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How does torsion-level keep the car level on braking and acceleration?
#1
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humanpotatohybrid
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As I have read in the documentation regarding TL I understand that if you hit a bump in the front, the front wheels being pushed into the car transmits a moment to the rear wheels that pushes them away from the car, the effect being similar to the whole car hitting two smaller bumps at the same time. And the same effect happens when hitting a bump with the rear axle.

What I don't yet understand is how this system reduces pitching on acceleration and braking. A horizontal force on the bottom of the wheels being below the center of mass, a moment is created on acceleration that pitches the car down on the side opposite the direction of acceleration. But wouldn't this effect be transmitted thru the TL system the same way that bumps are, in that one axle going down would raise the other? Is the effect simply due to the moment about the center of mass being much less on normal acceleratory conditions than a sharp bump?

Posted on: 7/15 15:41
Owner of '55 Clipper Deluxe. Engine is stuck-ish.
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Re: How does torsion-level keep the car level on braking and acceleration?
#2
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HH56
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The SAE publication covers a bit of the theory behind those questions. In actual practice I cannot see as much dive or squat reduction as they claim but maybe some is there. The article is on site if you have not had a chance to read it. https://packardinfo.com/xoops/html/downloads/SAE_Torsion.pdf

You might also search youtube for some videos on TL tests. There are a couple of videos where the effect is seen and a halfway funny one where they did a test with a Cadillac, another hi end car maybe Lincoln and the Packard over a notoriously rough Detroit RR crossing. The Packard saided thru while the other two had interesting experiences.

The comparison demo is also In the Allison DVD sold by Motor City Packards as well as a video of a 54 Packard going over the same tracks. IIRC the DVD said the 54 was owned by one of the engineers doing th demo so it was not driven anywhere near the same speed as the cars in the demo. That is probably a good decision because by the looks of things at a slow speed, the Packard would not have fared well either if it went much faster.

Posted on: 7/15 16:05
Howard
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Re: How does torsion-level keep the car level on braking and acceleration?
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Ross
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The essence of TL is that it gives the effect of suspending the car at its center of gravity--or at least near it. That is the goal of all interconnected suspensions from a Citroen 2CV to an Austin "landcrab". That way a bump results in upward translation of the whole vehicle rather than rotation about the CG. Thus the disturbance is far less.

Naturally small changes in load move the CG thus the leveler is necessary.

To your question, the rear torque arms pull the back of the car somewhat downward during braking and thrust it upward during acceleration. That helps counteract the CG being above the road surface.

A TL car in good fettle will actually rise slightly on brisk acceleration.

Posted on: 7/15 20:38
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Re: How does torsion-level keep the car level on braking and acceleration?
#4
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humanpotatohybrid
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Howard, thanks for the info but aside from your 3rd paragraph I've already read/watched through those resources. I'll try and find that train track video though.

Ross thanks for the explanation. However I'm still confused on this:
Quote:
the rear torque arms pull the back of the car somewhat downward during braking and thrust it upward during acceleration

What physically causes this effect? The reaction torque of the wheels on the suspension system?

Posted on: 7/15 21:55
Owner of '55 Clipper Deluxe. Engine is stuck-ish.
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Re: How does torsion-level keep the car level on braking and acceleration?
#5
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HH56
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Here is a video of a presentation given at a PAC meet by one of the other members of the suspension development team. Has quite a bit of info but can get a little dry in spots. https://vimeo.com/281386084

Posted on: 7/15 22:16
Howard
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Re: How does torsion-level keep the car level on braking and acceleration?
#6
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humanpotatohybrid
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Thanks for the link to the nice video, I've watched about 1/2 of it so far.

So it's explained a bit more clearly in the video and it appears to be what I said in my previous response. First, as Ross mentioned the TL system has the effect of simulating supporting the car at two points near its center of gravity, not four points as it is supported in reality. But then my question was, what keeps the car from tipping forward or back on its center of gravity on acceleration? The answer is that the torque arms in the rear were intentionally designed such that the wheel torque on forward acceleration reacts through the torque arms, twisting them to raise the rear end of the car, counteracting the inertial effects that would otherwise make the rear end drop. The opposite effect occurs when braking. For whatever reason, the torque effect is stronger than the effect it cancels, making the car raise slightly on acceleration and drop slightly on braking (the change in height in the rear being transmitted to the front thru the TL system).

In the image below, the blue arm is the torque arm. The green arrow is forward wheel spin, and the red arrow is the reaction torque on the axle/torque arm assembly, transmitted through the torque arm it has the effect of pushing the car body up and the axle down, as shown by the pink arrows.
Resized Image


Assuming this explanation is accurate then I now understand it as well as I was hoping to.

Posted on: 7/16 10:52
Owner of '55 Clipper Deluxe. Engine is stuck-ish.
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