The force vectors on a skateboard during an Ollie | Hacker News Reader

The force vectors on a skateboard during an Ollie

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194 pointsaatish52 comments

If you like this and you haven't seen 1000fps flatland tricks:

https://www.youtube.com/watch?v=aHrn3-Cb3iM

I was a decent skateboarder in my day but these tricks are an order of magnitude harder than anything that was happening in my day/era... the exception was rodney mullen. he was the only pro doing anything like that

I really dig what Killian Martin is doing in that regard: pushing limits, applied to an artsy end.

[0]: https://www.youtube.com/watch?v=D_h-7Kp87OE

[1]: https://www.youtube.com/watch?v=cbSbbY5ibas

Well Rodney Mullen invented almost all of the tricks. (Including the more famous ones like the kickflip and such)

Witness: https://www.youtube.com/watch?v=uEm-wjPkegE

I hadn't seen that. It really emphasizes how much precision of timing/movement is required in order to pull off a trick.

Also: http://vimeo.com/1340684

I really like the overlay of physics information in the video. It is so much easier to explain to students what acceleration is by showing it rather than asking students to imagine "the second derivative of the position function r(t)=(x(t),y(t))."

Check out the software they used: http://www.cabrillo.edu/~dbrown/tracker/

For anyone trying to learn how to do an ollie for the first time, the one thing that really helped me was this: do it on grass. I spent ages on concrete, and it is a lot harder. On grass the board doesn't roll and you don't get hurt if you fall.

A bit like learning to walk on your hands - start in a shallow pool. Once you get comfortable with the motion translating it to a different surface is not that hard.

Yeah, that's how I did it. Grass is helpful until you can get the board to come up close to level and land on it.

For some reason it took me much longer to learn how to drop in on a ramp. I built an 8-foot ramp in high school (no vertical) and it took me ages (weeks, IIRC) to drop in without immediately planting a foot and running or falling down the ramp.

When I finally nailed it I was so happy. Damn, I miss skateboarding.

I remember the first time I dropped in too. It is another one of those things where if you don't commit all the way, you fail. Once you get it you have it for life. I think having my friends get it first helped me with that one!

I'm impressed you built a ramp! Luckily there was a nice new outdoor skateboard park near my friend's house, we didn't have to build anything :)

Doesn't grass absorb most of the pop, though?

You may be thinking of longer grass? It is just important that the board doesn't move forwards or backwards while you are practicing.

I'm less impressed by the skateboarding tricks and more impressed by the physics software that can derive all this from a video.

The simple Ollie in that video is no where near as impressive as the tricks performed in the video lbotos posted (top comment currently). Unfortunately, I get the feeling the tracking software would have a hard time with any component of movement that isn't coplanar (including rotation).

Well tracking software has been around for a while. I did something like this for a highschool project...

I guess it's just something I'd never seen before, impressive at first sight! I did all my physics quite a few years ago and don't remember video analysis like this, although I guess it wasn't really my focus anyway.

There's no advanced physics software required to get the data in the video, he's just plotting the second derivative of a line he got by tracing the center of the wheel. F is proportional to a :)

Skateboarding is an incredibly fun sport. You should give it a try.

I never got any good ollies in high school. Watching the vids and reading the article I think I realized something I never understood. It looks like most of the skater's upward body acceleration is coming from using the the _left_ leg, with the right foot going along for the ride (and smacking the tail down). I always tried to get the major jumping lift from my right leg, while also somehow smacking the tail at the same time. I think I've always had it backwards!

The highest ollies come from jumping with both legs, and using the ankle of the foot on the tail (right if you're regular-footed) to flick the tail down and pop the board.

It's basically a two-foot jump; the hard part is getting the board to stay with your feet.

Jumping on a bike is also basically a two-foot jump, where you pull the whole bike up after yourself using the handlebars. There is no special "pull up the front first and then push forward" as many people believe. The initial jerk is just to get the handle bars above the COG so that you can actually lift the bike from there and to put your pedals above the balance point of the rear wheel so you can push down hard for the actual jump. This might seem strange at first but when you think of it, apart from the legs what parts of the body have enough force to jump yourself that high, it's not like you can magically jump higher just because you are standing on some weird contraption.

On a bike you do get some help from preloading the tyres to give you a bit of bounce but that's minor compared to the rest of the movement.

You don't actually need any fancy software to see what's going on if you've mastered the fine art of free body diagrams: http://www.tjradcliffe.com/?p=1374 (not related to skateboards, but those damned wobbly suitcases).

And the advantage of simple free body diagrams is they are amenable to relatively simple models, frequently involving simple harmonic motion: http://www.tjradcliffe.com/?p=1410

These are teachable skills, and while it's great we have software to help us out, I'm a sufficiently aged crustacean to see the value in learning to do these sorts of analyses formally, by observation and inference.

It looks like the skater drags his shoes against the "lip" of the skateboard to get a bit more "forward" direction on takeoff, pulling it upwards.

Ollie is the hardest thing to learn in skateboarding. It took me and my friends a long time. I think it's hard to explain it, so it's hard to teach.

It's rather that the initial smack or pop of the tail against the floor makes the board fly upward, which you can reproduce by just by stamping on the tail of your board. That was my first insight.

You can do a standing Ollie with one foot: stamp it and as the board starts going up you shove your same foot to the right to stabilize it. After mastering that, a two-foot Ollie was just a matter of practice. Also, the longer you delay putting any downward force in the nose, the more air you get. I feel like I should make a video of a standing single foot Ollie, as two feet complicate demonstration, I think.

The moving forward of the foot isn't for adding more forward velocity, a standing Ollie can get really high and land in the same place. A moving Ollie relies on your existing velocity, which should be pretty high.

I've not skated for about 10 years, but I got a board recently to see if I still had it. Ollie'ing is like riding a bike, never forget how to do it. :)

> I've not skated for about 10 years, but I got a board recently to see if I still had it. Ollie'ing is like riding a bike, never forget how to do it. :)

It's tough for me. I skated nearly every day of high school and not much since then. I got on a board a few months back and it was like starting all over again. The biggest difference that I noticed was my diminishing flexibility. I need to start managing that--my wife, a physical therapist, says that lack of flexibility in our 30's and 40's is highly correlated with likelihood of falls when we are elderly.

> my wife, a physical therapist, says that lack of flexibility in our 30's and 40's is highly correlated with likelihood of falls when we are elderly.

I think this is why a lot of people get injured as they get older. You just don't realize how much less time you're putting into something you used to be able to do really well, then you try something that used to be easy with less flexibility and bam. problems, heh.

I ran an obstacle course recently after having not done it in awhile. I was actually pretty surprised at how coordinated I still was (I'm 31, so I don't have all the issues I could yet), but I felt the tightness during and was sooooo sore the next day

I really need to start doing consistent balance drills and flexibility training

I really need to start doing consistent balance drills and flexibility training

If that's your aim, I can't recommend a regular yoga practice highly enough.

This seems absurdly obvious now that you've mentioned it :)

I'll have to seriously look into that

> The biggest difference that I noticed was my diminishing flexibility.

This is definitely the biggest change I've noticed. There is no way I could skate now like I did in college and high school as my body simply won't allow it.

Also important is that I doubt I'd recover from a nasty fall like I did when I was younger.

Lastly, even if I could physically move like I did before, the mental concern about the cost of a bad fall / breaking a bone / pulling a ligament / etc. would prevent me from landing any type of trick beyond the basics. One of my friends who skated with me a lot in college used to like saying that the biggest factor in pulling off a trick is "thinking that you're going to".

> There is no way I could skate now like I did in college and high school as my body simply won't allow it.

With some work you probably could. You'd be surprised by how many "older" dudes (40+) absolutely crush it at the climbing gym. It takes a damn lot of strength and flexibility for them to be able to do that.

To add to this, when it comes to rock climbing so much is down to finesse, and precision. As a mediocre climber, I find myself limited when on routes that I can't muscle my way out of; it's a conscious effort to practise planting my feet slowly and accurately, and coordinating my balance.

Climbing is such a wonderful activity precisely because it blends so many things; strength, agility, discipline, accuracy, not to mention shear mental focus.

Oh, and you get to play with lots of gear and gadgets. It's great for geeks. :-)

Good on you! I noticed I was initially afraid to just get moving on the thing. What about the skateboard falls? Partly why I stopped street skating was all the falls and ankle kinks. I still feel like I need to stage a fall on this new board just to break the ice. :-)

I wouldn't say it's the hardest, but it's definitely the first hard step.

I learned to ollie in the summer of 1987. It had taken me and my brother about 6 months to figure it out. The hardest part was training our brains to be ok with it. Once we landed a few, it was then fairly simple. But, it was the landing that was most difficult. We intuitively understood the motion and forces required, but we resisted letting our back foot follow through.

By 1989, we were able to do bike racks, picnic tables (with appropriate ramp angle), garbage cans. And, we eventually figured out the kick flip, which was the second hardest trick to learn (at first).

Now, the 360 flip to handrail grind - that's a freaking hard trick that requires some guts to let oneself do. And, it's one of those tricks that requires a good video camera nearby to establish proof.

>It's rather that the initial smack or pop of the tail against the floor makes the board fly upward, which you can reproduce by just by stamping on the tail of your board. That was my first insight.

I see this repeated over and over. Maybe I'm misinterpreting your comment, thought I've definitely seen it elsewhere such as this comment on the article:

>It actually looks like he "bounces" the board while starting the ollie, which provides a portion of the initial upwards velocity of the board.

Think about that for a moment. This is patently absurd. It's like trying to get several feet of air by crouching down and hitting the ground with a hammer. The collision is too elastic.

I think the confusion might arise because a really strong smack of the ground by the tail coincides with more torque (torque, right?). Of course, you can easily ollie if your tail hangs over nothing at all; it's probably even easier because you can rotate further.

IMHO, Terr's guess is exactly right. You get all the lift from pushing down the nose / dragging across the grip tape.

> IMHO, Terr's guess is exactly right. You get all the lift from pushing down the nose / dragging across the grip tape.

This is clearly not the case. The front foot guides the board, stops it rotating backwards, and is necessary to land the ollie, but it doesn't help get the board into the air.

The back foot pushes down (HARD) on the tail, the entire board rotates about the back axle until the tail hits the ground. At this point the front truck (which has significant weight (relative to the rest of the board)) is moving up and the rear truck is stationary, the momentum of the entire system is now up - the skater now (simply!) has to lift the rear foot at about the same speed as the tail lifts off the ground, and use the front foot to stop the board from over rotating back.

>Think about that for a moment. This is patently absurd. It's like trying to get several feet of air by crouching down and hitting the ground with a hammer. The collision is too elastic. //

It's not at all like hitting the ground with a hammer [but hammers will bounce on concrete FWIW and even flip end over]. It's more akin to a see-saw that isn't fixed down. If the force at the back of the board is sufficient to overcome the weight of the board pulling it down at the front then the board will rotate lifting the front. Once the back of the board hits the floor there is a new pivot point, the rear tip of the board (https://www.youtube.com/watch?feature=player_embedded&v=339k...). Now the board is rotating - in the clip - anticlockwise. But the front foot prevents this rotation and provides a further pivot.

You can do this experiment to demonstrate that a pivoted plank can jump. Take a ruler, place it on top of a rounded pen - this setup mimicking the rear wheel of a skateboard. Hit the ruler down at the short end and you'll see it flip up (it might take some practice!). Indeed the whole ruler, if it's short enough, might flip and continue rotating such that the long end goes up and over the short end - but this is because you don't have a front foot pushing that part down. Replacing the foot with your hand - as a barrier above the ruler preventing this continued rotation - leads to the ollie being imitated. The ruler jumps up with nothing other than a swift hit on the short end furthest from the pivot.

                                ( ) hand imitates
                                ( ) front foot
                                ( )
    ( )                         ( )   
    ( )
    ( ) hit
    ( ) down
    ---------------------------------
             O
             pivot, eg pen

Indeed, you don't even need the new pivot point of the back of the board either. If you smack the tail but don't drive it down the board will fly up into the air because the ground is resisting the pivot point of the wheels but the sky is not. The center of mass will start a linear trajectory once the pivot is defeated and I imagine that the angular velocity at the center of gravity will be mostly preserved once it takes off... My physics are rusty.

Yeah, you misinterpreted me. I didn't mean that just hitting something makes it fly up horizontally. I meant that if you simply stand on the tail of a board the nose points up and the whole board will fly upwards, like a seesaw. This is how you pick up your board if you're cool, stamp on the tail and it jumps into your hand. Small demo of that exact action: https://www.youtube.com/watch?v=JDU9wZUTLdU

The ollie is just that action with a foot sliding forwards to change the angle of the board.

Yea, I was being an idiot. I was interpreting some people's claims as the bouncing of the board being enough to propel the skater upwards. I guess nobody is saying that.

I really should know better since I can ollie. I think my mental model of the ollie put too much emphasis on the sliding.

Anyway, sorry.

I didn't say all the lift, I said "a bit more forward"...

The majority of the upwards movement seems to come from pushing down on the back, levering most of the board-mass upwards.

Big moment in my childhood was learning to 'pop an ollie'. From that moment on, you could separate yourself from others with these seemingly impossible standing jumps up kerbs and over obstacles. I didn't ever really get much higher than maybe a foot on my best day, but it was great fun.

Now I feel like finding a board just to see if I can. Sadly my Powell Peralta was lost with my Lego in some parental clean out while I was out being an adult that had flown the nest. I'd gladly trade my school photos and stupid artwork that was saved to get those back again.

We are dragging the shoe on our forward foot, but it's against the "grip tape" (made from crushed glass in a bed of adhesive). The force of friction from the shoe sliding forward against the grip tape is, in conjunction with the upward force from striking the tail of the board against the ground, sufficient to lift the board. Coordinating those two motions is the essence of an ollie.

The "a-ha" moment for me was concentrating on driving my rear foot into the ground.

No matter how you jump or drag your front foot, if you don't get a good solid "pop" on the tail, nothing happens. Felt weird at first but now it seems like it couldn't be any other way.

(where "now" is 10 years ago, when I last ollied anything... wtf)

It helps but it's not necessary. It's a game of shifting centers of rotation.[1]

    Kick the backend :: upward tilted lift, center : ~back wheels.
    Quickly slow the frontend :: upward tilted lift, center : ~front wheels.

[1] it requires suppleness in choreography, that's why any kid can do it, they're swift; the window of execution for both steps are very tiny. And they ask you to uncouple your mind, your feet and your center of gravity. Mind altering leap of faith it is.

I always thought this was the most logical explanation. Look at how high people can ollie,[1]. This wouldn't be possible if it they were just changing the axis of rotation. And this video here shows the skaters clearly pulling the board up with the grip tape/front lip [2]. Sliding your foot along the board is one of the basic steps for olling.

[1] http://www.youtube.com/watch?v=IYE-ZaXbzUg

[2] http://www.youtube.com/watch?v=339k4XEvbxY

Yes the Griptape is an essential part to make high ollies consistently. However ollie's are possible to do without griptape as well. Just a lot harder because you'll have a lot less control.

The harder you slam down the tail of the board the more friction you get, the easier it is to pull the board up/forward.

The motion capture shown is kind of limited. The wheels are tracked, but not the CG of the rider. Also, the video doesn't show the beginning of the ollie. It looks like, in this one, the rider did this entirely by loading up the springs in the trucks to get a bounce, rather than dragging the tail of the board.

It's a good physical simulation problem. Now that Gazebo has a real physics engine (by Mike Sherman from Stanford) rather than ones from video games, it should be possible to do an ollie in a robot simulation. (The game-type physics engines use an impulse/constraint model for contacts, which is unrealistic when you have a collision followed by sliding.)

> by loading up the springs in the trucks to get a bounce

There are no springs in skateboard trucks. The force comes from pushing down on the tail and then shifting your front foot up the nose.

There are bushings which will provide a small amount of spring, but your general points stands that the force comes from pushing down on the tail and shifting the front foot up to the nose to pull the skateboard up.

No, it doesn't make sense to track the center of mass of the skateboarder here. If you're interested specifically in the forces on skateboard, you want to track the center of mass on the skateboard. If you include the skateboarder in your system and track the new modified center of mass, you will only see the effect of gravity, being the only remaining external force.

Tracking the centre of mass of the skateboarder would give you an idea of how to get additional height on your ollie. Essentially sucking up your legs, bending at the waist and throwing your arms down will allow you to clear higher obstacles.

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