OK... light wheels vs heavy wheels

[Tom]

So, I got a co-worker who thinks heavy wheels oughta be real good cause while it's hard to accelerate them, once you're up to speed you aren't accelerating any more. I'm trying to explain that when you're climbing, you're accelerating all the time, cause gravity's trying to accelerate you backwards.

Any good source of info on how I tell this guy he's a dummy, or do I have to go out and find the calculations on my own?

[Jeremy]

Centripetal Acceleration

If an object is traveling in a circle then it is accelerating, even if its speed is constant. This is because velocity is a vector, and so a change in direction is a change in velocity, and acceleration is the change in velocity per change in time. A turning type of acceleration is a sideways acceleration. Along the circle, this means that the acceleration is along the radius, pointing toward the center of the circle.

I pulled this off a physics website. I did a google search for "circular motion formulas" Lots of stuff came up.

Cheers,

Jeremy

[Bud]

While the centripetal acceleration can be expressed as mass-independent (v^2/r) the force required to achieve that acceleration is mass-dependent (m*v^2/r). Tell him that, yo.

[mosca]

Think of heavier wheels in terms of energy storage, in that it will take extra energy to: a) get the wheel spinning, i.e. flywheel effect, and b) get the wheel up a hill. In the first case, the extra energy you put in is stored as kinetic energy, and in the second case, the extra energy is stored as potential energy. In any case where energy is stored and released, the efficiency will be less than 100%, i.e. some energy will be lost. Probably a very small amount in these cases, but I think a lighter wheel will, technically, be slightly more efficient. I don't think it's necessary to consider acceleration in this case.

Otherwise I say let the guy keep riding his heavy wheels!

[Ti Designs]

So the question becomes "do you want your wheels to accelerate?". Physics people will say one thing, biomechanics people will say something else, cyclists will say a third thing. I'm a cyclist with a degree in physics, and coaching is a long term study of biomechanics - so I should probably shut up now.

But I woun't - sorry. From a physics standpoint, lower rotating weight one the outside of the wheel (it's that R squared thing) will accelerate best. If you take into account the constant changes in speed, it makes a big difference. I canceled my subscription to Cycling Science years ago because while they could do the math, they didn't understand what really happens on the road. Changes in pace mean sudden accelerations, added rotating weight means greater peak loads which means more fatigue - opps, we've gotten into biomechanics!

From a biomechanics standpoint you need to look at what muscles do well and what causes fatigue. They want to push through a load, then relax and get blood flow. Without the ability to relax and get blood flow the muscles quickly stop working (sit against the wall with your knees at 90 degrees for a few minutes, you'll see what I mean). Muscles also have an overload state, higher tension causes tears in the fibers and you feel fatigue (and soreness the next day). So the ideal case is where you can push through a load but never see real high peak torque. If there are rapid changes of pace, lower rotating weight limits the peak torque. climbing counts for lots of changes of pace as the bike speeds up and slows down each pedal stroke. If on the other hand you're doing a flat time trial and you're looking to hold the exact same speed the whole time, rotating weight acts as a flywheel, allowing the rest state for the muscles without a significant loss of energy in the system.


Hmmm, if you don't buy that load of crap, let's just say that expensive wheels always look faster...

[RPS]

So, I got a co-worker who thinks heavy wheels oughta be real good cause while it's hard to accelerate them, once you're up to speed you aren't accelerating any more. I'm trying to explain that when you're climbing, you're accelerating all the time, cause gravity's trying to accelerate you backwards.

Any good source of info on how I tell this guy he's a dummy, or do I have to go out and find the calculations on my own?I don't quite understand what you mean by accelerating all the time. If you start at the bottom of a climb at 10 MPH and hold that speed until you get to the top, how are you or your wheels accelerating; or at least accelerating in a meaningful way that adds to power requirements?

If I spin the front wheel of a bike in place, it is true that any given segment of the rim is constantly accelerating. However, this is meaningless because at the opposite side of the wheel there is another segment accelerating in the opposite direction, canceling out the forces.

[flydhest]

RPS,

The force of gravity. F=MA. Your body and bike are the M. The A comes from overcoming gravity. Gravity accelerates bodies at 9.8 m/s^2 in a fall. The acceleration is reduced on an inclined plane, but it's still there.

[Climb01742]

if i take my shoes and socks off, and use all 20 hand and foot digits, i bet i still can't figure this one out.

[davids]

if i take my shoes and socks off, and use all 20 hand and foot digits, i bet i still can't figure this one out.
Yeah, but you'll be able to convince us to buy either one!

[RPS]

RPS,

The force of gravity. F=MA. Your body and bike are the M. The A comes from overcoming gravity. Gravity accelerates bodies at 9.8 m/s^2 in a fall. The acceleration is reduced on an inclined plane, but it's still there.Sorry, I can't agree; you are taking this out of context. Unless you actually fall, in which case you will go from climbing to having gravity accelerate you until you find pavement (or some other object that will stop you from accelerating further).

[Ti Designs]

Sorry, I can't agree; you are taking this out of context. Unless you actually fall, in which case you will go from climbing to having gravity accelerate you until you find pavement (or some other object that will stop you from accelerating further).


There are some laws you should never break, the law of gravity is one of them. F=MA, we know what the M is, A is SIN(incline)*9.8M/Sec^2. The acceleration is there wether you think you're falling or not - sorry.

[Grant McLean]

If on the other hand you're doing a flat time trial and you're looking to hold the exact same speed the whole time, rotating weight acts as a flywheel, allowing the rest state for the muscles without a significant loss of energy in the system.


The "flywheel" thing came up on weighweenies a while ago too.

In the real world, how much can 100 or 200 grams of additional flywheel weight
help offset the deceleration of a 150-200 lb rider & bike? The fact is that aero drag is by
far the greatest cause of losing speed. (90% at 40 km/h?) I think this flywheel
thing is a red herring, since nobody is going to put 5 or 10 lbs of weight on
their wheels in the real world.

g

[rnhood]

Mostly good points here but, keep in mind that only when all things are otherwise equal will they apply. For instance, the heavier wheeel may be a stiffer wheel and, this will likely overshadow any advantage a few less grams will make, especially on that rear wheel when climbing. So the heavier wheel-set could very well be the more efficient wheels.

[Ti Designs]

The "flywheel" thing came up on weighweenies a while ago too.

In the real world, how much can 100 or 200 grams of additional flywheel weight
help offset the deceleration of a 150-200 lb rider & bike? The fact is that aero drag is by
far the greatest cause of losing speed. (90% at 40 km/h?) I think this flywheel
thing is a red herring, since nobody is going to put 5 or 10 lbs of weight on
their wheels in the real world.


weightweenies - I think the name says it all. One of my customers has a 13 pound bike, I ride a 26 pound fixed gear with tractor tires. He points out ways he can make his bike faster, I wait for him at the top of every hill...

Nobody is going to put 5 or 10 pounds of weight on their wheels? How 'bout the Mavic +/- or the Ambrosio 51,151??? The disk I use on the track is 8 pounds...

[sspielman]

..... I canceled my subscription to Cycling Science years ago because while they could do the math, they didn't understand what really happens on the road.....


I have never heard it summed up better.....you are my new hero.....

[RPS]

There are some laws you should never break, the law of gravity is one of them. F=MA, we know what the M is, A is SIN(incline)*9.8M/Sec^2. The acceleration is there wether you think you're falling or not - sorry.It must be the difference between an engineer and a physicist. According to your definition, if I stand completely still on the side of a 10 percent incline I’m accelerating, right? Whether I’m moving or not doesn’t matter. Whether I’m gaining speed or not also doesn’t matter.

I'll agree that gravity is there whether I'm falling or not, but it doesn't necessarily lead to acceleration (at least not in the context of this wheel-weight discussion).

[Louis]

For the sake of simplicity let's ignore drag, stiffness, handling and non-mass related effects.

If you could input power to the pedals smoothly throughout the pedal stroke then there won't be a accelerating / decelerating cycle per stroke. Either way you look at it, that effect is not terribly important, and only affected by the non-linearities in drag as you go just a little faster when the cranks are at an efficient angle. So let's also ignore that effect.

The bottom line then becomes more mass you have to haul up the hill. The heavier the wheel (and therefore the heavier the bike & rider) the more power you have to produce to maintain a given speed on a grade. If we're talking flats and constant speed, then the wheels could weigh 100 lbs and it wouldn't matter one bit (other than gyroscopic forces that would affect handling). Which is just another way of saying what has already been mentioned above by at least one person.

Louis

[Louis]

According to your definition, if I stand completely still on the side of a 10 percent incline I’m accelerating, right? Whether I’m moving or not doesn’t matter. Whether I’m gaining speed or not also doesn’t matter.

I'll agree that gravity is there whether I'm falling or not, but it doesn't necessarily lead to acceleration (at least not in the context of this wheel-weight discussion).

Gravity results in a force (F=ma) which you have to overcome in order to climb the hill. If the hill were in outer space or on a very small/low density planet gravity would be very small and you could climb the hill with little effort (and no drag !).

[Grant McLean]

weightweenies - I think the name says it all. One of my customers has a 13 pound bike, I ride a 26 pound fixed gear with tractor tires. He points out ways he can make his bike faster, I wait for him at the top of every hill...

Nobody is going to put 5 or 10 pounds of weight on their wheels? How 'bout the Mavic +/- or the Ambrosio 51,151??? The disk I use on the track is 8 pounds...

You've mixed a bunch of logic there.
I don't think you mean that you beat your customer becuase you're heavier,
right? Obviously, a faster rider on a heavier bike can beat a slower rider
on a light bike. What's that got to do with anything?

When i said: "real world" that was code for "ON THE ROAD". Of course there are
conditions where a heavy track disc makes total sense.

Tell you what, strap on that disc, and lets do mt washington next year.
Winner buys beers? (bring your wallet!) :)

g

[ergott]

It must be the difference between an engineer and a physicist. According to your definition, if I stand completely still on the side of a 10 percent incline I’m accelerating, right? Whether I’m moving or not doesn’t matter. Whether I’m gaining speed or not also doesn’t matter.

I'll agree that gravity is there whether I'm falling or not, but it doesn't necessarily lead to acceleration (at least not in the context of this wheel-weight discussion).

The force is still acting on you. You have friction at your aid. If that incline was wet ice you wouldn't be standing. The force of acceleration would overcome the friction.

[orbea65]

I personally believe that the bottom line is that in this application (cycling), the difference is negligiable (sp?).
Same with overall bike weight. It doesn't matter what yer bike weighs, the guy/gal in the best condition is gonna win, heavy wheels or not.

[Grant McLean]

The force is still acting on you. You have friction at your aid. If that incline was wet ice you wouldn't be standing. The force of acceleration would overcome the friction.

look at you mr. science.

someone was paying attention! :)

g

[RPS]

Gravity results in a force (F=ma) which you have to overcome in order to climb the hill. If the hill were in outer space or on a very small/low density planet gravity would be very small and you could climb the hill with little effort (and no drag !).Apparently I'm having a harder-than-normal time communicating today. Did I ever state that gravity would not work against the climber?

Is it not correct to differentiate between gravitational force and acceleration, which is what I've been trying to do? The fact that there is a gravitational force present doesn’t necessarily lead to acceleration.

I merely disagreed with the definition used by Ti Designs. If I stand still on the side of a 10 percent incline, I’m not accelerating. In that case the road acting through my feet pushes up with a force exactly equal to that of gravity’s so that I remain stationary. In my book that is not acceleration.

[Ti Designs]

You've mixed a bunch of logic there.
I don't think you mean that you beat your customer becuase you're heavier,
right? Obviously, a faster rider on a heavier bike can beat a slower rider
on a light bike. What's that got to do with anything?

My point was that weight weenies tend to overlook everything but the one number they can measure with a scale. I don't need to go to their web site to know what their conclusion was on the rotating weight of wheels...


When i said: "real world" that was code for "ON THE ROAD". Of course there are
conditions where a heavy track disc makes total sense.

Tell you what, strap on that disc, and lets do mt washington next year.
Winner buys beers? (bring your wallet!)

I think orbea65 wants to take you up on that one...


If you could input power to the pedals smoothly throughout the pedal stroke then there won't be a accelerating / decelerating cycle per stroke. Either way you look at it, that effect is not terribly important, and only affected by the non-linearities in drag as you go just a little faster when the cranks are at an efficient angle. So let's also ignore that effect.


IF you could input power to the pedals smoothly throughout the pedal stroke THEN we can ignore the effect. Sadly, you can't. Biomechanics do enter into cycling, as much as the math guys and weight weenies want to ignore it.

[Ti Designs]

I merely disagreed with the definition used by Ti Designs. If I stand still on the side of a 10 percent incline, I’m not accelerating. In that case the road acting through my feet pushes up with a force exactly equal to that of gravity’s so that I remain stationary. In my book that is not acceleration.

There is no acceleration, there is no gravity. The earth sucks...

[Grant McLean]

My point was that weight weenies tend to overlook everything but the one number they can measure with a scale. I don't need to go to their web site to know what their conclusion was on the rotating weight of wheels...



no arguement on that from here!!!

:beer:

g

[RPS]

It must be the difference between an engineer and a physicist. According to your definition, if I stand completely still on the side of a 10 percent incline I’m accelerating, right? Whether I’m moving or not doesn’t matter. Whether I’m gaining speed or not also doesn’t matter.

I'll agree that gravity is there whether I'm falling or not, but it doesn't necessarily lead to acceleration (at least not in the context of this wheel-weight discussion).I see part of the problem, and it's my fault. That's what I get for trying to use a tone that would not sound harsh.

What I meant was: It must be the difference between an engineer and a physicist. According to your definition, if I stand completely still on the side of a 10 percent incline I’m accelerating, right? Whether I’m moving or not doesn’t matter, right? Whether I’m gaining speed or not also doesn’t matter, right?

I was trying to communicate a point without sounding sarcastic. Unfortunately I missed on both points. Sorry, English is my second language.

[quehill]

This might not have been true in the past, but nowadays I think there's a huge apples and oranges issue lurking in wheel/rim weights. I've ridden (but sadly don't own) a pair of very light tubular Hyperons and I own and put a ton of miles on a pair of Bontrager Race X-Light Aero aluminum clinchers. The Bontragers are bricks, but because they're heavy at the rim (flywheel effect), super stiff (little energy lost to lateral flex), and aero, I prefer them on any ride where I don't spend a ton of time pointed up-hill.

For most cyclists today, a heavy/heavy rimmed wheel is an aero wheel is a laterally stiff (setting Shimano pre-builts to the side) wheel. The physicists may be able to prove that the flywheel effect doesn't mean much, but I think it's pretty well accepted that aerodynamics is a big deal.

Cheers.

[flydhest]

rps,
The point is, on a bike, you'll roll down if you don't put in force. You'll roll down at an increasing rate. That's the acceleration. There wasn't anything to agree or disagree with.

[mosca]

...when you're climbing, you're accelerating all the time, cause gravity's trying to accelerate you backwards.

I think this is incorrect - if you are riding in a straight line at a constant speed, you are not accelerating, regardless of the slope or the force of gravity. OK, my brain is maxed-out now...

[petitelilpettit]

I think this is incorrect - if you are riding in a straight line at a constant speed, you are not accelerating, regardless of the slope or the force of gravity. OK, my brain is maxed-out now...

You are correct, mosca. When anything is moving at a constant rate, the acceleration is zero. Accelleration only applies when an object is speeding up or slowing down. this is why it is called 'acceleration' and 'deceleration'.

Now on this topic, I agree fully with orbea65; you could have the best wheels on your bicycle, but it is all relevant to the rider. I had an instance where I was riding with one of my buddies, who happens to be about five foot nine inches and has a Scott CR1, so he has a 15lb bike with some lightweight wheels. I decide to challenge him on the ride to a hill climb. I'm six foot three and on a CDA with handbuilts (roughly 19lbs), and I had to ride slowly so he wasn't stranded on the climb. It's all about the motor on the bicycle.

Pettit

[flydhest]

petit and mosca,
While that way of defining things is accurate, the way most people are using the word accelerate is as a transitive verb. Gravity is accelerating you downward and you must acclerate the bicycle upward. On flat ground, wind resistance and rolling resistance are accelerating you backward, so you must accelerate the bicycle forward. When these two accelerations are balanced, the net is zero, so the measured speed relative to the ground is constant and that acceleration is zero. Go forward in a straight line on flat ground, you still have to impart force. That force will equal your mass times your acceleration. Since you know that the force won't be zero, you can conclude that the acceleration won't be either. F=MA.

[manet]

gravity is the same in both instances.

[Louis]

On flat ground, wind resistance and rolling resistance are accelerating you backward

Fly, in my world friction can't accelerate me in any direction. Sometimes I'm very happy that it is there, and when there is ice on the road I really miss it. Other times when it eats up energy that I would rather use getting to my destination I'm bummed, but never has friction actually accelerated me backwards...

Louis

[Jeremy]

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

I think that some people are confusing terms here. The common understanding that accelerate means to speed up, and decelerate means to slow down ignores the external forces trying to slow everything down. Anytime the external forces trying to decelerate your bicycle (gravity, rolling resistance, aerodynamic drag, etc...) exceed the external forces trying to accelerate your bicycle (gravity, wind... unfortunately there aren't very many), the rider must apply force to maintain a given velocity. This force is called acceleration. In the absence of any external forces, once a body is set in motion it will continue in a straight line at the given velocity forever. This doesn't happen in the real world. The force slowing you down (deceleration) must be countered by an equal and opposite force speeding you up (acceleration) to maintain a constant velocity. At any point while riding, if stopping pedaling slows you down, you must be accelerating to maintain your velocity. This is true whether you are climbing or not. I may be wrong here, but I think that I've got it right.

Jeremy

[mosca]

...the way most people are using the word accelerate is as a transitive verb.
I think most people are using the word acceleration when they mean force.
It's all about the motor on the bicycle.
Except in my case, where it's clearly my heavy wheels that are slowing me down...

[manet]

resistance is futile.

[BoulderGeek]

While all of you engineers were working these impressive formulae, there were three nubile female coeds for every male in my English classes.

:-D

It's just sour grapes, as I can't do math to save my life.

[zap]

Additionally, within each pedal stroke, there is acceleration and deceleration.

[mosca]

While all of you engineers were working these impressive formulae, there were three nubile female coeds for every male in my English classes.

:-D

It's just sour grapes, as I can't do math to save my life.

My formidable math skills tell me that you've got more women than you need - think any of those coeds are looking for a middle-aged bald guy in California?

[flydhest]

I think most people are using the word acceleration when they mean force.

Except in my case, where it's clearly my heavy wheels that are slowing me down...


Mosca, no, they were using it not to mean force, but force/mass, or put slightly differently, implicitly they were accounting for the (multiplicative) difference between the two. Like when people talk about weight, but they mean mass. Wow, in fact, that's a perfect example, because it's the reverse sort of. Weight is mass accelerated by gravity, so weight is the force.

Louis, friction . . . suppose you're going at 20 miles an hour. Friction can cause you to slow down, eventually to zero. That's (negative) acceleration. I don't see why friction can't do it? It is, of course, a passive force, but it's there nonetheless.

[RPS]

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

I think that some people are confusing terms here. The common understanding that accelerate means to speed up, and decelerate means to slow down ignores the external forces trying to slow everything down. Anytime the external forces trying to decelerate your bicycle (gravity, rolling resistance, aerodynamic drag, etc...) exceed the external forces trying to accelerate your bicycle (gravity, wind... unfortunately there aren't very many), the rider must apply force to maintain a given velocity. This force is called acceleration. In the absence of any external forces, once a body is set in motion it will continue in a straight line at the given velocity forever. This doesn't happen in the real world. The force slowing you down (deceleration) must be countered by an equal and opposite force speeding you up (acceleration) to maintain a constant velocity. At any point while riding, if stopping pedaling slows you down, you must be accelerating to maintain your velocity. This is true whether you are climbing or not. I may be wrong here, but I think that I've got it right.

JeremyThe first highlighted sentence is completely right; the second is not.

Force and acceleration, although related by physical laws, are different animals. They don’t even have the same units of measure.

Force is quantified by most Americans in pounds (actually pounds force so as not to confuse with pounds mass). Hopefully we won’t go down that road also, which will certainly take this topic further downhill.

Acceleration is the rate of change in velocity as a function of time; and is normally measured by Americans in feet per second per second, or ft/sec/sec.

[Jeremy]

RPS,

I think you don't understand the distinction that many are making. Yes, acceleration is the rate of change in velocity as a function of time. But, if you stop pedaling and start to slow down, then the external forces cause you to decelerate. You must apply force to counter this. That force must be equal to the force slowing you down. This is an accelerating force. Imagine riding at a constant 20 m/ph, then stop pedaling. If you slow down (decelerate), then the moment before you stopped pedaling you must have been accelerating to maintain the constant velocity. If the tendency of a body is to decelerate, then an equal and opposing force must be present to maintain velocity. It is true that force and acceleration are "different animals", but the force I described in the second highlighted sentence is a force equal and opposite to a decelerating force. It must then be an accelerating force.

Jeremy

[mosca]

Mosca, no, they were using it not to mean force, but force/mass, or put slightly differently, implicitly they were accounting for the (multiplicative) difference between the two. Like when people talk about weight, but they mean mass. Wow, in fact, that's a perfect example, because it's the reverse sort of. Weight is mass accelerated by gravity, so weight is the force.

I see what you mean, acceleration can be used to define an amount of force, but it is more commonly used to define a rate of change in velocity. I thought this use of the term acceleration was leading to some confusion earlier in the thread.

[Louis]

Louis, friction . . . suppose you're going at 20 miles an hour. Friction can cause you to slow down, eventually to zero. That's (negative) acceleration. I don't see why friction can't do it? It is, of course, a passive force, but it's there nonetheless.

As long as we agree that friction/drag results in a force which resists your desire to move I think we're fine. When we start to mix forces due to acceleration and forces due to other physical effects we have to be careful about what is accelerating you vs what is applying a force. In terms of cause and effect friction does not apply an acceleration in the same manner that gravity applies an acceleration.

You sum the forces then see what happens when they are all added up. You don't sum accelerations.

Extra-credit: If anyone can explain in a simple-to understand manner what causes gravity without recourse to discussion of warped space and marbles and bowling balls on a rubber sheet they get 10 points. :)

[Jeremy]

In terms of cause and effect friction does not apply an acceleration in the same manner that gravity applies an acceleration.

Isn't this exactly what wind resistance (friction) does in the case of the terminal velocity of a falling object? When the force of friction = the force of gravity, velocity is constant?

Jeremy

[Louis]

Isn't this exactly what wind resistance (friction) does in the case of the terminal velocity of a falling object? When the force of friction = the force of gravity, velocity is constant?

Agreed.

[mosca]

RPS,

I think you don't understand the distinction that many are making. Yes, acceleration is the rate of change in velocity as a function of time. But, if you stop pedaling and start to slow down, then the external forces cause you to decelerate. You must apply force to counter this. That force must be equal to the force slowing you down. This is an accelerating force. Imagine riding at a constant 20 m/ph, then stop pedaling. If you slow down (decelerate), then the moment before you stopped pedaling you must have been accelerating to maintain the constant velocity. If the tendency of a body is to decelerate, then an equal and opposing force must be present to maintain velocity. It is true that force and acceleration are "different animals", but the force I described in the second highlighted sentence is a force equal and opposite to a decelerating force. It must then be an accelerating force.
If your pedaling force is equal (in magnitude and opposite in direction) to the total force of drag, then your velocity will remain constant, therefore I don't think it's clear to define it as an accelerating force, since there is no change in velocity therefore no acceleration.

Sorry for acting like the semantic police today...

[RPS]

RPS,

I think you don't understand the distinction that many are making.
JeremyJeremy, trust me on this -- I understand the distinction being made quite well, but if it is wrong in some cases, I feel that it would not be in anyone's interest to simply agree. Just because many are saying something doesn't make it right, does it? I'm not trying to be difficult -- I'm just trying to share information that is not common to most people who haven't studied it.

Perhaps if you look at it backwards as previously suggested it will make more sense.

A=F/M (i.e. -- acceleration equals force divided by mass).

To be correct, we must look at "THE SUM OF ALL FORCES", not just one or two. We can't focus solely on gravity. It doesn't work that way.

Therefore, if we have a mass of known quantity (bicycle, rider, clothing, water, etc...) and we also know that many forces are applied to that "mass" from many directions (including but not limited to gravity, road, wind, friction, etc...), then we can add them up (keeping the direction of the forces in mind) and then we can estimate the acceleration.

If the sum of the forces is zero, then acceleration is zero, and the rider and bike will proceed at constant velocity. If the sum of the forces is positive, the bike will accelerate. If the sum of the forces is negative the bike will decelerate.

[fierte_poser]

If your pedaling force is equal to the total force of drag, then your velocity will remain constant, therefore I don't think it's clear to define it as an accelerating force, since there is no change in velocity therefore no acceleration.

Sorry for acting like the semantic police today...

+1.

Force is force. I would never describe a force as an accelerating force or a decelerating force. It is simply a measure of, well, force. It also has a direction (vector). Depending on your frame of reference and choice of axes, you might have a positive acceleration or a negative acceleration depending on the direction of the net force vector! :banana:

If the net force (sum of all forces) on a particular object = zero, then, by definition, acceleration = 0. :beer:

[Jeremy]

Yes, I understand all of that. I never tried to focus solely on gravity. Maintaining a given velocity, whether climbing or descending, requires a constant application of force. If F=MA and F>0 then A>0, no? Specifically in the case of climbing at a constant velocity, does not A represent "the sum of all the forces" working against the rider over mass? I think that we are talking in circles.

Jeremy

[manet]

.for robert

[flydhest]

partial derivatives . . . it's how the cool kids communicate.

Louis, if I could do that, I'd be a millionaire writing pop-culture physics.

[Tom]

and if acceleration is defined as a change in velocity over time and velocity is a vector (speed and direction) then any point on the rim is accelerating all the time because it is constantly changing direction not flying off the rim in a straight line... so the wheels are always accelerating and if the more mass the more force, no?

[Bud]

At the risk of sounding like a pedant, I feel like I might have failed society in my 7 years as a high school physics teacher. There are a TON of misconceptions (or "alternative conceptions") floating around in here.

Three very common ones are:
-> Weight=mass (as Fly pointed out)
-> Acceleration is a force
-> An object's speed must be changing for it to have (or experience) an acceleration

We all ride bikes and are therefore heavily influenced by the actual practice of cycling. When trying to relate the theoretical (such as Newton's Laws) it is difficult, especially when our experience contradicts the scientific explanations others are presenting (a classic venue for application of Conceptual Change Theory, but that's another story).

Rather than try to write some elegant explanation which will convince everybody (not possible since I have no idea what type of people you all are and what you bring into this thread), I would like to propose that you consider the acceleration of the wheel itself- that was the direction of the original post, and it might simplify things if we think about only that and not inclines, wind, etc. A wheel is ALWAYS accelerating, even if its speed is constant. Considering this fact, a more massive wheel will require a larger centripetal force than a lighter wheel accelerating (centripetally) at the same rate (angular displacement per unit time^2). (as Tom pointed out).

Remember- physics is phun. :cool:

[ergott]

Let's see if I have this right...

http://upload.wikimedia.org/math/6/e/3/6e306f943fc864e7ee41a1b3a7f16172.png

Let's turn this equation counterclockwise. The acceleration you apply will reach a terminal velocity that is based on your mass and the frictional forces opposing. Since mass is part of the numerator, increasing mass will increase the terminal velocity. Remember you are keeping every other number the same.

However, heavy wheels will take take longer to reach their terminal velocity. They will also require more force to stop so deceleration will take longer as well.

HS physics, while very interesting, was long enough ago to start talking decades;-) I'm probably completely wrong.

[stevep]

get the expensive wheels.
they're better.

[RPS]

A wheel is ALWAYS accelerating, even if its speed is constant. Considering this fact, a more massive wheel will require a larger centripetal force than a lighter wheel accelerating (centripetally) at the same rate (angular displacement per unit time^2). (as Tom pointed out).

Remember- physics is phun. :cool:No doubt this is 100% correct; any segment of a wheel rim is constantly accelerating. Unfortunately, this information is being used in some cases out of context to mean something that it is not.

At constant speed, the larger centripetal forces of a heavier wheel merely place greater forces on components like the rim so that the wheel doesn't fly apart. As a wheel spins at constant speed the rim material is put in tension to counter these centripetal forces. Personally, I don't know why I would care about this unless I was concerned about my wheels flying apart.

Not all forces contribute to rider power requirements. Some do, others don't. We need to know which actually work against the rider, and under what conditions (which may be transient).

[Mikej]

[QUOTE=RPS]No doubt this is 100% correct; any segment of a wheel rim is constantly accelerating. Unfortunately, this information is being used in some cases out of context to mean something that it is not.

Would not the bottom of the wheel not be accelerating (contact patch at road and tire are really going zero mph, unless there is a loss of traction) and the top be moving twice as fast as the center?

[mosca]

and if acceleration is defined as a change in velocity over time and velocity is a vector (speed and direction) then any point on the rim is accelerating all the time because it is constantly changing direction not flying off the rim in a straight line... so the wheels are always accelerating and if the more mass the more force, no?

Yes, if you consider a part of the wheel independently. On a complete wheel, or any object rotating at a constant rate around its center of mass, the sum total of all these forces is zero, therefore, at constant speed, total acceleration for the wheel is zero.

Would not the bottom of the wheel not be accelerating (contact patch at road and tire are really going zero mph, unless there is a loss of traction) and the top be moving twice as fast as the center?

The bottom of the wheel would have a velocity of zero relative to the ground. If you isolate a segment of the wheel, it would have an acceleration (centripetal) due to the rotation around the axle, but again, acceleration for the whole wheel would be zero.

This sort of topic really requires accurate use of terminology, i.e. force, velocity, acceleration should not be considered interchangeable, especially if anyone is still god forbid trying to glean any useful content from this discussion.

[RPS]

[QUOTE=RPS]Would not the bottom of the wheel not be accelerating (contact patch at road and tire are really going zero mph, unless there is a loss of traction) and the top be moving twice as fast as the center?Mike, I'm sorry, I’m not sure I understand the gist of your question.

I think I agreed that any given segment (like where the valve stem is located as an example) is constantly accelerating as the wheel spins at constant speed. That's exactly what accounts for it going from essentially zero MPH at the bottom (relative to the ground) to twice as fast as the rider (and rest of bike) at the top.

However, this change in absolute speed (relative to the ground) of any given segment of the wheel is countered by one that is 180 degrees on the other side of the wheel. Hence, as the valve stem goes from zero speed at the ground to twice as fast as the bike at the top, another segment of equal mass went from twice as fast as the bike at the top to zero speed at the ground. The net of it all is that the rider doesn't see it as a power drain – energy is not consumed (or stored in other forms) solely because of this (provided we are still talking about riding at constant speed).

If I didn't understand the gist of your question, I apologize.