Perpetual Motion back on the scene again!

  As an objective scientific / engineering type, I’ve tried hard…

….to get my head around these new ‘Osymetric’ chainring jobbies.  They’re an elliptical chainring which (apart from possibly trashing your knees like the old Biopace things used to) are claimed to give you 10% more power and 10% less fatigue.

  While I can sort of understand the latter claim, the first claim doesn’t appear to make any sense to me.  I await some proper research, but until then….

There’s a trivial law of nature called the 1^st Law of Thermodynamics which states that, basically:

 “Whenever heat is added to a system, it transforms to an equal amount of some other form of energy.”

  This added energy does one or both of two things: (1) Increases the energy in the system and (2) does external work if it leaves the system. 

  More scientifically:

              ΔU = Q + W

  Where ΔU is the internal energy of a system which is equal to Q, the heat added to the system during the process plus W, which is the work done to the system during the process. If heat is taken from the system then Q is negative, and if work is done by the system then W is negative.

  The first law is essentially a statement of conservation of energy.

In a nutshell - energy can be neither created nor destroyed.

Perpetual Motion Machines

  The idea is that you give the wheel a shove and it will go round and round forever, perhaps even accelerating. You could even use the extra energy created to power something!

  The Victorians (and, to be fair, even Leonardo da Vinci) got themselves a bit bogged down with this. You can see the clever concepts that went into the above machines but at the same time you can see intuitively that it’s not possible.

  Perpetual motion machines that contravene the laws of thermodynamics are designed to produce work without an energy input.  This isn’t possible as energy can be neither created nor destroyed. Were such devices possible, they would inevitably bring great riches to their creator and would solve world energy production problems forever!! The prospect of vast wealth perhaps offers some explanation as to why perpetual motion machines have been pursued even after they have been proven impossible!

  You can see where this is going already I suspect!

  Spot the similarity between the above machines and the Osymetric ring? Hmmmm…..

  So what’s being claimed is that you apply say 300W worth of power to the pedals and that gets amplified to 330W through the Osymetric ring?  Sorry, that breaches the laws of physics. IMPOSSIBLE!!

  Once again, in a nutshell - energy can be neither created nor destroyed.

  I’ve done what many scientists do and created my own ‘Thought Experiment’ to visualise that this can’t work. It’s basically another Perpetual Motion machine, based on wobbly shaped chainrings.

Captain Cosmic's Perpetual Motion Machine

  According to the theory of elliptical chainrings and their magical generation of free power, with just one push of the pedal at 'X', the machine will continue spinning and accelerating rapidly forever.  In fact, the effect would be rather like an old fashioned handstart of an aircraft propellor!  The thing is, you know that just can’t happen, so why imagine that the same chainrings will develop more power for you?

  I think what is causing the confusion is the same reason why 99% of motorsport enthusiasts can’t understand (let alone describe) the difference between engine torque and engine power.

  Let me try….

  Torque is a rotational Force. A force can be seen as an instantaneous phenomenon, like whacking a cricket ball with a bat. The size of force applied to the cricket ball by the bat determines how quickly it accelerates. In the same way, the amount of torque applied by the engine determines the rate at which the car accelerates at that instant.

  Power is the amount of energy expended over a given length of time. A common analogy in teaching is to ask students to imagine two identical mountain climbers going up a cliff. Both weigh the same, but one gets to the top twice as fast as the other. They have both expended the same amount of Energy to overcome gravity but one has produced twice as much Power.

  In other words, he’s applied his energy to the rock face twice as fast as his colleague. The more Power he can produce, the faster he will climb. Similarly, the more Power a car can produce, the faster its top speed.

  So Power has a time element to it. In fact the basic equation is:

  POWER = ENERGY / TIME

  In rotational systems, power is the product of the torque (turning force) τ and angular velocity ω,

P = τ ω

  where ω measured in radians per second (1 Degree = 0.0175 Radians)

  So let’s apply this to the wobbly chainring:

  At that dead spot just past top dead centre, the elliptical chainring takes on a smaller radius, exactly the same effect as momentarily dropping to the small ring.  The Force applied to the pedal by your foot is the same, but there’s an increase in the lever effect (or Moment) caused by the smaller chainring radius.  So more Torque is applied to the rear wheel. So far, so good…

  But, you are effectively now in the small ring, so the back wheel has travelled less distance for the same amount of pedal movement. Put another way, the time taken for the wheel to cover the same distance as before has increased. So more Force / more Time = same Power. Or more Torque x less angular momentum = same Power

  The opposite happens as the pedal falls through the sweetspot at 90 degrees. The radius of the elliptical chainring now increases, again equivalent to switching up to an extra large big ring.  Force applied to the pedal by your foot is still the same but the increase in radius of the chainring results in a decrease in the pedal Moment. Now less Torque is applied to the back wheel but it has effectively travelled further for the same pedal movement.  Hence time taken for the wheel to cover the same distance has decreased. So less Force / less Time = same Power.  Or less Torque x more angular momentum = same Power.

  After a complete 360 rotation of the cranks, the pedals have gone exactly the same distance as a conventional chainring, the backwheel has gone exactly the same distance and your muscles have applied exactly the same force to the pedals.  So the power output over that 360 rotation is, er, exactly the same.

  Okay, so what’s possibly being claimed is a 10% increase in Torque at that dead spot on the pedals. No problem with that. BUT – the increased Torque is being applied 10% slower (10% less angular velocity), so the Power remains the same.

  P = 1.1 τ  x  ω/1.1

Cancel the 1.1 out and you get….

P = τ ω

  Exactly the bleedin’ same again!

  With me?  However, you can see where, with practice, this redistribution of torque might have some benefit physiologically.  The smoothing out of the biomechanical disadvantage at the dead spot (where you now create more torque with less angular velocity) and the biomechanical advantage of the sweetspot (where you now create less torque with greater angular velocity) may be what causes the 10% reduction in leg fatigue.

 Now that makes some sort of sense, yes, even I can see that!

  Reducing the effort & resulting muscle fatigue needed to get over that flat spot at the top of the downstroke is perhaps where these chainrings may help, but they cannot give you extra power.

  Also, if all else fails, think about it this way - The shape of your chainring cannot possibly increase the maximum amount of fuel your body can incinerate aerobically.

 That’s down to VO2 Max!!