride the edge with your shoulders lined up with the curb. At the end, let the board roll off the end and land with bent knees, rolling away. A common problem is landing with only the back truck on, which tips the nose down onto the top. The fix is to ollie a little higher and level the board in the air. Another is landing too far onto the top. Then come in closer to the edge before you pop.

In a 5-0 (say five-oh), only the back truck grinds. The nose stays up in the air and the front truck hangs above the edge. If you have practiced a manual, balancing on the back wheels while rolling on flat ground, you already know the feeling. A 5-0 is a manual done on an edge instead of on the ground.

The steps begin like a 50-50. Roll toward the curb nearly parallel at a slow, steady speed. Ollie up toward the edge, but keep a little more weight on the back foot as you land, so the back truck meets the corner first and the nose stays up. Your back foot sits over the back bolts, not on the very end of the tail. Your front foot stays light over the front bolts, steering the nose level.

The hard part is the balance. If you lean too far back, the tail drags on the curb and the board stops. If you lean too far forward, the front truck drops and the trick turns into a 50-50. Aim to keep the nose a few inches up and steady. At the end, press the front foot down a little as the board rolls off, so both trucks come down together on the ground.

The approach is the path you ride toward the obstacle. For a 50-50 or 5-0, come in at a shallow angle, nearly parallel to the edge, but slightly toward it. A good picture is a door opened only a little. If you ride straight at the curb, the board will hit the edge and stop, or fly over the top. If you ride exactly parallel and far away, you cannot reach the edge when you pop.

Speed matters as much as angle. A friction force slows the board the whole time it grinds, so a board that arrives slowly stops partway. A board that arrives too fast is hard to steer onto the edge and hard to control at the end. Start at an easy rolling speed, a little faster than a brisk walk, and add a little speed on each try once you can land on the edge.

You can also choose a side. In a frontside grind your chest and toes face the edge as you approach. In a backside grind your back and heels face it. Both are normal, and each skater may feel more comfortable on one side first. Pick one side, practice it until it feels steady, then learn the other.

In a boardslide, the board sits across the rail, crosswise, with the rail between the two trucks. The middle of the deck’s underside slides along the metal. A low flat bar is the classic place to learn it, because the bar is narrow and slick. The board points across the bar while you slide along it.

Here are the steps. First, roll toward the flat bar nearly straight on, but just to one side of its end. Next, ollie and turn your shoulders and the board a quarter turn, so the board crosses over the bar. Then land with the bar under the middle of the deck, between your feet. Keep your knees bent and your weight right over the bar, with your head above it.

To come off, turn your shoulders back a quarter turn at the end of the bar, so the board points forward again, and roll away. A common problem is leaning away from the direction you are sliding. Your feet then shoot forward and you sit down. The fix is to keep your weight over the bar and your eyes on its end. Another is landing with a truck on the bar, which hangs up the board; ollie a little higher to clear it.

The nose is the front end of the deck, past the front truck. In a noseslide, you ollie up, turn the board a quarter turn, and set only the underside of the nose on the ledge. The rest of the board hangs off the ground side, and the back wheels stay in the air. A waxed ledge works better than a round rail for learning it, because the nose sits flat on top.

Here are the steps. First, roll toward the ledge nearly parallel at an easy speed. Next, pop an ollie and turn your shoulders and the board a quarter turn toward the ledge. Then press your front foot onto the nose so the nose lands on the top edge. Most of your weight should sit over the front foot. Keep your back foot light on the tail and your knees bent.

To come off, push lightly with the front foot and turn your shoulders back a quarter turn, so the board lines up with your path and rolls away. A common problem is not getting enough weight on the nose. The board then slips off right away. The fix is to lean forward over the front foot and look toward the end of the ledge. Another is turning too far, past a quarter turn, so you come off pointing the wrong way.

The tail is the back end of the deck, past the back truck, the part you pop for an ollie. A tailslide is the back-end partner of the noseslide. You ollie, turn the board a quarter turn, and set the underside of the tail on the top edge of the ledge. The front of the board points out over the ground and the front wheels hang in the air.

Here are the steps. First, ride toward the ledge nearly parallel at an easy, steady speed. Next, ollie a little higher than usual, because the tail must reach the top of the ledge. Then turn your shoulders and the board a quarter turn and press the tail down onto the edge with your back foot. Keep most of your weight over the back foot, with your front foot steadying the board over the front bolts.

To come off, turn your shoulders back a quarter turn and let the board drop to the ground pointing forward, with knees bent. A common problem is the tail landing below the top of the ledge, against its side. The board then stops hard. The fix is to pop higher and keep the front knee bent so the board levels out. Many skaters find the tailslide harder than the noseslide, because the slide happens behind them, where they cannot see it as easily.

Your body tends to follow your eyes. If you stare down at your front foot during a grind, your head drops and your shoulders tip forward. That moves your weight off the balance point, and the board slides off early. Coaches tell skaters to look at the end of the ledge or rail instead, the spot where they plan to roll off.

Looking ahead also lines up your shoulders. For a 50-50 or 5-0, your shoulders should point along the ledge, the same direction as your path. For a boardslide, noseslide or tailslide, your shoulders turn with the board for the slide, then turn back for the exit. Your head and eyes lead the turn, and your shoulders and hips follow.

The exit is how you leave the obstacle. On a low curb, the simplest exit is rolling off the end and landing with bent knees, both feet over the bolts. Plan your exit before you start the trick, and look at that spot from the very beginning. A skater who knows where the trick ends rides it with more confidence than one who is only thinking about getting on.

To bail is to leave the board on purpose when a trick goes wrong. Every skater bails many times while learning a grind. The safest bail is a step-off: as soon as the board stops or tips, step both feet onto the ground on the side away from the obstacle and jog a few steps. The board may roll away, so check that no one is in its path.

Sometimes there is no time to step off. Then fall small. Bend your knees and get low, so you drop a short distance. Tuck your chin. Let your body roll onto the meaty side of your shoulder, your hip or your back instead of stopping all at once. Slide on your knee pads if you fall forward. Do not throw out locked, straight arms to catch yourself. Wrist injuries are among the most common skateboarding injuries, and wrist guards help prevent them.

Fear is a normal signal, not a weakness. If a trick scares you, make it smaller. Go back to the standing drill, lower the speed, or practice ollying onto the curb top again. A short practice session with many small, controlled tries builds a grind faster than a few big, scary ones. Stop when you are tired, because tired legs land badly.

Grinds and slides build on each other, so the order you learn them matters. A common path starts with a steady rolling ollie, then ollying onto the curb top, then the 50-50. The 50-50 teaches you to land on an edge with your weight centered. Next comes the 5-0, which adds the manual’s balance on the back truck.

Slides usually come after you are comfortable on your trucks. A boardslide on a low flat bar teaches the quarter turn on and off. The noseslide and tailslide add turning plus putting most of your weight over one end. Each new trick reuses a skill you already have: the ollie, the quarter turn, a manual’s balance, or looking at the end of the obstacle.

Keep each practice short and focused. Choose one trick and one side, frontside or backside, for a session. Warm up with ollies on flat ground. Count your clean tries instead of your falls. When one trick works on a low curb most of the time, it is ready for a slightly longer or slightly higher ledge at the skatepark, one small change at a time.

When metal trucks scrape along a concrete edge, they slow down. The force doing this is friction, a force between two surfaces that are touching. Friction always points against the sliding. If the board slides forward along the ledge, friction on the trucks points backward. That backward push is what eventually stops every grind and slide.

Physicists split friction into two kinds. Static friction holds two surfaces still against each other, like the grip between your shoe and the grip tape. Kinetic friction acts while two surfaces slide past each other, like the trucks on a ledge. For most pairs of surfaces, kinetic friction is a bit weaker than the most static friction can give. That is why a stuck board is harder to start sliding than to keep sliding.

Friction comes from the surfaces themselves. Up close, even a smooth rail has tiny bumps and pits. As the truck slides, those bumps catch, scrape and break, and the surfaces briefly stick together. A rough surface catches more. A slick coating, like wax, fills the pits and lets the surfaces slip. Isaac Newton’s first law explains the rest: the board would keep moving if no force pushed against it.

When you stand on a board on a ledge, gravity pulls you down. The ledge pushes back up. That upward push is called the normal force. Normal here means at a right angle, because the force points straight out of the surface. On a level ledge, with the skater not bouncing, the normal force exactly balances the weight.

Weight is the pull of gravity on a mass. It equals mass times g, where g is 9.8 meters per second squared near Earth’s surface. Weight is a force, so it is measured in newtons, written N. One newton is about the weight of a small apple. Mass is measured in kilograms and stays the same anywhere; weight would change on the Moon.

Here is a worked example. A skater and board together have a mass of 60 kilograms. Weight = m times g = 60 kg times 9.8 m/s squared = 588 N. On a level ledge, the normal force is also 588 N. If the skater pushes down hard or lands from an ollie, the normal force jumps above 588 N for a moment, and so does the friction.

Experiments show a simple pattern for sliding friction. Press two surfaces together twice as hard and the friction roughly doubles. Scientists write this as f = μN. Here f is the friction force, N is the normal force, and μ, the Greek letter mu, is the coefficient of friction. The coefficient is a plain number with no unit. It describes how grabby a pair of surfaces is.

The coefficient belongs to a pair of surfaces, not to one. Steel trucks on waxed steel have one value. The same trucks on rough concrete have a larger one. For example, a waxed metal rail might be around 0.2 and bare rough concrete around 0.5. These are example numbers, not fixed constants; real values change with wax, dirt, heat and wear.

Here is a worked example. The 60 kg skater from before grinds a ledge with an example coefficient μ = 0.30. The normal force is 588 N. Friction f = μN = 0.30 times 588 N = 176.4 N, about 176 N. That backward push is a little less than a third of the skater’s weight. Notice that the size of the trucks’ contact patch does not appear in the formula. For many surfaces, friction depends on the push, not on the area touching.

Newton’s second law says force equals mass times acceleration, F = ma. Acceleration is how fast speed changes, in meters per second, each second. When friction is the only push along the ledge, it causes a deceleration, a slowing down. Divide the friction force by the mass and you get how quickly the board loses speed.

Put the pieces together. Friction f = μN, and on a level ledge N = mg. So f = μmg. The slowing is a = f divided by m = μmg divided by m. The m on top and the m on the bottom cancel, leaving a = μg. A heavier skater pushes harder on the ledge and gets more friction, but also has more mass to slow down. The two effects cancel exactly.

Here is a worked example. With μ = 0.30, the slowing is a = 0.30 times 9.8 m/s squared = 2.94 m/s squared. Each second, the board loses about 2.94 meters per second of speed. A board that starts the grind at 3.0 m/s stops after t = 3.0 divided by 2.94, about 1.0 second. That matches what skaters see: a grind on a bare ledge lasts about a second unless they bring more speed.

If you know how fast the board starts and how quickly it slows, you can find how far it goes. For steady slowing from speed v down to zero, the slide distance is d = v squared divided by 2a. On a level ledge, a = μg, so d = v squared divided by (2μg). Here v is the speed in meters per second as the grind starts.

Here is a worked example, step by step. A skater starts a 50-50 at v = 3.0 m/s on a ledge with an example μ = 0.30. First, square the speed: 3.0 times 3.0 = 9.0. Next, find the bottom: 2 times 0.30 times 9.8 = 5.88. Then divide: d = 9.0 divided by 5.88 = 1.53 meters. The board grinds about one and a half meters before friction stops it.

The formula also works backward. Suppose you want to grind a ledge 2.0 m long with an example μ = 0.25. Solve for v: v squared = 2 times μ times g times d = 2 times 0.25 times 9.8 times 2.0 = 9.8. So v = the square root of 9.8, about 3.1 m/s. Arrive slower than that and the board stops before the end.

Look at the formula again: d = v squared divided by (2μg). The speed is squared. That means distance does not grow in step with speed. It grows much faster. Double the speed and the distance becomes 2 times 2 = 4 times as long. Triple the speed and it becomes 3 times 3 = 9 times as long.

Here is a worked example with μ = 0.30, so 2μg = 5.88. At 1.0 m/s, d = 1.0 divided by 5.88 = 0.17 m. At 2.0 m/s, d = 4.0 divided by 5.88 = 0.68 m. At 4.0 m/s, d = 16 divided by 5.88 = 2.72 m. Going from 2 to 4 m/s, the speed doubled and the distance went from 0.68 m to 2.72 m, four times as far.

This is why skaters say speed is your friend on a long ledge, and why the same idea also calls for care. More speed carries a grind farther, but it also means more energy to manage if the trick goes wrong. A safe plan adds a little speed at a time. If your grind keeps stopping halfway, a small increase in speed often fixes it, because a small change in speed makes a bigger change in distance.

In the formula d = v squared divided by (2μg), μ sits on the bottom. When the bottom gets smaller, the answer gets bigger. Cut μ in half and the distance doubles. This is the pattern Deshawn and Ana measured: same speed, same ledge, but the waxed half let the trucks slide about twice as far.

Here is a worked example. Both skaters arrive at 3.0 m/s, so v squared = 9.0. On bare concrete, with an example μ = 0.40, the bottom is 2 times 0.40 times 9.8 = 7.84, so d = 9.0 divided by 7.84 = 1.15 m. On the waxed half, with an example μ = 0.20, the bottom is 2 times 0.20 times 9.8 = 3.92, so d = 9.0 divided by 3.92 = 2.30 m. Half the μ, twice the distance.

Wax lowers μ by putting a soft, slippery layer between the metal truck and the hard edge. The truck glides over the wax instead of catching on bumps in the concrete or paint. As skaters grind, the wax gets scraped away and pushed off the ends, so μ creeps back up. That is why the first grinds after waxing feel fast and later ones slow down.

Skateparks build edges from a few materials, and each grinds differently. Many ledges have a ledge cap, a strip of steel angle iron along the top corner. Steel is smooth and hard, so trucks slide along it well, and the steel protects the concrete from chipping. Round steel rails and flat bars are also slick, especially after a little wax.

Bare concrete is rougher. Its surface has sand and small stones in it, which catch the trucks and raise the coefficient of friction. Fresh concrete can feel sticky until skaters wear it smooth and wax it. Concrete also crumbles at the corner over time, so a bare edge can get chipped, which makes the board hang up.

Paint changes over time. A newly painted curb can feel slick at first because the paint fills the rough pits in the concrete. As skaters grind it, the paint wears through in patches, and those bare spots grab. For example, a skater may feel the board slow suddenly when the trucks reach a chipped patch. Checking an edge with your hand before skating it is a smart habit.

A moving board carries kinetic energy, the energy of motion, equal to one half times mass times speed squared: KE = ½mv². When friction stops a grind, that energy does not vanish. It turns into heat in the trucks and the edge, and a little into sound. That is the rough scraping noise of a grind. Rub your hands together fast and you feel the same effect.

Here is a worked example. A 60 kg skater starts a grind at 3.0 m/s. KE = 0.5 times 60 times 3.0 squared = 0.5 times 60 times 9.0 = 270 joules. A joule is the SI unit of energy. Friction does work equal to force times distance: 176.4 N times 1.53 m = about 270 joules. The two numbers match, because friction used up all the kinetic energy.

Friction also causes wear, the slow loss of material from rubbing. Trucks are usually made of aluminum, which is softer than steel, so grinding steel rails shaves tiny amounts off the hangers over time. Slides wear the wood under the nose, tail and middle of the deck. Wax wears off ledges. A worn deck can get thin or chipped, so checking your board for cracks is part of grinding safely.

A skater wants to 50-50 a ledge 2.0 m long. The ledge is waxed steel, with an example μ = 0.25. She arrives at 2.5 m/s. Will she reach the end? First, square the speed: 2.5 times 2.5 = 6.25. Next, find the bottom: 2 times 0.25 times 9.8 = 4.9. Then divide: d = 6.25 divided by 4.9 = 1.28 m. That is short of 2.0 m, so the board stops about 0.7 m before the end.

What speed would carry her all the way? Use the formula backward: v squared = 2μgd = 2 times 0.25 times 9.8 times 2.0 = 9.8. The square root of 9.8 is about 3.13. She needs about 3.1 m/s as she starts the grind. That is only about 0.6 m/s more than before, yet it carries her 0.7 m farther.

She has two choices. She can bring a little more speed, or she can lower μ with fresh wax. If wax brings μ down to 0.20, then at 2.5 m/s the bottom becomes 2 times 0.20 times 9.8 = 3.92, and d = 6.25 divided by 3.92 = 1.59 m. Wax helps, but here it is not quite enough on its own. A small speed increase is the stronger fix.

Every physics formula is a model, a simplified picture that matches reality in some ways. The formula d = v squared divided by (2μg) assumes a level edge, a steady μ and a board that slides the whole way. Real grinds break those rules in small ways. Knowing where a model stops helps you use it well.

Landing on the edge is one example. When the trucks hit the ledge after an ollie, the normal force spikes above the skater’s weight for a split second. Friction spikes with it, so the board loses extra speed right at the start. Balance is another. If the skater leans and the board tips, the grind ends early, no matter what the formula says.

Slope changes the math too. On a gently sloped flat bar at a skatepark, gravity pulls the board along the slope and partly cancels friction, so the board slows less. On a sloped edge going up, gravity adds to the slowing. The coefficient also changes along a real edge as wax thins and paint wears. Even so, the main ideas hold: more speed and less friction mean a longer grind.