How much does weight influence performance?

[javadude]

 

Iban Mayo and Mauricio Soler is in the leading group up 10km from the summit of the Col du Calibier. Soler realises he has still got a piece of newspaper tugged in from the previous descent. So he pulls it out and starts reading it. Coincidentally Le Equippe published this thread in the paper (How random is that?). Soler starts reading it and realises it  is the biggest load of sh!T he has ever read. He throws it away' date=' he gets up and attacks haard!

 

Mayo tries to follow and Soler is like STFU!!$ and attacks again. Mayo is dropped and lots of stuff is running through his mind... "Omg im on EPO, my bike is 2KG below the UCI limit , my wheel is 200grams.. gees my freakin Powertap training program..heelp! How is Soler doing it!?! He is not riding for a pro tour team! He can't afford a powertap!! his bike is heavier than mine.. i weigh lighter than him.. OMG!!

 

Soler takes the victory without knowing a watt from a heart beat!!

[/quote']

 

Exactly, and dont forget to wear your lucky socks.

 

[Willehond]

epoh,

Pantani also did not know a watt from a joule, or even a heartbeat (he would not wear a monitor).  Unfortunately he also did not know the difference between EPO and cocaine, but that is a different story and ALSO not the point of this thread...

[gianni]

1.25% added to a riders time per 1kg extra gianni2007-08-30 15:19:02

[buckstopper]

And what if the 1kg is all muscle and improves my power to weight ratio?

I don't think you can make a sweeping statement like 1.25% extra for 1kg

[gianni]

*comments deleted due to not needing further aggro from arguing with some tightass**

 

 

 

 

gianni2007-08-30 16:01:42

[bruce]

Technically Titus is correct' date=' but so is bruce..

It all depends on relativity. Even if your speed is constant on an incline, you're accellerating because gravity has an accelleration of 9.8m/s2. So if you've moving up at any speed, you're accellerating, albeit not relative to the surface you're travelling on

[/quote']

 

fandacious, I'm afraid you are wrong.  Firstly, newtonian mechanics (i.e. the equations governing motion) are entirely accurate for all bodies moving at velocities up until fairly close to the speed of light.  At speeds approaching the speed of light, the theory of relativity becomes useful - so in the case of a bicycle, the special case of relativity boils down to standard Newtonian mechanics.  Bottom line, acceleration is the rate of change of velocity (expressed in calculus as a = dv/dt), so if your velocity is constant, acceleration is zero!!

 

A body that is free falling under the influence of gravity will eventually reach terminal velocity.  The point at which the force of gravity is balanced by the force of wind resistance opposing the force of gravity.

 

A very good explanation can be found at:

http://en.wikipedia.org/wiki/Motion_graphs_and_derivatives

or at

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l3a.html

 

[GoLefty!!]

 

Sias' date=' I can't blame you for repeating the stuff you read in bike magazines and hear from other people who heard it from someone who read it in a bike magazine. In the PR business, they know that if someone hears something three times from three different sources, it is considered as the gospel truth, and they design PR (publicity) campaigns around that.

 

But back to physics. The notion that weight saved on a wheel is somehow more valuable than weight saved on the bike or body, is pure nonsense. The effect is so miniscule, in the order of less than one half a percent for an 85kg rider and bike losing say 400grams on the wheel, that it is completely negligible. Weight is weight, save it and you gain. Weight (saved) on a wheel is jsut weight saved.

 

[/quote']

 

Johann, I loved your discussions on Rotor blades and dispelling their scientific mumbo jumbo, but sometimes your arrogance is slightly misplaced and over the top.  Accusing Sias of being duped by "stuff you read in bike magazines" is a bit rough, not to mention condescending...

 

If you want a credible source of the rotational weight theory, look at research done by Ed Burke (I hope you know who he is.)

He did studies on so called rotational element (wheels and pedal-and-shoe systems) indicate that saving 250g on each shoe (easily done) would save 1.3 percent of rotational power at constant speed on a flat road, plus 0.4 percent extra on climbs!  This translates to 1.5 to 1.7% extra power required by a cyclist.  Not bad for "weight is weight" hey?

Burke suggests that a pound added to the wheel rim is equal to two pounds on the bike frame.  I know what you think about that, but his reasoning on energy required to provide the kinetic energy for rotational and linear motion makes sense to me, so before you fob it off as "stuff you read in magazines", make sure that credible sources have not actually done studies first.

 

Remember that wind resistance slows you down, for which you have to compensate by accelerating the wheels forward and around, in my mind it makes sense that rotational elements will require extra force to accelerate.  Stating that rotational systems take ZERO force to accelerate against resistance does not cut it in my physics book.  I would have flunked applied mathematics if I claimed that...

 

Goodluck trying to convince the arrogant **** that those studies holdany water. Afterall, in the Bornman universe, Highschool physics is all you need to understand the effects of rotational mass on performance.

 

I gave up "discussing" anything with that  **** a while ago.

 

 

[sias]

Mmmm Johan, lyk my ek is nie die enigste een wat dink jy is 'n... toemaar, daar's kinders op die site...

Lekker naweek vir jou...

sias2007-08-30 23:10:05

[Johan Bornman]

Mmmm Johan' date=' lyk my ek is nie die enigste een wat dink jy is 'n... toemaar, daar's kinders op die site...

Lekker naweek vir jou...

[/quote']

 

Dear Sias, Willehond and others.

 

I'm sorry that you seem to think my comment about reading stuff in bicycle magazines was condescending. It certainly wasn't my intention to disparage you and I apologise if I rubbed you up the wrong way. I reserve my views of bicycle magazines though.

 

I don't know everything and where neccessary I ask or research.

 

I asked who Asker Jeukendrup is because I dont know. I hope when others ask questions they don't get the same treatment.

 

The originator of this post, Xak1, asked an interesting question and lots of people have attempted to answer it. I commented on one aspect of the question, namely the statement that weight saved on wheels, the so-called rotational weight,  is somehow twice as valuable as weight saved elsewhere on the bicycle.

 

I happen to disagree with that "twice as important" statement and have attempted to quantify the issue. Each time I produce the maths, the argument is redefined by someone who doesn't like what they see and we start all over again.

 

I appears to me that at the end of the day we will disagree on how much a bicycle actually accellerates during a race. I hold forth that

a) a bicycle accellerates very slowly and

b) that it doesn't do it quite as often and violently as most people believe.

 

Attempting to quantify that has proved fruitless. The last time that subject was raised, Tim Blegenhout (Lefty) suggested that a bicycle accellerates by 1kph per pedalstroke. Clearly that isn't possible because anyone can see at a glance that doing so requires far more energy than we can consume in food and from the supply of energy reserves stored in our body tissue. This makes 1kph per pedalstroke thus above the absolute upper end of the guestimate. If we consider how much energy reserve we have after a race - and we know we have energy left because we can still maintain body heat, walk, think etc - we clearly use up less energy than what some people argue. Arriving at a figure for how much we accellerate has to be meaningful. The tank of energy is limited and no use coming up with a theoretical figure that's bigger than the tank.

 

We just can't quantify how often and how much a bicycle accellerates during a given journey. The best we can do is aim for a given journey with given gradients and speeds.

 

But back to my sweeping notion that a saving in rotational weight is much the same as any weight saving:

 

Linear accellerating of a body to a given speed is a function of the mass of that body, start speed, end speed and the duration of time for reaching the new speed.

 

Rotational accelleration of a body to a given speed is the same, the difference being that the speed is calculated slightly differently, but the energies required are very much the same. At the speeds we're talking about it is safe to say that rotational accelleration is the same as linear accelleration.

 

A bicycle combines the linear and rotational components.

 

Therefore, accellerating two bodies of the same overall mass, where the one has a rotational component of X grams less than the other, are very, very similar - negligible, I argue. But, races are won on a hair's breadth, sometimes.

 

My answer to Xak1's question: Weight is a big factor in accelleration but we don't accellerate enough for the type of weight we can realistically save on the wheel's circumference to make a meaningful difference.

 

In short, weight is weight.

 

JB

 

 

 

 

 

 

[Sean]

*comments deleted due to not needing further aggro from arguing with some tightass**

 

 

 Actually Gianni' date=' I thought your comment "Lose weight Fatso" was far more useful to me that all the Physics101 (or 201, or even 301) posts prior to this

 

Seeing as how I could lose about 10kg of weight without affecting muscle mass (thats a polite way of saying I ate all the pies) I wonder how much money one of our racing whippets would have to spend to achieve the same loss? They'd probably float away...  

 

[/quote']

[Johan Bornman]

[

  

If you want a credible source of the rotational weight theory' date= look at research done by Ed Burke (I hope you know who he is.)

He did studies on so called rotational element (wheels and pedal-and-shoe systems) indicate that saving 250g on each shoe (easily done) would save 1.3 percent of rotational power at constant speed on a flat road, plus 0.4 percent extra on climbs!  This translates to 1.5 to 1.7% extra power required by a cyclist.  Not bad for "weight is weight" hey?

Burke suggests that a pound added to the wheel rim is equal to two pounds on the bike frame.  I know what you think about that, but his reasoning on energy required to provide the kinetic energy for rotational and linear motion makes sense to me, so before you fob it off as "stuff you read in magazines", make sure that credible sources have not actually done studies first.

 

[/quote]

 

Ed Burke, yup, I have his book on my shelf. I also have a Sport Medicine book of his somewhere.

 

If I remember correctly, Dr Burke was the guy who preached cardiac health and then keeled over from a heart attack.

 

He is also the guy who, amongst other things, reccommends anti-inflammitories for tennis elbow - something that doctors generally cringe at. His method for finding the optimum dose of asprin (to treat tennis elbow): take more and more asprin until your ears ring, and then back off a little bit.

 

His all-time gem was convincing a US track cycling team to inflate their tyres with Helium to make them save weight and hence ride faster.

 

However, let the man speak for himself (and I trust you paraphrased him correctly):

 

"[loosing weight on a shoe].....would save 1.3 percent of rotational power at constant speed on a flat road."

 

There is no such thing as "rotational power" and, any phycisist would understand that extra weight on a flat road doesn't require more power to maintain.

 

Already I hear the flamethrower ignitors click so let me quickly qualify the last statement: we assume that the lighter and heavier shoe have the same aerodynamic profiles. 

 

ED Burke, yes I remember him. I also remember how many bicycle magazines slavishly published his stuff.

[Gumpole]

I dont know if this is of any relevance but try this:

get your front wheel and hold it in your 2 hands by the ends of the axle. lift it up, twist it, move it around in the air.

Now while holding get somebody to spin it as fast as possible... now try and twist it move it etc.

 

It is a lot more difficult, why the weight is the same.. or is this aerodynamics at play???

[Man with no name]

I was hoping my little tiff earlier this year with him-who-shall-not-be-named would grab "fight of the year" at the annual Hubber nominations, but this thread is thread-tening to take that title away from me (or should I unselfishy say "us")......

[Johan Bornman]

I dont know if this is of any relevance but try this:

get your front wheel and hold it in your 2 hands by the ends of the axle. lift it up' date=' twist it, move it around in the air.

Now while holding get somebody to spin it as fast as possible... now try and twist it move it etc.

 

It is a lot more difficult, why the weight is the same.. or is this aerodynamics at play???

[/quote']

 

You are observing the gyroscopic effect of a turning wheel. Any force you apply to the axle of that spinning wheel comes out 90 degrees downstream of the spin.

 

This can be nicely demonstrated by attempting to hold the stationery wheel by one end of the QR only. You can't. Now spin it, you'll see you can easily balance it on one end only, however you have to keep on turning around. That's because the upward force of your hand on the QR makes the wheel turn on its axle 90 degrees further. By turning you counteract the constant attempt to turn and it stays in your hand.

 

I'm sure someone can say this more eloquently but try.

 

 

[xak1]

Have a look at http://www.analyticcycling.com/WheelsClimb_Page.html.  I played around with the model, and it seems that this model supports Johan Bormann's view.  I kept everything the same, but made in the one instance, the bike heavier and the wheels lighter, and the other the wheels heavier and the bike lighter.  On a 13 km climb, with a 10% gradient, both riders get to the top at the same time.  I think the biggest impact that weight on wheels will have is to overcome inertia, but once at speed, a heavier wheel may have more momentum.  Look at time trial bikes - their wheels are heavier, but more aerodynamic.  One can take this argument very far, as the question can then be asked what about 29er's vs 26er's in MTB.  As the rim and tyre weight on a 29er is further from the center, will it mean a bigger weight effect - probably not, as then we would have seen smaller and smaller wheels.  In the end, for practical real world sitaution, I tend to lean towards the weight is weight view.

[Willehond]

xak1, I tend to agree with the constant speed theory, but my assertion is that we as cyclists NEVER travel at constant speeds.  We are always accelerating or decelerating on a micro level due to pedaling inefficiency, wind changes, bunch surges etc.  That is why I believe it has less of an effect on ime trial bikes and riders and more on crits for instance.  I also think more energy is required to accelerate a heavy wheel as opposed to a light one (apart from the obvious linear movement of the weight), even if this is quite a small force, but the weight must make a difference.