Engineering Applying Physics to Vehicle Dynamics: Do Wheel Number Matter?

[erobz]

So you think that the torque input from your engine is completely transferred to acceleration, regardless of the other forces acting on the vehicle? If the front end of your vehicle is resting against a wall, do you think that having a rear wheel torque will give your vehicle an acceleration?

Now you are adding a wall. Another external force that is not present in the idealization.

Before anything else, you must first admit you are wrong. The proof is that even you can see that your modelization gives results that don't make sense.

Yes...I've admitted this over and over. It doesn't make sense! Clearly I must be wrong, but where specifically remains to be addressed... Because it works. The formulation works either way. I'm just a little circus monkey spinning the crank on the jack in the box!

The way you are doing it - one block with two free rotating axles in a single FBD - is not good. It doesn't give results that make sense. It's not used anywhere, ever.

I can take any part of the system I'd like to be the system. The laws of physics don't "break". They might be uninteresting for some choices, but they don't break.

I've shown that a widely published author and physicist has done the very thing I have. So don't say "Its never used".

I would tell you to do the FBD correctly and get back to us, but I already did it for you, for each method. All you have to do is accept that these are the right methods to do the work properly and try to understand them.

Your model will yield to the same conclusion as mine under the same assumptions. Yours just initially has extra stuff in it. They are fundamentally the same models.

 

[jack action]

Your equation:
$$a = \frac{2TR}{mR^2+4I}$$
Your drawing:

The force at the rear wheel is $F= \frac{T}{R}$. So rewriting your equation:
$$a = \frac{2FR^2}{mR^2+4I}$$
Where's the front wheel force in that equation?

How can this equation be good if the force at the front wheel is not present?

If the force at the front wheel is equal and opposite to the force at the rear wheel, shouldn't the acceleration be zero? If it is greater, shouldn't the vehicle decelerate?

From my post #37:
$$T= T_{in} - (F_r + F_d)R$$
$T_{in}$ is what you simplify as $T$, and $F_r$ (rolling resistance) is the force at the front wheel shown on your model. I admit that I added the drag force $F_d$ just to show you that forces acting on rigid bodies that are connected somehow to the rear axle (i.e. both the vehicle's body and the front axle) does influence the net force, thus the acceleration as well.

 

[erobz]

Your equation:
$$a = \frac{2TR}{mR^2+4I}$$
Your drawing:

View attachment 314627​

The force at the rear wheel is $F= \frac{T}{R}$. So rewriting your equation:
$$a = \frac{2FR^2}{mR^2+4I}$$
Where's the front wheel force in that equation?

How can this equation be good if the force at the front wheel is not present?

If the force at the front wheel is equal and opposite to the force at the rear wheel, shouldn't the acceleration be zero? If it is greater, shouldn't the vehicle decelerate?

From my post #37:
$$T= T_{in} - (F_r + F_d)R$$
$T_{in}$ is what you simplify as $T$, and $F_r$ (rolling resistance) is the force at the front wheel shown on your model. I admit that I added the drag force $F_d$ just to show you that forces acting on rigid bodies that are connected somehow to the rear axle (i.e. both the vehicle's body and the front axle) does influence the net force, thus the acceleration as well.

You are adding a force that doesn't exist. Where is $F$ in that diagram?

Here are the EOM.

Front wheels:

$$ \circlearrowright^+ \sum \tau = R f_f = I \alpha = I \frac{a}{R} $$

Rear Wheel:

$$ \circlearrowright^+ \sum \tau = T - R f_r = I \alpha = I \frac{a}{R} $$

Total Vehicle:

$$ \rightarrow^+\sum F = 2 \left( f_r - f_f \right) = Ma $$

 

[jack action]

You are adding a force that doesn't exist. Where is $F$ in that diagram?

The force at the rear wheel. Sorry, it is not labeled in your drawing.

 

[erobz]

The force at the rear wheel. Sorry, it is not labeled in your drawing.

Thats is not correct though. $\sum \tau = I \alpha$ must be satisfied on the rear wheel (and the front wheel). Under the no-slip condition this is what follows:

Rear Wheel:

$$ \circlearrowright^+ \sum \tau = T - R f_r = I \alpha = I \frac{a}{R} $$

There is no other $F$ that is causing a torque, that has already not been listed.

And just to be clear, $f_r$ is not rolling friction. It is static friction acting on the rear wheel. The $r$ subscript refers to rear, not rolling. There is no rolling friction in this problem.

 

[jack action]

So why isn't your acceleration set up as:
$$a = \frac{(2T-Rf_f)R}{mR^2+4I}$$
(The only reason I put a $2$ in there is that you assume that $T$ was for each wheel in one of your equations but you didn't in the other. So it is kind of confusing)

Or as I put it in my post #37 (ignoring the drag force $F_d$ I added):
$$a = \frac{F_{net}}{m_e} = \frac{\frac{T}{R}}{m\left(1+ \frac{4I}{mR^2}\right)} = \frac{\frac{T_{in}-F_r R}{R}}{\frac{1}{R^2}(mR^2+ 4I)} = \frac{(T_{in}-F_r R)R}{mR^2+ 4I}$$
Do you see the resemblance?

 

[erobz]

So why isn't your acceleration set up as:
$$a = \frac{(2T-Rf_f)R}{mR^2+4I}$$
(The only reason I put a $2$ in there is that you assume that $T$ was for each wheel in one of your equations but you didn't in the other. So it is kind of confusing)

Because it's not the solution to the system of equations.

There is only $T$ on each rear wheel, not $2T$.

 

[jack action]

Why are you ignoring $f_r$ in your final solution? (It is the rolling resistance, right?)

 

[erobz]

Why are you ignoring $f_r$ in your final solution? (It is the rolling resistance, right?)

No its the static friction on the rear wheel. I stated that in #65?

 

[jack action]

Where is the rolling resistance?

 

[erobz]

Where is the rolling resistance?

The model does not need "rolling resistance" the static friction is independent of rolling resistance. This is just basic physics...its not meant to examine tire deformations. It's a purely theoretical application of Work/Energy without resistive loss, or $\sum F = ma$ without resistive loss.

 

[jack action]

The front wheel has clearly a rolling resistance. There is a friction force that goes backward. Where is that force in your equation?

 

[erobz]

The front wheel has clearly a rolling resistance. There is a friction force that goes backward. Where is that force in your equation?

Its not rolling resistance. Its static friction $f_f$.

Front wheels:

$$ \circlearrowright^+ \sum \tau = R f_f = I \alpha = I \frac{a}{R} $$

The front wheels are not being driven by an active torque applied to the front axle, but they are none the less accelerating if the vehicle is accelerating ( angular ) and the no-slip condition applies. The static friction is responsible for the angular acceleration of the front wheels. Without it, they just slide without rotating.

 

[Lnewqban]

I want this to be over as much as anyone else. So I'm going to be as concise as possible about what I find confusing still:

$$\int F_{net} \, dx = \int 2 \left( f_r - f_f \right) dx = 0 $$

Will anyone explain this?

I see debate and understanding progress.
Could you explain that equation on your own words?
Sorry, I don't understand the function of fr and ff in it.

 

[jack action]

Its not rolling resistance. Its static friction ##f_f#.

We have already shown that this static friction is a response to the rolling resistance.

The front wheels are not being driven by an active torque applied to the front axle

So you have a backward force applied to a wheel with no reaction torque? Nope. Something is reacting to that force. That front wheel torque wheel center force is ultimately coming from the rear wheel torque, passing through the body.

but they are none the less accelerating if the vehicle is accelerating ( angular ) and the no-slip condition applies.

Since the force is backward, it should contribute to decelerating, not only the front wheel itself but the whole vehicle, including the rear wheel.

 

[erobz]

I see debate and understanding progress.
Could you explain that equation on your own words?
Sorry, I don't understand the function of fr and ff in it.

$f_f$ and $f_r$ are the static friction forces acting on the vehicle. They are the only forces acting on the vehicle externally in the direction of motion. Just like the example out of my old physics textbook that we examined earlier. Yet... if static friction cannot do work, by definition that integral must be $0$.

 

[erobz]

We have already shown that this static friction is a response to the rolling resistance.

Its a steel wheeled car. virtually undeformed steel wheels on a steel track. rolling resistance is nill.

Since the force is backward, it should contribute to decelerating, not only the front wheel itself but the whole vehicle, including the rear wheel.

Your right. It does in the following $\sum F$ equation:

Total Vehicle:

$$ \rightarrow^+\sum F = 2 \left( f_r - f_f \right) = Ma $$

 

[Lnewqban]

$f_f$ and $f_r$ are the static friction forces acting on the vehicle. They are the only forces acting on the vehicle externally in the direction of motion. Just like the example out of my old physics textbook that we examined earlier. Yet... if static friction cannot do work, by definition that integral must be $0$.

Thank you very much, erobz.

Just like it happens in the picture shown in that book that you posted earlier, using the magnitudes of those forces at the contact patches, which are relatively easy to calculate, is a very convenient and traditional way to compute the work in or out a vehicle.

As you are an intelligent person trying to understand the apparent conflict of the equation related to that way, I am simply trying to reason that, by definition of work (force times distance), those static forces themselves are unable to perform any work (add to or subtract from the car any kinetic energy).

Nevertheless, the magnitudes of those forces are a direct reflection of the actual physical processes happening inside and outside that vehicle (heat, expansion of gases, transmission reduction, torque at shaft, pad friction on brake rotors, etc.).

Mathematically, there is no contradiction in that equation.
IMHO, physically, there is.

 

[erobz]

Thank you very much, erobz.

Just like it happens in the picture shown in that book that you posted earlier, using the magnitudes of those forces at the contact patches, which are relatively easy to calculate, is a very convenient and traditional way to compute the work in or out a vehicle.

As you are an intelligent person trying to understand the apparent conflict of the equation related to that way, I am simply trying to reason that, by definition of work (force times distance), those static forces themselves are unable to perform any work (add to or subtract from the car any kinetic energy).

Nevertheless, the magnitudes of those forces are a direct reflection of the actual physical processes happening inside and outside that vehicle (heat, expansion of gases, transmission reduction, torque at shaft, pad friction on brake rotors, etc.).

Mathematically, there is no contradiction in that equation.
IMHO, physically, there is.

Thank you for being honest. Something is strange here in that model and we aren’t quite sure what. That is how I’m interpreting your opinion. I hope I'm not projecting.

 

[jack action]

Its a steel wheeled car. virtually undeformed steel wheels on a steel track. rolling resistance is nill.

Steel wheels do deform. There is no rolling possible without deformation. The simple fact that you have the static friction present means that it was created by some other opposing force or torque.

Your right. It does in the following equation:

Yes, but you are treating each $a$ in your equations as if they were independent of each other. They are not. $a$ is the acceleration of the vehicle but also the acceleration of each of your wheels. Furthermore, $a =\alpha R$ where $\alpha$ is the angular acceleration of your front and rear wheels. (They could have different angular acceleration if they had different radiuses.)

This means that if a wheel decelerates (both longitudinally and angularly), the other wheels must also decelerate, and the vehicle as a whole must decelerate. How is that represented in your equations in post #10?

Here is the problem with your 3 equations in post #10. There are 2 ways to look at it:

1. Mine. $f_f$ is known. This means that the only unknows are $a$ and $f_r$. Every other variable must be known. So you have 3 equations with 2 unknowns, so one of your equations is redundant. But we know that they all have pertinent information not available in the other equations. That cannot give a good result. You need more equations to solve this (post #28).
2. Yours. $f_f$ is unknown. Then you have 3 equations with 3 unknowns, namely $a$, $f_r$, and $f_f$. If you solve your system of equations in post #10, I end up:

$$f_r = \frac{T}{R}\left(1- \frac{I}{\frac{1}{2}MR^2 + 2I}\right)$$
$$f_f = \frac{T}{R}\frac{\frac{I}{R^2}}{\frac{1}{2}M + \frac{2I}{R^2}}$$
$$a= \frac{\frac{T}{R}}{\frac{1}{2}M + \frac{2I}{R^2}}$$

Doesn't that look weird that the front static force $f_f$ depends on the wheel inertia? What law of physics is implied here? If I assume $I = 0$, then it disappears:
$$f_r = \frac{T}{R}$$
$$f_f = 0$$
$$a= \frac{2T}{MR}$$
And something is clearly wrong with the factor of $2$ with the acceleration.

 

[erobz]

Steel wheels do deform. There is no rolling possible without deformation. The simple fact that you have the static friction present means that it was created by some other opposing force or torque.

Yes, but you are treating each $a$ in your equations as if they were independent of each other. They are not. $a$ is the acceleration of the vehicle but also the acceleration of each of your wheels. Furthermore, $a =\alpha R$ where $\alpha$ is the angular acceleration of your front and rear wheels. (They could have different angular acceleration if they had different radiuses.)

This means that if a wheel decelerates (both longitudinally and angularly), the other wheels must also decelerate, and the vehicle as a whole must decelerate. How is that represented in your equations in post #10?

Here is the problem with your 3 equations in post #10. There are 2 ways to look at it:

1. Mine. $f_f$ is known. This means that the only unknows are $a$ and $f_r$. Every other variable must be known. So you have 3 equations with 2 unknowns, so one of your equations is redundant. But we know that they all have pertinent information not available in the other equations. That cannot give a good result. You need more equations to solve this (post #28).
2. Yours. $f_f$ is unknown. Then you have 3 equations with 3 unknowns, namely $a$, $f_r$, and $f_f$. If you solve your system of equations in post #10, I end up:

$$f_r = \frac{T}{R}\left(1- \frac{I}{\frac{1}{2}MR^2 + 2I}\right)$$
$$f_f = \frac{T}{R}\frac{\frac{I}{R^2}}{\frac{1}{2}M + \frac{2I}{R^2}}$$
$$a= \frac{\frac{T}{R}}{\frac{1}{2}M + \frac{2I}{R^2}}$$

Doesn't that look weird that the front static force $f_f$ depends on the wheel inertia? What law of physics is implied here? If I assume $I = 0$, then it disappears:
$$f_r = \frac{T}{R}$$
$$f_f = 0$$
$$a= \frac{2T}{MR}$$
And something is clearly wrong with the factor of $2$ with the acceleration.

None of this is unexpected with the model.

$f_f$ and $f_r$ are a reaction force to the acceleration. The underlying no-slip criterion here is simply $f_f < \mu_s N_f$ and $f_r < \mu_s N_r$

If you let $I \to 0$ the wheels ( front or back ) provide no resistance to the vehicle's acceleration. A wheel without inertia takes no torque to accelerate.

The $2$ comes from the last equation. The torque acting on each wheel causes the reaction $2 f_r$. e.g. there are $2 f_r$ forces accelerating the car forward.

That is all internally consistent.

What is not internally consistent is the work done by the static forces should equal zero, but...

$$ \int F_{net} dx = \int 2 \left( f_r - f_f \right) dx \neq 0 $$

 

[hutchphd]

When you ( @erobz ) originally hijacked the post in #8 your question was whether the static friction forces did work on the car

Does static friction force do work in this case? It seems like the frictional forces must be doing work, but it often seems like this is "explained away" in some way I still don't fully understand.

and the answer was no. There is no net energy flowing from the contact. When there are contact forces between two bodies we only worry about net work done.
This is equivalent to asking how much work Elmer's glue does. The static friction "force" is merely a placeholder TBD by the rest of the problem. It is useful because it is usually accompanied by a limit at which the problem has a nonlinear response. It does no work itself because it is static by definition. It is unfortunate to call it a friction...I prefer stiction. In this problem essential work is done by the engine and dissipated by various rotational and linear dissipative friction. So there is a net force on the car which you can call static friction if you must but the Work is more complicated as @jack action has repeatedly noted.
In the canon of introductory physics we should ban "static friction" and revise the whole notion of "dissipative forces"

 

[erobz]

In the canon of introductory physics we should ban "static friction" and revise the whole notion of "dissipative forces"

Yeah, it's not my fault I discovered the math doesn't jive the way it is typically presented. I'm taking a beating here defending my sanity.

 

[hutchphd]

I stopped worrying about the sanity part long ago...makes it easier

 

[erobz]

I stopped worrying about the sanity part long ago...makes it easier

Is it too soon to ask if I can be mentioned in a footnote somewhere when the theory is revised!

"Stubborn [insert expletive] challenges us to revise something no one cares about anymore" could be a good title of an insight?

 

[hutchphd]

Also sanity is grossly overrated !
When I was teaching it I thought about some revisions but nothing ever "jelled". Personally I just revert to more fundamental stuff when necessary. Energy and momentum are conserved although some people find further subdivision important. Carnot did do a few useful things after all

 

[Lnewqban]

Rewinding back to post #8:

Does static friction force do work in this case? It seems like the frictional forces must be doing work, but it often seems like this is "explained away" in some way I still don't fully understand.

If I'm doing a force balance to solve the OP's problem, there are frictional forces acting as external forces on the vehicle. The rear frictional forces assisting acceleration and the front tires frictional forces opposing it. These forces must be doing the work...they are the only external forces (in the direction of motion). But I so often hear about static frictional forces not doing work because the point of contact between the tire and the road is instantaneously at rest w.r.t the road. Am I still confused, because I feel a bit unsure of my understanding?

...
But then again...in both cases (accelerating/not accelerating) the point of contact is instantaneously at rest w.r.t. the ground. So, I still am unsure what the actual resolution is! It all seems like a bit of a circular argument. It feels like whether or not the static frictional force does work or not is whether or not the body is accelerating, and whether or not the body is accelerating is whether or not the static frictional forces are doing work.

In the absence of external forces doing the work of accelerating the car, couldn't that energy come from an internal or onboard source, like accumulated potential energy in a spring, or compressed air, or steam, or battery?

 

[erobz]

Rewinding back to post #8:
In the absence of external forces doing the work of accelerating the car, couldn't that energy come from an internal or onboard source, like accumulated potential energy in a spring, or compressed air, or steam, or battery?

I don't think, because a body can't apply a net force to itself.

 

[Lnewqban]

But that has been the principle of all automobiles, trucks, ships, locomotives, airplanes, old and new: to move at the expense of accumulated work or energy that is contained inside it.

That energy can be the chemical energy of fuel or batteries, converted to useful mechanical work via an engine or motor.
Is that energy (and its resulting internal output torque) the one doing the useful work.

These are the guts of a Tesla model S.
That grey plank is a battery capable of accumulating 100 Kw-h of energy.
The red motors can transfer enough of that energy onto the tires in 3.0 seconds as to accelerate the car's mass of 2,000 kg from 0 to 100 km/h.

 

[erobz]

But that has been the principle of all automobiles, trucks, ships, locomotives, airplanes, old and new: to move at the expense of accumulated work or energy that is contained inside it.

That energy can be the chemical energy of fuel or batteries, converted to useful mechanical work via an engine or motor.
Is that energy (and its resulting internal output torque) the one doing the useful work.

These are the guts of a Tesla model S.
That grey plank is a battery capable of accumulating 100 Kw-h of energy.
The red motors can transfer enough of that energy onto the tires in 3.0 seconds as to accelerate the car's mass of 2,000 kg from 0 to 100 km/h.

View attachment 314662

That’s all fine. They just need something to “push off” (or mass to eject) to accelerate their own center of mass.
I just noticed an inconsistent model, which either is indication of a bad static force model, or we accept the fact that we can’t really do much if we have don’t have anything to interact with.

Perhaps just accepting that duality is the simplest resolution. Just treat this as looking at it from the universes perspective, instead of the “thing” in it. After all $\Delta U_{universe}=0$ . The static friction force does work on the universe. That has to be the case. And that’s actually what’s being calculated here. Maybe static friction is saved…from nothing. Because nothing was going so change anyhow.

Anyhow, I expect to be burned at the stake the more I talk! I’m going to bed.