How to Pump Your Bike, According to Physics

Mar 25, 2021
by Seb Stott  

Have you ever wondered how pumping works? Why pushing downwards into the ground helps to propel you forward? I did, but for a long time I was satisfied with a very common explanation that went something like this: when you're pushing into the down-slope of a roller, you're making your bike heavier, and when you let the bike come towards you as you go up the next roller, the bike is lighter. That's what propels you forward - it's almost like you're turning gravity up and down to your advantage.


But a couple of things made me realize this explanation was wrong. And this misunderstanding was causing my pumping technique to suffer.

The first was the realization that you could pump around corners. Not just to generate traction or to do a sweet cuttie; pushing into a berm can actually generate speed. In fact, it's possible to pump while snaking side to side on flat ground, as demonstrated in this Skills With Phil video, which is great fun by the way. But a turn isn't downhill or uphill. So how does that work?

Pumping a turn is just like pumping a trough.

The second revelation came when a coach told me I was pumping too early. Instead of pushing into the steepest part of the down-slope, he got me to wait a fraction longer and push into the curved transition at the end of the slope. The difference was palpable.

When I thought about it more, pushing into the down-slope can't be what's pushing you forward. Imagine you're going down a hill with a constant gradient. Gravity accelerates you down the slope because the vertical force of gravity is not at right angles to the slope. But, if you extend your legs and push into the ground, you're pushing at right angles to the ground underneath your wheels, which doesn't help propel you forward at all. A lot of pumping explanations talk about pushing vertically downward onto a down-slope, but because the only thing to push against is the floor, the reaction force you're generating is always at right angles to the ground, and to direction you want to accelerate.

Physics 101

So how is it that pumping helps to generate speed in a corner or through a set of rollers? It's all to do with conservation of angular momentum.

This is a classic physics demonstration where someone spins around with heavy weights in both hands. Then, as he pulls the weights in towards the center of rotation, he spins faster. On first glance, it might look like the weights only rotate faster because they're describing a smaller circle, but the weights are actually speeding up.

The same is true of a pirouetting ice-skater, a spiral coin machine, the orbits of the planets, or the water draining from your bathtub: as the radius decreases, the speed increases. This is explained very neatly by the law of conservation of angular momentum. For a mass that is moving around a circle or a corner with a constant radius, the angular momentum is given by its mass, times the radius of the turn, times its velocity. (Angular momentum=MVR). In the absence of friction or other external forces, this quantity always stays the same.

So if the radius of curvature decreases (like the weights getting closer to the axis of rotation) then in order to conserve angular momentum, the velocity has to increase. In fact, if you halve the radius of the circle, the velocity must double. Let's apply this to pumping.

Imagine you're in a berm or a trough which has a radius of five meters. It has a center of curvature, the point that's always five meters away from any point on the curve. If you extend your arms and legs to push away from the ground, your center of mass will move towards the center of rotation. So as far as your body weight is concerned, the radius of curvature has decreased, and by conservation of angular momentum, your speed will increase. So when you're pumping through a set of rollers, it's the radius of curvature of the rollers that's important, not the down slope or the up slope. Picture the concave, U-shaped part of the rollers - the trough between the peaks. That is when you want to extend your arms and legs because you can move your center of mass towards the center of curvature about which the ground is curving. Similarly, when you're going over the crest of the rollers, you want to contract your arms and legs such that you move towards the center of curvature, which is now somewhere beneath the ground.

Push through the trough, compress over the crest.

This is why I used to suck at pumping. You don't want to extend your arms and legs when you're on the steepest part of the down slope, but the most tightly curved part of the transition. Sometimes the difference in timing can be quite subtle, but if you have a long down-slope before the transition, it pays to be patient and push only once the ground starts to curve. Similarly, it's the crest of the roller where you want the bike to come up towards you, not the steepest part of the up-slope.


A more thorough explanation

But the law of conservation of angular momentum is not really an explanation: a law is just a relationship between some variables, not a cause. So if you want a better explanation of why angular momentum is conserved, check out this video from Vsauce.


To summarize, if you have a mass going around a circle with one radius, and then you pull that mass in towards a narrower radius, (like the rider pumping into a berm, or an ice skater pulling her arms in), it doesn't go instantly from one radius to a narrower radius, it moves in a spiral pattern. And when that happens, the path it follows is no longer perpendicular to the line drawn from the mass to the center of rotation. That means if you pull towards the center of rotation, you're no longer pulling that mass perpendicular to its direction; you're pulling it slightly ahead, and that is what causes the mass to speed up. It's being pulled slightly forwards as well as radially inwards. This explains why a mass will speed up when it's drawn towards the center of curvature.

On a bike, as you push away from a berm, your center of mass moves in a spiral instead of an arc with a constant radius. And when you do that, you're pushing not only towards the center of the turn; you're also pushing your mass slightly forwards.

The transition giveth and the transition taketh away

It's worth remembering that the opposite is also true. If you allow your center of mass to move towards the ground in a compression, you will lose speed, just as an ice skater who extends her arms and legs away from the axis of rotation will slow down. Centrifugal force will naturally cause your arms and legs to collapse slightly unless you make a conscious effort, and your suspension will compress too, pulling you further from the center of curvature. So if you're on a full suspension bike you have to work even harder to counter this and to maintain speed. But remember that even if you only maintain your speed by pumping through a turn or compression, that's a win compared to the speed you would lose if you rode passively.

Also, pumping is not "free speed". The work you do pushing yourself away from the ground against the centrifugal force is what provides the energy that accelerates you down the trail. Anyone who's ridden a few laps of a pump track knows this energy is not "free".

Of course this doesn't just apply to a pump track. I find pumping most fun when finding natural corners and compressions to gain speed from. Just remember to look for the curves not the slopes.


132 Comments

  • 318 2
 I wish Taj could have illustrated my textbooks in school. I think I would have learned a whole lot more.
  • 37 0
 It's never too late
  • 13 2
 That wouldn't have worked for me since the only time I used my textbooks was when I needed a pillow in detention.
  • 10 0
 The best text book I ever had, or have seen since, was called the Cartoon Guide to Physics. It takes you all the way to particle physics, with the most incredibly fun and persuasive drawings. See if you can find a copy!
  • 11 2
 Completely random, but I keep finding it crazy that I have the same first name as Taj. It trips me out every time I see his name on a Pinkbike article.
  • 4 0
 Agreed. I’m ok at pumping, but the combination of Taj’s graphics and Seb’s explanation just made something click in my brain. Can’t wait to try this out
  • 2 0
 @hangdogr: you a character from a Nickelodeon show?
  • 4 1
 @tajpatel: How do you think @mikekazimer and @mikelevy feel each time they read each other's articles?
"Hey, wait, I didn't write that!"
  • 3 6
 The illustration quality is great and worthy of a textbook - unfortunately any physics teacher would cry at the misunderstanding of the subject that is being promoted in this article.
  • 6 0
 @SteveWatts: would you be so kind as to elaborate on what is wrong, rather than being smug without substance?
  • 1 0
 @dominic54: I love stuff like that, just bought a copy
  • 74 3
 Sorry, long post. Now y'all have got me worked up. I've been obsessed with pump tracks and the physics behind riding them for well over a decade, so I apologize in advance for what's about to come. I've done my best to keep it less technical.

Excellent illustrations from Taj, as usual, by the way! The physics of pumping is not well understood by the MTB community and it's truly refreshing to hear someone who understands that it's not about "being light on upsides and heavy on downsides". It's not about kinetic and potential energy transfer, and nor is pumping "free speed" by any means! It's also not exactly about conservation of angular momentum in the MTB case. This is different form the "ice skater" analogy because the ice skater does not add energy externally to the entire system. They don't do any "positive work" on the whole system. But in the case of pumping a roller or berm, the rider certainly does! By pressing hard into the ground in the right way, the rider is actually adding energy to the system, IE doing positive work on the whole system by pushing against the ground. That's how the rider's speed (linear, not angular) can progressively increase. Taj also makes the very keen observation that the rider can do the converse, ie extracting energy from the system (and slow their speed) if they "allow the trail to pump them".

Taj, you've made a similar revelation that I did many years ago about pumping. However for me, it was an excellent video from Lee McCormack (from Lee Likes Bikes) at the time, where he uses a BMX to show that you can pump perfectly flat ground to increase your speed. That was a real eye-opener for me and like you, I realized I was pumping all wrong! Lee's video helped me to understand that pumping is not exactly about getting heavy and light and not about upsides and downsides. Heck, you don't even technically need gravity either to do it!

The best explanation I can make for how to pump is this: Pumping to generate speed simply requires the rider to make a "positive displacement under load" of his/her center of mass toward the center of curvature while going through this curved path.

Remembering back to high school physics, the rider is basically using their legs and arms to push and do "positive work" on the bike-rider system and the result is an increase in the kinetic energy (speed) of the system.  So if you want to increase speed while turning left on flat ground, start in a low crouch, lean, and while arcing left and the load on your arms/legs increases due to the curved path, you progressively move your body left (further from your wheels) while you are arcing left.  That's it.   You've done positive work and you'll increase your speed.   For the case of rollers in a straight line, you start in a crouch as you enter the valley between rollers.  As soon as you start entering the "concave up" portion of a valley, your arms and legs start taking more load due to the curved path taken (centripetal force as Taj notes it).   You progressively push against this new force you are feeling and effectively move your body mass toward the center of the curved path taken, which is further from the track surface basically) and finish your elevation when the valley is not concave up anymore. So it really has nothing to do with the front side and back side of rollers and doesn't actually have anything to do with gravity either.

Is pumping free speed?  Definitely not as you well know if you've ridden pump tracks!  When you take an arced path under load at speed, say in a valley or in a berm, your body weight basically becomes "magnified" in proportion to the speed you are going, and how tight that arc is. And you need to use your body to fight this load and also make a positive displacement against it.  As you go faster and faster, the load on your body through all these concave arcs becomes higher and higher.  It's kind of like doing squats with weights, with a really sadistic personal trainer.  They guy is yelling at you to do these squats faster and faster, but every time you speed up your squat, he goes and adds more weight on your shoulders!  On the pump track it's not enough to just resist this increasing load, but you actually must move your body weight against it, ever more quickly if you want to keep generating more speed.  Free speed?  No, far from it, especially as your speed increases!

The Zero-Gravity Intergalactic PumpTrack Thought-Experiment:  When human beings have outgrown our planet, have no fear because pump tracks can still work without gravity.  Imagine an all-steel pump track where the rider has magnets in their rims instead of rubber tires so you will stay in contact with the track at all times.  You're going to need good clip pedals too here despite all your friends telling you flats are so much better for technical riding.
You get on your bike, give yourself a little push, clip in, and much to your amazement, you realize you can still pump this track to generate speed.  Even without gravity, the technique still works the same way.  You start in the valley in a crouch and elevate your body while the track is concave up, and voila, your speed goes up.   Thankfully you bought some good clip pedals which keep your shoes stuck to the bike while going over the crests of those rollers.  You think to yourself: Hey, when I go over a roller, I feel this new pulling force in my arms and my legs trying to separate me from the track since there is no gravity to keep me down anymore.  You find this interesting and it makes you wonder if you can use that new force you feel to your advantage:  If I am extended upward after finishing my "valley pump" and if I fight this new pulling force I'm feeling when I'm going over a roller (through the convex portion)  and if I use my legs and arms to pull myself toward the track, will that also help me to generate more speed?  To your amazement it does.  In space on the mag-track, you learned that you can effectively "push-pump" the valleys but also "pull-pump" the crests of the rollers too.  You can accelerate at an incredible rate, far beyond what you could do back on earth.  You are pretty impressed with yourself and you realized you really have to develop those leg muscles for pulling up though.

Okay that was super long. Thanks to those who got to the end of that!
  • 25 1
 I like the magnetic pump track thought experiment. The only bit I disagree on is that ice skater does do work when pulling her limbs towards the axis of rotation against centripetal force. This is the same as what's going on with pumping. Conservation of momentum does not mean conservation of energy. In fact, if momentum is conserved but inertia is decreased and velocity is increased, the kinetic energy has also increased. (E=L^2/2I, where L is angular momentum and I is the moment of inertia, so for given L, energy must increase as I decreases.) In both the ice skater and the pumping case, work is done against centripetal force to increase kinetic energy.
  • 6 0
 @seb-stott: great explainer, for your next trick can you apply this to the physics of sending it off a lip? A lot of the problem inexperienced riders have with jumps is “sucking up the lip” and finding they don’t make the landing. Meanwhile pros can be going a lot slower and send it to the moon. A lot of the same pumping principles apply.
  • 4 1
 @kpickrell Wow. This comment is way more to the point than Seb's article. I have very few objections to what is said here. I believe what Seb wrote is a real phenomenon, but one that has a minor effect on generating speed. The "positive displacement under load" is THE GREATEST summary on pumping physics I have ever heard.

I would add an analogy to skating, but not to doing pirouettes, but just skating forwards. There too you are making a positive displacement under load, that is with exerting more than your body weight to do positive work. This is very close to the magnetic pumptrack thought experiment, but results in oscillating motion from side to side as well as pulsing acceleration forwards.

What is worth to note is the pitch of your skates or your transition. Exerting force to a plane at 30° to your desired speed vector as opposed to 45° is like pushing a harder gear. as at 45° 1" of vertical movement gives 1" of horisontal. But pumping the end of a transition or berm will have an angle less than that, and you will advance more in line with your desired speed vector than perpendicular to it.

The curvature provides you with centripetal force, to keep you "glued to the ground" while you pump. This increases your ability to push the ground for longer with more force -> doing more work. Now you just need to pump as late as possible as hard as possible. But not so late, that your final velocity vector is not parallel with the ground but upwards producing a "small hop".

@seb-stott I reckon you should update the article with @kpickrell 's comment.

@verskis Mietipäs kriittisemmin
  • 1 0
 @Altron5000: Sounds interesting!
  • 1 0
 @kpickrell:
Your comment is on point. I never really thought of it like that. I usually do pumping corners technique to warm up and it generates so much speed without rollers. Great comment.
  • 5 0
 @seb-stott: You are right that the skater does work (positive work too!) when pulling their limbs in, but that is work done "internal to the system". Angular momentum will be conserved as you say in this case. They are not somehow pushing against the ground to increase their rotational speed. No energy is somehow added to "the system" in this case. With the MTB rider pumping case, the bike/rider is the system and this system is not spinning. The rider does similar positive work by pushing their arms and legs (say in the valley in a roller) but the big difference is that the ground sees a load that is used to add energy which is external to the "bike/rider" system. Also Seb, sorry as I read this article as Taj being the author, so I apologize for that!

@sokantoivo: Your analogy to ice skating to propel yourself forward is a really good one. Yes, it is just like the mag-track idea turned on its side. You also have observed that when accelerating forward while skating your skates are actually taking an arc, or at least they need to be in order to use the pumping effect to accelerate you. When you're on your right skate, the arc is concave left and you need to move your center of mass progressively "left" relative to the skate while going through this arc. Doing this, you're adding external energy to the system by pushing against the ice. These arcs when skating can be very subtle and I doubt that most skaters even realize it is there. In this case there is no spinning and no angular momentum involved really. It's pretty interesting to watch this effect in speed-skating. You can see the top racers conserving their energy and their legs by making small movements to pump and maintain speed until the last lap. There they top racers that have been able to save their legs, can often pull far away from the field through more pronounced pumping "sprint" at the end.
  • 2 0
 @Altron5000: yes. I'm just starting to figure out how to do that, and it's a gamechanger. an article on this would be very much appreciated!
  • 2 0
 Delightfully dorky discussions going on here. Never though about it from a standpoint of conservation of angular momentum

I've always viewed it like this: If you sit static on your bike, your Center of Gravity (CG) will go up and down with the rollers. Your speed will increase as you descend to the troughs and it will decrease as you ascend to the crests. If you work to keep your CG exactly level (moving your body up and down with respect to the bike), your ground speed will remain (roughly) constant.

To accelerate you must push down and raise your CG while going down the backside of the roller. You are accelerating the bike the downward and countering that acceleration with your mass/inertia, which is why the zero-gravity example from above would still work. You run out of leg length quickly doing this which is why manualing through taller rollers is helpful. Part of the chainstay length is added to your legs.

For a fun visualization for the mechanical nerds out there, look at a radial piston hydraulic motor. youtu.be/xUF64qmUKv4?t=36

Of course, this is all purely academic. What matters is that it is really fun when you get it right!
  • 3 0
 @kpickrell and @seb-stott: really enjoyable discussion! I think the situation is a bit more complicated than either of you are presenting - specifically that there is more than one process at work. For instance, while you can pump without gravity, and can eg pump a turn, that doesn't mean that when riding with gravity there isn't a benefit to absorbing the top of a roller - reducing the amount by which your COM must raise - and thereby maintaining a greater fraction of your energy in kinetic form (rather than potential). At the same time, that argument doesn't trump the arguments you two are making; it's just a complex situation.
The other complication being ignored here, I think, is that we're riding on a two wheeled platform that has some physical length that is often comparable to the curvature of the surface it is on. We can apply torque to the platform and create lift on one wheel and added down force on the other. I think it's pretty common to see good riders almost lifting their front wheel off the ground as it starts to go up the face of a roller. That may correlate also with a raise in their COM (doing work against the load), but I think there is more to it than that.
I think it's possible to create a thought experiment where one can generate forward speed in a scenario like this, in the absence of both gravity... and any initial speed. If we go purely with the doing work against a load theory, then with no speed comes no centrifugal load, and therefore nothing to do work against. But imagine the magnetic pump track in space again, and have the rider with their front wheel on the up-face of a steep roller, and their rear wheel on the flat run in. I contend that the rider could push on the bars perpendicular to the face, and pull down with their legs perpendicular to the run-in (thereby loading their legs), and then when they spring up from their legs, and hold tight to the bar, the direction of that impulse can have a non-zero component in the direction parallel to the up-face and therefore generate "forward" movement for the rider. I think of this as a "conflicted-plane" situation, where one can push against one plane and generate movement parallel to the other (different) plane. Again, this comes into play when there is curvature, and likely gets confused in our minds with the simple up/down against the load approach. An easy way to visualize this is an astronaut in the corner of a room on the space station, in the absence of gravity, but within arm's reach of both walls that are right-angles to each other. We can all imagine how one could push off the wall of one's choosing to create movement parallel to the other wall.
  • 1 0
 @timotheysski @Altron5000: Phil Metz on his YT channel, skills with Phil has some great videos where he explains how to do this. You should maybe start with his bunny hop videos as they explain quite a bit of what he is doing when he is jumping. It's interesting watching him follow his girlfriend at slow speed while she barely leaves the track yet he is getting plenty of air.
  • 70 1
 Cartoons do a better job explaining this than most videos
  • 18 0
 i sometimes picture myself as a frog when i ride my bike. my helmet is always red tho. what frog wears a blue helmet?!?
  • 41 4
 Wow, this is the best article ever on pumping. I have learned to pump a bike almost two decades ago without really knowing why it works (and I am an engineer Big Grin ).
It feels really good to get a proper explanation to it, my mind is blown now!
  • 36 0
 seems like good info to know. probably gonna forget in an hour and go back to sucking at pumping
  • 25 0
 THIS is content I want to see!
  • 19 0
 I’d always thought of pumping from a conservation of energy approach. You decrease your COM at the top so you lose less KE to PE and decrease your COM at the bottom to lose PE and gain KE. This approach gives a much more nuanced explanation and gives better insight into timing. Given all this, I’m sure I’ll continue to suck at it!
  • 26 1
 I failed P.E.
  • 3 0
 This is about right, and so is the article. Imagine dropping a marble onto a flat surface; it ends up with zero horizontal velocity. Now imagine dropping a marble into a quarter-pipe, perfectly aligned withe the vert. It converts all of its potential energy to horizontal kinetic energy, and ends up with a lot of horizontal velocity. Rollers and berms are between the quarter pipe and the flat surface. So, you want to have raised yourself up, and already be falling downward before the downslope, so your downward speed gets converted to forward speed when you hit the downslope. On the lower part of the roller, pushing down will also give you some speed, as described in the article.
  • 3 1
 Not really - you dont pump by lowering COM at the bottom, but rather the opposite - you push your bike down, increasing COM. Also if you only converted your PE to KE you would not gain any speed throughout several rolls, as gained KE will be again lost when you raise back.
  • 1 0
 @SJP: the big difference between you and a marble is that you can do work. Pumping is work (e.g. me gasping for breath after 3 pump track laps), that energy that you’re putting into the system is effectively extra forward momentum. You can pump indefinitely on a skateboard ramp. But you will get tired.
  • 1 0
 @mwart Conservation of energy isn't a great explanation because total energy (kinetic + potential) is not conserved. As you push into the transition against centrifugal force, you're doing work which adds kinetic energy - this is what makes you faster from an energy standpoint. This video goes into more detail on this -https://www.youtube.com/watch?v=8AJKN3QTfoE

Also, when you're more compressed at the top and extended at the bottom, you're actually decreasing the change of potential energy because the difference in the height of your center of mass is less.
  • 1 0
 @Altron5000: Someone raised the marble.
  • 20 0
 The frog didn't strap his helmet...
  • 7 0
 Nice bumbag, tho...
  • 4 0
 @evildos: True, bumbags are more important than a helmet. Wink
  • 14 0
 Very interesting read. Makes sense tho. I've always said that growing up skateboarding, made me better at pumping and cornering when I got into bikes later in life. A lot of the same "feel" applies in my opinion. Not just in the skatepark or at the pumptrack either. Out on the trail as well. Once ya have the "feel" for pumping transitions down. You can apply it to anything!
  • 4 0
 Same with snowboarding. I feel like it’s much easier to grasp what’s being explained here on a board than a bike.
  • 16 0
 It's my favorite way to be just as out of breath at the bottom of the hill as the top.
  • 13 0
 I'm a fan of the Friday fails whereby people "hump" rollers rather than pumping them by bouncing off the top of each. The rat a tat tat of a good set almost saved as they hang on... Good stuff.
  • 12 0
 I remember the first time I saw BMX riders on a half-pipe and being like "how can they keep getting higher without pedaling"? The explanation that at least made me realize it wasn't magic was that it was the same concept as a kid going higher and higher on a swingset by pumping their legs and leaning their bodies (most kids learn this on the playground).

Growing up riding halfpipes definitely made pumping on an MTB a little more natural.
  • 2 0
 Same for me for making pumping a bike feel natural, except it was with driving a surfboard down the line of a fast peeling wave.
  • 2 0
 The swingset is a good analogy!
  • 16 1
 *laughs in Engineering degree*

...

*Cries right after*
  • 13 0
 I've been doing it all wrong, thank you, this is amazing
  • 12 0
 Very cool article! I like Taj's illustrations too.
  • 8 0
 If you're trying to learn to pump, the timing is the hardest thing to get right. It's much easier to time it right if your speed into a roller is slow. Slower entry means a larger time window when you should be sucking up and pushing down. When your going faster you and your bike are in at the optimal spot for the motion for a lot less time, hence it's more difficult to time it. Also using a hard tail or a rigid dirt jumper, (or a bmx) will make it much easier to tell when you've done it right. Once you've got the timing right, build up the entry speed gradually.
  • 3 0
 Paradoxically, the slow speed is also the hardest, as it strongly punishes imperfections and you basically stop and you usually compensate incorrect timing by putting more energy in, so it makes it even more frustrating and discouraging.
I find it a bit easier to learn to have a bit more speed (i.e. to get across 2-3 rollers without pumping) to get some early success and confidence, then maybe get slower to practice precision.
  • 8 0
 Wow my mind is completely blown, what a great article, thanks! Makes a whole lot of sense actually. I think real life pumping definitely uses a combo of both the conservation of angular momentum as well as your own GPE, when pre hopping into a roller for more speed for example?
  • 2 0
 Glad you liked it. I think you can think of pre-hopping as the opposite of pushing into the concave trough at the bottom, in that you're "going light" in order to move your center of mass down towards the center of curvature, which is underneath the ground on the convex part of the crest.
  • 8 0
 Brillant article, excellent illustrations.

Here Pinkbike is like a friendlier more colorful and helpful version than the nerding–out flame wars/circlejerks in ze German Mtb–News.de comment board, where every minor academic error gets dissected to death and endlos discussions ensue.

Well done team
  • 5 0
 Thanks!

I fully anticipate every error to be "dissected to death", however...
  • 8 0
 This single article has been more helpful in helping me understand pumping and what to shoot for then dozens of youtube videos and articles.
  • 6 0
 @seb-stott One thing that might be interesting is to compare pros versus amateurs during basic tricks by recording their motion either using a camera (you must have many), xbox-near infrared with a body fit, or a Vicon set up (the small dots you put on your body). That way, you can find out and compare what part of their body are they using "most" to achieve a particular action. You might not need lots of people, but could be interesting to compare. The premise is that the pros would be more efficient.

You can use this method (among others) that, full-disclosure, I worked on with relatively little setup: "Angular momentum primitives for human turning: Control implications for biped robots"
  • 6 1
 Sorry Seb, but this is false. Conservation of angular momentum only applies when you are spinning, like a flip. To explain pumping draw a free-body diagram of a wheel on a decline. The contact patch isn't directly under the axle, so a downward force has nothing to directly oppose it. This creates a force imbalance and resulting acceleration. This is also why bmx bikes pump so good and 29ers roll over things well.
  • 5 0
 Pretty sure this is wrong. When you jump from flat ground, you begin by bending your knees so that you can push down into the ground. The ground pushes back, propelling you into the air. The pushback is in the opposite direction that you are pushing, so on flat ground that is up. When pumping, that pushback on the downslope propels you forward in the direction you are already moving, causing you to speed up. The steeper the downslope, the more forward momentum you gain. The reason that Phil can generate speed on flat ground is because the turning that he's doing allows him to angle his bike so that he can push against the ground at an angle, which provides some forward momentum in the opposite direction.
  • 5 0
 I was at a pump track last weekend and my kid asked, with a smartass grin, why I was so tired after I had only done 1 full lap. Point taken, I will never say free speed again.
  • 1 0
 I've been riding pump tracks for at leat 6y now still the same, I'm so tired after only 1 or 2 laps it's ridiculous
  • 4 0
 Great read @seb-stott ! The comic book style and the simple delivery of the information should hopefully help riders coach themselves better and help coaches coach better.

It seems, especially with the advent of YouTube "coaches" and "how-to" videos, that we are awash in a sea of poor coaching with no foundation in any of the sciences, whether that's pedagogy, motor learning or physics. So we just get this constant re-hashing of coaching what they see (using internal focus cues of move arms/legs like this or "elbows out" etc...) or copying others poor coaching (get light/heavy to pump etc...)!

If a coach can blend some motor-learning theory with these simple physics cartoon lessons I think the standard of riding and coaching the world over would improve. Maybe there's a market for comic-book coach education?
  • 5 1
 This article is technically incorrect, and the original understanding at the top of the article is more correct.

Pushing down into the bike when it is sloping downhill or before the apex of a corner generates forces normal to surface (more traction when pumping a corner) as well as forces that are pointing in the direction we are traveling, giving the acceleration. The angular momentum explanation is way off base.
  • 5 1
 CONSERVATION of ANGULAR momentum? LOL

When you pump you ADD force with your muscles to accelerate your FORWARD motion.

Force vectors explain how you gain forward acceleration on a slope and more force equals more acceleration.
  • 8 2
 Way beyond me...I just ride
  • 6 0
 The Frog def shreds harder than me.
  • 3 0
 My friends and I learned pumping by taking our brakes and chains off at skateparks, dirt jumps or pump tracks. A lot more consequential, but incredibly effective and a ton of fun haha.
  • 4 1
 Pumping means the permanent development and transformation of potential and kinetic energy. The only relevant forces are gravity and the body movement of the rider. It has nothing to do with rotation or centrifugal forces!
  • 11 1
 If you set up your frames of reference properly, you'll get the same result with both mathematical approaches
  • 2 0
 @pmhobson: You could even argue that styleguide is just presenting a more abstract view of the same problem. Total energy of the system is just PE+KE, from which you can break out the KE into the integral of all the forces acting upon the bike over time. Doing so necessarily will incorporate the centripetal forces incurred through pumping, hence the rotation within the bike+rider system.
  • 7 1
 I agree. The above article is wrong. Yes conservation of momentum shows that the speed of rotation increases the closer the center of mass is moved to the center of rotation, but this is in order to maintain the actual velocity of the mass. If this was how pumping worked you would lose all of your percieved gains from moving your mass closer to the center of rotation, as soon as the rotation (the transition of the slope) ended.
  • 2 0
 @gabriel-mission9: Exactly, The skater or whatever spins faster as their moment of inertia is decreased, as soon as they open up they spin slower again. This is not the same as pumping. You keep almost all of the speed you gained by pumping.
  • 2 0
 Great explanation of the somewhat complicate phenomena. Took me a few minutes to understand. It would have been nice if there were an image showing the two different centers of curvature for crest and trough of a pump track. But all in all great stuff. Now time to test it on the trails!
  • 3 0
 Lovely but I'd love to learn more on the physics of pumping berms, I'm an idiot and don't know how it works, I just do it and it "works"
  • 2 0
 Turn the frog picture 90 degrees and look at the curves that are drawn on it, the physics are the same. You pump the transition to make the radius shorter. Shorter radius = faster. The only difference is where the transition is.
  • 3 0
 Can you please do an article on the physics of how a brake rotor can damn near cut your finger off? Errr....Asking for a friend.
  • 1 0
 @bgilby Been there bud! Damn near chopped the tip of my index finger off once with an 8" Hope rotor!! And the wheel wasn't spinning very fast at all!!
  • 1 0
 Ahhh yeah my “friend” has a hot tip for this one. Always remember to look when tightening a thru axle, especially when the wheel is spinning.
  • 3 0
 This is such a rad explanation and also provides insight to good technique versus bad. Hopefully we can get some more scientific explanations of skills!
  • 2 0
 It’s super hard to teach how to pump.
I’ve tried to teach my son and friends and I believe it purely comes from experience as well as technique.
Although my son would rebel to me if I said the sky was blue. ????
  • 3 2
 "because the only thing to push against is the floor"

What about gravity? You can always push against that, even on a consistent (not curved) downslope. In fact pushing against gravity (or inertia, for pumping a berm or flat ground) is what makes a pump work: because the slope\berm\ground isn't perpendicular to the momentum, any force against the momentum\gravity will have a force vector mirrored across the normal of the slope (a line perpendicular to the tangent at any given point), so some of pushing "down" force becomes pushing forward, but this doesn't need to happen on a curved surface. The curve just helps stretch the pump forces because the vector pushing back directly against you pushing against gravity\intertia slowly increases, giving some feedback to help indicate you can keep pushing. On a linear sloped landing you have to push more evenly and "slowly" (don't blow the pump all at once), but you can still pump without a curve.

I think the reason you needed to pump later is that you run out of pump before the bottom, not because pumping the curved part of the transition is inherently better.
  • 3 0
 I thoroughly enjoyed this article, but have a technical question pursuant to illustration #1 -

are those Mario Kart speed boost chevrons or Excitebike speed boost chevrons?
  • 1 0
 Or WipeOut 3 speed boost chevrons?
  • 1 0
 When we're talking about pumping things like roots does the same physics apply? Surely there is an effect of acceleration when you push down onto an angular piece of ground, the downward force being sent forwards by the angle of the ground and therefore increasing your forward velocity. Are we saying that the conservation of angular momentum is the only thing going on here or is it just a greater component?
  • 1 0
 I've always puzzled over the pumping of berms as per my original training/description of how pumping 'works'....now i know why. Cheers Seb.

Obvs gonna be sciencing the shit out of those rollers on the Verderers descent soon to prove it all to myself :'-)
  • 1 0
 More of this sciencey shit that was a really good read and I learned something. Usually I keep going back and forth form my school work and the PB article but i had to read this one all the way though. Also Taj's drawings were as always very helpful funny and awesome.
  • 1 0
 Draw a diagram, this would fail high school physics: "A lot of pumping explanations talk about pushing vertically downward onto a down-slope, but because the only thing to push against is the floor, the reaction force you're generating is always at right angles to the ground, and to direction you want to accelerate."
  • 1 0
 Therefore the fastest roller shape one can build is...? Is a sine wave really the fastest roller shape? Should the radius of the crest match the radius of the trough or should they be different? What is the fastest wavelength/height ratio?
How and why do rollers at the ends of berms work?
  • 1 0
 It is not like the concept of pushing down on the downslope is all wrong. In reality, there are both effects combined. Imagine a pumptrack with more of a saw tooth profile rather than smooth transitions. You will still generate speed by pushing on the downslope and pulling up before the upslope.
  • 1 0
 Now that thought was just as bright as a 20-watt light bulb Should've pumped it when I rocked it Niggaz so stingy they got short arms and deep pockets This goes on in some companies With majors, they're scared to death to pump these Wu-Tang, Protect ya neck
  • 1 0
 Not much of a reader, but I've seen really great videos from Kyle and April teaching how to do it. This one talks about rowing and anti-rowing and some physics behind it, which I find informative:

www.youtube.com/watch?v=_R2zHfzqINQ
  • 1 0
 @seb-stott: Thanks for this. Very informative.

I've recently been looking into optimum curve geometry for jumps and have somewhat fallen down the rabbit hole of designing back-to-back clothoid curves with minimum radius to target a certain g force as research from the skiing world shows it could probably help create more beginning-friendly jumps. Would be really interested to hear your thoughts on this topic and the physics/design of jumps.

Keep up the good work.
  • 1 0
 Awesome cartoons, but ummmm.... doesn't the roller end at some point? By this math, as you exit the roller and the radius goes to infinity, you would lose all the speed you gained in the roller. That's not what happens, because that's not what's happening. I'm happy that the mental game you played with yourself got you timing the pump track well, but it's probably best to keep it real when you are "educating" the rest of us.
  • 1 0
 Ride down a constant downslope (no curvature) and pump repeatedly. Then try it again without pumping at all. You’ll see that pumping increases your velocity. Doing it on a bike with small wheels and no suspension will be more effective.

Plenty of comments explaining how the author’s point in the beginning is incorrect:
“if you extend your legs and push into the ground, you're pushing at right angles to the ground underneath your wheels, which doesn't help propel you forward at all”

The motion of your body when pumping is not at a right angle to the ground.

The conservation of angular momentum idea is valid as well, and if you can reduce your angular inertia by moving your CG toward the center, you will rotate faster.
  • 3 0
 I need to go to the pump track and see if this is how I pump and change my technique if its not.
  • 4 0
 New mantra: "be the frog"
  • 3 0
 Watch TAJ pump the "troughs" of these rollers at 0:15 in this 1995-ish video:
www.youtube.com/watch?v=myzdNqwnsB0
  • 3 0
 I remember an engineer telling me it’s impossible to gain speed by pumping. I laughed.
  • 1 0
 I mean it’s cool and all, but did you account for relativistic effects? Just kidding, great read. Fun to think about these things more seriously, and as a Physics undergrad this is right up my alley.
  • 1 0
 Sometimes I get a stronger pump by a slight delay between arm pump and leg pump and finishing the leg pump with an ankle stretch out/fling made more effecting with front foot on peddles not arch.
  • 2 0
 This article and the referenced Skills with Phil video might be the best thing that happened to my riding skills in a long time. More of these please!
  • 1 0
 This is not an equal comparison ,as the rabbit ???? is on a full suspension & the frog ???? is on a hard tail, (possibly a SC Chamaeleon)
of course he’s gonna be faster
Duuhh. Smile
  • 1 0
 This was very informative -- I remember trying to figure this out in my first physics class, unable to find an answer. I've forgotten about that curiosity since then, until now !
  • 1 0
 More nerdlyness to go along with the video I mentioned above...

What’s the Best Way to Swing a Playground Swing?
bit.ly/3uf2xbZ

And if you just want to go straight to the maths...
bit.ly/3wjJ3oO
  • 3 0
 I wish I had a bum bag as cool as that frog. Taj is the best.
  • 3 0
 We just got a new pump track, so timely for me. Thanks!
  • 2 0
 Pump pump the jam when the beat is pumping - I love pumping. changed my whole style once I got it.
  • 1 0
 I used to be able to pump on trails, then I had a short bike with 26" wheels. Now I have a long bike with 29" wheels and I can't pump for sh!t.
  • 2 0
 Reads article:

First thought: Can someone please explain why the rabbit is wearing a robe at the pumptrack??
  • 1 0
 Great, now the trails will be full of edgy senders magically picking up speed everywhere instead of just a couple here and there.
  • 2 0
 He's right that pumping a turn is like pumping a trough.
  • 2 0
 Tomorrow pump track. From theory to practice:-)
  • 2 0
 Really enjoyed this one! Great job
  • 1 0
 Can we just give credit where credit is due and everyone head over and watch Lee McCormack explain this stuff?
  • 1 0
 lee mccormack is great, but he doesn't explain pumping like the article does.
  • 5 0
 @Jashfoo: I agree with the physics concepts in this piece, but I don't believe conservation of angular momentum is the *cause* of pumping propulsion. It can certainly be used, but I don't think it's a primary factor for a great rider.
  • 1 0
 @leelikesbikes: Aaaahhhh!!! The man himself!! Yes, I agree with you, the more I thought about it, I thought that it has to be one piece of the puzzle. It would be interesting if someone would draw a free-body diagram (is that what they're called?) showing all the different forces that go into pumping at the different stages. ...Or I can stop being nerd and practice at the pump track.
  • 1 0
 @leelikesbikes Nerdly for sure, but an interesting video to add to the discussion.

How Swings Work - Sixty Symbols
www.youtube.com/watch?v=UXo6WvHRs_I
  • 2 0
 Hey! Vsauce Michael here.
  • 1 0
 What do you think of this Seb's point? Does it account for the fact that a rider delivers added force through body movement?
  • 1 0
 This is really interesting content, thanks @seb-stott , glad your on pinkbike, looking forward to more articles!
  • 3 2
 This explanation is completely incorrect - if Pinkbike has any editorial standards the article will be removed.
  • 2 0
 Please explain! Intuitively I also don't think it's right but I haven't seen anyone articulate a good argument why it's not.
  • 1 0
 fuck this, too complicated, gonna go ride my bike...
  • 1 0
 Long Legged Larry for Mayor!
  • 1 0
 The frog looks poppy
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