Re: {Chrysler 300} Torsion Bar Diameters
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Re: {Chrysler 300} Torsion Bar Diameters



I doubt that you removed any metal. We spent most of 2 days in October 1978 with a hand grinder at Road Atlanta trying to remove enough material to soften a round sway bar by removing metal over the center section to a slightly D shaped bar. Sand paper wouldn't do anything but remove paint.

Paint thickness 0.005 (0.0025 on radius?) probably nominal and wont contribute to stiffness. Better to have the metal only dimension.

On 10/25/2023 1:31 PM, James Douglas wrote:

Ed,

 

Thanks for the math lesson. I am aware of everything you said.

 

I plan on getting the car weight this week to see what the total weight is and the front-rear weight. Curious as to the modern, as opposed to the old rule of thumb, which today is 10% of the front-end weight for spring rate. That is generally much stuffer than the old days. I do not want to go there and the new Firm Feel Springs are 1.06 which I thing may be too much.

 

I sanded one spot on a bar and removed the paint on a bar and got a .005 difference. Hence the warning to not sand on the bars for establishing the diameter.

 

I plan on getting new springs for the rear. Eaton has the rate and load data for both the 4-bbl and RAM cars.

 

I just want to get a handle on the t-bars, their part numbers and the corresponding diameters.

 

James

 

 

From: chrysler-300-club-international@xxxxxxxxxxxxxxxx <chrysler-300-club-international@xxxxxxxxxxxxxxxx> On Behalf Of Edward Mills
Sent: Wednesday, October 25, 2023 11:17
To: chrysler-300-club-international@xxxxxxxxxxxxxxxx
Subject: Re: {Chrysler 300} Torsion Bar Diameters

 

First, torsion bars are designed for rotation. Torque in ft-lbs (NOT POUNDS) is related to rotation (in degrees or radians NOT INCHES). To think of a stiffness at the wheel, you MUST INCLUDE ALL THE OTHER COMPONENTS - LINKAGE / DIMENSIONS necessary to convert the twist to a linear (up and down) motion. These will also affect the net stiffness at the wheel. So if you are talking in terms of 125 or 130 POUNDS stiffness (force) you need to know the rest of the geometry including where the force is applied and where the linear motion is calculated.

Second, the torsion equation is Rotation = (Torque x Length) /  (G x J) where G is the Shear Modulus - a material constant and J is a function of the cross section. In the case of a round bar J = 1/2 x Pi x Radius to the 4th power. G is essentially a constant for steel (as opposed to Aluminum or other materials). So you are correct, a small change in radius can be significant to the rotational component of the motion. 

In the case of a 1.000 bar vs an 0.980 bar, the difference is the 1.000 inch bar is torsionally 8.5% stiffer. A 1.06 bar vs an 0.982 bar is torsionally about 35% stiffer - and to me that would be a concern.

But if you are debating an 0.983 vs 0.982, the difference in the torsion bar is only 0.4% stiffer (probably within tolerances).

Third, the difference in sanding paint off is negligible. You want the steel dimension as even a few thousandths of paint does not contribute to the stiffness. Further, spring steel is tough - you are not going to affect the diameter of a hardened steel bar with sandpaper (don't ask how I know). If you do, believe me, you have other problems.

Hopefully someone can get you the measurements you need - but be aware changing the wheel rate (stiffness) on one end of the car without changing both ends proportionally will also affect the handling. Stiffening just the front may add understeer while stiffening just the rear (or softening the front) can add oversteer. Small changes may be negligible but a 35% increase in front stiffness could add to the understeer that most cars of that vintage had.

Best, Ed

On 10/25/2023 10:07 AM, 'James Douglas' via Chrysler 300 Club International wrote:

Hi All,

 

I know I have asked this before. In the next few weeks, I will rebuild the front end of my 1964 300K single 4BBL.

 

What I need from the group is a very careful measurement of the front torsion bars for a 1962 300H, 1963 300J or 1964 300K RAM. When I say careful, I mean it as factory torsion bars are produced with diameter increments of only 0.020. A small increase in size makes a big increase in spring rate.

 

Even when cleaning a bar to take a measurement, if one uses sandpaper or the like to get through the paint, that can remove 0.020.

 

I am hoping two or three of you can take some lacquer thinner and clean a spot on your bars, assuming you have access, and take calibrated caliper and measure your bar. I think it should be someplace around one inch.

 

Some bars that I have out of a car are:

 

709 Bar = 0.983

708 Bar = 0.982

 

I have been told that a 1962 300H also has the 606 and 606 bars which should be the same as 300J or 300K RAM. That bar is reported to be 1.01 inch and has a rate of 130 pounds. My problem is that one source says that the 1962 rate is 130 while the and the 1963 is 125 all the while the part numbers are the same (606 and 607)

 

Given that a .002 change in diameter makes a big change in spring rate, I am trying to do exactly what the factory did and not guess or play around. To date, I have not been able to find a set of 606 and 607 torsion bars. The folks at Firm Feel have a new set they made, at 40 inches long which is 1.060 which I fear may be too much.

 

Decades ago, I ran across a 300 parts car that I purchased that had the RAM-Trailer Towing package on it and I took it all off and used in that 300K hardtop. It was a big improvement in handling without being too harsh. That is what I want to achieve now.

 

This is why I am asking for help. A good list of the bars and the exact diameters would not be a bad thing to have.

 

Thanks all, James

 

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