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
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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