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Or would more power go through the pedals if the rider held the frame still/upright?

Or is there little difference? How would it be measured and what are the forces acting on the bike when rocking while sprinting?

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    Great question. Might be related to another question: does adding power through the arms and the upper body while moving the bike side to side help to add more power during sprints? – Sam Apr 4 at 17:40
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    From personal experience, rocking the bicycle is a product of intense muscle output. You are simply exerting more force which is difficult to properly counter and therefore your bike rocks. Preventing your bike from rocking will give you more stability, control over your movements, and therefore give more energy to your pedaling. – Brian Blumberg Apr 4 at 17:53
  • If you want a visual explanation, think about how a water hose works. If you spray the water everywhere, without control, there is little force to the water. However, if you press your finger onto the opening and try to accurately spray water in a single direction, there will be more force. In both cases, there are equal amounts of water (i.e. muscle power), but one is simply using the water more efficiently. – Brian Blumberg Apr 4 at 17:56
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The rocking motion does not help with power output, but it is the way you maintain balance with least effort.

When one is riding in straight line, the system of bike and rider rotates almost freely around the axis that goes through the points where tires contact the road (there are steering forces to maintain the balance, but these are very small compared to pedaling forces). Because there is nothing to brace against, the torques around this line have to even out. Because pedals are not directly above the line, pushing them down needs some torque to counter it. When sitting down, the most natural support is the combination of handlebars and saddle (if you wondered why saddles extend forward between riders' legs, this is one of the reasons). Standing up to pedal allows using more force on the pedals and loses the support from saddle at the same time. This requires more force handlebars, but also allows another option: Rocking the bike from side to side brings the pedal that is pushed downwards closer to the centerline, which reduces the torque that needs to be countered.

From mechanics standpoint, the arms don't do any work when holding the bike upright. Holding the bike upright by twisting the handlebars wouldn't indeed take any effort if you could just lock your arms like suspension lockout. Unfortunately muscles don't work that way, so maintaining the bike upright while pedaling standing takes more effort than rocking the bike around.

This method could be used to control balance when sitting in saddle, too. The problem is that when sitting, you need to wiggle your lower body to keep up with the saddle and the upper body in other direction to maintain balance. You can do it if you really want, but it is not comfortable or efficient.

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  • Thanks, the balance and centre line points make a lot of sense - I'm 6ft5 so quite gangly and unstable when out of the saddle but that's probably my lack of sprinting experience; if I 'lock' my arms and keep the bike upright it feels much more stable and powerful but that's probably just perception, if I increased my confidence while rocking the bike I would probably be able to go faster. – Wilskt Apr 4 at 23:19
  • I used to do the same when I was beginner. The bike geometry and tires also affect this: some bikes tend to pull to sides when you rock them, but good racing bikes tend to be stable. – ojs Apr 5 at 8:18
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It is not the rocking movement that gives you more power. The rocking is a result of applying power in a diferent way.

When you stand up on the pedals you are able to apply all of your wheight onto one of the pedals, but as it is on one side of the bike it will be necesary to compensate. Your arms will instinctively apply force in the oposite direction.

These actions however do not elliminate the fact that your relaitve center of mass will change throghout the movement, so your body and bike oscilate side to side, in order to keep it over the line between your tires.

If while standing on the pedals you also pull up on the handlebars, you will be able to apply more downward force to the pedals than just your weight. However, for it to be affective you must not counteract that force with the other foot (the one going up). You also must keep your torso "aligned" with your legs, otherwhise, part of your efort is also wasted on not bending yourself sideways.

Anecdotally, I've been able to keep up with partners trying to get away by means of pedalling stand up, while I remain on the saddle. My conclusion is that yes, pedalling stand up will help produce more power, but also, It must be performed correctly for it to be effective.

There is another way in wich it helps, and it is on long rides, or long climbs. Stand up pedalling uses a slightly different muscle group, thus can be a relief for some muscles while alowing you to continue moving forward.

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  • So if, instead of rocking, I hold myself and the bike in a centred position using my arms and upper body, how would that affect the amount of power I can put through the pedal? The thought I started with is that rocking side to side and the imbalances and corrections that implies is a waste of energy, so is it better to hold the rider still? – Wilskt Apr 4 at 18:08
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    @Wilskt kind of. I do not have the means to measure. But if your body is swaying left to right, that's energy not used on going forward. I see that happening to a lot of riders. Now, the fact that you spend some energy going side to side does not mean that the energy used to go forward does not increase, just means not all of available energy is used in the right direction. – Jahaziel Apr 4 at 18:17
  • Just rocking does not have to spend much energy at all. A pendulum can swing for a long time without spending almost any energy. Similar situation happens when a frame flexes. The energy is not lost, most of it is returned by the frame like from a spring. youtu.be/BH_AL4rxrp8?t=226 Maybe the rocking does cost significant energy after all. However, that must be proved more carefully. This argument is way too oversimplified. – Vladimir F Apr 4 at 18:28
  • @VladimirF very true, it could be that the upper body energy required to keep the bike still could greatly outweigh any potential benefit in getting more power to the pedals. – Wilskt Apr 4 at 18:39
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I ride a road bike and can rock the bike fine. I also ride a recumbent, where all of the rider's weight is on the seat, and "rocking" the bent is impossible.

The upshot is that rocking the road bike allows the engagement of the core muscles which is everything up the trunk/torso, as well as the shoulders and arms.

A very approximate test showed that I was able to maintain ~250W on a road bike for some minutes, but achieving the same forward speed on my bent was under a half of that power. However I couldn't go a lot faster, because on the bent the core muscles aren't really used.

By comparison, imagine hypothetically riding your road bike hard, but no-hands. How much power could you put down in a no-hands "sprint" ? This would be like riding without engaging any core muscles. note - don't actually do this.

In short, rocking the bike helps engage extra muscles for more power and therefore more sprint speed.

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    Paragraph 3, do you mean you would struggle to maintain 250w on a recumbent because there are less muscles available to you in that position? – Wilskt Apr 5 at 11:29
  • @Wilskt yes exactly. Since the core muscles are unable to contribute, total power output is lower. Speeds remain about the same because of the drastic drop in wind resistance compared to an upright bike. – Criggie Apr 5 at 20:24

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