carguycw

No, you are wrong. Example: picture a bicycle with no suspension. If the rider brakes really hard, he/she will be catapulted forwards over the handlebars. (I think most people have had this happen to them when they were kids. ) This is due to weight transfer. In this case, all of the weight has been transferred off of the rear wheel. It happens regardless of the vehicle's suspension, or lack thereof.

When a car or bicycle accelerates or decelerates, it is subject to 2 horizontal forces- a force acting on the wheels in one direction, and a parallel equal force acting on the vehicle's CG in the other direction (refer to Newton's Third Law). In the case of braking, the force on the wheels acts backwards and the force on the CG acts forwards. Since the CG is above the ground, the two counteracting forces create a torque, or twisting force, that acts on the vehicle. This is what causes weight transfer, which causes nosedive.

Hope this clears things up.

Scarmiglio

Why would there be less forward weight transfer due to ride height? I don't see how ride height and weight transfer relate.

carguycw

Here's another analogy...

Picture a person pushing on a refrigerator. The higher up they push, the easier it is to tip the refrigerator over. In other words, applying the same amount of force higher up on an object produces more weight transfer.

Rando

OK I get now. Thanks for drive it into my stubern(sp) head.

B8 Turbo

You are right, but I'm talking based on straight line. I understand your explanation. Now that I'm using coil-overs and harder spring rates, the nose dive doesn't occur so huge that it made me felt insecure about the brake system. Anyway, nowadays my brakes are superb except the pad.....need to get EBC Green stuff once my current pads are used up.

fossil boy

i think this is due to the Angle of Momentum (?). Objects in motion are NOT the "rigid" blobs we model for mathematical equations. The wheels (being the round things on the car in contact w/ the road), actually become a pivot point, thus the distribution of mass, weight transfer and all that other jazz, is not Linear (but gee, the equations are so easy to solve!), but rather, Curvi-linear trajectories....

NuCom Ron

Whose car in here has the most horsepower? What is it, 150? These are not fast cars......they are nice looking cars. What the hell kind of brakes do you need on a 2700 pound car?
I have an 03 MP5 w/ 4 wheel disc. Stops better than any car that I have ever owned.....So let's just leave facts

a) Mazda's engineers know far more about braking than we do
b) If cross-drilled rotors were necessary, they would have 'em
c) We all want cross-drilled rotors b/c they look kewl....period.

Hopefully, this will end this mastermind discussion.

carguycw

No, they are not "Curvi-linear trajectories"...

The forces on a car braking in a straight line ARE simple. There is nothing complicated about it. There is no curvi-linear nonsense involved. It is simple, high-school Newtonian physics.

PUT THE CRACK PIPE DOWN!!

fossil boy

Look, I don't have either a crack pipe or a physics book handy, however "helpful" they might be...

First, most objects in motion are not following two dimensional mechanics - we solve them that way because the approximation is close and it is relatively easy. Truly, though, via my mental imagery of the problem, your initial observation seems correct.
The vehicle is following two basic trajectories (yes, I'm simplifying things): Forward, and then, the downward motion of braking.
Yes, we could likely come very close to the weight transfer issue using Vectors. But, consider this: during braking the car is approaching zero velocity, thus, all the issues of nose-dive(weight transfer) etc. are changing as momemtum decreases, thus, the relationships between the forward motion and downward motion are not proportional, but rather, differential. Thus the actual weight transfer changes during the course of braking as does nose-dive and the car's speed. Therefore, my statement of "curvi-linear trajectories" was an attempt to describe this aspect - the fact that these are not straight-line motions, but those which actually do change with respect to forward momentum, braking resistance, etc. Further, the differential shift of weight via the springs/shocks etc, are arcuate b/c these are not proportional relationships.
Think back to the RWD muscle cars set up for 1/4 mile runs. Usually, these are "jacked-up" in the rear for weight transfer over the power wheels (sounds like a kiddie toy) to help with traction. While the car may "squat" during take-off, the back does lift up somewhat during the rest of the run - it's basically the same phenomena (but backwards) and indicates that the relationships between weight transfer, changes in ride height(downward) and forward momentum are differential, thus, in my mind at least, slightly arcuate paths taken by the vehicle rather than straight.
And yes, as stated, these can likely be resolved using vectors, etc., but, that is a human approach and not necessarily a complete picture. The problem can become as complex as we choose to make it; since there is always torsion, unlevel surfaces, and innumerable other variations we could attempt to confuse ourselves with.
Oh and just to play homage to the actual topic: I would find a main advantage to slotted rotors to be the decrease of wheel mass, which may improve efficiency/acceleration!
BTW man, I've enjoyed the discussion - it's been too long since I've had to consider Physics (and please don't make me do Calculus, I'm on vacation)
Hope you had a good X-mas!

carguycw

What he said.

To make everything clear here, we are talking about STEADY STATE braking- in other words, the rate of deceleration doesn't change. If this is the case, the weight transfer remains fixed. This is a reasonable way to represent braking force because, unlike during acceleration, the brakes on most modern cars are powerful enough to lock up the wheels at just about any realistically attainable speed.

Do not confuse this situation with acceleration; this is comparing apples to oranges. The rate of deceleration while braking is basically fixed by the grip of the tires, which does not change with decreasing speed unless the car has a significant amount of aerodynamic downforce (let's not go there right now). Therefore, the car always slows down at the same rate. However, a car's rate of *acceleration* always falls off at high speed because the torque multiplication of the transmission decreases each time you upshift, reducing the amount of force pushing the car. This is NOT comparable to braking unless your car is powered by an engine that exerts the same amount of force at all speeds- in other words, a jet or rocket engine.

FWIW the reason why enthusiasts in the 60's would jack up their muscle cars is simple- it raises the car's center of gravity, which increases the weight transfer to the rear wheels during acceleration, improving traction. (It's the reverse of my earlier post explaining how lowering a FWD car reduces braking-induced weight transfer to the front wheels.)

Also, let's not confuse the issue by throwing in outside factors, like uneven surfaces, aerodynamic downforce and drag, etc. Of course these things have an effect on real-world braking. However, my point (and Jesse's) remains the same: at a given rate of deceleration, the amount of weight transfer is FIXED.

Have a nice day.

fossil boy

Hi and Happy New Year...
Apparently, we will be continually at odds here, but, in any case I do appreciate the dialogue.
My initial post in this topic, carguycw, was in support of your position that a lowered vehicle has better stopping power than a comparable one which is higher. As someone else replied, this is due, in part to a higher center of gravity for the taller vehicle which creates more weight transfer during braking.
My point, solely, is that this is caused by the fact that the car is moving rather than stationary. As we now have a taller v. lower moving objects, the raised vehicle then, will transfer the weight in a more pronounced fashion because of a rotational component that is centered between the front and rear wheels. I attributed this to an increased angle of momemtum at the roof-line (just as a reference) for the taller vehicle as compared to the lower vehicle.
I do believe that one facet of a vehicle's suspension is to moderate these types of rotational forces. The front/rear springs serve as boundary conditions or limits to the threshold forces necessary for weight transfer as well as impart a defined range of internal rotation (front to rear).
BTW, my ideas of these types of rotational components are also likley the very reason why top heavy/tall SUV's are more prone to roll over in hard turns - in those instances, the lateral angle of momemtum is also higher than for lower vehicles.
And of course, some of these aspects could be improved by utilizing improved springs/suspension with greater limits to compression/tension which would reduce the actual transfer of energy.
Now, I am not sure if braking force is constant or not, so I'll back away from that point (good job, Pseudo!). However, if we are curious if the braking versus velocity is proportional or not, then perhaps, by plotting Speed v. Braking distance for various speeds, we could see if the results fall close to a linear relationship.
This fairly well concludes my thoughts; sorry to bring an "old topic" into the New Year.

carguycw

After making a futile attempt to comprehend everything you're saying in this last post, I have to conclude that you've got things all mixed up. I think you've thought up your own really complicated theory to explain something that's really simple. Really, this is Vehicle Dynamics 101. There are no "Curvi-linear trajectories", "angles of momentum" [there is no such thing BTW], "threshold forces" or "greater limits to compression/tension which would reduce the actual transfer of energy" involved here. [ENERGY is irrelevant when we're talking about FORCE, or do you really understand the difference?] All of the examples you'ce cited (vehicle braking, jacked up drag cars, and SUV rollovers) can be explained with really simple equations that depend only on the mass, physical dimensions of a vehicle, and the forces acting on the vehicle.

I've consulted the oracle (i.e. Google.com) and come up with a pretty darn good, simple explanation of Vehicle Dynamics 101. Please read this series before posting further on this topic.

http://www.miata.net/sport/Physics/index.html

If this still doesn't make sense to you, please study some basic physics and try again.

Peace.

fossil boy

I am so sorry you are having difficulty trying to comprehend my rationale – perhaps it is because your unusually large head is located within your exceptionally tight ***. Such constrictions, literally or figuratively, impede the clarity of one’s thoughts, especially that of imagination and creativity. This may also explain your repeated condescending attitude, as per previous posts, and the apparent over-inflated feeling of self-importance that your writings exude. What I find particularly appalling is the means in which you attempt to belittle someone who was merely trying to explain (with logic, reason and some physics) a concept you initiated and (oddly, at this point at least) one that I do agree with. Please keep in mind, I have sincerely tried to be civil and diplomatic regarding these posts, but given what I interpret as belligerence on your part, I am left with little recourse other than simply ignoring you.

As a general statement, let’s first briefly discuss the philosophical concepts of Math, Physics and Nature. Nature, as I am defining it here, is the system that we, as humans, attempt an understanding. The quest for answers and defintion seems to be a strictly human trait, and it is imperative to realize that all subsequent results and knowledge are biased by the human viewpoint. This perspective, in turn, is limited by the breadth of our knowledge. Indeed, this becomes a vicious circle, of sorts – one that is only broken by creative and imaginative thinking on our part (which is precisely why it is rather unwise to stifle such!).

Most physical sciences initiate investigations at the observational level. From observations of natural systems, we then formulate hypotheses as explanations of particular events. These hypotheses, if proven reliable, may eventually evolve into theories or laws. The escalation in status of our “explanation” is determined solely by how effective our hypothesis is in predicting the outcome of future experiments, and, how precise the results are. Keep in mind, then, that what has been created is a MODEL. This model becomes the basis for our experimentation and is what we actually test for we cannot sample and test the specific system itself and certainly not the whole. But rather, we identify and isolate only particular aspects of Nature, and, via various assumptions and conditions, hold the remainder constant.

The model which science creates is usually based upon mathematical relationships. It is important to note, and appreciate, that mathematics too, is merely a model, being a human-derived tool. Certainly, our adherence to Base-10 math exemplifies this, as its origins stem only from the number of fingers that we have. To our discussion, Newton’s contributions also include aspects of Calculus, which he created to explain and prove some of his Laws. Now the point here is not to discount centuries of advances in math and physics, but to put forth the notion that even our common system of math may not be perfect, and indeed, other systems do exist (e.g. the non-linearity of Fractal Math).

Now if we ASSUME our system of math is correct, or at least, appropriate to the problem we are considering, we then find other dilemmas when formulating hypotheses/models. Since we cannot attempt to model the complete system, we must begin to parameterize those aspects that we can identify. Again, this identification codified by our knowledge base – we can only identify that which we understand. Further, if we are adept at identifying those parameters and successfully do so, then often the issue becomes a matter of quantifying those parameters. Worse yet, even if we can take into account every aspect of a system, and actual assign some numerical value to each, we then find ourselves with equations that are either exceedingly complex or completely unsolvable. Thus, we do make a bevy of assumptions, some out of basic human ignorance; some out of necessity. The result is often a much-simplified model of some complex phenomena. And truly, the extent of simplification (and also “Assumptions”) for a given problem is directly related to the level of mathematical expertise held by the investigator/user.

Thus when you previously alluded to the problem being steeped in basic “High School Physics”, you are somewhat correct. But bear in mind, that version was intended to be simple. And straight-line Physics relationships can indeed do much for yielding an approximate answer, one that may very well be accurate enough for some applications. Such simplified approaches may not, however, describe the system completely. An “answer”, even if correct, is not unique – there may be many pathways that give equivalent results. Thus, even a correct response does not mean that one has an exact understanding.

Now, let’s address our quandary. I did skim through the website you attached (so in your lofty opinion, I suppose this would qualify the legitimacy of my response), and there are a few interesting nuances I’d like to illuminate for you.

Here’s a quote from page1:

“The braking forces create a rotating tendency, or torque, about the CG. Imagine pulling a table cloth out from under some glasses and candelabra. These objects would have a tendency to tip or rotate over, and the tendency is greater for taller objects and is greater the harder you pull on the cloth. The rotational tendency of a car under braking is due to identical physics.”

This probably doesn’t need much explanation, but as I recall, rotational forces can be described as curvi-linear.

“Let us continue analyzing braking. Weight transfer during accelerating and cornering are mere variations on the theme. We won't consider subtleties such as suspension and tire deflection yet. These effects are very important, but secondary.”

Hmmm…. Might the suspension parameters ultimately be the limits one would use if modeling this problem using Integrals?

Paraphrased quotes from page 3

“
“Physics is the science of measurement. Perhaps you have heard of highly abstract branches of physics such as quantum mechanics and relativity, in which exotic mathematics is in the forefront. But when theories are taken to the laboratory (or the race course) for testing, all the mathematics must boil down to quantities that can be measured. In racing, the fundamental quantities are distance, time, and mass. This month, we will review basic equations that will enable you to do quick calculations in your head while cooling off between runs….

…Technical people call this style of calculating ``back of the envelope,'' which is a somewhat picturesque reference to the habit we have of writing equations and numbers on any piece of paper that happens to be handy. You do it without calculators or slide rules or abacuses. You do it in the garage or the pits. It is not exactly precise, but gives you a rough idea, say within 10 or 20 percent, of the forces and accelerations at work.
And now you know how to do back-of-the-envelope calculations, too.”

OK, here he basically acknowledges that indeed, the equations (Models) used are indeed simplified, not a complete description of the Forces acting within a vehicle. The point being, that the simple relationships are sufficient for approximating a reasonable value, but, NOT an attempt to describe the process entirely.

Oh, and for the proportional braking issue which you guys also trounced upon me, here’s page 11:

“We can immediately see from this table (and, indeed, from the formulas) that it is the distance, not the time, that varies as the square of the starting speed v. The braking time only goes up linearly with speed, that is, in simple proportion.

The numbers in the table are in the ballpark of the braking figures one reads in published tests of high performance cars, so I am inclined to believe that the second way of looking at the problem is the right way. In other words, the assumption that the brakes are better than the tires, so long as they are not overheated, is probably right, and the assumption that brakes dissipate energy at a constant rate is probably wrong because it leads to the conclusion that braking takes more time than it actually does.”

Braking time is proportional according to him, NOT the distance! Thus, your source doesn’t exactly agree with you, now does it?

Lastly, I apologize for such a lengthy and somewhat snide post. Mr. Walton, you and Pseudo obviously are tremendous resources for factual automotive (and physics) knowledge. I admire the extent of details you are able to produce (Hell, it seems you know much more about the history and mechanics of my GT-S than I do). Truthfully, I’ve learned much from both of you. I do get quite peeved, however, when you attack and chastise the thoughts and opinions of others, especially in such a pompous manner. I think that we can all be much more productive if we take the time to listen and learn from each other rather than only seeking the opportunity to pounce and “prove someone wrong”. All I attempted in my initial post in this thread was to provide a viable mechanism. And even if I were wrong (which I am not, SUCKER!!!!) it wouldn’t justify the tone of your response. But then again, that’s only my opinion.

Hopefully, we can put this to rest now. Have a beer on me – Cheers!

carguycw

OK, this is my last post on this, because this is becoming the Thread that Would Not Die.

Fossil Boy: I apologize for my occasional bouts of snideness. I am somewhat of a smartass, it's a bit of a personality flaw of mine. However, I think that a quote from an old Paul Newman movie is in order: "What we have here is failure to communicate."

First, I know that the issue is more complicated than just a couple of forces acting on a rigid "blob" because, in reality, the tires will deflect, the center of gravity will move due to nosedive, aerodynamic drag and lift forces come into play, etc. etc. etc. However, I am an engineer, not a scientist, and I like to keep things simple. To me, spending 5 minutes to solve one equation and get a solution that's 95% correct is better than spending all day solving complex differential equations to get a solution that's 98% correct. That's just me.

Second, the people on this forum are mostly newbies with little knowledge of math or physics. For them, the simpler the explanation, the better. I usually try to avoid snideness, and try to make my explanations as straightforward as possible. I was just getting frustrated at the fact that this thread wouldn't die. Another bad habit of mine.

Third, I still think you were overcomplicating things. I was trying to keep the discussion centered ONLY on the forces acting on the car, not the mathematical relationship of velocity vs. distance, the rate of energy dissipation, or other stuff like that. [BTW I never said that braking distance is proportional to velocity- I was trying to concentrate ONLY on force. But I digress.] Yeah, you might be interested in all that stuff, but I thought that was beyond the scope of the discussion, which brings me to my fourth point...

After your last post, I actually agree with most of your points. However, I was unaware of it until the last post. I am saying this with as little snideness as possible... you need to study up on technical writing. No offense, but I really couldn't understand what your first couple of posts were actually saying. Like Pseudo mentions, you need to make your definitions more clear. I have done a LOT of reading in my time, and your first several posts, although I'm sure they made perfect sense to you, made VERY little sense to me. Basically, it sounded like you were spouting off a bunch of 50-cent buzzwords ("curvi-linear trajectories", "threshold forces", etc.), intermingled with philosophical ramblings about Nature and Man, but without any explanation in plain English. I misinterpreted your high-falutin' writing style for an attempt to make me look like an idiot, without really providing any explanation of why I was wrong- and I tend to react badly to assaults on my intelligence (another annoying personality trait of mine.) Sorry for the misunderstanding, but please understand that this is a public forum, try to keep your explanations simple and limit your words to 3 syllables or less.

Again, peace. May the next discussion be more fruitful. I am signing off now.

TheMAN

and this is becoming one of the threads that's giving me a migrane... who deleted my post earlier? I hope everyone knows that posts are *not* to be deleted unless it's really bad