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Pound for pound what makes more of a difference?

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Pound for pound what makes more of a difference?

Old 08-18-12, 07:51 AM
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Originally Posted by cyccommute View Post
By the way, you see this is cars as well and motorists are as interested in losing weight off the wheels as much as bicyclists. Motorists, specifically race car teams, are talking about losing weight from all rotating parts...that's the cams, cranks, axles, wheels and anything else that goes spins.
Racers (of any sort) are already spending huge amounts of money/time to compete against other racers (who are also seeking any advantage). That means tiny improvements in performance worth a lot. But that doesn't make the improvements not tiny.
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Old 08-18-12, 07:58 AM
  #52  
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Originally Posted by wphamilton View Post
I vote for taking it off the bike first, even for overweight folks. Why not go faster and uphill easier while losing weight?
Well, because it usually costs money. And the increase in performance is tiny.

The ironic thing is that many people worried about small differences in weights rarely ride in the drops. Riding in the drops would do much more than removing 5 lbs from the bike and it's free.
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Old 08-18-12, 09:26 AM
  #53  
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There are some problems with that article and our current discussion. He states that "the 0.3kg/0.66lb difference in wheels...is so small compared to your body mass that the differences in wheel inertia will be [imperceptible]". I fully agree but remember what I said: "5 lbs off the wheels would make a huge difference". Take the same ride on a bike equipped with steel wheels and the amount of effect that the wheels have wouldn't be nearly as small.

Additionally, his tests would be better with a broader data base. I get the impression that he is drawing conclusions from a few rides he made himself. Vary the riders, days, bikes, wind conditions, wheel weights, etc., then do the analysis on the data.

Originally Posted by gregf83 View Post
You're a little confused on your physics here. If you were able to pedal smoothly there would be zero acceleration going on regardless of whether you were riding in a vacuum. As most people don't pedal smoothly there is a very small acceleration when your pedals are at 3 o'clock and you are applying maximum force.
I'm not confused about the physics at all. You seem to be however. Let's start with a question: Why do you have to pedal at all? If you were in a vacuum and on a frictionless surface, you wouldn't need to pedal at all to maintain a given speed. If you have pedal you are constantly putting energy into the system in the form of force on the drivetrain. This is the outside forces that are working on the system. If you have to use force to push against another force to keep the vehicle moving, you are accelerating. Force always has an acceleration component.

You can pedal as smoothly as you like but the fact that you have to pedal to maintain a given speed says that you are accelerating in the face of external forces.

Originally Posted by gregf83 View Post
As far as heavier wheels or a heavier bike making a difference, conservation of energy applies and if you read up on it you will find that additional weight on the bike or in the rims will make negligible difference in the power required. The additional power will be due to a slightly higher rolling resistance from the additional weight.
Um...no. The Law of Conservation of Energy says that the overall energy of a closed system remains constant over time. A bicycle rolling down the road isn't a closed system. The universe in which it operates is closed but the bicycle...and even the world it operates in...isn't.

I believe that you are thinking of the Law of Conservation of Momentum but even that is incorrect. That law, again, only applies in a closed system, i.e. a system that isn't being acted upon by outside forces. The outside forces in this case are air resistance, friction of the surface and friction in the system itself.

Originally Posted by gregf83 View Post
Note that even if you added 10 kg to the bike or wheels it would require less than 3W of additional power at 30kph. The 5 lbs talked about in the OP would add less than 1W which would be very difficult to measure let alone feel.
Those numbers seemed to have come out of no where. In a real world example, a 10kg load on a bike will significantly impact the performance of the bike. I've done way too much loaded touring to be fooled by that kind of statement. I can 'sprint' on a loaded touring bike but I'm not going to 'sprint' too fast nor for too long. If you really think that adding 10 kg to a bike doesn't have much effect, I welcome you to enter said bike in your nearest bike race and see where you finish.

Originally Posted by njkayaker View Post
It's a reasonable assumption. For a really heavy bike, loosing 5 pounds would matter even less.

Obviously, all things being equal, a lighter bike is better. Another reason a bike has "extra weight" is because it's cheaper. The tiny benefit for most riders of spending large amounts to save a hard-to-achieve 5 pounds off of a road bike isn't really worth it.
Originally Posted by njkayaker View Post
Racers (of any sort) are already spending huge amounts of money/time to compete against other racers (who are also seeking any advantage). That means tiny improvements in performance worth a lot. But that doesn't make the improvements not tiny.

Really? You tour. Would you rather carry a 7 lb tent or a 2 lb tent? A 5 lb bag or a 2 lb bag? A 2 lb stove or a 6 oz stove? A 2 lb sleeping pad or no pad? Tourist go through the exercise of trying to shed the kind of weight we are talking about...and a whole lot less...all the time. Many of us are willing to trade weight for comfort even to silly levels like sawing the ends off tooth brushes because we realize that weight does matter.

Many of us who tour spend obscene amounts of money to save 2 lbs. I know that I have. A Eureka Timberline for example, weighs in at the portly 7 lb but costs $100 and is a rugged tent. I have two of them (a worn out one for parts and a newer one). I don't use the Timberline anymore because I spent $300 to buy a Fly Creek UL1 to save 5 lbs. I spent about $100 to replace the $5 aluminum cook set made from regular pots without handles with an MSR set that weighs about 1/2 as much. I carry a down bag that cost twice as much as the Dacron bag I used to use. I can give you example after example from my own personal camping gear set up.

Yes, any of us could probably lose 15 or 20 lbs and carry the heavier equipment but why? We are going to opt for the lighter weight equipment because, in the end, the weight of the bike is important. If we lose the weight, we are still going to opt for the lighter equipment.
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Old 08-18-12, 09:38 AM
  #54  
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Originally Posted by Wait For Me View Post
For all of us that are more then 10% body fat. What makes more of a noticeable difference? Shaving 5lbs of the bike or 5lbs off the rider?
OP should specify a "noticeble difference" in what metric?

In overall enjoyment in life, losing excess body weight has it all over the a notable difference in bicycling "performance", unless bragging about bicycle specifications and/or seconds shaved off a meaningless scorecard have great significance to the cyclists. My "performance" in all manners of life, including bicycling has greatly improved by losing 34 pounds in six months. Didn't cost a cent. Try losing 34 pounds of bicycle weight for free.
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Old 08-18-12, 09:42 AM
  #55  
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Originally Posted by cyccommute View Post
I don't know where you came up with the 15% thing. But you are wrong about cruising at a constant speed. Since we live in an atmosphere and with friction and with variations in altitude, we are never 'cruising at a constant speed'. We are constantly accelerating in response to air resistance and tire friction. Since we are constantly accelerating, the wheels weight has a higher impact on that acceleration.

If you don't like the studded tire example, try the steel wheel vs aluminum wheels. If you can score them, compare the steel to carbon fiber. By the way, my studded tires a knobbed tires that replace knobbed mountain bike tires. Similar rubber, similar tire patterns, similar tire stiffness. The studded tire has a huge impact over the knobbed mountain bike tires.
Put your bike on a stand and shift it to the hardest gear. Now put two fingers on one of your pedals and spin the back wheel. You might be surprised to find how little effort it takes to accelerate a bicycle wheel to 30 mph faster than you ever would riding it. This effort can be reduced, but, of course, not eliminated, by spending mega$$$ on carbon wheels, special spokes, etc.

The distinction between rotating weight and overall weight is real but grossly exaggerated.

In hilly terrain without stop signs or other reason to apply the brakes, the flywheel effect of rotating weight can be beneficial and act as a force to effectively level the terrain slightly.

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Old 08-18-12, 09:46 AM
  #56  
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Originally Posted by cyccommute View Post
The problem is that all those small accelerations aren't balanced by the momentum. In a vacuum, without friction and on a perfectly flat surface, momentum will never change. Try getting the bike rolling and then don't pedal. Eventually the bike will stop rolling without continued input of energy...even on a flat surface. And the world, as MechBgon has so wisely stated elsewhere, is seldom flat. Every time you have to put energy into the system to maintain speed, you are accelerating against some force that is slowing your bike down. And every time you push on the pedals, you are spinning the weight of your wheels around while accelerating them.

By the way, you see this is cars as well and motorists are as interested in losing weight off the wheels as much as bicyclists. Motorists, specifically race car teams, are talking about losing weight from all rotating parts...that's the cams, cranks, axles, wheels and anything else that goes spins.
And the bike with heavier wheels will roll farther. In other words the deceleration is less, this improvement corresponding exactly to how much lower your acceleration was.

If you don't agree with this, try this: leave the decelerations out of it, assuming for the sake of argument that you gain nothing from the extra momentum. Put the numbers to how much extra power it takes to accelerate in each pedal stroke, heavy wheel vs light one. With any reasonable estimation of how much the speed varies in each stroke, after adding them all up you'll find it's insignificant.
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Old 08-18-12, 09:48 AM
  #57  
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Originally Posted by njkayaker View Post
Racers (of any sort) are already spending huge amounts of money/time to compete against other racers (who are also seeking any advantage). That means tiny improvements in performance worth a lot. But that doesn't make the improvements not tiny.
Those who are obsessed with weight should take an enema before riding. Could easily shed several hundred grams by doing this.

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Old 08-18-12, 10:50 AM
  #58  
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Originally Posted by Don in Austin View Post
Put your bike on a stand and shift it to the hardest gear. Now put two fingers on one of your pedals and spin the back wheel. You might be surprised to find how little effort it takes to accelerate a bicycle wheel to 30 mph faster than you ever would riding it. This effort can be reduced, but, of course, not eliminated, by spending mega$$$ on carbon wheels, special spokes, etc.

The distinction between rotating weight and overall weight is real but grossly exaggerated.

In hilly terrain without stop signs or other reason to apply the brakes, the flywheel effect of rotating weight can be beneficial and act as a force to effectively level the terrain slightly.

Don in Austin
Good point! It should also be pointed out that the energy required to accelerate you whole 15 - 30 lb bike up to 30 mph is actually not that great... at least compared to your 150 - 300 lb body.

Riding a bike loaded with 20 lbs of panniers and luggage is a different exercise than riding an unladen bike while wearing a 20 lb back pack (or carrying 20 lbs of food in a basket compared to eating all the food and gaining 20 lbs before you ride).

On a relatively smooth surface (any road) the heavier bike will hold momentum and you can shift you body around easily when you need to. On a rough surface (a singletrack trail with rocks and roots) it is much more difficult as the direction of the bike needs to change often to clear obstacles, so carrying the weight on your body becomes more convenient - you can stand up once and let your legs absorb the movement of the bike over the obstacles and keep your body relatively still. I belive this is the rationale for riding 20 lb XC mountain bikes while wearing an 8 lb hydration pack with tools and rain gear and food. And it is probably why most people riding long distances let the bike carry the cargo and ride without too much luggage on their bodies.

On flat ground, weight makes little-to-no difference, regardless of where you carry it. Yes, it takes more energy to accelerate the weight, but you have more momentum so you slow down less for any given amount of drag, so you actually have less accelerating to do.
Going down a hill, I know that my heavier rider+bike combo (close to 300 lbs if I am carrying anything besides me and the bike) is faster than lighter rider+bike combos. But since it is harder to go up the hill, it is unlikely I will gain back the ground lost during the climb to the 150 lb guy on the 16 lb bike.
Going up a hill, if you are seated and pedal smoothly, then extra weight will slow you down, but it does not matter where you put it - on the bike frame, on your body, or on the wheels - it will slow you down the same amount. If you stand up to pedal or move yourself or the bike around a lot then the effect of the location of the weight on climbing becomes less clear.
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Old 08-18-12, 11:04 AM
  #59  
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It's a reasonable assumption. For a really heavy bike, loosing 5 pounds would matter even less.
I disagree. In the winter and spring, my training bike usually rolls out at about 45 pounds, thanks to studded tires, a trunk bag and one or two panniers. And ditching one pannier at the base of Greenwood Road prior to a hill interval does make a difference. In fact, the day I'm specifically thinking of (April 4th), I made a sizeable dent in my previous PR on Strava. And that was just static weight, not rotating.

I now hold the KOM mark for that climb (with my XC-racing mountain bike, no less) and if I have to recapture it from someone, I'll be throwing on the 265-gram road tires. When I stomp, they go. On my KOM ride, that was basically how it went... keep hammering the pedals around without dropping out of my powerband. Because once I begin to drop out of my powerband, I have to downshift, and that's the end of that. Light wheels simply have less resistance to the power strokes, allowing me to keep my gear when I'm at the ragged edge. If you quantify the difference that allows this to happen, it might not look like a lot on paper. Whatever. I think it was Albert Einstein who said the most powerful force in the universe is compound interest and it makes a good analogy here. Just a few percent can have a cumulative effect.

Anyhow, empirical results are empirical results, and these are relevant to the OP's question. When my co-worker notices me taking the pannier off my training bike before leaving work, he knows I'm going for some kind of record assault

Put the numbers to how much extra power it takes to accelerate in each pedal stroke, heavy wheel vs light one. With any reasonable estimation of how much the speed varies in each stroke, after adding them all up you'll find it's insignificant.
Try switching a mountain bike from typical tires to 1.25" folding Paselas with Conti Race 650 Light tubes, then do some stop-&-go traffic. Drag-race the cars. It's not insignificant in real life, quite the opposite.

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Old 08-18-12, 11:18 AM
  #60  
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I think the bigger issue is that at a certain point (different for us all) the trade off of price and trouble per weight off the bike becomes unrealistic at which point weight off the rider takes precedence. Where that point is could be at either end of extremes.


There is no one general answer that can be made as the sweet spot depends on your finances, physical conditioning, skills and who knows what else.
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Old 08-18-12, 12:30 PM
  #61  
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Easy for me. I can lose the equivalent of $10K worth of bicycles before I need to drop weight from my current bike. Dropping 5# from my butt is only a start.
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Old 08-18-12, 01:51 PM
  #62  
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Originally Posted by I-Like-To-Bike View Post
OP should specify a "noticeble difference" in what metric?

...
Libraries of Congress per Newton-meter.
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Old 08-18-12, 03:36 PM
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Originally Posted by mechBgon View Post
I disagree. In the winter and spring, my training bike usually rolls out at about 45 pounds, thanks to studded tires, a trunk bag and one or two panniers. And ditching one pannier at the base of Greenwood Road prior to a hill interval does make a difference. In fact, the day I'm specifically thinking of (April 4th), I made a sizeable dent in my previous PR on Strava. And that was just static weight, not rotating.

I now hold the KOM mark for that climb (with my XC-racing mountain bike, no less) and if I have to recapture it from someone, I'll be throwing on the 265-gram road tires. When I stomp, they go. On my KOM ride, that was basically how it went... keep hammering the pedals around without dropping out of my powerband. Because once I begin to drop out of my powerband, I have to downshift, and that's the end of that. Light wheels simply have less resistance to the power strokes, allowing me to keep my gear when I'm at the ragged edge. If you quantify the difference that allows this to happen, it might not look like a lot on paper. Whatever. I think it was Albert Einstein who said the most powerful force in the universe is compound interest and it makes a good analogy here. Just a few percent can have a cumulative effect.

Anyhow, empirical results are empirical results, and these are relevant to the OP's question. When my co-worker notices me taking the pannier off my training bike before leaving work, he knows I'm going for some kind of record assault



Try switching a mountain bike from typical tires to 1.25" folding Paselas with Conti Race 650 Light tubes, then do some stop-&-go traffic. Drag-race the cars. It's not insignificant in real life, quite the opposite.
you have a HUGE difference in rolling resistance. It's not just weight. Not a valid comparison. For highway speed I would rather be on road tires with lead weights on the spokes than MTB tires, even if the effective rotating weight of the road tires was more with the lead added.

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Old 08-18-12, 03:53 PM
  #64  
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Originally Posted by cyccommute View Post
I'm not confused about the physics at all. You seem to be however. Let's start with a question: Why do you have to pedal at all? If you were in a vacuum and on a frictionless surface, you wouldn't need to pedal at all to maintain a given speed. If you have pedal you are constantly putting energy into the system in the form of force on the drivetrain. This is the outside forces that are working on the system. If you have to use force to push against another force to keep the vehicle moving, you are accelerating. Force always has an acceleration component.

You can pedal as smoothly as you like but the fact that you have to pedal to maintain a given speed says that you are accelerating in the face of external forces.
Sorry, you'll need to review your high school physics. Acceleration of the bike and rider is a function by the net force acting on the bike. In the case of pedalling on the flats at a constant speed the force you apply to the pedals is balanced by the forces resisting forward motion, i.e. wind resistance, rolling resistance etc. At constant speed, by definition, there is no acceleration. a = dv/dt. No change in velocity means no acceleration.

Now as I stated earlier there are actually very small accelerations going on with each pedal stroke, since we don't pedal with constant torque, but those accelerations are minimal and not relevant to this discussion.
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Old 08-18-12, 03:53 PM
  #65  
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Originally Posted by Don in Austin View Post
Put your bike on a stand and shift it to the hardest gear. Now put two fingers on one of your pedals and spin the back wheel. You might be surprised to find how little effort it takes to accelerate a bicycle wheel to 30 mph faster than you ever would riding it. This effort can be reduced, but, of course, not eliminated, by spending mega$$$ on carbon wheels, special spokes, etc.

The distinction between rotating weight and overall weight is real but grossly exaggerated.

In hilly terrain without stop signs or other reason to apply the brakes, the flywheel effect of rotating weight can be beneficial and act as a force to effectively level the terrain slightly.

Don in Austin
Yep. On a stand it doesn't take much effort to spin up a wheelset. But measure the effort between a steel wheel and an aluminum wheel. I won't talk about carbon because I've never had the experience but I have had experience with both steel and aluminum. The steel wheel requires noticably more effort even on the stand than a lightweight aluminum does.

But this example is as artificial as the frictionless vacuum example. In the real world where friction actually exists and where the world works against the flywheel effect of the heavier wheel, the weight of the wheels is important and loads of money is spent to reduce that weight. If the weight of the wheels didn't matter or if you got some benefit from a flywheel effect, don't you think that the pros would be riding bikes with lead wheels?

Originally Posted by wphamilton View Post
And the bike with heavier wheels will roll farther. In other words the deceleration is less, this improvement corresponding exactly to how much lower your acceleration was.

If you don't agree with this, try this: leave the decelerations out of it, assuming for the sake of argument that you gain nothing from the extra momentum. Put the numbers to how much extra power it takes to accelerate in each pedal stroke, heavy wheel vs light one. With any reasonable estimation of how much the speed varies in each stroke, after adding them all up you'll find it's insignificant.
You can leave the decelerations out of the equation as you can leave the air out of the equation and the friction out of the equation. I'll not argue that momentum will keep an object moving to infinity in the absence of those forces. But that is a simplified model that only serves to show how momentum works. In the real world, you have to take those into account and they have a real impact. Frictional force, for example, has a greater effect on the heavier object because the mass of the object is larger. You have to put more force into the object to keep it moving in the face of the larger force pushing back. There is no free lunch.

Originally Posted by mechBgon View Post
Try switching a mountain bike from typical tires to 1.25" folding Paselas with Conti Race 650 Light tubes, then do some stop-&-go traffic. Drag-race the cars. It's not insignificant in real life, quite the opposite.
That's probably the best example that people can wrap their heads around because they are doing 'real' acceleration...friction and air resistance require 'real' acceleration as well but it's harder to grasp. Start from a standing stop with heavy wheels and accelerate as fast as you can. Then do the same with light wheels. Fill the tires with water and do the same. If, as people are saying here, the weight of the wheels doesn't matter, then why would the water filled wheels be incredibly difficult to get up to the same speed? Alternatively, why would the water filled wheels take far more energy to get up to the same speed over the same distance?
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Old 08-18-12, 04:05 PM
  #66  
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Originally Posted by cyccommute View Post
Y
You can leave the decelerations out of the equation as you can leave the air out of the equation and the friction out of the equation. I'll not argue that momentum will keep an object moving to infinity in the absence of those forces. But that is a simplified model that only serves to show how momentum works. In the real world, you have to take those into account and they have a real impact. Frictional force, for example, has a greater effect on the heavier object because the mass of the object is larger. You have to put more force into the object to keep it moving in the face of the larger force pushing back. There is no free lunch.
You're not following. "You" can even leave the decelerations out if you want (actually you are) and it still doesn't make much difference. You have the physics wrong.
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Old 08-18-12, 04:09 PM
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Originally Posted by cyccommute View Post
Yep. On a stand it doesn't take much effort to spin up a wheelset. But measure the effort between a steel wheel and an aluminum wheel. I won't talk about carbon because I've never had the experience but I have had experience with both steel and aluminum. The steel wheel requires noticably more effort even on the stand than a lightweight aluminum does.

But this example is as artificial as the frictionless vacuum example. In the real world where friction actually exists and where the world works against the flywheel effect of the heavier wheel, the weight of the wheels is important and loads of money is spent to reduce that weight. If the weight of the wheels didn't matter or if you got some benefit from a flywheel effect, don't you think that the pros would be riding bikes with lead wheels?
But they do use considerably heavier wheels in certain events - think disk wheels that weight up to a pound more than moderately lightweight wheels but are used in the highest speed events. No-one is saying heavier wheels are an advantage, just that they are only a disadvantage in certain situations... in other situations they make no difference.



Originally Posted by cyccommute View Post
You can leave the decelerations out of the equation as you can leave the air out of the equation and the friction out of the equation. I'll not argue that momentum will keep an object moving to infinity in the absence of those forces. But that is a simplified model that only serves to show how momentum works. In the real world, you have to take those into account and they have a real impact.
Agreed.

Originally Posted by cyccommute View Post
Frictional force, for example, has a greater effect on the heavier object because the mass of the object is larger. You have to put more force into the object to keep it moving in the face of the larger force pushing back. There is no free lunch.
^Nonsense. Once it is up to speed, a wheel with 5 lbs of lead weights glued to the rim will have the same forces acting against it as the identical rim without the weights. And the forces that do act on the wheel - bearing, aerodynamic, and rolling resistance (which are identical for the two wheels) is less able to slow down the heavier wheel because it has more momentum.


Originally Posted by cyccommute View Post
That's probably the best example that people can wrap their heads around because they are doing 'real' acceleration...friction and air resistance require 'real' acceleration as well but it's harder to grasp. Start from a standing stop with heavy wheels and accelerate as fast as you can. Then do the same with light wheels. Fill the tires with water and do the same. If, as people are saying here, the weight of the wheels doesn't matter, then why would the water filled wheels be incredibly difficult to get up to the same speed? Alternatively, why would the water filled wheels take far more energy to get up to the same speed over the same distance?
Nobody is arguing that weight affects acceleration - but it has little-to-no affect on speed when you are not accelerating. And your previous comments about 'constantly accelerating' are less true with a heavier wheels as it takes more of the outside forces (aerodynamic, bearing, and tire friction) to slow the wheel down because of the added momentum.

Also, the example above proves very little because it is not a controlled experiment. Swapping from a fat knobby tire to a supple high-pressure road tire will give more benefits from reduced rolling resistance (energy lost to tire deflection) than from reduced weight. Certainly the road tire will feel much faster, and i agree that during acceleration, a big part of that is definitely the weight, but, just as you cannot ignore deceleration and air resistance, you cannot ignore the rolling resistance caused by the fatter softer thicker tire.
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Old 08-18-12, 04:23 PM
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Originally Posted by Don in Austin View Post
you have a HUGE difference in rolling resistance. It's not just weight. Not a valid comparison.
Actually it is a valid comparison. The rolling resistance starting from a standstill (stoplight launches in traffic, for example) is quite small with the tires inflated to a fairly high pressure. In that instance, nearly all my power is going into acceleration. When the studded tires are running at ~25psi on icy/snowy days, it's true that they have serious rolling resistance in addition to their weight. AWESOME training tires, I must admit... they make my legs strong like ox

For highway speed I would rather be on road tires with lead weights on the spokes than MTB tires, even if the effective rotating weight of the road tires was more with the lead added.
Ironically, I go about as fast on this setup as I do on my sport-touring bike. Four consecutive course records on our local paved-&-gravel century:


Continental RaceKing 2.2 Supersonic. Good tires. Also featured: 315-gram NoTubes Alpine XC-race rims. For this event, I also used my Speedplay Zero Ti pedals as shown.

Dunno if you can view this on Strava or not, but this is a sample of that bike's road speed, solo, out-and-back on a 32-mile route that includes a 5-mile ascent: http://app.strava.com/activities/15815111 22.0mph average, not bad for a middle-aged has-been According to my fancy scale, I'm at 16.4% body fat.

Swapping from a fat knobby tire to a supple high-pressure road tire will give more benefits from reduced rolling resistance (energy lost to tire deflection) than from reduced weight. Certainly the road tire will feel much faster, and i agree that during acceleration, a big part of that is definitely the weight, but, just as you cannot ignore deceleration and air resistance, you cannot ignore the rolling resistance caused by the fatter softer thicker tire.
Well, there's tires, and then there's tires. The ones shown above are as supple in your hands as a Grand Prix 4000. 55mm wide, 460 grams. Amazing tires.

Reverting to the actual topic: rider training is certainly foremost, so if you're out of training, get going on that. Eat healthy, hit a healthy weight, get enough recovery, and that's all a big win. But if your bike has un-functional weight, you can do something about that and improve your overall power-to-weight ratio there immediately. 5 pounds was used as a hypothetical number, and that's not easy to remove from some bikes, but for some of us that could take the form of leaving our U-lock at work instead of hauling it around. For the Midnight Century event I mentioned above, I saved almost 5 pounds by cacheing two 34-ounce waterbottles on the course for quick pickup after the last major ascent, instead of hauling them 60 miles from the start.

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Old 08-18-12, 04:23 PM
  #69  
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Originally Posted by gregf83 View Post
Sorry, you'll need to review your high school physics. Acceleration of the bike and rider is a function by the net force acting on the bike. In the case of pedalling on the flats at a constant speed the force you apply to the pedals is balanced by the forces resisting forward motion, i.e. wind resistance, rolling resistance etc. At constant speed, by definition, there is no acceleration. a = dv/dt. No change in velocity means no acceleration.

Now as I stated earlier there are actually very small accelerations going on with each pedal stroke, since we don't pedal with constant torque, but those accelerations are minimal and not relevant to this discussion.
High school physics is good for demonstrating the principles because you can leave out all those bothersome details that get in the way of understanding the principles. In the high school physics world, you can get on the bike, give the pedals a mighty shove and the bike will move at a constant speed until it is acted upon by an outside force. Since you've removed the outside force, there is no velocity change and no acceleration in any direction. I have no problem with that explanation nor that model. It's simple to explain but maybe a little difficult for most to understand.

But in the real world those bothersome details have real consequences. Let's look at what you said above: you agree that you are applying force to the bike to balance the forces resisting forward motion, so do I. Once balanced you can maintain a constant speed but what do you have to do to maintain that constant speed? Unlike the high school physic model, you have to constantly apply force to the pedals to maintain that balance and the constant speed. What happens when you stop applying that force to the pedals? You can't just set the bike on its way and expect it to roll forever because the bike is being acted upon by outside forces changing it's speed and momentum. The bike will slow down, i.e. experience a negative acceleration because of the forces acting against the bike and rider. The net velocity change of the system is zero while you are applying a force to counteract the negative forces but because you have to constantly apply force to the pedals to maintain that balance, the rider is having to constantly accelerate the bicycle. Otherwise the bike would have to stop moving.
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Old 08-18-12, 04:27 PM
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Originally Posted by wphamilton View Post
You're not following. "You" can even leave the decelerations out if you want (actually you are) and it still doesn't make much difference. You have the physics wrong.
I'm certainly not following this post. Where have I left out the decelerations?
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Old 08-18-12, 04:56 PM
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Originally Posted by mechBgon View Post
Try switching a mountain bike from typical tires to 1.25" folding Paselas with Conti Race 650 Light tubes, then do some stop-&-go traffic. Drag-race the cars. It's not insignificant in real life, quite the opposite..
If you get on the brakes, you waste the momentum. Getting up to speed at the start, there's a difference. I think I said that from the get-go.

Pedaling along, the difference is insignificant, even in real life.
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Old 08-18-12, 04:58 PM
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Originally Posted by cyccommute View Post
I'm certainly not following this post. Where have I left out the decelerations?
Where does the rotational energy go, if not in opposing air resistance and rolling resistance? Your reasoning leaves that out, hence it leaves out the effects of the decelerations.
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Old 08-18-12, 04:59 PM
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Originally Posted by cyccommute View Post
The net velocity change of the system is zero while you are applying a force to counteract the negative forces
correct.
but because you have to constantly apply force to the pedals to maintain that balance, the rider is having to constantly accelerate the bicycle.
incorect.
Otherwise the bike would have to stop moving.
incorrect.

In a steady state scenario the velocity of the bike is a constant and there is no acceleration unless a net force acts on the bike. If the bike were constantly accelerating as you suggest the velocity would change.
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Old 08-18-12, 05:01 PM
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Originally Posted by LarDasse74 View Post
But they do use considerably heavier wheels in certain events - think disk wheels that weight up to a pound more than moderately lightweight wheels but are used in the highest speed events. No-one is saying heavier wheels are an advantage, just that they are only a disadvantage in certain situations... in other situations they make no difference.
Lots of people here have said that heavier wheels are an advantage. A comment about the "the flywheel effect on rolling hills" comes to mind. If heavier wheels make no difference, why do we all...or at least almost all...ride lightweight wheels? A steel wheel is M1 Abrams tank strong and would certainly have advantages in certain bicycling activities but you don't find too many of them out there. You could use a steel wheel in touring, for example, and pull enough tension on the rim that you'd never have to worry about broken spokes. A strong steel wheel on a mountain bike, especially a gravity machine, would be wonderful. But why do we not use them? Because they are a bugger to get rolling and keep rolling.

Originally Posted by LarDasse74 View Post
^Nonsense. Once it is up to speed, a wheel with 5 lbs of lead weights glued to the rim will have the same forces acting against it as the identical rim without the weights. And the forces that do act on the wheel - bearing, aerodynamic, and rolling resistance (which are identical for the two wheels) is less able to slow down the heavier wheel because it has more momentum.
First you'd have to get it up to speed. That wouldn't be too easy. Everyone concentrates on momentum once at speed but forgets that friction is a force that is proportional to the mass of the wheel as well as the other component of the force, the acceleration. Rolling resistance is related to the friction forces between the tire and the road and as such would be higher in a wheel that has more mass. Other forces acting on the system are going to be proportional to the mass of the system as well by the very definition of a force.

Originally Posted by LarDasse74 View Post
Nobody is arguing that weight affects acceleration - but it has little-to-no affect on speed when you are not accelerating. And your previous comments about 'constantly accelerating' are less true with a heavier wheels as it takes more of the outside forces (aerodynamic, bearing, and tire friction) to slow the wheel down because of the added momentum.
Again it comes down to what happens if you stop pedaling. You can get a bike up to a constant speed but you have to 'work' to maintain that constant speed. If you were in a true state of constant speed, you wouldn't need to input any energy into the system to maintain that speed. Outside conceptual physics model (or inside the earth's atmosphere) you can't maintain a constant speed on any object because you always have to put energy into the system to maintain the speed. There are systems that have less friction and less air resistance than other systems but in the end, you still can't maintain a constant velocity without continual force input. If you are putting force into the system, you are accelerating some part of the system.

Originally Posted by LarDasse74 View Post
Also, the example above proves very little because it is not a controlled experiment. Swapping from a fat knobby tire to a supple high-pressure road tire will give more benefits from reduced rolling resistance (energy lost to tire deflection) than from reduced weight. Certainly the road tire will feel much faster, and i agree that during acceleration, a big part of that is definitely the weight, but, just as you cannot ignore deceleration and air resistance, you cannot ignore the rolling resistance caused by the fatter softer thicker tire.
I didn't say anything about knobbies vs smooth tires. My example would be a pretty good controlled experiment...not that I was aiming for a controlled experiment.
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Old 08-18-12, 05:05 PM
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Originally Posted by wphamilton View Post
Where does the rotational energy go, if not in opposing air resistance and rolling resistance? Your reasoning leaves that out, hence it leaves out the effects of the decelerations.
I'm still not following because I didn't say that the rotational energy doesn't go into opposing air resistance and rolling resistance. Of course it does. And you have to keep supplying it or the bicycle stop rolling precisely because the energy goes into opposing the decelerating forces.
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