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Post by Bracketracer on Oct 11, 2017 19:12:37 GMT -5
So, the first statement is correct. It would be harder to start and stop the motion of the higher moment of inertia (weight to the outside). The second statement, it is opposite. The same force or energy would cause smaller and slower oscillation in the system with a higher moment of inertia. Assuming that Z axis is vertical, X axis is parallel to the track lane, and Y axis is perpendicular to the track lane:
OK, because the direction of the wobble is actually side to side (moving + or - in the Y axis as the car tries to rotate about it's COM), not in rotation (the Z axis tips left and right like a tight rope walker would experience) how does having the weight on a pinpoint dead in the center or spread wider affect the oscillation?
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Post by Chuy on Oct 11, 2017 22:19:27 GMT -5
So, engineering convention typically describes rotational motion around the axis it spins on. So yes, although the motion is in the x-y plane, it is usually described to rotate around the z axis. In the end, they really are arbitrary directions and conventions, what is important is that we agree to one and stick to it.
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Post by Professor Moriarty on Oct 11, 2017 23:07:05 GMT -5
So, engineering convention typically describes rotational motion around the axis it spins on. So yes, although the motion is in the x-y plane, it is usually described to rotate around the z axis. In the end, they really are arbitrary directions and conventions, what is important is that we agree to one and stick to it. I’ll see your axis and raise you a direction! Which rear wheel are we talking about that crashes into the rail? |
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Post by Chuy on Oct 11, 2017 23:39:49 GMT -5
I don't actually think it is hitting, i think it is weaving
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Post by Chuy on Oct 12, 2017 9:34:17 GMT -5
Well, the physics behind it is a differential equation with seven degrees of freedom requiring eigenvalues and numerical methods to solve. In other words, I am having to study a specific problem with a different way and equation of solving each scenario that engineering and physics have no solution to the equation. Having to rely on multiple iterations of "guesses" to find the answer. I'll be reading for a while....
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Post by Chuy on Oct 12, 2017 9:49:10 GMT -5
There are lots of possible solutions bouncing in my head right now from rear wheels that are "too" well balance to cause a matching natural frequency in each with no ability to dampen. It's like holding a "G" note on the E string of a guitar that makes the G string vibrate also. Definitely an interesting problem
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Tungsten
Oct 12, 2017 10:57:41 GMT -5
via mobile
Post by Bracket Also on Oct 12, 2017 10:57:41 GMT -5
I don't actually think it is hitting, i think it is weaving Chuy, you can hear when the wheel strikes the rail. I have no doubt what it is when I hear that sound. |
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Post by Chuy on Oct 12, 2017 17:10:53 GMT -5
Thank you, that feedback helps. So does it run true, slap, and then wiggle??
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Post by Professor Moriarty on Oct 12, 2017 18:55:44 GMT -5
Pulling teeth!
given the number of questions I have.... and the rate at which the answers are trickling in...
I have a feeling that we are going to be on this thread for a looooong time.
Now... I am not saying that the wheel didn't hit... and maybe your alignment was handed down to you by the great builder himself...
but considering that others have had success with side weights... and a few vendors found it worth their time to produce their own styles...
Perhaps we can narrow down what some of the others were doing to keep the rears off the rail.
Which type of side weights were you using? The original Puma Weights? The Cub Weights?
Did you try a few different weighting scenarios with the remaining weight needed?
Why were you suggesting to Pinewood CNC that he create a D4D style car (which obviously uses side weights)?
Do you suspect that John and Joel also saw this wiggle/slap but decided to foist them onto unsuspecting customers when they were first being sold?
Have you tried just a single weight on the dominant side?
I seriously have about 25 more questions off the top of my head.
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Tungsten
Oct 12, 2017 19:19:44 GMT -5
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Post by Chuy on Oct 12, 2017 19:19:44 GMT -5
I agree, this generates a lot of questions. Does anyone have a wiggler that they are willing to let me look at?
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Post by jspearm1983 on Oct 12, 2017 20:22:08 GMT -5
Couldnt you do two sizes of these? One a little wider to go behind the rear axle (and leave no wood on the car body edges), and then one a little more narrow to fit between the two 1/8th inch sides of the car body? If you didnt want the extra weight of the rear plate that is full car-width, you could taper the back of it for a teeny tiny aero advantage.
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Tungsten
Oct 12, 2017 22:15:05 GMT -5
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Post by Bracket Also on Oct 12, 2017 22:15:05 GMT -5
Thank you, that feedback helps. So does it run true, slap, and then wiggle?? For me it got off the hill fine, the wobble built up until it was banging the rail by about the middle of the flat. |
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Tungsten
Oct 12, 2017 22:37:55 GMT -5
via mobile
Post by Bracket Also on Oct 12, 2017 22:37:55 GMT -5
Pulling teeth! given the number of questions I have.... and the rate at which the answers are trickling in... I have a feeling that we are going to be on this thread for a looooong time. Now... I am not saying that the wheel didn't hit... and maybe your alignment was handed down to you by the great builder himself... but considering that others have had success with side weights... and a few vendors found it worth their time to produce their own styles... Perhaps we can narrow down what some of the others were doing to keep the rears off the rail. Which type of side weights were you using? The original Puma Weights? The Cub Weights? Did you try a few different weighting scenarios with the remaining weight needed? Why were you suggesting to Pinewood CNC that he create a D4D style car (which obviously uses side weights)? Do you suspect that John and Joel also saw this wiggle/slap but decided to foist them onto unsuspecting customers when they were first being sold? Have you tried just a single weight on the dominant side? I seriously have about 25 more questions off the top of my head. Joe, am I the only one that reported an instability when running cavity weights? We did run both the full weight and the half weights as well as the prototypes you sent. I had to go back and read the thread where I posted D4D's car. It was in response to a drawing of pinwoodcnc's where I thought he had come up with an original idea that needed a few tweaks. D4D's car was a TWO OUNCE Governor class car and therefore could get by with his homemade lead cavity weights and very little additional ballast. I don't know how closely pinewoodcnc follows league racing and I thought seeing that car might help guide him to a faster car of his own design. Some people did have success while using cavity weights, true, but I did not. Why? I don't know. Why aren't they using them now? THAT I could probably answer but you probably wouldn't want to hear it and I doubt it would be a productive conversation. Did I try a half weight on one side? No, but I still have the car you built like that and if you remember it was gawdawful slow. |
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Tungsten
Oct 12, 2017 22:41:18 GMT -5
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Post by Bracket Also on Oct 12, 2017 22:41:18 GMT -5
Oh, and BR asked me to add that the guys that were faster than him with the Puma weights on were faster than him when they weren't running Puma weights so it's likely it wasn't the weights that made them fast.
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Post by Professor Moriarty on Oct 12, 2017 23:52:23 GMT -5
Thank you for taking the time to answer a few of these questions, Mr. D.
I think that you have cleared things up for us over here.
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