it's amazing how that tired ass, misinformed crap about inertia/rotating weight can still linger...
people who want to sound like they know what they're talking about but have no idea...
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
I have... many times.
When it comes to acceleration and 26 vs. 29, all that matters is wheel mass. Talk of moment of inertia or the "flywheel effect" or leverage are all bogus.
Moment of Inertia is an abstract concept that is tossed around mostly by people who like to use big words. I'd bet the builder, while he might deserve respect, does not know what the formula for MoI is or how to measure it or what the value of it is for those wheels compared to others. If he did he wouldn't talk about it like he does. I'd also bet that he hasn't read any of the research that shows that even great changes in MoI do not have measurable impacts on performance.
The bottom line is that, like many other technical concepts, MoI is WAY over-hyped and 99% of the times it is brought up it is irrelevant.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
I have... many times.
When it comes to acceleration and 26 vs. 29, all that matters is wheel mass. Talk of moment of inertia or the "flywheel effect" or leverage are all bogus.
Moment of Inertia is an abstract concept that is tossed around mostly by people who like to use big words. I'd bet the builder, while he might deserve respect, does not know what the formula for MoI is or how to measure it or what the value of it is for those wheels compared to others. If he did he wouldn't talk about it like he does. I'd also bet that he hasn't read any of the research that shows that even great changes in MoI do not have measurable impacts on performance.
The bottom line is that, like many other technical concepts, MoI is WAY over-hyped and 99% of the times it is brought up it is irrelevant.
I'm not exactly sure what moment of inertia is, but the way the builder writes about inertia seems to make perfect sense. As we know, otherwise identical rims, tires, and spokes are always heavier in the 29er version. Therefore, unless using more advanced technology, 29er wheels will have greater inertia and slower acceleration compared to 26" wheels using the same components.
So I wouldn't disagree with the builder on either of his statements: 1) given the nature of a larger wheel with more material, 29ers are more accelerationally (made up) challenged 2) all things equal, a wheel with the lower inertia will accelerate better.
The builder never mentions moment of inertia, but his logic seems sound to me.
As we know, otherwise identical rims, tires, and spokes are always heavier in the 29er version. Therefore, unless using more advanced technology, 29er wheels will have greater inertia and slower acceleration compared to 26" wheels using the same components.
So I wouldn't disagree with the builder on either of his statements: 1) given the nature of a larger wheel with more material, 29ers are more accelerationally (made up) challenged 2) all things equal, a wheel with the lower inertia will accelerate better.
The builder never mentions moment of inertia, but his logic seems sound to me.
Except you haven't considered the extent to which the differences matter.
In rough terms, wheels might make up 1/3 of the total inertia of the bike and the bike only 1/6 - 1/8 of the total inertia of the system. That means wheel inertia is maybe 4-5% of total inertia. A 29er wheel will be heavier with identical construction by roughly 10%, but that means 1/2% to the total system. It's of no consequence.
When you read the author say how critical it is to use ultralight spokes to reduce inertia, you know he's ignorant of the real issues.
A 29er wheel will be heavier with identical construction by roughly 10%, but that means 1/2% to the total system. It's of no consequence.
I see your point. However, I think a 10% difference in wheel weight, centered mostly in the rim and tire, is definitely noticeable and significant. It's roughly the equivalent of going from a 600g to 500g tire. Plus, though you state the difference is only 1/2% of the total system, doesn't the fact that it's rotating mass bump that number up?
I guess what meltingfeather is really getting at is that the bump isn't as much as people often think. It's an interesting point. Fundamentally, the statements of the builder in question are sound (and make sense logically), but it would be interesting to find out just how much of a difference a change in rotational weight actually makes in energy expenditure compared to an identical change in static weight.
I see your point. However, I think a 10% difference in wheel weight, centered mostly in the rim and tire, is definitely noticeable and significant. It's roughly the equivalent of going from a 600g to 500g tire. Plus, though you state the difference is only 1/2% of the total system, doesn't the fact that it's rotating mass bump that number up?
It is noticeable but not due to its contribution to overall inertia. Significant I'm not so sure.
I factored in the fact that it's rotating in my initial estimate of 1/3 of total inertia of the bike. Yes it does matter. The rim and tire matter twice as much as any other weight. Spokes are too light to really matter, nothing else on the bike is much different from anything else.
Originally Posted by Durga
I guess what meltingfeather is really getting at is that the bump isn't as much as people often think. It's an interesting point. Fundamentally, the statements of the builder in question are sound (and make sense logically), but it would be interesting to find out just how much of a difference a change in rotational weight actually makes in energy expenditure compared to an identical change in static weight.
Detailed calculations of that have been posted here before. They are pretty straightforward once you realize you don't have to worry about moments of inertia.
meltingfeather: When it comes to acceleration and 26 vs. 29, all that matters is wheel mass. Talk of moment of inertia or the "flywheel effect" or leverage are all bogus.
The stuff about the spokes is a bit of bogus, but moment of inertia is very real and it relates directly to rotational mass (mostly rim and tire/tubes). Just like mass affects momentum, rotational mass affects moment of inertia. And the greater the rotational mass the more rotational force will be required to change its rotational rate. Also the further out the object's mass is, the more rotational inertia the object has, and the more rotational force (torque) is required to change its rotation rate.
I am sure you feel the difference between 2100g wheels with 825g rubber and 1550g wheels with 500g rubber. In fact the difference is huge, although we are talking about 1/3 lighter set of wheels.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
meltingfeather: When it comes to acceleration and 26 vs. 29, all that matters is wheel mass. Talk of moment of inertia or the "flywheel effect" or leverage are all bogus.
The stuff about the spokes is a bit of bogus, but moment of inertia is very real and it relates directly to rotational mass (mostly rim and tire/tubes). Just like mass affects momentum, rotational mass affects moment of inertia. And the greater the rotational mass the more rotational force will be required to change its rotational rate. Also the further out the object's mass is, the more rotational inertia the object has, and the more rotational force (torque) is required to change its rotation rate.
And how do you think any of this affected his component choice?
I understand what MoI is and how it relates here, which is why I made the comments I did. The fact is, he built a light wheelset for a weight weenie bike. Spicing the explanation up with technical terms is just blowing smoke and I call it like I see it. I guarantee you he didn't consider or even look at MoI in quantitative terms in spec'ing the wheelset, he just picked the lightest components he could.
At the end of the day, what I said about wheel mass, diameter, and acceleration is still a fact of life. A 29er wheel and a 26" wheel of the same mass will not accelerate any differently in terms of actual effort. Discussion of perceived effort is something entirely different and, incidentally, a topic on which MoI has exactly zero bearing. There is no special consideration of MoI that a 29er wheel needs vs. 26".
Originally Posted by serious
I am sure you feel the difference between 2100g wheels with 825g rubber and 1550g wheels with 500g rubber. In fact the difference is huge, although we are talking about 1/3 lighter set of wheels.
huh?
Last edited by meltingfeather; 04-24-2012 at 02:16 PM.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
I guess I could see how the "moment of inertia" might influence acceleration (overcoming the resistance the mass presents to a change), but I'm in the whole "more rotating mass tracks better on my fully rigid rig" camp, more specifically, blissfully ignorant and using the size of my smile as my gauge of what works for me
Meltingfeather: A 29er wheel and a 26" wheel of the same mass will not accelerate any differently in terms of actual effort
Wrong! The difference in effort is similar to 2 less teeth on a sprocket. As I said, the farther the mass is from the center, the harder it is to accelerate.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Meltingfeather: A 29er wheel and a 26" wheel of the same mass will not accelerate any differently in terms of actual effort
Wrong! The difference in effort is similar to 2 less teeth on a sprocket. As I said, the farther the mass is from the center, the harder it is to accelerate.
So you don't understand the rolling wheel OR how gearing works?
What you said about moment of inertia is true, but what you don't realize is that moment of inertia doesn't matter in a rolling wheel. While increasing the wheel radius does increase the moment of inertia, it decreases angular velocity at the same time. Those simultaneous changes cancel each other out, leaving only the change in mass. Furthermore, only the mass in the rim and tire matter.
As for your gearing comment, you obviously just made that up but "2 teeth" isn't a change in gearing. You are missing a denominator.
I am an engineer and I know exactly what I am talking about. First, forget about a rolling wheel, it is an accelerating wheel we worry about, ok? Changing the size of the wheel (from 26 to 29 in) is similar to changing the size of the sprocket (decreasing by about 2 teeth), from a gearing ratio. How difficult is that to understand?
So if you ride 32:16 on 26er, you will be 32:18 on 29er, to keep the effort similar in terms of acceleration. Unfortunately, if all materials are the same (rim, tube, tire), there will be more of it on a 29er wheel (there is more diameter to cover), hence the additional weight, and that will be felt in acceleration, but can be addressed by additional changes in gearing ratios.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
I am an engineer and I know exactly what I am talking about. First, forget about a rolling wheel, it is an accelerating wheel we worry about, ok?
No, not OK. A bicycle wheel rolls always. There is never a reason to "forget" about that so as to come to the wrong conclusions.
Originally Posted by serious
Changing the size of the wheel (from 26 to 29 in) is similar to changing the size of the sprocket (decreasing by about 2 teeth), from a gearing ratio. How difficult is that to understand?
It's easy to understand, just irrational. You yourself just listed a numerator and a denominator for wheel size (29" and 26") yet not for the change in sprocket size. How difficult is that to understand?
Originally Posted by serious
So if you ride 32:16 on 26er, you will be 32:18 on 29er, to keep the effort similar in terms of acceleration.
You have now changed the subject. The subject was wheel inertia, now it's the effect of wheel size on gearing. Yes, a 29er changes gearing over a 26er by roughly 10%. That has nothing to do with wheel mass, moments of inertia, or acceleration.
Originally Posted by serious
Unfortunately, if all materials are the same (rim, tube, tire), there will be more of it on a 29er wheel (there is more diameter to cover), hence the additional weight, and that will be felt in acceleration, but can be addressed by additional changes in gearing ratios.
Yes, that has been recognized from the very beginning. It is only the extra mass of the wheel that matters, not the diameter. That's what MF said that you took exception to. It is also of very little consequence and would not be appropriate to "address" it with gearing, much less with an incomplete representation of gearing.
I am an engineer and I know exactly what I am talking about. First, forget about a rolling wheel, it is an accelerating wheel we worry about, ok? Changing the size of the wheel (from 26 to 29 in) is similar to changing the size of the sprocket (decreasing by about 2 teeth), from a gearing ratio. How difficult is that to understand?
So if you ride 32:16 on 26er, you will be 32:18 on 29er, to keep the effort similar in terms of acceleration. Unfortunately, if all materials are the same (rim, tube, tire), there will be more of it on a 29er wheel (there is more diameter to cover), hence the additional weight, and that will be felt in acceleration, but can be addressed by additional changes in gearing ratios.
So you've changed the topic, and I said nothing about gearing previously, but you agree that, given the same gearing and wheel mass, a 29er will not require additional effort?
Since you're an engineer and know what you are talking about, forgive me if it sounds odd for me to tell you that effort (energy, work, etc.) and gearing are not one and the same as you state. Gearing affects the distribution of force and time (small force + long time or large force + short time), not the effort (work done).
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
I did not change the topic. I am trying to relate wheel size changes to gearing changes for those that still do not understand that diamater (just like rotational weight) make a difference. And I am relating gearing to torque, because acceleration is the real issue (but craigsj still needs to learn that fact of life ).
Also you said: "but you agree that, given the same gearing and wheel mass, a 29er will not require additional effort?" It has to be "gear inches" that has to be the same not just gearing. But I am certain this is what you are getting at, right?
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
I did not change the topic. I am trying to relate wheel size changes to gearing changes for those that still do not understand that diamater (just like rotational weight) make a difference. And I am relating gearing to torque, because acceleration is the real issue (but craigsj still needs to learn that fact of life ).
you absolutely did change the topic. read your first post. nothing about gearing... all about inertia and mass and greater torque to change rotation rate. incidentally, you did not in that "explanation" acknowledge the critical point that radius affects rotation rate in such a way as to remove radius alltogether from the energy requirement calculation (my whole point).
Originally Posted by serious
Also you said: "but you agree that, given the same gearing and wheel mass, a 29er will not require additional effort?" It has to be "gear inches" that has to be the same not just gearing. But I am certain this is what you are getting at, right?
you are being pedantic, but at least you understand what I'm saying.
you still didn't answer the question...
and you still seem to be equating gearing (gear inches or whatever you want to call it) with effort. are you telling me it takes less effort (energy) to climb a hill if you do it in a lower gear?
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
I did not change the topic. I am trying to relate wheel size changes to gearing changes for those that still do not understand that diamater (just like rotational weight) make a difference. And I am relating gearing to torque, because acceleration is the real issue (but craigsj still needs to learn that fact of life ).
If you are now going to claim you were arguing gearing differences all along, then you have utterly failed to understand even the slightest bit of the conversation that occurred before you. Don't tell someone he's wrong when, first off, he is not and, second, you haven't bothered to understand what he is talking about.
You know, if you shift up to your big chainring your wheel gets harder to "accelerate" as well. How does what spoke you choose change that?
Here is an MTBR thread where this is discussed at length. Both MF and I participated.
Here is a repost of comment #118 made by smilinsteve. I chose it since neither MF or I wrote it:
"Yeah I was hung up on this for a while, so I’ll explain it. Starting from the link you posted, the energy of a wheel rolling down a ramp is equal to is rotational plus translational energies:
E(trans)=½mv²
E(rot)=½Iω²
As craig pointed out, it does take more energy to spin a larger wheel to a certain rotational velocity, but the larger wheel spins more slowly for a given translational velocity. So the extra energy to spin it is canceled by the less angular velocity it needs.
Mathematically, Melting Feather showed the equation that the total energy of a rolling wheel is dependent on the mass and the translational velocity, not the radius. He didn’t derive this formula though, so I’ll do that now.
Put some energy into a wheel to make it roll and that energy is converted to translation and rotation:
E = ½mv² + ½Iω²
I’m going to use moment of inertia I = mr², as for a hoop or hollow cylinder
And ω= angular velocity (rad/sec), which is related to translational velocity for a rolling wheel (not slipping) by ω= V/r (1 rotation = 2pi radians and circumference =2pi*r)
Substituting:
E= ½mv² + ½mr²ω²
E= ½mv² + ½mr²( v²/ r²)
Notice the r²’s cancel out in the second term and you are left with
E=mv²
So, you can see the energy it takes to get a wheel up to certain translational velocity depends on its mass. More mass more energy. Higher velocity, more energy. The size of the wheel doesn’t matter assuming the distribution of the mass of the wheel stays the same ( a different distribution of mass would change the moment of intertia formula and therefore the result)."
and you still seem to be equating gearing (gear inches or whatever you want to call it) with effort. are you telling me it takes less effort (energy) to climb a hill if you do it in a lower gear?
Less Torque! As in the force required to accelerate a wheel.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Less Torque! As in the force required to accelerate a wheel.
less torque over a longer period (due to lower velocity) = the same amount of work (effort)
you are starting to throw red herrings and confuse the topic. i think i've explained in very plain terms what i mean, so i'll leave you to your antics.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
Thank you tl1, that is the point of this sentence: The size of the wheel doesn’t matter assuming the distribution of the mass of the wheel stays the same
As you indicate the mass distribution is certainly not the same.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
So all else being equal, how is a 29 inch wheel/tire assembly going to ever have less mass than an equivalent 26 inch wheel/tire?
what do you mean by, "all else equal?"
either the mass is equal or the rim and tire specs are equal... they are mutually exclusive.
i think it's pretty obvious that the same spec wheel in 29er will weigh more than the 26" version... i forget when humans typically learn that, but it's in the single digits, age wise, I think.
my point is that talk of moment of inertia and "flywheel effect" is irrelevant.
my goal is to help dispel the myth that 29er wheels are somehow harder to accelerate due to diameter. mass is all that matters.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
my point is that talk of moment of inertia and "flywheel effect" is irrelevant.
my goal is to help dispel the myth that 29er wheels are somehow harder to accelerate due to diameter. distribution of mass is all that matters.
Here's the equation for the kinetic energy of a rotating wheel...
and a simple explaination of why MF is right...
"One other interesting point from this equation is that for a bicycle wheel that is not slipping, the kinetic energy is independent of wheel radius. In other words, the advantage of 650C or other smaller wheels is due to their lower weight (less material in a smaller circumference) rather than their smaller diameter, as is often stated."
This supports my statement! Especially the last sentence, since mass distribution is certainly different on a larger wheel. Too funny.
Mass distribution is not different between wheel sizes. It is remarkably consistent, even between road and MTB wheels. Do some calculations...if you are capable.
my point is that talk of moment of inertia and "flywheel effect" is irrelevant.
my goal is to help dispel the myth that 29er wheels are somehow harder to accelerate due to diameter. mass is all that matters.
1) when spin bikes talk about their superior flywheel, they basically mention the weight. So flywheel effect is very real. Except for spin bikes it is the moment of inertia a heavier wheel maintains that is attractive since it better emulates coasting. So much for irrelevant.
2) of course mass is all that matters, but in the real world, a larger diameter bike wheel will have more mass. But I regret mentioning diameter now.
FoShizzle, look away if you don't like this ... or STFU.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
1) when spin bikes talk about their superior flywheel, they basically mention the weight. So flywheel effect is very real. Except for spin bikes it is the moment of inertia a heavier wheel maintains that is attractive since it better emulates coasting. So much for irrelevant.
Spin bikes don't have rolling wheels, something you still don't understand yet. You are arguing just to argue when the math has been presented to you.
Unlike a spin bike, MTBs don't have a great deal of "flywheel effect". A trail bike has perhaps 5% of its inertia in the wheels, only half of that due to "flywheel effect". A spin bike has no inertia associated with the rider's weight so it makes it up bu exaggerating the weight on the rims.
Originally Posted by serious
2) of course mass is all that matters, but in the real world, a larger diameter bike wheel will have more mass. But I regret mentioning diameter now.
Tell that to the guy that wrote post #47:
Meltingfeather: A 29er wheel and a 26" wheel of the same mass will not accelerate any differently in terms of actual effort
Wrong! The difference in effort is similar to 2 less teeth on a sprocket. As I said, the farther the mass is from the center, the harder it is to accelerate.
What you should be regretting is saying something so wrong and then trying to defend it.
There are plenty of real world examples of 29" wheels that aren't heavier than many 26" wheels. When of identical construction the difference will only be 10%, and a 10% increase in 5% of total inertia is trivial anyway.
1) when spin bikes talk about their superior flywheel, they basically mention the weight. So flywheel effect is very real. Except for spin bikes it is the moment of inertia a heavier wheel maintains that is attractive since it better emulates coasting. So much for irrelevant.
spin bikes?!?
lol
now i know you're trolling.
Last edited by meltingfeather; 04-26-2012 at 08:02 AM.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
craigsj: Spin bikes don't have rolling wheels, something you still don't understand yet.
OMG, can you stretch you mind a little bit (same for you meltingfeather)? What is the difference between the friction created by a flywheel and brake pad and a normal wheel and ground (other than a different coefficient of friction, of course). A wheel does not have to actually roll on the ground to comply with the laws of physics.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
craigsj: Spin bikes don't have rolling wheels, something you still don't understand yet.
OMG, can you stretch you mind a little bit (same for you meltingfeather)?
More than you, I suspect, but I'm not interested in changing the subject.
Originally Posted by serious
What is the difference between the friction created by a flywheel and brake pad and a normal wheel and ground (other than a different coefficient of friction, of course). A wheel does not have to actually roll on the ground to comply with the laws of physics.
You continue to demonstrate an unwillingness to even consider the relevant physics. The wheel rolling on the ground is important because it defines a relationship between velocity and angular velocity, not because of any friction with the ground. The spin bike does not have this relationship so it is irrelevant.
A real engineer, when confronted with the simple equations of smilinsteve above, would STFU. It is a simple matter, well documented and understood for longer than any of us have been alive, and you look foolish continuing to argue against it.
The wheel rolling on the ground is important because it defines a relationship between velocity and angular velocity, not because of any friction with the ground. The spin bike does not have this relationship so it is irrelevant.
For the last time, I don't care about the velocity the bike. Only about the rate of change in velocity. We call it acceleration. This is what is greatly affected by additional rotational mass. Same for deceleration of course.
Your 29er zealotry is embarrassing and clouding your judgement too. 29ers don't need that much defense. They just need wheels that are as light as reasonably possible.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
For the last time, I don't care about the velocity the bike. Only about the rate of change in velocity. We call it acceleration. This is what is greatly affected by additional rotational mass. Same for deceleration of course.
Your 29er zealotry is embarrassing and clouding your judgement too. 29ers don't need that much defense. They just need wheels that are as light as reasonably possible.
I have a set of ZTR Race 29er wheels that weigh 1365 gr & Maxxlite 29er tires that weigh 345 gr each, is there a set of 26" wheels/tires that are lighter than that ? Let's say that the 26" set of wheels including tires are the same weight as the 29" & the bikes also weigh the same, which will accelerate the quickest ?
I have a set of ZTR Race 29er wheels that weigh 1365 gr & Maxxlite 29er tires that weigh 345 gr each, is there a set of 26" wheels/tires that are lighter than that ? Let's say that the 26" set of wheels including tires are the same weight as the 29" & the bikes also weigh the same, which will accelerate the quickest ?
It depends on the distribution of the mass, which affects the inertia of the wheel. Additionally, the smaller the wheel, the higher the rolling resistance (all else being equal.)
And if you have a weightweenie 29er wheelset, chances are the 26" version is lighter simply because it requires less material.
I have a set of ZTR Race 29er wheels that weigh 1365 gr & Maxxlite 29er tires that weigh 345 gr each, is there a set of 26" wheels/tires that are lighter than that ? Let's say that the 26" set of wheels including tires are the same weight as the 29" & the bikes also weigh the same, which will accelerate the quickest ?
Wow, that is a light wheel set. I hope the Maxxlite 29 tires will hold up! Of course there is a lighter set. The 26er Maxxlite is 285g and certainly NoTubes can build lighter 26er wheels.
But if there is a 26er set of the same weight then it is a wash. This is the point that the zealots are making, but in the real world 26in wheels can always be made lighter than 29in wheels.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
For the last time, I don't care about the velocity the bike. Only about the rate of change in velocity. We call it acceleration. This is what is greatly affected by additional rotational mass. Same for deceleration of course.
I'll remind you of your post #47 for a third time:
Meltingfeather: A 29er wheel and a 26" wheel of the same mass will not accelerate any differently in terms of actual effort
Wrong! The difference in effort is similar to 2 less teeth on a sprocket. As I said, the farther the mass is from the center, the harder it is to accelerate.
So, first off, there is no "addtional rotational mass". You are, once again, trying to change the argument. You claimed that wheels of equal mass accelerate differently due to their size.
In order for a "rate of change of velocity", which you condescendingly remind us is known as acceleration, to exist there needs to be a corresponding rate of change of angular velocity in the wheels. A larger wheel will have a larger moment of inertia but angular acceleration changes as well. That is why you are wrong. No one cares what the actual velocity is, just so long as you compare apples to apples.
If you would spend one moment looking at the mathematical explanation provided you then you might not continue to look so dumb. Nowhere in the final answer is there a radius term so, as an engineer, you should be able to figure out what that means. All that matters is mass, not diameter, just as MF said in the beginning.
But if there is a 26er set of the same weight then it is a wash. This is the point that the zealots are making, but in the real world 26in wheels can always be made lighter than 29in wheels.
Glad you finally came around.
Happy to be labeled a "right answer zealot" by people like you.
It depends on the distribution of the mass, which affects the inertia of the wheel...
Sorry, but it doesn't. That's what meltingfeather, craigsj and others have been saying. You can have a 29" wheel with a tungsten rim and a magnesium hub, and a 26" wheel with a magnesium rim and a tungsten hub. If they weigh the same and the wheel isn't slipping the energy needed to accelerate them is identical. The distribution of the mass has no effect.
Sorry, but it doesn't. That's what meltingfeather, craigsj and others have been saying. You can have a 29" wheel with a tungsten rim and a magnesium hub, and a 26" wheel with a magnesium rim and a tungsten hub. If they weigh the same and the wheel isn't slipping the energy needed to accelerate them is identical. The distribution of the mass has no effect.
The moment of inertia varies depending on where the mass is distributed. While this is likely negligible for most bike wheels, it is easily measurable in a lab setting.
From wiki:
"The further out the object's mass is, the more rotational inertia the object has, and the more rotational force (torque, the force multiplied by its distance from the axis of rotation) is required to change its rotation rate."
Going back to craigsj's post showing smilinsteve's equations, read the last sentence:
"The size of the wheel doesn’t matter assuming the distribution of the mass of the wheel stays the same ( a different distribution of mass would change the moment of intertia formula and therefore the result)."
There's a very straightforward and simple explanation found here:
All of the explanations provided in this thread were based on the assumption of constant velocity, equivalent mass, and equivalent mass distribution between the 26" and 29" wheels.
craigsj: If you would spend one moment looking at the mathematical explanation provided you then you might not continue to look so dumb. Nowhere in the final answer is there a radius term so, as an engineer, you should be able to figure out what that means.
That is because those calculations ignore the friction where the wheel touches the ground. If that factor was included (as it is in the real world), then radius absolutely matters.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
The moment of inertia varies depending on where the mass is distributed. While this is likely negligible for most bike wheels, it is easily measurable in a lab setting.
From wiki:
"The further out the object's mass is, the more rotational inertia the object has, and the more rotational force (torque, the force multiplied by its distance from the axis of rotation) is required to change its rotation rate."
Going back to craigsj's post showing smilinsteve's equations, read the last sentence:
"The size of the wheel doesn’t matter assuming the distribution of the mass of the wheel stays the same ( a different distribution of mass would change the moment of intertia formula and therefore the result)."
There's a very straightforward and simple explanation found here:
All of the explanations provided in this thread were based on the assumption of constant velocity, equivalent mass, and equivalent mass distribution between the 26" and 29" wheels.
All of the explanations provided in this thread were based on the assumption of constant velocity, equivalent mass, and equivalent mass distribution between the 26" and 29" wheels.
The part you were going to show is how different the MoI constants are for 29er & 26" mtb wheels, or how such a difference could even be reasoned to exist.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
the part you were going to show is how different the moi constants are for 29er & 26" mtb wheels, or how such a difference could even be reasoned to exist. :d
either the mass is equal or the rim and tire specs are equal... they are mutually exclusive.
i think it's pretty obvious that the same spec wheel in 29er will weigh more than the 26" version... i forget when humans typically learn that, but it's in the single digits, age wise, I think.
my point is that talk of moment of inertia and "flywheel effect" is irrelevant.
my goal is to help dispel the myth that 29er wheels are somehow harder to accelerate due to diameter. mass is all that matters.
There's some tire models that have a 26 inch model and a roughly equivalent width 29 inch model and the same for rims. The 29 inch version of either will always have more mass. Spokes will be longer for a similar gauge and lacing pattern in a 29 inch wheel thus heavier, as will the inner tube (if used) or the quantity of sealant and the rim tape. For a given width the 29 inch tire/wheel will always have more mass.
So it seems you are describing a distinction without a difference when complaining about the follies of "moment of inertia" and "flywheel effect" as the 29er will always have more mass unless you downsize everything. Then if you do you'll find yourself on a cyclocross tire/wheel not a 29 incher wheel. I'm one of those loons though that doesn't have a problem with a wheel/tire assembly with more mass or even a bicycle with (gawd forbid) long chainstays.
Ok, not ALL of you are right, but none of you are going to admit that you are wrong. I just know by feel that my 29er, while roughly 4lbs lighter than my all mountain 26er feels like it accelerates at about the same rate, with racier tires and less suspension. Obviously that weight difference is spread throughout the bike, not just in the wheels/tires.
My advice: Go RIDE your bikes.
Last edited by thrasher_s; 04-27-2012 at 10:42 AM.
That is because those calculations ignore the friction where the wheel touches the ground. If that factor was included (as it is in the real world), then radius absolutely matters.
Now you're speaking in absolute nonsense.
What is the speed of the tire relative to the ground where the tire contacts the ground? That's right, it's zero. How much friction is there? How could it possible matter regardless? You are an idiot.
So it seems you are describing a distinction without a difference when complaining about the follies of "moment of inertia" and "flywheel effect" as the 29er will always have more mass unless you downsize everything.
Not at all. There is a real difference.
If moment of inertia did matter as people mistakenly believe, rotating inertia would increase with the cube of radius once mass is factored in, not just proportional to radius (due to increase in weight). That means people think inertia increases 33% when, in fact, it is only 10%. It is smaller than people think either way but it is not a "distinction without a difference".
The first myth is that 29ers are slower to accelerate because of the larger wheels. That is demonstrably false. The second myth is that the first myth is true because of the moment of inertia of the larger wheel. That is a total physics falsehood.
There's some tire models that have a 26 inch model and a roughly equivalent width 29 inch model and the same for rims. The 29 inch version of either will always have more mass. Spokes will be longer for a similar gauge and lacing pattern in a 29 inch wheel thus heavier, as will the inner tube (if used) or the quantity of sealant and the rim tape. For a given width the 29 inch tire/wheel will always have more mass.
So it seems you are describing a distinction without a difference when complaining about the follies of "moment of inertia" and "flywheel effect" as the 29er will always have more mass unless you downsize everything. Then if you do you'll find yourself on a cyclocross tire/wheel not a 29 incher wheel. I'm one of those loons though that doesn't have a problem with a wheel/tire assembly with more mass or even a bicycle with (gawd forbid) long chainstays.
all i'm saying is that those terms, which i'm tired of typing, are irrelevant.
talk grams all you want. there is a prevalent misconception that the phenomena you describe play a role (intrinsically tied to diameter) when they do not.
i ride what feels good to me. period.
i discuss the physics of bicycles because i'm an engineer with a penchant for details and there are a lot of discussions both here and in cycling circles across the world that include ridiculous and factually inaccurate treatment of "physics."
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
my point is that talk of moment of inertia and "flywheel effect" is irrelevant.
my goal is to help dispel the myth that 29er wheels are somehow harder to accelerate due to diameter. mass is all that matters.
The bike's builder never mentions moment of inertia or the flywheel effect - only that 29ers are more accelerationally (there's that word again) challenged, most likely as a result of the extra mass that's typically associated with a larger tire/rim/etc.
If you did not take the author's comments that way I understand your taking issue with him. However, I'm pretty sure you two are on the same page.
I didn't mean to offend any non-ripped racers or stamp collectors! Being a weight weenie is a completely harmless hobby (unless you consider harm to one's bank account). But, with the exception of your LBS, most people don't think highly of someone who sits on a $10k bike and doesn't know how to use it. It may not be fair but that doesn't make it less true.
You learn something everyday! I thought that BMI and body fat percentage are the same but I stand corrected. I think a 6' rider with 8 % body fat would be around 158# or so. Skinny but able to stand. But we digress...
he he... Im 6,1 witn a body fat under 8% and 185lbs
The bike's builder never mentions moment of inertia or the flywheel effect - only that 29ers are more accelerationally (there's that word again) challenged, most likely as a result of the extra mass that's typically associated with a larger tire/rim/etc.
Sorry, but this is wrong. The author clearly believes that poorer acceleration is an inherent trait of 29ers. He is a weight weenie so if it were simply about weight, he wouldn't bother to make a point of the wheel size.
29″ers are terrific, everyone here knows this, but they also have weaknesses such as the poorer acceleration. And I wanted to deal with this on my specific design bike. For this reason, the focus first of all has been on the construction of the wheels. Here I wanted to build a wheel set with the lowest possible inertia for good acceleration,...
For the spokes it had to be the Superspoke by Sapim / Tune, because the rotating mass had to be so small.
29ers do not have poorer acceleration, heavy wheels do. The author doesn't understand this and neither do a lot of people here.
If you were to talke 2 identical 26" bikes and secretly add 10% mass to the rims of one of them, I am quite confident that few, if any, riders could tell the extra mass in a blind test, yet if you did the same test substituting a 29" wheelset, most everyone could tell the difference. Part of that is that it couldn't be blind, but most of it would be the change in steering and handling. They feel a difference, see a difference, then convince themselves that the larger wheels must be slower because they think they will be. The difference is far less than 1%, an order of magnitude below what people are typically able to notice. It is a myth born out of people's naive imaginations and sustained by their refusal to consider basic facts. Facts are the enemy of opinions; that's why they are so resisted here.
P.S. Just to make the point more clear, I'll add this. As I've said before, the additional inertia added by the larger wheel size is about 0.5%. The additional effort to steer the front wheel, though, is 33%. A rider sees a larger wheel, feels the extra effort required to steer the bike, then assumes the wheels are slower to accelerate. Bad assumption.
What is the speed of the tire relative to the ground where the tire contacts the ground? That's right, it's zero. How much friction is there? How could it possible matter regardless? You are an idiot.
Hey genius, that friction between tire and the ground, created by a rider on a bike, is the equivalent of adding mass to a tire. The larger the tire radius, the harder it will be to accelerate it when turned by a sprocket connected to a crank. Again focus on acceleration, not velocity.
You keep talking about the fictional tire that does not have any weight on it and does not have any friction when you want to roll it. In addition you talk about wheels that keep the same mass as they get larger. All abstract BS to satisfy those formulas where radius gets cancelled out. Guess what, that never happens in a real situation.
You do not have the capacity to understand basic concepts or you are too stubborn. Either way, it is sad.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Hey genius, that friction between tire and the ground, created by a rider on a bike, is the equivalent of adding mass to a tire.
Removing all doubt, aren't you?
There is no friction between the tire and the ground unless the tire is slipping. Even in the event there is slippage, it is in no way "the equivalent of adding mass to a tire". Friction results in the loss of energy, mass results in the storage of energy.
Originally Posted by serious
The larger the tire radius, the harder it will be to accelerate it when turned by a sprocket connected to a crank. Again focus on acceleration, not velocity.
Not that the equivalent mass exists, because it doesn't, but the extra radius would have no effect on it for reasons already discussed ad nauseum. You simply don't understand and apparently never will. Inertia of a rolling wheel is NOT impacted by its radius, period.
Originally Posted by serious
You keep talking about the fictional tire that does not have any weight on it and does not have any friction when you want to roll it. In addition you talk about wheels that keep the same mass as they get larger. All abstract BS to satisfy those formulas where radius gets cancelled out. Guess what, that never happens in a real situation.
Don't know what you mean by a tyre having no weight on it.
Tires don't have friction when you "want to roll it".
Wheels don't keep the same mass as they get larger, but larger wheels can have the same mass as smaller ones.
Those "formulas" are physical laws, and that BS you refer to are hypotheticals that help you understand what the laws mean.
Originally Posted by serious
You do not have the capacity to understand basic concepts or you are too stubborn. Either way, it is sad.
Look who's talking. You can't even tell the difference between a moment of inertia and a gear ratio.
The bike's builder never mentions moment of inertia or the flywheel effect - only that 29ers are more accelerationally (there's that word again) challenged, most likely as a result of the extra mass that's typically associated with a larger tire/rim/etc.
Since you're "not exactly sure what moment of inertia is," (your words), maybe that's why you didn't understand his comments in that context. It is clearly what he is talking about (e.g., "inertia" & "rotating mass" are dead giveaways).
Originally Posted by Durga
If you did not take the author's comments that way I understand your taking issue with him. However, I'm pretty sure you two are on the same page.
I did take issue with his BS about rotating mass and inertia... then you asked me to explain and I did. We are not on the same page because he is confused about basic physics while being the one that brought it up.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
There is no friction between the tire and the ground unless the tire is slipping
Bahahaha. No wonder you are clueless. Static friction mean anything to you?
Don't know what you mean by a tyre having no weight on it.
Your precious equations, DO NOT reflect a weighted tire. That is why they assume "no friction". But in your world there is no friction. Bahahaha.
Tires don't have friction when you "want to roll it".
Hop on your bike dumbass. What do you think is the resistance you feel at the crank? Bahahaha.
Now get off the internet go home and experiment with riding around with wheels of different sizes but same weight. But if you cannot find such wheels here is the equivalent: CHANGE THE ****ING GEARS to experience the SAME EFFECT. And for christ's sake, don't just roll around. We are not arguing about moment of inertia any more. ACCELERATE the ****ing bike to see what we are talking about.
Maybe I do need to get laid.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
There is no friction between the tire and the ground unless the tire is slipping
Bahahaha. No wonder you are clueless. Static friction mean anything to you?
Static friction and friction are not the same thing. From Wikipedia:
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
No relative motion, no friction. I am eager to hear how static friction effects kinetic energy, though. By all means...
Originally Posted by serious
Don't know what you mean by a tyre having no weight on it.
Your precious equations, DO NOT reflect a weighted tire. That is why they assume "no friction". But in your world there is no friction. Bahahaha.
They aren't my equations, they are THE equations which were presented by others, as a matter of fact. Please, though, elaborate on how a weighted tire with static friction requires more work to accelerate. This should be good.
Originally Posted by serious
Tires don't have friction when you "want to roll it".
Hop on your bike dumbass. What do you think is the resistance you feel at the crank? Bahahaha.
It is my mass trying to remain at rest, and you call yourself an engineer.
Originally Posted by serious
Now get off the internet go home and experiment with riding around with wheels of different sizes but same weight. But if you cannot find such wheels here is the equivalent: CHANGE THE ****ING GEARS to experience the SAME EFFECT. And for christ's sake, don't just roll around. We are not arguing about moment of inertia any more. ACCELERATE the ****ing bike to see what we are talking about.
Once again you prove you don't understand the difference between gearing and inertia. It's really pathetic considering your overblown ego.
There is only one argument here, the effect on wheel size on inertia. It's clear, even to you, that you can't win that argument and I'm not interested in your moronic changes of subject. I am not motivated to sort through the misunderstandings of someone unwilling to learn.
first inertia & mass, then gearing, then diameter... no wait OK mass again, then friction...
what next? unicorns?!?
let's not forget SPIN BIKES!!!
Originally Posted by serious
Maybe I do need to get laid.
no doubt
you are confused. you are wrong.
i personally think you are trolling at this point, based just on how fast you change the subject and spew irrelevant information.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
The guy who hasn't taken physics since junior year of high school gets what Craig and MF are saying.
The entire system, as I imagine it, is a function of translating the force of your foot on the pedal to the tire pushing back against the ground. The lever arm of a larger wheel would change that force. Your "two teeth" statement, Serious, about the difference between 26ers and 29ers eliminates that variable, leaving a direct comparison.
Now, I see where Serious is going wrong. I think he's still imagining the wheels floating in space. There, the only thing a rider needs to overcome to get a wheel spinning is the tendency for an object to stay at rest. In that situation, I can imaging the wheels accelerating differently, although I'm not sure I'd be able to feel it.
However, going back to the "two teeth" argument, if the same force on the pedals generates the same force at the point where the tire contacts the ground, regardless of wheel size, the bike (with rider) will accelerate forward at the same rate. As riders, we're not trying to overcome the tendency for our wheels not to spin. We're trying to overcome the tendency for our entire system - bike, rider, pack, the morning's pizza/beer - to stay at rest. A heavier setup will require more force, but it's not the wheel size that makes the difference. It's the number of slices you ate and beers you drank (the overall system weight).
After the last time we discussed this (ad nauseum), being averse to mashing through a bunch of equations I remember I went down in my basement and made up a 26er rim with some screws in the holes to weigh the same as a 29er rim. I then rolled them down a ramp side by side and they accelerated at exactly the same rate. I did it at least 20 times. The radius had no impact on acceleration of the rolling wheels - only the mass mattered.
I'm not mentioning any names, but there's def. some mis-information trickling through this thread. It would be nice if it stopped. Seriously.
However, going back to the "two teeth" argument, if the same force on the pedals generates the same force at the point where the tire contacts the ground, regardless of wheel size, the bike (with rider) will accelerate forward at the same rate. As riders, we're not trying to overcome the tendency for our wheels not to spin. We're trying to overcome the tendency for our entire system - bike, rider, pack, the morning's pizza/beer - to stay at rest. A heavier setup will require more force, but it's not the wheel size that makes the difference. It's the number of slices you ate and beers you drank (the overall system weight).
Yes, this.
Weight in the tire and rim is a little more important because it has to move along with the bike AND it has to rotate, but the vast majority of the effort goes to move your body, about 95% for a rider of around 170 pounds.
So basically speaking. If I have a wheel set where I have a light rim and heavier hubs it will accelerate the same as a wheel with a heavier rim and lighter hub if both wheels have the same mass? This is contrary to everything I have ever been taught about wheels and rotational weight. I always thought that the lighter the rotational weight at the outer edge of the rim will accelerate faster than if the lighter weight is focused at the center of the wheel. Now I am not an engineer nor do I possess any sort of technical degree so now I am just confused as hell. Fun to read though.
So basically speaking. If I have a wheel set where I have a light rim and heavier hubs it will accelerate the same as a wheel with a heavier rim and lighter hub if both wheels have the same mass? This is contrary to everything I have ever been taught about wheels and rotational weight. I always thought that the lighter the rotational weight at the outer edge of the rim will accelerate faster than if the lighter weight is focused at the center of the wheel. Now I am not an engineer nor do I possess any sort of technical degree so now I am just confused as hell. Fun to read though.
True if the wheel is not on the ground. The placement of the mass makes a difference if the wheel is spinning in air. The ground contact changes everything.
So basically speaking. If I have a wheel set where I have a light rim and heavier hubs it will accelerate the same as a wheel with a heavier rim and lighter hub if both wheels have the same mass? This is contrary to everything I have ever been taught about wheels and rotational weight. I always thought that the lighter the rotational weight at the outer edge of the rim will accelerate faster than if the lighter weight is focused at the center of the wheel. Now I am not an engineer nor do I possess any sort of technical degree so now I am just confused as hell. Fun to read though.
No, they will not accelerate the same. The closer the wheel's weight is to the axis of rotation, the faster it will accelerate.
Let's say you have two cylinders of equal radius and mass, one of them solid and one of them hollow. If you roll them down a frictionless incline, the solid cylinder will reach the bottom first due to a different moment of inertia. (I = Mb^2 vs I = 1/2Mb^2)
This discussion is getting pretty funny. Sex life of physicists and unicorns!
In real life I think none of this really matters, especially for the vast majority of normal riders.
If you want to intuit the the effect of mass distribution on a spinning body then think of the spinning ice skater who tucks and starts spinning faster. Same amount of stored energy but by redistributing her mass more towards the center of rotation she spins faster. Ergo if she wanted to spin at the tucked speed but with her arms extended she would need to add energy to accomplish that.
Without worrying about different sized wheels, use the above example to convince yourself that redistributing mass on a given wheel while keeping the wheel size the same changes the amount of stored energy for a given rotational speed. The maximum stored energy occurs when all of the wheel's mass is located at the rim. It should be obvious that stored energy relates to rider effort and torque.
I will probably be sorry for not leaving this alone....
No, they will not accelerate the same. The closer the wheel's weight is to the axis of rotation, the faster it will accelerate.
Let's say you have two cylinders of equal radius and mass, one of them solid and one of them hollow. If you roll them down a frictionless incline, the solid cylinder will reach the bottom first due to a different moment of inertia. (I = Mb^2 vs I = 1/2Mb^2)
That's a false comparison. The force acting upon a cylinder, rolling down a hill without slipping, is a friction force acting on the outer surface of the cylinder. That's an extrinsic retarding force in opposition to gravity. We're talking about intrinsic, accelerating forces that the rider is applying to the cranks. It's completely different.
If you want to intuit the the effect of mass distribution on a spinning body then think of the spinning ice skater who tucks and starts spinning faster. Same amount of stored energy but by redistributing her mass more towards the center of rotation she spins faster. Ergo if she wanted to spin at the tucked speed but with her arms extended she would need to add energy to accomplish that.
It is not analogous to a wheel because the spinning body has no translational movement related to radius, but even this example can be used to make the point.
the energy is the same in the two cases.
small radius = spin fast... large radius = spin slower... same energy.
now relate the difference in spin rate to the difference between a small wheel and a large wheel moving along the ground at the same speed.
small radius = spin fast... large radius = spin slower... same energy.
Originally Posted by borabora
Without worrying about different sized wheels use the above example to convince yourself that redistributing mass on a given wheel while keeping the wheel size the same changes the amount of stored energy for a given rotational speed. The maximum stored energy occurs when all of the wheel's mass is located at the rim. It should be obvious that stored energy relates to rider effort and torque.
wheel size is the whole point.
i just realized that you're on a tangent due to the confusion in the thread (i think).
Last edited by meltingfeather; 04-27-2012 at 11:38 AM.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
That's a false comparison. The force acting upon a cylinder, rolling down a hill without slipping, is a friction force acting on the outer surface of the cylinder. That's an extrinsic retarding force in opposition to gravity. We're talking about intrinsic, accelerating forces that the rider is applying to the cranks. It's completely different.
I have to agree with jtmartino and borabora regarding the hub question. It appears to me that moving the mass of the hub outward will decrease the wheels angular velocity and translational velocity. I see no offsetting benefit.
if all a 29er wheel did was raise the gearing "2 teeth" you just gear down the cassette(already been done) and you're equal. Equal mass= strength would be lovely but make a 29 inch wheel the same mass as a 26er and i'll bend it easier, make a tire the same mass and i pinch it easier , make the frame or fork that holds the wheel the same mass and i'll snap it easier. Top it off 29 inch wheels increase wheelbase which makes the bike harder to quickly lift up and over stuff.Little stuff that a 29er can rollover easier are of no concern to me. A lot of the things that 29ers don't do as well are not things you have to do to get down the trail efficiently, a lot of the limitations are inefficient things that are fun to do that i want to do while riding down the trail.
Also, it doesn't matter where the force comes from. We're talking wheels here. The same principles would apply if both wheels were on bikes instead of rolling down a hill. We are not talking about the input force, but the resistance to change that affects acceleration.
If both wheels (cylinders) were on a bike, the solid one would still accelerate faster.
This is because the moment of inertia constant, defined as k, varies depending on the object's shape and mass distribution. This is what meltingfeather wanted me to show calculations for earlier.
So basically speaking. If I have a wheel set where I have a light rim and heavier hubs it will accelerate the same as a wheel with a heavier rim and lighter hub if both wheels have the same mass? This is contrary to everything I have ever been taught about wheels and rotational weight. I always thought that the lighter the rotational weight at the outer edge of the rim will accelerate faster than if the lighter weight is focused at the center of the wheel. Now I am not an engineer nor do I possess any sort of technical degree so now I am just confused as hell. Fun to read though.
Hub weight doesn't really matter. Spokes do some but they are light. What really matters is the rim and tire. With real parts, the variation probably isn't too much though.
So basically speaking. If I have a wheel set where I have a light rim and heavier hubs it will accelerate the same as a wheel with a heavier rim and lighter hub if both wheels have the same mass?
No... that's not the point at all.
Originally Posted by jarwes
This is contrary to everything I have ever been taught about wheels and rotational weight. I always thought that the lighter the rotational weight at the outer edge of the rim will accelerate faster than if the lighter weight is focused at the center of the wheel. Now I am not an engineer nor do I possess any sort of technical degree so now I am just confused as hell. Fun to read though.
I think this is a point of confusion. Bike wheels generally have a very tight range of mass distribution... even across different types of wheels. To make an argument that a 29er requires more energy, you would have to demonstrate that it has a significantly different MoI CONSTANT from a 26" wheel, which is not the case. The constant is the number that comes before the mr^2 in the MoI equation. From the picture below, you can see that different shapes have different MoI constants.
For bike wheels, it comes down to somewhere between a hoop (1) and a solid disc (0.5).
So any valid argument that is attempting to conclude that a 29er wheel of the same mass as a 26" wheel requires more energy to accelerate would have to go after the MoI constant. Notice that the constant is separate from radius and mass, which are accounted for. So you can't just say, "a 29er wheel has a different mass distribution because the weight is further out." That part is what falls away when you consider energy required.
If you could somehow show that a 29er wheel MoI is 0.78mr^2 while a 26" wheel MoI is 0.75mr^2, you would have a theoretical case that would then fall on it's face when you compare the differences in quantitative terms.
All of the arguments that have been attempted so far are invalid.
Last edited by meltingfeather; 04-27-2012 at 01:29 PM.
Originally Posted by pvd
Time to stop believing the hype and start doing some science.
When you are going to explain how all those equations relate to a rider accelerating on a bike, we can talk. Until then, here is a simple explanation that jarwes might be able to better relate to:
Wheel size: 20 inch (recumbent
Chainring: 53
Sproket: 16
Gear Inches: 60.6 (one turn of the cranks)
Wheel size: 700c (29er)
Chainring: 53
Sproket: 16
Gear Inches: 87.1 (one turn of the cranks)
Lets assume that the wheels are equal in weight. Which will require more torque to accelerate? And I am not trying to change the subject. I am trying to stop the stupidity of your MoI which is irrelevant to real world riding. We got that!
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Hell, I don't even get it at this point. I'm still stuck on that stupid MOI thing, and I can't even calculate the constant difference between 26" and 29"!
Bringing gearing, friction, rider weight, weather conditions, etc. is crazy talk at this point.
If nothing else, people should take away one thing stated by both you and craigsj - mass matters, not wheel size.
This thread is really great and all but does this inertia stuff is great, but I have a more fundamental question about this "fully trail worthy show bike".
1. Does anyone think this bike looks "fun" to ride?
To me, the only thing that would make that bike more "fun" are Stan's the crow tires
Hell, I don't even get it at this point. I'm still stuck on that stupid MOI thing, and I can't even calculate the constant difference between 26" and 29"!
Haha. I've done these calculations but it was a few years ago and I discarded the spreadsheet. The results were pretty consistent from 0.70 - 0.74 even including a road wheel. Obviously road wheels can vary a lot but that's not important here. For MTB wheels, you typically always have the same type of construction with a fairly heavy tire and rim of similar proportions. It doesn't change much.
To do this calculation, you need to identify each part used in the wheel (spokes are one part collectively and so are nipples), estimate the distance from the center of each mass to the axle, multiple the distance by the mass of the part, and sum all those together. Finally, divide the result by the total mass. You will get a value between 0 and 1 if you do it right.
This thread is really great and all but does this inertia stuff is great, but I have a more fundamental question about this "fully trail worthy show bike".
1. Does anyone think this bike looks "fun" to ride?
To me, the only thing that would make that bike more "fun" are Stan's the crow tires
I think you should ask this back in the original thread. A bike of that weight is an achievement but it doesn't interest me.
how stupid are you man? The gearing is constant so if ignore it. The point is clear, the situation real and the fact you cannot even admit what will be the result is pathetic.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Wheel size: 20 inch (recumbent
Chainring: 53
Sproket: 16
Gear Inches: 60.6 (one turn of the cranks)
Wheel size: 700c (29er)
Chainring: 53
Sproket: 16
Gear Inches: 87.1 (one turn of the cranks)
Lets assume that the wheels are equal in weight. Which will require more torque to accelerate? And I am not trying to change the subject. I am trying to stop the stupidity of your MoI which is irrelevant to real world riding. We got that!
Originally Posted by serious
how stupid are you man? The gearing is constant so if ignore it.
Rarely do you see someone here so willingly stupid. You pose a question where the only difference in two examples is gear inches and then claim that gearing is constant so it should be ignored. Once again, you can't even understand what gearing is.
You really should stop posting. Not just here but in general.
Rarely do you see someone here so willingly stupid. You pose a question where the only difference in two examples is gear inches and then claim that gearing is constant so it should be ignored. Once again, you can't even understand what gearing is.
You really should stop posting. Not just here but in general.
Stop posting in general? Really? Read on and stop being such a little girl.
Strictly speaking the gearing in both cases is the same (53:16). The difference is the radius of the wheel. My whole point in this discussion is that the size of the wheel is another way to change relative "gearing". This is why a larger wheel will ALWAYS be harder to turn if everything else is equal. Not to mention that it will inevitably carry more weight too.
The fact that you guys focused so hard on MoI trying to convince everyone that wheel size makes no difference is completely misleading. You disconnected your conceptual wheel from bike, from rider, from the ground to make your point but could not relate it to actual, real world bike riding.Congratulations, well done.
My rides:
Lynskey Ti Pro29 SL singlespeed
KHS Team 29
S-Works Roubaix SL3 Dura Ace
KHS CX 550 cyclocross
Strictly speaking the gearing in both cases is the same (53:16).
No it is not, "strictly speaking" or otherwise. The gearing of a bicycle involves more than just the gears, that's why the "gear inches" in your example show a difference. As I already said, all this proves is how ignorant you are.
My whole point in this discussion is that the size of the wheel is another way to change relative "gearing".
Your most rapid flip flop yet. Now the wheel is part of "relative gearing", a term you invented so you could switch to the right side of this argument.
Gearing and inertia are unrelated as has been said many times. Gearing can be changed, that's what gears are for. Gearing is not what people complain about when they say 29ers are slow to accelerate.
Originally Posted by serious
The fact that you guys focused so hard on MoI trying to convince everyone that wheel size makes no difference is completely misleading.
No one ever said size "makes no difference". Size does not factor into the rotating inertia of the wheel.
craigsj, your feigned indignation at my posts is hilarious, which is why I keep toying with you.
And there is the explicit admission of your true motivations.
I sincerely hope others keep this in mind every time they see future garbage you post. You offer no understanding, insight, OR respectable motivations. You are what's wrong with online forums.