Sigh... first off, let me apologize that the examples I've tried to use to illustrate what gernby's drivetrain is undergoing have been difficult to understand. Unfortunately, my ability to communicate the concept effectively to a layperson lags behind my personal understanding of the subject.

By the way, for those who still believe inertia doesn't exist, you want want to do a little research into the modeling of dynamic systems. A good topic to start with would be D'Alembert's Principle and how it is applied to the modeling of masses in dynamic systems.

Let's try one last example here. Should you be interested in further research on the subject, I would suggest Modeling and Simulation of Dynamic Systems, by Dr. Robert L. Woods and Dr. Kent L. Lawrence. You might recognize Dr. Woods' name - he's the driving force behind the most successful Formula SAE Team in history, and the man for whom the SCCA Dr. Bob Woods Cup Award was recently named. For what it's worth, I've studied rather extensively under both Dr. Woods & Dr. Lawrence.

Now, onto the final example I'm going to post in this farcical thread. Take two arbitrary masses resting on a flat plane, and connect them with a spring. Now, place a force on the first mass, acting to pull the two masses apart. What happens? The first mass is accelerated in the direction of the force vector. This force is in turn transmitted through the first mass to the spring. The spring elongates, but in turn passes the force along it's length to the second mass, where it acts to also accelerate the second mass.

Now here's the interesting part. As the first mass moves, the spring is not a rigid link, therefore the force felt by mass two will not be equal to the force felt by mass 1 at any given moment until the system has come to equilibrium (a condition we will only achieve if we have a constant or linear system input and a damped system). The spring will stretch as it is loaded, and the force felt by the second mass will be determined by the distance the spring is stretched and the spring constant.

Don't believe me? Here's a simple way you can illustrate it to yourself in the real world. Take a couple of Hot Wheels or other small, wheeled objects. Attach one to the other using a small, light spring out of a pen or some other household object. Pull on the first car. Does the second car move with the same acceleration? During the transient response phase, it most definitely does not. What happens to the spring? It is stretched to some length greater than its "at rest" length.

Now, if you add a damper to the spring, the transient oscillations of the system will eventually be reduced to zero, and the steady-state response of both masses will be the same. However, the steady-state response isn't what we're concerned with, because the spring will see its highest loading and deformation during the transient system response phase.

So what happens if I keep the first mass the same, the spring the same, but double the second mass? Simple - the amplitude of the transient load and deformation oscillations will be doubled. In layman's terms, the maximum amount that the spring will be stretched during the transient response will be twice that of the previous example. You can model this as well, by rigidly attaching a 3rd Hot Wheels car to the second one in our little real world example. Double the second mass, double the maximum transient extension of the spring.

OK, so why do we care about this? Well, in the real world, where no part of gernby's car, trailer, or drivetrain is infinitely rigid, the best way to approximate the behavior of a mass in a dynamic system is by breaking it up into thousands of discrete, interconnected, spring/mass/damper systems. This is the best way to simulate the deflection and distortion of elastic materials (like aluminum, steel, iron. etc.) in response to loads and stresses.

Unfortunately, the simple rigid body dynamics that Blitz and gernby keep referring to do not have the capacity to account for or properly model the deflection and distortion of the various components that make up this real world and decidedly non-rigid system.

To put it a little more simply - the spring in the above is a rough approximation of gernby's drivetrain - double the load, double the distortion that the components will feel during the transient phase of the dynamic system response. Hopefully that additional distortion is below the design limits of the system.

On a second to last note, gernby - you called my expertise on the subject into question a couple of pages back. Let me state my credentials (such as they are) for the record: I'm am a senior undergraduate student in Mechanical Engineering, who will graduate next spring with a BSME and a minor in Materials Science with a focus on Failure Mechanics. Contrary to what some of you have suggested, I do know what I'm talking about.

Perhaps, since you seem to be such an authority on the subject, you'd care to share your credentials with our viewing audience?

Lastly, on the somewhat related subject of fatigue failure and your hitch - I did some work under one of my professors over the summer for General Electric, researching fatigue failures on turbine blades. One of the things I learned during the course of this work was that initial fatigue cracks are often way too small to detect with the naked eye, and - depending on the material and the magnitude of the overload condition - can progpagate fast enough that you'll have absolutely no visual warning of an imminent failure condition. In other words, just 'cause you can't see any cracks, don't mean it ain't gonna break on you the next time you exceed the design load.

That's it guys - respond all you want. I've wasted enough time on this thread over the last couple of days, and I wont waste any more. In parting, let me just say that I hope for gernby's sake that the additional loads he is subjecting his drivetrain and hitch do not cause him any grief while he continues to throw caution to the wind. Good luck, gernby - you're gonna need it.

Intuitive physics... indeed.

 

As a fellow boat owner (and one who refuses to buy a proper tow vehicle)... I just want to say this was a highly entertaining thread.

 

your example is still showing the acceleration as being constant. in your example the accelartion is equal, so the force greater due to the greater mass. but, it's not, acceleration can be controlled. we are accelerating slower, and not trying to do a 14sec quarter mile.

again, double the mass, 1/12 the acceleration (0-5 vs 0-60 in 6sec), the spring will deflect less because the force will be 1/6. you are assuming the acceleration is constant. Its not, it is less, so the force is way less.

as for credits, i'm an Actual Mechanical Engineer working with as structures and mechanical systems (hydraulic, and pneumatic systems) for the an Airline.

you should show this thread to your professors, and ask their option.

as for the fatique issue, turbine blade failure senarios are different. i would imagine the cycle duration less than a second. towing a boat, cycles are every time you stop and go. you can detect the crack before it becomes critical, unlike a turbine blade.

 

just want to add this:

using your example with the hot wheels.
-do the first run normal. measure the distance the spring elongated.
-add the third car
-pull the front hot wheel slowly just slightly to about half the distance you first elongated the spring the first time. you will notice that the 2 rear cars will start to move, not at the same rate as initially, but slower, but they will still move. with out elongating the spring lonr.

do you get it now. acceleration is less.

 

Iain, I think you are doing a fantastic job modelling the forces and stresses of the frame and hitch pulling the boat. I see nothing in your previous 2 long winded "explanations to lay persons" that even remotely approach that of the drivetrain. Neither mention the force being applied to move the objects being constant as it is with the drivetrain when pulling the boat. Actually, for your models to work, the forces could not be equal.

As for credentials, I doubt mechanical engineers study Engineering Physics I (kenetics, motion, accelleration, energy, F=MA, etc.) more than once during the curriculum, do they? I know in my engineering curriculum (Telecommunications) we never studied anything twice, but we took the same calculus based Physics class as every other engineer at Texas A&M (including MEs). Honestly, it was my best course throughout my entire college carreer. I'm fully aware that your knowledge and expertise in fatigue and mechanical failure are vastly greater than mine, but that really isn't what we are disputing. We are disputing the force that the drivetrain has to tollerate.

You know, since you believe you are dealing with such inferior intellects here, maybe you should point out the error in our thinking directly instead of recreating more and more theoretical models that I don't believe relate to the drivetrain one bit. So far, the most comfortable model that I've come up with is not a model at all. The ACTUAL situation is that it only takes about 400 lbs to accellerate the boat up the ramp (slowly), another 400 lbs to accellerate the car up the ramp (also slowly), and the drivetrain is designed to put out over 2000 lbs of force. I'm not even getting close to the design limits of the drivetrain. Please ... where's the flaw in that?

BTW, did you watch the video? Does it look like the engine is even having to struggle? I would be surprised if the peak output from the engine is over 80 ft-lbs. That's just a bit over half what it has to endure during a WOT accelleration run in 1st gear.

 

Ahh- they're too busy comparing Slinkies with pogo sticks to notice that we're here. We can talk about them all we want out in the open, and they won't even notice it. It has become painfully obvious to me, however, during the course of this thread, that one of the strong points in an Engineer's curriculum is certainly not spelling They are a good source of entertainment, though

 

Precisely. Yet you keep going on and on and on and on and on.....

 

When did you have time for this.....with "yell" practice and all?

I got in trouble for calling it "yelling" practice once!!

 

Dude, the video is hilarious. As for any potential damage to your car, I guess time will tell (screw engineering/physics models*). Thanks for sharing!


*no offense to the engineers/physics gurus here, u guys are smart and i enjoyed the mental sparring

 

All of the arguing over the drivetrain and flex is irrelevant. Most cars and trucks are over engineered from that standpoint anyway.

My boat and trailer weighs 9000lbs dry and about 9500lbs most of the time that I tow. Keep in mind, I tow with a vehicle rated to 9100 lbs.

The problem has never been starting/stopping or even pulling her out of the water. It is lateral stability when cruising down the highway.

The suspension will not hold up to highway speeds for long and could potentially cause a jackknife. It is simply too unstable, especially with a poor tongue weight. tongue weight should be 7 to 10 percent of the total. In this case, 210 to 300lbs.

But, I think I read that gernby is only going short distances. So, who cares? If it works, great. Don't worry about what anyone else thinks. But, if I see you on the highway going 65mph, I will be staying waaay back

Good luck!!

Eric