Source: http://www.youtube.com/watch?v=ymrGyUj5oQg&feature=youtube_gdata
Water sports Rafting Whitewater kayaking Whitewater canoeing
Thursday, March 31, 2011
Seasons: Winter
Banks Mag Winter 2011 Video Issue #2
Brian Ward discovers an unexpected and new-found love for water in its frozen and expanded form.
A short film by Skip Armstrong & Ryan Bailey
Music: Doorways by Radical Face
Available on iTunes
itunes.apple.com/us/album/doorways/id375677026?i=375677035
Cast: The Banks Mag, Skip Armstrong
Frostgun Invitational 2010: New Zealander Wins 10,000 Euro Prize Money
Frostgun Invitational 2010: New Zealander Wins 10,000 Euro Prize Money
The New Zealander Russ Henshaw has won one of the most important Big Air Events of Europe, the Frostgun Invitational in Val Thorens. Not only has he proven himself as a skiier, but now he also has an extra 10,000 euros of prize money in his pocket.Source: NEWS/Freeskiing/Frostgun_Invitational_2010_Russ_Henshaw_Wins_0637.htm
Xterra World Tour 2010: Next Stop Saipan
Xterra World Tour 2010: Next Stop Saipan
Following the great championship in South Africa, racers now prepare to set their minds and bodies to the limit in the ninth annual Xterra Saipan Championship Race.Source: NEWS/Extreme_Sports/Xterra_World_Tour_2010_Next_Stop_Saipan_Preview_0583.htm
Wild New Route Planned For Absa Cape Epic 2011 MTB Race
Wild New Route Planned For Absa Cape Epic 2011 MTB Race
The Absa Cape Epic is the most prestigious mountain bike race in the world and 1,200 cyclists are expected to participate in the raw, natural locations of Saronsberg (Tulbagh), Worcester, and Oak Valley.Source: NEWS/Biking/Absa_Cape_Epic_2011_Mountain_Bike_Race_Africa_Preview_0803.htm
WWA Wake Park World Series: Wakeboarders Gear Up For June
WWA Wake Park World Series: Wakeboarders Gear Up For June
The dates and locations are in for the 2010 WWA Wake Park World Series. In its third year, this is the first series of its kind with an international field of professional wakeboarders setting up against their competitors at cable parks throughout the world.Source: NEWS/Wakeboarding/WWA_Wake_Park_World_Series_2010_Preview_0576.htm
The Wheel Deal Part 1: Wheel Size
Many extreme sports rely on wheels of one type or another, including skateboarding, mountain boarding, inline skating, street luge, BMX and FMX. Different situations require different types of wheels, depending on the terrain and the types of riding you're doing.My favorite extreme sport is skateboarding, so this post focuses primarily on the options available in skate wheels. But the physics involved applies to any wheeled sport.
If you skate, you know that there are lots of wheels designs on the market from tiny, rock-hard wheels for street skating to giant, gummy wheels for old school cruising. Why are some wheels better for certain uses and not so good for others? As you probably guessed - it all comes down to physics.
Wheel Size
Among the many things you need to consider in choosing the best wheel for your riding is size. Most pro street skaters opt for small wheels. It's a good choice. Small wheels are fast on smooth surfaces such as skate park concrete, wood ramps, and most of the boxes, benches and banks you're likely to hit. But on asphalt or chewed up concrete, little wheels are much slower than big wheels. Just about every skater has at one time or another had the unpleasant experience of running across a pebble or crack that stops their board dead in it's path, sending the rider for a rough tumble. Those sorts of sudden stops are more likely if you ride tiny wheels.
So, what's size got to do with it? Well, here's a little sketch to show you what's going on.
The small red cirle represents a wheel with a diameter of about 50 millimeters, typical of lots of street and park wheels. The big black circle is like a large (95 millimeter) cruising wheel. This sketch shows the wheels just as they hit the edge of a 20 millimeter step (in this picture, I'm imagining the wheels rolling to the right), which is the sort of thing you might run across as you ride over the joints between sections of a typical sidewalk.The arrows show the direction of the force that results from the wheels hitting the obstacle. As you can see, the black arrow points up and to the left. That means some of the force pushes the wheel upward and some of it pushes back.
The red arrow is mostly pointed to the left and just a bit up, which means most of the force exerted by hitting the step goes into slowing the wheel, and the board it's attached to.
Of course, most of the bumps and cracks you'll run across in real life are a lot smaller than this. Even for smaller obstacles, though, more force will go into slowing a small wheel down than would go into slowing a larger wheel. You'll still get a force pushing the larger wheel upward, which makes for a rough ride, but at least it doesn't do as much to sap your speed (or stop you in your tracks).
If you race down a big hill made of asphalt, you end running over lots of little bumps that seriously slow small wheels, but aren't such a problem for big ones.
Are big wheels always better than small ones? Not at all. In fact, small wheels are usually MUCH faster than large wheels on smooth surfaces. Want to know why? Check out The Wheel Deal Part 2 in my next post to find out one reason that small wheels are better (sometimes).
Source: http://feedproxy.google.com/~r/blogspot/uUCU/~3/WRmZPn7vfuU/wheel-deal-part-1-wheel-size.html
Wednesday, March 30, 2011
GEAR TEST WITH MARK BRONTSEMA AND SAM WELLS, ROCK CLIMBERS; Because Climbing Is Social
Rock climbers Mark Brontsema and Sam Wells test out four climbing helmets for fit, comfort, durability and price; photos
Why is Snow so Slippery?
Snow sports are a blast. I prefer snowboarding, but I would never miss out on a toboggan ride or a turn on one of those old fashioned snow saucers (What genius conceived of those anyway? Is there anything more insane that cruising down a hill on a disk with no obvious way to steer or stop? Of course, I love it anyway.) As embarrassing as it is, I've even been known to to ride an inner tube now and then.The key to all those sports - as well as skiing, snowlerblades, ski bikes, and cafeteria lunch trays - is that things slide well on snow. You could leave it at that and simply go outside to have fun, but I just had to know a bit more.
If you hunt around, you'll see that there are at least two possible reasons why snow is slippery. One common explanation is that the melting point of ice rises as you squeeze it. Water ice is unusual in that way. This could explain why ice skates work. When you stand on skates, it creates very high pressures under the sharp blades. The pressure raises the melting point of the ice until it creates a thin layer of water, which is very slippery. (This is, however, not a universally accepted explanation.)
Pressure might work for ice skaters, but it's not much help for sleds, skis, snowboards, or any other device that slides on a large, flat surface. Because your weight is distributed over a big board, the pressures are very low. At best, you might raise the melting point of the ice by a degree or so, but if the temperature of the air and snow outside is more than a few degrees below freezing, you won't actually melt any snow with pressure.
Another possibility I've heard on occasion is that the friction of the board sliding on the snow creates heat, which melts some of the snow and creates a thin lubricating layer of water. Now, this explanation sounded just absurd to me. But I didn't want to dismiss it until a estimated just how much heating you might get from snowboarding down a hill.
You know what? I was stunned -- STUNNED -- to find out that cruising down a hill creates lots of heat. At a relatively modest 36 kilometers an hour (22 mph) down an equally modest hill (imagine a 15 degree slope or so), someone about my size (75 kilograms) creates about 300 watts of heat power due to friction between the snowboard and the snow. That's three times as much power as put out by a typical light bulb. If you've ever touched a lit incandescent bulb, you know that's hot. Theoretically, friction could heat the bottom of the board by nearly 40 degrees Celsius (about 100 degrees Fahrenheit)! In the real world, the bottom of your board will never get that hot. It will only warm up to the point that it melts the snow. It takes energy to melt snow, and the melting ends up using the energy that would heat your board to any higher temperature than 0 degrees C (32 degrees Fahrenheit).
But suppose you were snowboarding on a day when temperatures went as low as at -40 degrees C, even moving slowly would generate enough heat from friction to melt the snow and provide a very thin layer of water for you to slide on. (Of course, -40 C is just about the lowest temperature ever recorded in Alaska, so you'd probably get an awful case of frostbite before you got to the top of the lift.)
So there you have it - it's friction, not pressure, that melts snow below your board and let's you whiz down the hill at a breakneck pace. Wacky.
For those of you who like the details, the math that led me to this conclusion is below.
The Math and Physics
You can change the parameters of the problem to model any sliding sport you like, but I'm using a snowboard in my analysis.
The first thing to do is figure out how much work is done when you slide on a snowboard. Work is defined as force times distance. Specifically, we're interested in the frictional force multiplied by the distance traveled.
The force of friction is just the force pushing the snowboard down onto the snow multiplied by the coefficient of friction (u).
Because the snowboard is moving downhill, the force pushing against the snow due to the mass of the snowboard and rider is reduced by the cosine of the hill slope.
Friction force = m g u cosine(theta)
m = rider's mass (I'm using 75 kilograms)
g = acceleration due to gravity (0.8 meters per second squared)
u = coefficient of friction for a waxed board sliding on snow (about 0.04, according to Wikipedia)
theta = slope of the hill (I'm using a modest 15 degrees)
If you multiply this by the distance traveled on the hill, you get the total energy expended on the trip. If instead, you multiply the force by the velocity of the snowboard, you get the work per unit time. That's the same as the power (watts in SI units).
Power = m g u cosine(theta)* v
v = velocity (for this problem, I'm using 10 meters/second, which is about 36 kph, or 22.5 mph)
Plug all that in, and you find a power output of about 284 watts to slide down the hill.
The equation for heat conduction through a slab of material (such as a snowboard deck) is
Power = dQ/dt = k A (T2-T1)/L
k = thermal conductivity (about 0.25 Watts/(meters*degrees Kelvin))
A = area of board touching the ground (about .25 square meters for a typical board)
T2 = temperature on the hot side of the board (in Kelvin)
T1 = the temperature on the cold side of the board
L = the thickness of the board (I'm using one centimeter, 0.01 meters)
When I rearrange this to solve for the temperature difference between the two sides of the board (T2-T1) and plug in the numbers, I get temperature difference of about45 degrees.
As I mentioned above, that's the maximum temperature difference between the two sides, but the hot side should never get above the freezing point of water because the heat generated by friction would have to melt all the snow before it could lead to higher temperatures. That's because of the phase transition from ice to water that occurs at the freezing point (It's the same reason that water with any ice in it at all will have a temperature of exactly 0 degrees Celsius. You can confirm this by putting a pot of snow on the stove and turning up the heat. The temperature will rise to 0 degrees C and stay there until all the snow is melted.).
So, if you're out snowboarding when the air temperature (and top of your board) are at -5 degrees Celsius, the temperature of the bottom of your board be about 0 degrees Celsius when you're moving along at 10 meters/second. In fact, it will always be at about 0 degrees Celsius if you're moving at almost any reasonable speed, and there will be a very thin layer of water under it due to all the frictional energy you're generating by sliding down the mountain.
Source: http://feedproxy.google.com/~r/blogspot/uUCU/~3/98TqO5Qa59k/why-is-snow-so-slippery.html
Slippery When Wet: Drew Duval
This is the third of 7 trailers from Shasta Boyz Productions NEW film, Slippery When Wet. Each trailer will feature an athlete from the movie and give some insight into each characters lifestyle. The sequel to Wet Dreams, this film will take viewers throughout the United States, Mexico, Hawaii, Japan and get to know some of the most talented athletes in the industry. This trailer features some of the finest white water all in CALIFORNIA. Next up is long time hucker and Cali native, Drew Duval. Get ready for the next big thing from SBP. Trailer music by 2pac "California Love" (Rusko Remix) shastaboyz.com
Cast: Shon Bollock
The 2011 Season Of The Telekom Extreme Playgrounds Starts In Duisburg
The 2011 Season Of The Telekom Extreme Playgrounds Starts In Duisburg
The Telekom Extreme Playgrounds is back for another spectacular season of biking in 2011. Duisburg is set to welcome a long list of international BMX and mountain bike champions, plus three bands are expected to rock the stage.Source: NEWS/Biking/Telekom_Extreme_Playgrounds_2011_Duisburg_Preview_0823.htm
2010 Subaru Freeskiing World Championships In Snowbird Utah
2010 Subaru Freeskiing World Championships In Snowbird Utah
Current Subaru Freeskiing World Tour leader Griffin Post has high hopes for the 2010 Subaru Freeskiing World Championships in Snowbird, Utah. With his high ambitions, winning just the event is simply not enough.Source: NEWS/Freeskiing/2010_Subaru_Freeskiing_World_Championships_In_Snowbird_Utah_Preview_0614.htm
Tuesday, March 29, 2011
Holden Racing Team Clipsal 500 Wrap Up
2010 European Downhill Cup In �pič�k
2010 European Downhill Cup In �pič�k
The first race in the history of the iXS downhill series in Eastern Europe was being held in the Czech Republic. �pič�k, which is situated in the "B�hmerwald" national forest, presented a perfect event on a high organizational level.Source: NEWS/Biking/European_Downhill_Cup_2010_Spicak_Results_0754.htm
A Boulder-Climbing Paradise, Where Sacredness Meets Sport
To far-flung devotees of the intensely physical rock-climbing style known as bouldering, Hueco Tanks, Tex., is known for the grandeur and challenge it offers.Source: http://www.nytimes.com/2009/04/14/sports/othersports/14boulder.html?partner=rssnyt&emc=rss
LG Snowboard FIS World Cup 2011: Finale In Arosa, Switzerland
LG Snowboard FIS World Cup 2011: Finale In Arosa, Switzerland
The LG Snowboard FIS World Cup will organize two snowboard cross races on the 24th and 25th, one halfpipe competition on the 26th, and a parallel giant slalom on the 27th to finish things off. Some of the Crystal Globes are still up for grabs.Source: NEWS/Snowboarding/LG_Snowboard_FIS_World_Cup_2011_Arosa_Switzerland_Preview_0850.htm
ASA Big Air Triples 2010: Get Tickets Now For June 19th
ASA Big Air Triples 2010: Get Tickets Now For June 19th
For the first time ever, the ASA Big Air Triples welcomes skateboarders to the event, which previously has only showcased BMX riders. Tickets are now on sale for the event in June.Source: NEWS/Skateboarding/ASA_Big_Air_Triples_2010_Get_Tickets_Now_For_June_19th_Preview_0669.htm
Monday, March 28, 2011
Sir Richard Branson to cross the English Channel
Sir Richard Branson is turning 60 and to celebrate, he's crossing the English Channel!
Champions League 360 - Real Salt Lake - The Semifinals - Part 1
Go behind the scenes with Real Salt Lake as the team prepares for the first leg of its semifinal series with Costa Rican side Deportivo Saprissa.
The North Face Park and Pipe Open 2011: Logans Win On Same Skis
The North Face Park and Pipe Open 2011: Logans Win On Same Skis
After dreaming of this day since they started skiing, a brother and sister team both achieved top honors at the 2011 The North Face Park and Pipe Open Series.Source: NEWS/Freeskiing/The_North_Face_Park_and_Pipe_Open_2011_Results_0815.htm
How to Skateboard
How to Skateboard
Do you love reading skateboarding articles and watching videos of the pros? Wish you could land the tricks with ease like they do, but you just don't know how to get started?Sunday, March 27, 2011
Winter X Games 15: Freeskiing Photo Gallery
Winter X Games 15: Freeskiing Photo Gallery
Just days ago, some of the world's greatest freeskiiers demonstrated their awe-inspiring skills at the 15th Winter X Games. Our photographer was on site and captured the moments worth remembering.Source: NEWS/Freeskiing/Winter_X_Games_15_Freeskiing_Photo_Gallery_2011_0826.htm
Climbing the Walls in Brooklyn
Since opening last fall, Brooklyn Boulders, an 18,000-square-foot gym, has become a destination for New York rock climbers of all levels.Source: http://travel.nytimes.com/2010/05/28/travel/28boulders.html?partner=rssnyt&emc=rss
Skimming Physics
The Delaware coast has great skimboarding beaches. They're so good, in fact, that the 2007 East Coast skimboarding championships were held at Dewey Beach, Delaware a few weeks ago. My son and I stopped by after the waves died down and were too small to surf.
In case you're not familiar with skimboarding, it's one of the coolest and least recognized extreme sports. Check out this video from the 2004 championship, which was also held at Dewey Beach.
Skimboarding, surfing, wakeboarding, and kiteboarding are among the popular skimming sports. In all four sports, athletes glide across the surface of the water on one type of board or another. (Skimming is also known as planing, but who would want to practice a sport called "planeboarding?")
Most boats plow through the water, pushing the water they pass through to either side. It's tough work, which means that they move very slowly compared to the sorts of speeds we're used to on dry land. Twenty knots (about 23 miles per hour) is a pretty good clip for a boat. To go faster, or reduce the amount of energy it takes to move, watercraft have to skim.
In unpowered skimming sports, particularly surfing and skimboarding, speed is less an issue than conserving energy. The more the board pushes water aside, the more quickly the surfer or skimmer will lose energy and slow down.
So, how does skimming work? In water more than a few inches deep, a surf board or skim board glides along the surface of the water with a small angle of attack -- that means that the board is not perfectly level relative to the water surface, but instead raised up a little more at the front, and lowered down a bit at the back.
You can get the idea if you stick your hand out of the window of a moving car. Tilt the leading edge of your hand up, and you will feel lift pulling upward, tilt it down and the force pushes your hand down. (It's still called 'lift' when it pushes down, except that now it's a "negative lift" or "downward lift." I know, it's not a good term, but it's what we're stuck with because most engineers and physicists who first started looking into this stuff were interested in upward forces, not downward. In race car engineering, they tend to call negative lift "down force.")
When a surfer is moving fast on a wave you can see the trail they leave behind that results from the board pushing water straight down below the board. Unlike a boat, surfers and other skimmers don't create a v-shaped wake (at least not much of one, provided their speed is high).
A boat's wake comes from pushing water sideways, which is a very inefficient way to get around in the water. That's because, when it comes down to it, a boat's primary function is to stay afloat. Moving from place to place is secondary.
Most surfboards, skimboards, and related skimming equipment barely float at all. When surfers sit on their boards waiting for waves, their boards tend to be mostly under water.
Once they get moving, a surfboard or skimboard doesn't float so much as fly. That is, the board's buoyancy doesn't matter at all once the board is moving fast enough. What's important is the size of the board, or more specifically the surface area of the board. The more surface area available, the more lift the board will have. The more lift the board has, the slower you have to go to start skimming over the water instead of plowing through it.
The cool thing about skimboards, in contrast to all the other skimming sports, is that they also use another phenomenon to glide along. They benefit from something called the ground effect.
When skimming on a very thin layer of water, about a centimeter or so, the board pushes the water down but the ground gets in the way. Instead, it has to flow out toward the sides of the board. But water, like all fluids, resists flowing through small spaces. The term for this is viscosity. Molasses is very viscous and flows slowly, air is not very viscous and flows quickly, water is somewhere between the two.
Because of the ground effect, skimboarders can glide for a very long time over shallow water. As soon as they enter water more than a centimeter deep, the board acts like a surfboard instead of a skimboard.
The same thing happens to airplanes. When they are high up, they have lots of air below them, and they fly by pushing air down with their wings. When they come in for a landing, the air can't escape from under the wings and the plane begins to act like a skimboard on shallow water. Pilots are familiar with the ground effect. In fact, it helps when landing because it feels as though they are coming down on a cushion. On the other hand, if a novice pilot isn't prepared for the extra cushion of the ground effect, they can end up skimming over the runway and missing their landing altogether.
The ground effect makes skimboarding unique among extreme sports (or any other type of sport, as far as I know). No other sport takes advantage of this particular bit of physics.
So go skimming! It feels very cool to glide along on one of those things, and it gives you something fun to do at the beach when the waves are too small to surf.
In case you're not familiar with skimboarding, it's one of the coolest and least recognized extreme sports. Check out this video from the 2004 championship, which was also held at Dewey Beach.
Skimboarding, surfing, wakeboarding, and kiteboarding are among the popular skimming sports. In all four sports, athletes glide across the surface of the water on one type of board or another. (Skimming is also known as planing, but who would want to practice a sport called "planeboarding?")
Most boats plow through the water, pushing the water they pass through to either side. It's tough work, which means that they move very slowly compared to the sorts of speeds we're used to on dry land. Twenty knots (about 23 miles per hour) is a pretty good clip for a boat. To go faster, or reduce the amount of energy it takes to move, watercraft have to skim.
In unpowered skimming sports, particularly surfing and skimboarding, speed is less an issue than conserving energy. The more the board pushes water aside, the more quickly the surfer or skimmer will lose energy and slow down.
So, how does skimming work? In water more than a few inches deep, a surf board or skim board glides along the surface of the water with a small angle of attack -- that means that the board is not perfectly level relative to the water surface, but instead raised up a little more at the front, and lowered down a bit at the back.
You can get the idea if you stick your hand out of the window of a moving car. Tilt the leading edge of your hand up, and you will feel lift pulling upward, tilt it down and the force pushes your hand down. (It's still called 'lift' when it pushes down, except that now it's a "negative lift" or "downward lift." I know, it's not a good term, but it's what we're stuck with because most engineers and physicists who first started looking into this stuff were interested in upward forces, not downward. In race car engineering, they tend to call negative lift "down force.")
When a surfer is moving fast on a wave you can see the trail they leave behind that results from the board pushing water straight down below the board. Unlike a boat, surfers and other skimmers don't create a v-shaped wake (at least not much of one, provided their speed is high). 
A boat's wake comes from pushing water sideways, which is a very inefficient way to get around in the water. That's because, when it comes down to it, a boat's primary function is to stay afloat. Moving from place to place is secondary.Most surfboards, skimboards, and related skimming equipment barely float at all. When surfers sit on their boards waiting for waves, their boards tend to be mostly under water.Once they get moving, a surfboard or skimboard doesn't float so much as fly. That is, the board's buoyancy doesn't matter at all once the board is moving fast enough. What's important is the size of the board, or more specifically the surface area of the board. The more surface area available, the more lift the board will have. The more lift the board has, the slower you have to go to start skimming over the water instead of plowing through it.
The cool thing about skimboards, in contrast to all the other skimming sports, is that they also use another phenomenon to glide along. They benefit from something called the ground effect.
When skimming on a very thin layer of water, about a centimeter or so, the board pushes the water down but the ground gets in the way. Instead, it has to flow out toward the sides of the board. But water, like all fluids, resists flowing through small spaces. The term for this is viscosity. Molasses is very viscous and flows slowly, air is not very viscous and flows quickly, water is somewhere between the two.
Because of the ground effect, skimboarders can glide for a very long time over shallow water. As soon as they enter water more than a centimeter deep, the board acts like a surfboard instead of a skimboard.
The same thing happens to airplanes. When they are high up, they have lots of air below them, and they fly by pushing air down with their wings. When they come in for a landing, the air can't escape from under the wings and the plane begins to act like a skimboard on shallow water. Pilots are familiar with the ground effect. In fact, it helps when landing because it feels as though they are coming down on a cushion. On the other hand, if a novice pilot isn't prepared for the extra cushion of the ground effect, they can end up skimming over the runway and missing their landing altogether.
The ground effect makes skimboarding unique among extreme sports (or any other type of sport, as far as I know). No other sport takes advantage of this particular bit of physics.
So go skimming! It feels very cool to glide along on one of those things, and it gives you something fun to do at the beach when the waves are too small to surf.
Source: http://feedproxy.google.com/~r/blogspot/uUCU/~3/cYeGDtq4mXo/skimming-physics.html
Wenger Patagonian Expedition Race 2010: Part 1
Wenger Patagonian Expedition Race 2010: Part 1
At the Wenger Patagonian Expedition Race, one of the hardest adventure runs in the world, the participants have to constantly push it to the limit.Source: NEWS/Extreme_Sports/Wenger_Patagonian_Expedition_Race_2010_Part_1_0698.htm
iXS European Downhill Cup 2010: Julien Camellini Dominates In Pila, Italy
iXS European Downhill Cup 2010: Julien Camellini Dominates In Pila, Italy
One of the most beautiful races of the European series was staged last weekend in Pila. The cup�s penultimate race attracted 320 riders from 15 countries.Source: NEWS/Biking/iXS_European_Downhill_Cup_Pila_Italy_2010_Results_0760.htm
Saturday, March 26, 2011
The Wheel Deal Part 2: Wheel Size (again)
Bigger wheels are faster on rough surfaces, as I pointed out in my last post, because more of the forces that they experience as they roll over bumps pushes the wheel up rather than pushing backwards and slowing the wheel and rider down.So, why not just ride the largest wheels you can find? Unfortunately, when it comes to skateboarding and inline skating, larger wheels simply don't roll as well on smooth surfaces. The problem is that the urethane they're made of changes shape when you ride.
A simplified model of a skate wheel might look like this, where the jagged lines are meant to be springs that represent the compressible urethane. That's not to say that there are actual springs inside the wheels. It's just that the urethane acts much like springs, and it's simpler to understand the way wheels work if you pretend that they're are made of springs.As you can see in this sketch (I've exaggerated things a lot to make it easier to visualize), a wheel deforms as you ride along, resulting in a flat spot where the wheel touches the ground.
Most skate wheels are solid cylinders of material, except for the hole running through the middle where the bearings sit and the axle passes through. The springy urethane compresses when your weight pushes down on it. If you look at two wheels that are identical in shape except that one is large and the other is small, and both are made of exactly the same urethane, the larger wheel will deform more under the same weight.
This is where we can use physics to understand what's going on a little more precisely. Physicists think about springs in terms of something called the spring constant (usually symbolized with the letter k). The higher the spring constant, the more force you need to stretch or compress the spring.
If you connect two springs end to end, the total spring constant goes down (from k to k/2). In other words, it will be easier to stretch two springs connected in a row than it is to stretch just one. (You can test this yourself by tying rubber bands end to end.)As you can see in this sketch, attaching two springs side by side increases the total spring constant (from k to 2k), making them harder to stretch.If you cut a spring in half, it will double the spring constant. It's like taking the two springs connected end-to-end and getting rid of one, which doubles the spring constant from k/2 to k.
This is relevant to skate wheels because you could always make a small wheel by shaving down a big wheel. If you were to do that with the model of a wheel that I drew above, you're essentially shortening the springs. This makes the wheel stiffer (which is to say, less springy). Your weight pressing down on a small wheel will not deform the wheel as much because it's effective spring constant is much higher than it would be for a wheel that's identical in very way except for its larger size.
This makes larger wheels slower because compressing and stretching springs, or springy urethane, takes energy. With a perfect spring, you get all the energy back as it springs back to its natural shape. But no springs are perfect, and urethane is usually far from perfect.
Urethane is fairly resilient, which means that once it's deformed it bounces back into shape and gives back some of the energy that deformed it, just like stretching and releasing a spring. Depending on the exact formula of the urethane, a portion of the energy is always lost. Most of the lost energy turns into heat that warms the wheel and escapes into the air. If you squish a skate wheel you can expect to get back no more than 75% of the energy you put into it, and usually you get back a lot less. As a rough estimate, the deformation that comes with rolling on a urethane wheel will cause a large wheel to lose twice as much energy as one half its size. That's what makes larger wheels slower.
There are several ways to reduce the amount of energy lost due to the squishing of skate wheels. One common solution is to replace some of the urethane with a rigid core, like this Spitfire wheel.
Another possibility is to make the wheels wider, like these old school wheels.
Take a look at the cutaway sketch below that shows why wider wheels are less squishy. By widening the wheels, you're adding more springs (well, springy urethane anyway) in parallel. If you recall the diagram above, adding more springs side-by-side increases the total spring constant and makes it harder to stretch or compress the springs.
The wheel on the right is three times wider, and should be three times more rigid than the wheel on the left.
So, if you want bigger wheels that will roll as fast as smaller wheels, you have to make them wider. That leads to other problems. For one thing, wheels that are wide and have big diameters are heavy. That's not so good for all the ollie-based street moves, but fine for ramp, bowl and downhill skaters.
Another problem, which can be bad for all sorts of situations, is that the wider you make a wheel the harder it is to corner. If you try to ride in a circle, the outer edge of the wheel travels farther than the inner edge. Because both parts have to roll at the same speed, either the outer part of the wheel ends up turning too slowly or the inner part turns to quickly. That can lead to lots of wear and tear on the wheels, as well as extra friction that will slow you down whenever you change direction.
I've already explained why you need large wheels for riding on rough surfaces. Now you can see why smaller wheels are better for skating park concrete and obstacles. So, why not get REALLY tiny wheels for skating on smooth surfaces? Unfortunately, when wheels get too small other problems start to crop up. This post is already long enough, so I'll tell you about those issues some other time.
Source: http://feedproxy.google.com/~r/blogspot/uUCU/~3/OYiWOAttVFs/wheel-deal-part-2-wheel-size-again.html
ASA Big Air Triples 2010: Get Tickets Now For June 19th
ASA Big Air Triples 2010: Get Tickets Now For June 19th
For the first time ever, the ASA Big Air Triples welcomes skateboarders to the event, which previously has only showcased BMX riders. Tickets are now on sale for the event in June.Source: NEWS/Skateboarding/ASA_Big_Air_Triples_2010_Get_Tickets_Now_For_June_19th_Preview_0669.htm
King of Wake Releases 2010 Schedule
King of Wake Releases 2010 Schedule
Get ready for the world's best pro wakeboarding series, which kicks off this April. King of Wake has announced where and when the most elite pro wakeboarders will compete for the crown.Source: NEWS/Wakeboarding/King_of_Wake_Releases_2010_Schedule_0617.htm
Why is Snow so Slippery?
Snow sports are a blast. I prefer snowboarding, but I would never miss out on a toboggan ride or a turn on one of those old fashioned snow saucers (What genius conceived of those anyway? Is there anything more insane that cruising down a hill on a disk with no obvious way to steer or stop? Of course, I love it anyway.) As embarrassing as it is, I've even been known to to ride an inner tube now and then.The key to all those sports - as well as skiing, snowlerblades, ski bikes, and cafeteria lunch trays - is that things slide well on snow. You could leave it at that and simply go outside to have fun, but I just had to know a bit more.
If you hunt around, you'll see that there are at least two possible reasons why snow is slippery. One common explanation is that the melting point of ice rises as you squeeze it. Water ice is unusual in that way. This could explain why ice skates work. When you stand on skates, it creates very high pressures under the sharp blades. The pressure raises the melting point of the ice until it creates a thin layer of water, which is very slippery. (This is, however, not a universally accepted explanation.)
Pressure might work for ice skaters, but it's not much help for sleds, skis, snowboards, or any other device that slides on a large, flat surface. Because your weight is distributed over a big board, the pressures are very low. At best, you might raise the melting point of the ice by a degree or so, but if the temperature of the air and snow outside is more than a few degrees below freezing, you won't actually melt any snow with pressure.
Another possibility I've heard on occasion is that the friction of the board sliding on the snow creates heat, which melts some of the snow and creates a thin lubricating layer of water. Now, this explanation sounded just absurd to me. But I didn't want to dismiss it until a estimated just how much heating you might get from snowboarding down a hill.
You know what? I was stunned -- STUNNED -- to find out that cruising down a hill creates lots of heat. At a relatively modest 36 kilometers an hour (22 mph) down an equally modest hill (imagine a 15 degree slope or so), someone about my size (75 kilograms) creates about 300 watts of heat power due to friction between the snowboard and the snow. That's three times as much power as put out by a typical light bulb. If you've ever touched a lit incandescent bulb, you know that's hot. Theoretically, friction could heat the bottom of the board by nearly 40 degrees Celsius (about 100 degrees Fahrenheit)! In the real world, the bottom of your board will never get that hot. It will only warm up to the point that it melts the snow. It takes energy to melt snow, and the melting ends up using the energy that would heat your board to any higher temperature than 0 degrees C (32 degrees Fahrenheit).
But suppose you were snowboarding on a day when temperatures went as low as at -40 degrees C, even moving slowly would generate enough heat from friction to melt the snow and provide a very thin layer of water for you to slide on. (Of course, -40 C is just about the lowest temperature ever recorded in Alaska, so you'd probably get an awful case of frostbite before you got to the top of the lift.)
So there you have it - it's friction, not pressure, that melts snow below your board and let's you whiz down the hill at a breakneck pace. Wacky.
For those of you who like the details, the math that led me to this conclusion is below.
The Math and Physics
You can change the parameters of the problem to model any sliding sport you like, but I'm using a snowboard in my analysis.
The first thing to do is figure out how much work is done when you slide on a snowboard. Work is defined as force times distance. Specifically, we're interested in the frictional force multiplied by the distance traveled.
The force of friction is just the force pushing the snowboard down onto the snow multiplied by the coefficient of friction (u).
Because the snowboard is moving downhill, the force pushing against the snow due to the mass of the snowboard and rider is reduced by the cosine of the hill slope.
Friction force = m g u cosine(theta)
m = rider's mass (I'm using 75 kilograms)
g = acceleration due to gravity (0.8 meters per second squared)
u = coefficient of friction for a waxed board sliding on snow (about 0.04, according to Wikipedia)
theta = slope of the hill (I'm using a modest 15 degrees)
If you multiply this by the distance traveled on the hill, you get the total energy expended on the trip. If instead, you multiply the force by the velocity of the snowboard, you get the work per unit time. That's the same as the power (watts in SI units).
Power = m g u cosine(theta)* v
v = velocity (for this problem, I'm using 10 meters/second, which is about 36 kph, or 22.5 mph)
Plug all that in, and you find a power output of about 284 watts to slide down the hill.
The equation for heat conduction through a slab of material (such as a snowboard deck) is
Power = dQ/dt = k A (T2-T1)/L
k = thermal conductivity (about 0.25 Watts/(meters*degrees Kelvin))
A = area of board touching the ground (about .25 square meters for a typical board)
T2 = temperature on the hot side of the board (in Kelvin)
T1 = the temperature on the cold side of the board
L = the thickness of the board (I'm using one centimeter, 0.01 meters)
When I rearrange this to solve for the temperature difference between the two sides of the board (T2-T1) and plug in the numbers, I get temperature difference of about45 degrees.
As I mentioned above, that's the maximum temperature difference between the two sides, but the hot side should never get above the freezing point of water because the heat generated by friction would have to melt all the snow before it could lead to higher temperatures. That's because of the phase transition from ice to water that occurs at the freezing point (It's the same reason that water with any ice in it at all will have a temperature of exactly 0 degrees Celsius. You can confirm this by putting a pot of snow on the stove and turning up the heat. The temperature will rise to 0 degrees C and stay there until all the snow is melted.).
So, if you're out snowboarding when the air temperature (and top of your board) are at -5 degrees Celsius, the temperature of the bottom of your board be about 0 degrees Celsius when you're moving along at 10 meters/second. In fact, it will always be at about 0 degrees Celsius if you're moving at almost any reasonable speed, and there will be a very thin layer of water under it due to all the frictional energy you're generating by sliding down the mountain.
Source: http://feedproxy.google.com/~r/blogspot/uUCU/~3/98TqO5Qa59k/why-is-snow-so-slippery.html
Subscribe to:
Posts (Atom)