Basic Physics Equations (Bike Design)
05-25-06, 03:53 PM
#26
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Originally Posted by MrB
I agree with dhan in that statics will help you quantify forces within the frame. Actually, I would suggest a basic statics knowledge to anyone in an even slightly technical field. An understanding of it opens up a lot of doors to many engineering principles. If you are good at picking up tech principles without a whole lot of filler, you might want to look at the statics version of Schaums Outlines. Their series is great for crash coursing and great too as a companion to a textbook. Whatever you do, don't get the statics T.B. by R.C. Hibbeler. It will get you nowhere if you try to learn it on your own.
There is a lot missing though with just statics & dynamics. I think if you have a general understanding of statics you should be able to wing alot of the planning aspect of the project with a Computer Aided Engineer software. They can do a ton of calculations with the user almost entirely in the dark and produce simple answers too. If you are fairly good with a computer, I would suggest you look at either SolidWorks or ProEngineer. They are both rather interesting pieces of software but pricey(around 5 grand retail). Engineering computer labs at universities always have a CAE program installed. If you are not in acedemia, go to the engineering dpt. and ask around with some students, but don't look suspicious.(a lot of these student are always looking to make a buck) These labs usually stay open pretty late so you might be able to get done all what you need to do. But if you if you like what you see, find a student or famiy member to buy the academic version(around $200) for you. Many univ. have this software available at their book stores. I wonder if going this route is less illegal than going full out pirating. I know a couple people who have done this. I guess by paying the company something, they think it is so much better.
There is a lot missing though with just statics & dynamics. I think if you have a general understanding of statics you should be able to wing alot of the planning aspect of the project with a Computer Aided Engineer software. They can do a ton of calculations with the user almost entirely in the dark and produce simple answers too. If you are fairly good with a computer, I would suggest you look at either SolidWorks or ProEngineer. They are both rather interesting pieces of software but pricey(around 5 grand retail). Engineering computer labs at universities always have a CAE program installed. If you are not in acedemia, go to the engineering dpt. and ask around with some students, but don't look suspicious.(a lot of these student are always looking to make a buck) These labs usually stay open pretty late so you might be able to get done all what you need to do. But if you if you like what you see, find a student or famiy member to buy the academic version(around $200) for you. Many univ. have this software available at their book stores. I wonder if going this route is less illegal than going full out pirating. I know a couple people who have done this. I guess by paying the company something, they think it is so much better.
Thank you for your advice, but I need help with those specific equations, and as soon as possible, please.
And I am with Mike Burrows in that I will not be using a computer (beyond net research) for my project. It is important to posses knowledge of basic physics rather than complacently rely on a computer, as so many do.
Last edited by Ideologue; 05-26-06 at 01:13 AM.
05-25-06, 04:03 PM
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Originally Posted by mecheng
I concur with the above suggestions that statics knowledge would be helpful.
For one thing, you should also consider the moment balance on the bike, not just force balance. The former should also factor in the inertias of the wheels which will give you a more encompassing picture vis a vis bike performance versus componentry. The problem with your current set of equations is that you're treating the bike as a mass particle. It is not. The bike "system" can (and should) be broken down into a set of elements, such as wheels, fork, frame, drivetrain, saddle, rider, etc. Each of these elements should be analyzed separately (Free body diagrams help A LOT in this). Then you put the components back together. By Newton's 3d law, for every action there is a reaction. So for example, where the front wheel joins the fork there will be a force ON the fork FROM the wheel, and there will be an equal and opposite force FROM the fork ON the the wheel. But when you're factoring in the movement of the bike, you're dealing with dynamics, not statics. I suggest going to your local library and picking up a dynamics book. Alternatively, if you'd like to get your own copy, I would highly reccomend this book. Superbly written.
I'm not sure what exactly you mean by "designing a bike". Are you using a stock frame or are you building your own? If the latter, although you could approximate the tubes as simple beams and work forces at the joints that way, this is hardly an accurate method. Finite Element Analysis (FEA) is more appropriate in such a case, because of the nature of induced stresses in the tubes and rather complicated joint points. This route will almost certainly rely on a software package such as SolidWorks, ProE, or Ansys to name a few. Academic versions are considerably cheaper than "full" versions.
But if you're just putting the bike together from a set of components, I don't see why you would need all the equations. Just build the thing
P.S. I just got my Bachelor's in Mechanical Engineering, so that's why I'm playing a smartass
For one thing, you should also consider the moment balance on the bike, not just force balance. The former should also factor in the inertias of the wheels which will give you a more encompassing picture vis a vis bike performance versus componentry. The problem with your current set of equations is that you're treating the bike as a mass particle. It is not. The bike "system" can (and should) be broken down into a set of elements, such as wheels, fork, frame, drivetrain, saddle, rider, etc. Each of these elements should be analyzed separately (Free body diagrams help A LOT in this). Then you put the components back together. By Newton's 3d law, for every action there is a reaction. So for example, where the front wheel joins the fork there will be a force ON the fork FROM the wheel, and there will be an equal and opposite force FROM the fork ON the the wheel. But when you're factoring in the movement of the bike, you're dealing with dynamics, not statics. I suggest going to your local library and picking up a dynamics book. Alternatively, if you'd like to get your own copy, I would highly reccomend this book. Superbly written.
I'm not sure what exactly you mean by "designing a bike". Are you using a stock frame or are you building your own? If the latter, although you could approximate the tubes as simple beams and work forces at the joints that way, this is hardly an accurate method. Finite Element Analysis (FEA) is more appropriate in such a case, because of the nature of induced stresses in the tubes and rather complicated joint points. This route will almost certainly rely on a software package such as SolidWorks, ProE, or Ansys to name a few. Academic versions are considerably cheaper than "full" versions.
But if you're just putting the bike together from a set of components, I don't see why you would need all the equations. Just build the thing
P.S. I just got my Bachelor's in Mechanical Engineering, so that's why I'm playing a smartass
Thank you for your recommendations.
I am designing this bike from the ground up. I am redesigning many of the components including a new type of derailleur gear changer. I am not using tubes, it is to be a carapace design. As I posted above, it is a power-assist bike (the motor(s) will be removable).
As I also stated above: I regret saying that I need the equations for a bike design (maybe I will make a new thread and omit that information). Gaining clarification on the validity of the formulae I posted within this thread is very important to me. I do not understand why practically no one is addressing them. Opinions are welcome but they are not what I seek.
As you are now a BEng you should easily be able to confirm which of the equations I posted are right (if any) and which are wrong. If you are willing to do that for me I would be very grateful.
Last edited by Ideologue; 05-25-06 at 04:59 PM.
05-25-06, 04:11 PM
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Originally Posted by Avalanche325
Acceleration up an inclined plane is: F = Mgsinθ + MA Gravity is one of the forces involved.
Constant speed up an inclined plane: F = Mgsinθ. Again gravity is one of the forces.
I remember F=CrrgM for rolling resistance. But that is not accelerating. I'm rusty on this stuff.
Constant speed up an inclined plane: F = Mgsinθ. Again gravity is one of the forces.
I remember F=CrrgM for rolling resistance. But that is not accelerating. I'm rusty on this stuff.
Thank you very much for that. I was wondering if gravity ('g') should be used or not in the inclined plane constant speed equation. Some say yes but according to the Newnes Engineer's Reference Book (10th edition 1965) the force to pull an object up an inclined plane (ignoring friction) is: F = W X sinθ. It does not mention 'g' at all. So that is the cause of confusion for me.
05-25-06, 04:15 PM
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Originally Posted by sch
www.analyticcycling.com goes into this from the rider, not so much the bike designer point of view.
They have all this stuff thought out on their site. It is fee based, IIRC. The bike frame forum would be a bit more appropriate for this question and there is a large community of frame builders out there some of whom are quite analytic in their approach and have written their observations up.
Steve
They have all this stuff thought out on their site. It is fee based, IIRC. The bike frame forum would be a bit more appropriate for this question and there is a large community of frame builders out there some of whom are quite analytic in their approach and have written their observations up.
Steve
Thank you. I have seen that site before but did not find it all that helpful. Perhaps I am reading it wrongly?
05-25-06, 04:22 PM
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Originally Posted by Phantoj
Attacking an engineering problem by first assembling a bunch of equations is a poor approach. You really need two equations: ΣF=ma and ΣM=Iα.
The right approach to the problem is to isolate different things and determine just which forces are acting on those things. First, isolate the bike. What forces are acting on the bike as a whole? First consider the locations of those forces:
1. Tire contact patches
2. Seat
3. Hand grips
4. Pedals
You can calculate the magnitude of these forces with the two equations above for several conditions, or "load cases".
Then, you can isolate smaller and smaller parts of the frame and consider and balance the forces acting on that part of the frame for each of the load cases.
... but there are several catches.
The right approach to the problem is to isolate different things and determine just which forces are acting on those things. First, isolate the bike. What forces are acting on the bike as a whole? First consider the locations of those forces:
1. Tire contact patches
2. Seat
3. Hand grips
4. Pedals
You can calculate the magnitude of these forces with the two equations above for several conditions, or "load cases".
Then, you can isolate smaller and smaller parts of the frame and consider and balance the forces acting on that part of the frame for each of the load cases.
... but there are several catches.
I disagree. Going on my prior knowledge and the design work I have created thus far (and various other reasons) these equations are critical to the success of what I am doing. Of course other things are being considered; because I did not post on them here does not mean they are being overlooked.
I am not familiar with this equation: ΣM=Iα. And why did you use the ‘sum of’ symbol? It seems unnecessary to me.
05-25-06, 04:42 PM
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Originally Posted by supcom
With all due respect, if you do not have much knowlege of physics and presumably no knowlege of stress analysis or materials science, I see no possible way for you to accomplish your goal of designing an optimum frame.
With all due respect, pre-conceived notions seem to be dictating your erroneous thoughts. I am having trouble clarifying the very basic equations. I would bet that most bike builders do not even consider the equations I have requested help with. I have even spoken to microlight builders who design aircraft on ‘common sense’ and select materials because they think they will be up to the job! And many of these people are successful.
Material science is one thing, basic equations are another. All I need is a kick-start back into the subject of physics. Determining component size, such as shaft wall thickness, to ensure they can cope with the forces they will be exposed to is relatively easy. Many equations exist and are easy to find (unlike the ones I am asking for help with) that can be used to determine critical component dimensions.
It is not wise to judge too readily. There are billionaire business people who do not know the difference between net and gross!
05-25-06, 04:44 PM
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Originally Posted by Phantoj
If you're not departing too radically from current designs, you can copy what everybody else is doing - it's probably pretty close to optimum.
Blasphemy.
05-25-06, 04:49 PM
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Originally Posted by mayukawa
I think those equations that you've listed are basically useless. I assume that you'll be designing the frame of the bike and not all the other components of the bike? Are you designing a race bike? I would assume so if you're trying to minimize weight as the primary goal. I think the engineers who design bikes have to design it for a theoretical rider of certain weight. Then they will have to design X amount of strength into the bike (depending on material used). Of course they will make that stronger than that minimum for safety margin. I think the hardest part about designing the bike isn't about plugging and chugging numbers into equations, but rather determining what kinds of forces (tension, compression, bending, torsion) the bike will experience and at what locations on the bike. After you know that, then you can derive equations. Yes, you'll most likely need to derive your own equations if you use non-round cross-sectional tubing. But even then, no equation will let you determine how to design a bike for a certain feel.
I am not designing a race bike. I am trying to keep weight down as, of course it is the sensible thing to do, but also because I do not want a bike that weighs 100kg. I need the equations (that I posted here to ask for help with) in order to determine the forces you mentioned!
05-25-06, 04:53 PM
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Allow me to rephrase my question:
Please help me with these equations. Are they right or are they wrong?
Acceleration up an inclined plane: F = Mgsinθ + MA or F = Msinθ + MA
Constant speed up an inclined plane: F = Mgsinθ or F = Msinθ
Rolling resistance at a given speed: F = CrrWS
Rolling resistance at a given rate of acceleration: F = CrrWS² or F = CrrWS + MA
Air resistance at a given speed: F = ½PV²CdA
Air resistance at a given rate of acceleration: F = ½PV²CdA + MA
Overcoming inertia for a given rate of acceleration: F = MA
Key:
F = force
M = mass
g = gravity (9.81m/s²)
θ = angle (of inclined plane)
A = acceleration
Crr = coefficient of rolling resistance
W = weight
S = speed
Cd = coefficient of drag
V = velocity
P = air density
Please help me with these equations. Are they right or are they wrong?
Acceleration up an inclined plane: F = Mgsinθ + MA or F = Msinθ + MA
Constant speed up an inclined plane: F = Mgsinθ or F = Msinθ
Rolling resistance at a given speed: F = CrrWS
Rolling resistance at a given rate of acceleration: F = CrrWS² or F = CrrWS + MA
Air resistance at a given speed: F = ½PV²CdA
Air resistance at a given rate of acceleration: F = ½PV²CdA + MA
Overcoming inertia for a given rate of acceleration: F = MA
Key:
F = force
M = mass
g = gravity (9.81m/s²)
θ = angle (of inclined plane)
A = acceleration
Crr = coefficient of rolling resistance
W = weight
S = speed
Cd = coefficient of drag
V = velocity
P = air density
05-25-06, 08:36 PM
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I would separate each component, b/c you have some redundant information and misleading equations.
You are also forgetting that it takes energy (and force) to create rotation [wheels and crank].
You do need gravity in the formula for going up a hill. The equation without g includes W, which is weight, which is also mass times gravity in some parlance.
Acceleration: MA [can be positive or negative]
Acceleration (rotating): Torque = Inertia * angular acceleration
Moving uphill: Mgsinθ
Moving downhill: -Mgsinθ
Rolling resistance: CrrWS [I can't verify this]
Air resistance: ½PV²CdA [I can't verify this, but agree that it goes as V^2]
So, to get force, you would have to add all of the above terms (that are appropriate). The one caveat, is that you can't just add the torque term to the force terms, b/c the other forces are linear.
Then verify your units.
Key:
F = force
M = mass
g = gravity (9.81m/s²)
θ = angle (of inclined plane)
A = acceleration
Crr = coefficient of rolling resistance
W = weight
S = speed
Cd = coefficient of drag
V = velocity
P = air density[/QUOTE]
You are also forgetting that it takes energy (and force) to create rotation [wheels and crank].
You do need gravity in the formula for going up a hill. The equation without g includes W, which is weight, which is also mass times gravity in some parlance.
Acceleration: MA [can be positive or negative]
Acceleration (rotating): Torque = Inertia * angular acceleration
Moving uphill: Mgsinθ
Moving downhill: -Mgsinθ
Rolling resistance: CrrWS [I can't verify this]
Air resistance: ½PV²CdA [I can't verify this, but agree that it goes as V^2]
So, to get force, you would have to add all of the above terms (that are appropriate). The one caveat, is that you can't just add the torque term to the force terms, b/c the other forces are linear.
Then verify your units.
Key:
F = force
M = mass
g = gravity (9.81m/s²)
θ = angle (of inclined plane)
A = acceleration
Crr = coefficient of rolling resistance
W = weight
S = speed
Cd = coefficient of drag
V = velocity
P = air density[/QUOTE]
05-26-06, 01:09 AM
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Thank you for your help, Yes. I am left wondering why this professional engineering reference book omits the ‘g’ factor from it’s inclined plane equation? It is rather confusing to a (reborn) neophyte.
05-26-06, 06:35 AM
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I don't think that they are omitting it. They are assuming that you understand that W = mg.
Wikipedia has the same formula for rolling resistance w/ mg notation, but they do not have a velocity in there.
https://en.wikipedia.org/wiki/Rolling_resistance
The weight issue is further compounded in the english system of units, which has two pounds (pound mass and pound force). This is explained somewhat by the guys at mathematica.
https://scienceworld.wolfram.com/physics/Pound.html
Wikipedia has the same formula for rolling resistance w/ mg notation, but they do not have a velocity in there.
https://en.wikipedia.org/wiki/Rolling_resistance
The weight issue is further compounded in the english system of units, which has two pounds (pound mass and pound force). This is explained somewhat by the guys at mathematica.
https://scienceworld.wolfram.com/physics/Pound.html
05-26-06, 10:20 AM
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Thanks again, Yes. And I see that I made an error:
Error Correction:
I used the the 'A' symbol
twice in the air drag equations. I should have converted it into 'f'
to represent the vehicle's frontal area. So they should have been
written as follows:
Air resistance at a given speed: F = ½PV²Cdf
Air resistance at a given rate of acceleration: F = ½PV²Cdf + MA
Where:
F = force
M = mass
A = acceleration
Cd = coefficient of drag
V = velocity
P = air density
f = frontal area
Error Correction:
I used the the 'A' symbol
twice in the air drag equations. I should have converted it into 'f'
to represent the vehicle's frontal area. So they should have been
written as follows:
Air resistance at a given speed: F = ½PV²Cdf
Air resistance at a given rate of acceleration: F = ½PV²Cdf + MA
Where:
F = force
M = mass
A = acceleration
Cd = coefficient of drag
V = velocity
P = air density
f = frontal area
05-26-06, 10:34 AM
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Originally Posted by Ideologue
With all due respect, pre-conceived notions seem to be dictating your erroneous thoughts. I am having trouble clarifying the very basic equations. I would bet that most bike builders do not even consider the equations I have requested help with. I have even spoken to microlight builders who design aircraft on ‘common sense’ and select materials because they think they will be up to the job! And many of these people are successful.
Material science is one thing, basic equations are another. All I need is a kick-start back into the subject of physics. Determining component size, such as shaft wall thickness, to ensure they can cope with the forces they will be exposed to is relatively easy. Many equations exist and are easy to find (unlike the ones I am asking for help with) that can be used to determine critical component dimensions.
It is not wise to judge too readily. There are billionaire business people who do not know the difference between net and gross!
Material science is one thing, basic equations are another. All I need is a kick-start back into the subject of physics. Determining component size, such as shaft wall thickness, to ensure they can cope with the forces they will be exposed to is relatively easy. Many equations exist and are easy to find (unlike the ones I am asking for help with) that can be used to determine critical component dimensions.
It is not wise to judge too readily. There are billionaire business people who do not know the difference between net and gross!
The equations that are giving you so much trouble finding are some of the most fundamental equations in physics. They are available in just about any introductory physics textbook. Most of it is high school level stuff.
05-26-06, 10:40 AM
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Originally Posted by Ideologue
I am not designing a race bike. I am trying to keep weight down as, of course it is the sensible thing to do, but also because I do not want a bike that weighs 100kg. I need the equations (that I posted here to ask for help with) in order to determine the forces you mentioned!
How will these equations tell your the shock and vibration levels that your frame will need to withstand?
05-26-06, 11:18 AM
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Companies such as Boeing tend to rely on CAD, not a guy with a ruler, pencil and a memorized list of equations. I was not writing of Boeing, etc., but one only need spend two minutes online to learn of hundreds if not thousands of serious structural and other technical failures with commercial aircraft that have resulted in crashes and death. I would never set foot on a commercial plane. The only people who do should be people who are comfortable with the thought of a 60-second death plunge following the failure of a component the design of which was dictated more by the accountancy office than the engineer.
Today I spoke with a design engineer for a premier car manufacturing company. To prove my point (that I posted on previously) he was unable to help me. He said he covered these equations 20 years ago and had since forgotten them. Yes, the people who build safety-critical heavy engineering products rely on computers rather than themselves. Deep in their programming the computers know the equations, not many others!
I have over 10 physics books and engineering books (mostly borrowed from the library) and they do not cover the equations I seek help with (apart from F = MA). I guess the equations are just too simple for them to bother with! But if they are so simple why have not more people addressed them in this thread? Why have only a couple of posters actually offered help with them and for only a few of the equations? If they are so basic and easy why then have not more people posted the answers here?
So you are an aerospace engineer? I guess then, if you are, you should quite easily be able to post the correct equations here? Maybe you would like to do so to prove that you people still know the basics, just to reassure air-travelers and show that not all of you are subservient to CAD programs? If you could do that for me I would be very grateful. Perhaps you could explain the logic behind the answers too, just to show that you really understand what you are writing?
Today I spoke with a design engineer for a premier car manufacturing company. To prove my point (that I posted on previously) he was unable to help me. He said he covered these equations 20 years ago and had since forgotten them. Yes, the people who build safety-critical heavy engineering products rely on computers rather than themselves. Deep in their programming the computers know the equations, not many others!
I have over 10 physics books and engineering books (mostly borrowed from the library) and they do not cover the equations I seek help with (apart from F = MA). I guess the equations are just too simple for them to bother with! But if they are so simple why have not more people addressed them in this thread? Why have only a couple of posters actually offered help with them and for only a few of the equations? If they are so basic and easy why then have not more people posted the answers here?
So you are an aerospace engineer? I guess then, if you are, you should quite easily be able to post the correct equations here? Maybe you would like to do so to prove that you people still know the basics, just to reassure air-travelers and show that not all of you are subservient to CAD programs? If you could do that for me I would be very grateful. Perhaps you could explain the logic behind the answers too, just to show that you really understand what you are writing?
Last edited by Ideologue; 05-26-06 at 11:24 AM.
05-26-06, 11:20 AM
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Originally Posted by supcom
How will these equations tell your the shock and vibration levels that your frame will need to withstand?
I don't know, you tell me? Perhaps other equations are used for such calculations, such as ones I did not post on because they are very clear to me?
05-26-06, 11:27 AM
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This may seem like semantics to you, but it is not.
The force exerted by air resistance is F = ½PV²Cdf
The force required to accelerate (overcome inertia) is F=MA
The equation that you have written below is the force required to accelerate an object horizontally through the air or other inviscid fluid in the absence of rolling friction and gravity. That situation is just never going to happen on a bike, and is just not applicable to any real situation. Therefore, I would say that the formula is useless as is.
I also think that the equations that you are asking for help with are very basic fundamental equations that are discussed at various levels of detail in high-school and college courses. IMO, the equations that 'should' determine component size, shaft wall thickness, etc. are dramatically more complicated.
Also, you have not dealt with the rotational acceleration problem. This is also fairly easy (intro college physics course material), but significantly more complicated than what you are working on now. It also should not be neglected for a bike. That is why cyclists are more concerned with rotating mass (wheels) than mass on the bike frame.
The force exerted by air resistance is F = ½PV²Cdf
The force required to accelerate (overcome inertia) is F=MA
The equation that you have written below is the force required to accelerate an object horizontally through the air or other inviscid fluid in the absence of rolling friction and gravity. That situation is just never going to happen on a bike, and is just not applicable to any real situation. Therefore, I would say that the formula is useless as is.
Originally Posted by Ideologue
Air resistance at a given rate of acceleration: F = ½PV²Cdf + MA
Material science is one thing, basic equations are another. All I need is a kick-start back into the subject of physics. Determining component size, such as shaft wall thickness, to ensure they can cope with the forces they will be exposed to is relatively easy. Many equations exist and are easy to find (unlike the ones I am asking for help with) that can be used to determine critical component dimensions.
Also, you have not dealt with the rotational acceleration problem. This is also fairly easy (intro college physics course material), but significantly more complicated than what you are working on now. It also should not be neglected for a bike. That is why cyclists are more concerned with rotating mass (wheels) than mass on the bike frame.
05-26-06, 01:28 PM
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I don't get what you're trying to do. You say you want to optimize your design, but to do that you must first focus on the structure of what you're designing. It has to be structurally sound before you focus on performance issues (the equations that you listed). It's like designing a car where you try to optimize the coefficient of drag, but you didn't want to tackle the max cargo load capacity (for example). I think that's why a lot of the responders are focusing on static and dynamic forces that your design will experience.
05-26-06, 03:14 PM
#45
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I'm on my way to a Mechanical Engineering BS, and my courseloads have gone through statics, dynamics, fluids, CAD, among others. The help you're asking for involves all these disciplines, and more.
For instance, have you considered the fact that all these components you're trying to design are not solid, and rather, are all deformable? (Bending) Therefore, everything that's designed has an associated factor of safety associated with it; to put it simply, you design components to be at least X times as strong as the loads for which it is designed for. Because component failure is inevitable- all materials can be characterized as brittle or ductile. And even if ductile materials aren't stressed to an outright breaking point, permanent (plastic) deformation occurs which changes the original shape.
The deformation problems are what you should be concentrating on after getting a nice, regular, static problem. This is the stuff that CivE's and such contemplate when designing stationery bridges, trusses, etc, which you may argue is what a frame is. Which is why you need to have a basic design in order to use equations in the first place..
That's the big picture. The devil's in the details. While working on a Formula SAE car this year, one of the tierod ends (a bolt with an eye through it) sheared off during testing, taking a wheel with it. Would you be able to calculate the forces on your "tie rod end" - axles, points of contacts, etc? Never mind stress concentration factors (They increase when you have a groove in the material..) So, that's the shear part of kinematics; are you prepared to do those calculations as well? How safe is your design? What is the maximum load it can handle? Etc, etc.
This is what major bicycle manufacturers do to come up with new designs. In fact, I highly doubt that any major company ever guesses-and-checks new products anymore. The people you cite to come up with ultralight aircraft who do not do any calculations to come up with there designs are on the same level as the Wright brothers, only with modern-day tools. (The first attempts at heavier than air flight were fraught with danger...)
And regarding the design engineer you spoke of; even if he forgot the exact formulas, what difference does it make if he can go to his bookshelf and pull out his references? I shelled out a lot of bucks for these texts, and I'm thinking of keeping them for reference. Likewise, these programs of which you speak aren't magical; they require someone at the keyboard to actually know what they're doing to get meaningful results. (Pro Engineer, from personal experience, needs a guru at the keyboards to make sense of its idiotic interface)
If you're still determined to forge onward with your project, good luck to you and perhaps you could enlighten us with a basic sketch of your design. Just adding a watermark would validly copyright your creation, IIRC.
For instance, have you considered the fact that all these components you're trying to design are not solid, and rather, are all deformable? (Bending) Therefore, everything that's designed has an associated factor of safety associated with it; to put it simply, you design components to be at least X times as strong as the loads for which it is designed for. Because component failure is inevitable- all materials can be characterized as brittle or ductile. And even if ductile materials aren't stressed to an outright breaking point, permanent (plastic) deformation occurs which changes the original shape.
The deformation problems are what you should be concentrating on after getting a nice, regular, static problem. This is the stuff that CivE's and such contemplate when designing stationery bridges, trusses, etc, which you may argue is what a frame is. Which is why you need to have a basic design in order to use equations in the first place..
That's the big picture. The devil's in the details. While working on a Formula SAE car this year, one of the tierod ends (a bolt with an eye through it) sheared off during testing, taking a wheel with it. Would you be able to calculate the forces on your "tie rod end" - axles, points of contacts, etc? Never mind stress concentration factors (They increase when you have a groove in the material..) So, that's the shear part of kinematics; are you prepared to do those calculations as well? How safe is your design? What is the maximum load it can handle? Etc, etc.
This is what major bicycle manufacturers do to come up with new designs. In fact, I highly doubt that any major company ever guesses-and-checks new products anymore. The people you cite to come up with ultralight aircraft who do not do any calculations to come up with there designs are on the same level as the Wright brothers, only with modern-day tools. (The first attempts at heavier than air flight were fraught with danger...)
And regarding the design engineer you spoke of; even if he forgot the exact formulas, what difference does it make if he can go to his bookshelf and pull out his references? I shelled out a lot of bucks for these texts, and I'm thinking of keeping them for reference. Likewise, these programs of which you speak aren't magical; they require someone at the keyboard to actually know what they're doing to get meaningful results. (Pro Engineer, from personal experience, needs a guru at the keyboards to make sense of its idiotic interface)
If you're still determined to forge onward with your project, good luck to you and perhaps you could enlighten us with a basic sketch of your design. Just adding a watermark would validly copyright your creation, IIRC.
05-28-06, 01:37 PM
#46
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Originally Posted by Ideologue
Companies such as Boeing tend to rely on CAD, not a guy with a ruler, pencil and a memorized list of equations. I was not writing of Boeing, etc., but one only need spend two minutes online to learn of hundreds if not thousands of serious structural and other technical failures with commercial aircraft that have resulted in crashes and death. I would never set foot on a commercial plane. The only people who do should be people who are comfortable with the thought of a 60-second death plunge following the failure of a component the design of which was dictated more by the accountancy office than the engineer.
Today I spoke with a design engineer for a premier car manufacturing company. To prove my point (that I posted on previously) he was unable to help me. He said he covered these equations 20 years ago and had since forgotten them. Yes, the people who build safety-critical heavy engineering products rely on computers rather than themselves. Deep in their programming the computers know the equations, not many others!
I have over 10 physics books and engineering books (mostly borrowed from the library) and they do not cover the equations I seek help with (apart from F = MA). I guess the equations are just too simple for them to bother with! But if they are so simple why have not more people addressed them in this thread? Why have only a couple of posters actually offered help with them and for only a few of the equations? If they are so basic and easy why then have not more people posted the answers here?
So you are an aerospace engineer? I guess then, if you are, you should quite easily be able to post the correct equations here? Maybe you would like to do so to prove that you people still know the basics, just to reassure air-travelers and show that not all of you are subservient to CAD programs? If you could do that for me I would be very grateful. Perhaps you could explain the logic behind the answers too, just to show that you really understand what you are writing?
Today I spoke with a design engineer for a premier car manufacturing company. To prove my point (that I posted on previously) he was unable to help me. He said he covered these equations 20 years ago and had since forgotten them. Yes, the people who build safety-critical heavy engineering products rely on computers rather than themselves. Deep in their programming the computers know the equations, not many others!
I have over 10 physics books and engineering books (mostly borrowed from the library) and they do not cover the equations I seek help with (apart from F = MA). I guess the equations are just too simple for them to bother with! But if they are so simple why have not more people addressed them in this thread? Why have only a couple of posters actually offered help with them and for only a few of the equations? If they are so basic and easy why then have not more people posted the answers here?
So you are an aerospace engineer? I guess then, if you are, you should quite easily be able to post the correct equations here? Maybe you would like to do so to prove that you people still know the basics, just to reassure air-travelers and show that not all of you are subservient to CAD programs? If you could do that for me I would be very grateful. Perhaps you could explain the logic behind the answers too, just to show that you really understand what you are writing?
Ok, here are some hints. For air resistance, go buy an aerodynamics textbook. You'll find enough there to make your head swim. However, for anything other than simple shapes, determining the drag coefficient, Cd, is VERY difficult without either a wind tunnel or a super computer to run a Computational Fluid Dynamics (CFD) program. Either option is expensive. There are some estimates of Cd available on the internet, but these apply to the typical bike, not the specific bike that you want to design.
Rolling resistance is similar in that there is a constant that characterizes the specific tire that you are using. As far as I know, the only way to determine this constant is to measure it for a specific tire. Oh, and it's only valid for the road surface you test against.
However, these two equations do almost nothing for actually designing a bike. All these equations do is give you a way to estimate how fast your bike will go in a given circumstance. Most bike designers don't need to worry about these equations because everyone knows that a lighter bike (plus rider) goes up hills faster than a heavy one. Since the rider is FAR heavier than the bike, the bike is a very minor factor in the overall top speed.
Design of a bike is more about setting optimum geometry and designing a frame and components that are lightweight, yet don't fall apart and hurt someone. The equations you are working with don't tell you this. You need to learn how to do stress analysis. And that is far more than memorizing a set of equations. I suggest you start by enrolling in a mechanical engineering program.
