Spring rate
The number of pounds required to compress the spring 1 inch.
Stiffer springs require more weight to compress than soft springs.

You could also view this is the pounds of force that the spring pushes with after it's been compressed one inch.
It's the same thing.


Motion Ratio
The distance that the spring moves (compresses or expands) is not necessarily the same amount that the wheel moves.
This picture of a coil spring shows that the distance the spring moves (ds) is not the same as the distance that the wheel moves (dw).

Torsion bars are a different type of spring.
Because torsion bars ARE the pivot point, they have a motion ratio of 1 to 1 (1:1).

The length of the lower control arm is irrelevant to the motion ratio of a torsion bar.


However, the length of the lower control arm IS relevant to the pounds of force that the torsion bar applies at the wheel.
This amount of force at the wheel is called the wheel rate.

Imagine for a minute that the lower control arm is a lever that you are using to twist the torsion bar.
A longer lever makes it easier to twist the bar.
This means that a longer lower control arm will have a lesser wheel rate than a shorter lower control arm.

Here is a website with an explanation and formulas for calculating the wheel rate for any torsion bar.
It also has a calculator to do the math for you.
You'll need to know the length of your lower control arm,
the active length of your torsion bar,
and the diameter of your torsion bar.

https://swayaway.com/tech-room/torsion-bar-wheel-rate-calculator/

1962 to 1972 B bodies & 1970 to 1974 E bodies all use torsion bars with an overall length of 41 inches.
The hexes on each end of the torsion bar do not twist. Only the distance between the hexes twists. That distance between the hexes is the active length, and it's 39 inches.

In most cases, the active length of your bar and the length of your lower control arm will never change.
This calculator is primarily used for comparing the wheel rates of different diameter torsion bars.
It can also be used to compare the wheel rates advertised by different bar manufacturers.

Ok, time to measure the length of a lower control arm.

1962 to 1972 B bodies & 1970 to 1974 E bodies all use the same length of lower control arm.

On these cars, the main difference on the lower control arms is the position of the sway bar mounting tab IF the car was equipped with a sway bar at all.
1966 to 1969 sway bar tabs are positioned more outboard (towards the ball joint) than 1970 and later sway bar tabs.

If you plan to use the wider 1966 to 1969 sway bar with disc brakes, make sure that it doesn't hit the brake caliper when turning the steering all the way lock to lock.
It should work fine with 1966 to 1969 disc brakes, but if you're installing 1970 or newer brakes on an older car, you may have interference issues.

The 1970 and later lower control arms that use the narrow sway bar can be installed on older B bodies with careful drilling and clearancing to route the later sway bar through the K member instead of in front of it.


These pictures are of a 1970 to 1972 B body & 1970 to 1974 E body lower control arm with a sway bar tab.

The first picture shows pretty well how the measurements were taken from the centerline of the pivot to the center of the ball joint hole. 12 9/32 inches
I also measured from the centerline of the pivot to the center of the sway bar hole. 6 7/16 inches

The second picture looks a bit odd due to the perspective of the photo, but seeing it from this angle may help some people to envision the movement of the lower control arm.


I'M CERTAINLY NOT A SUSPENSION EXPERT, SO IF YOU SEE SOMETHING WRONG WITH THE MEASUREMENTS OR INFO, PLEASE SAY SOMETHING!

I went ahead and did the math...

Stock B & E body torsion bars
.86 diameter 102.4 pounds per inch wheel rate
.88 diameter 112.3 pounds per inch wheel rate
.90 diameter 122.8 pounds per inch wheel rate
.925 diameter 137.0 pounds per inch wheel rate

Firm Feel B & E body torsion bars
.845 diameter 95.4 pounds per inch wheel rate
.88 diameter 112.3 pounds per inch wheel rate
.94 diameter 146.1 pounds per inch wheel rate
1.00 diameter 187.2 pounds per inch wheel rate
1.06 diameter 236.3 pounds per inch wheel rate
1.12 diameter 294.5 pounds per inch wheel rate
1.18 diameter 362.9 pounds per inch wheel rate


Many people highly recommend Firm Feel torsion bars.
Firm Feel has done the work to make sure that the hex ends are correctly indexed (clocked) on larger than stock bars, so that you don't have ride height issues after installing them.

I notice that these rates don't quite match the Firm Feel advertised rates.
http://www.firmfeel.com/b_body_mopar_torsion_bars.html

My guess is that Firm Feels lower control arm measurement is about half an inch longer. Perhaps they measured all the way to the end of the lower control arm, instead of stopping in the middle of the ball joint mounting hole?
This illustrates why it would be a good idea to calculate the rates if you're comparing torsion bars from different vendors. How they measure things can affect their advertised wheel rate.


FOR THOSE OF YOU WHO ARE MORE EXPERIENCED, IS MEASURING TO THE END OF LOWER CONTROL ARM, INSTEAD OF TO THE CENTER OF THE BALL JOINT MOUNTING HOLE A BETTER METHOD?

So how do you choose which torsion bar size to install?

It would be a good idea to take care of a few other things before choosing your torsion bars.

1. Select the wheels and tires that you want to run.
Wheel size can limit your brake selection.
Tire sidewall profile affects handling.

2. Do any brake upgrades. Being able to stop is important.
11.75" rotors should fit in 15" wheels, or if you have bigger wheels, you can get bigger brakes.

3. If you want to remove any weight from the car, or shift any weight from the front to the back, now is the time.
Try to have 50% to 55% of the total car weight being supported by the front tires. You can check your work on a weigh scale at the local truck stop, dump, or recycling center. Get a weight with only the front wheels on the scale, and get another weight with the whole car on the scale.
(Front weight / total weight) x 100 = front weight %


Ok, now that the cars weight has been determined, you can select torsion bars.
If you're not sure what to choose, AR Engineering suggests a rule of thumb that torsion bar wheel rate should be 10% of your front end weight.
The torsion bars job is to support the weight of the car, so it makes sense that the cars weight should be considered when selecting torsion bars. Heavier cars need higher spring rates. Every car is different.

http://arengineering.com/tech/torsion-bar-tango/

Nice work! Thanks for the info.
Wes

The Sway Away calculator shows that for their applications, which is a kind of neat, but still not exact since they are factoring that for their sprint car applications. So there could be some slight variations to raw geometric calculations. However, what could be the impact on their t-bar rate calcs is whether you use the entire torsion bar length from end to end vs using only the active portion between hex ends up the radius. Since they point that out on their active length number, I'd imagine the Mopar version differs slightly from their sprint car version. What we don't know is the spline size they are using to determine their active length and how that translates to to the Mopar hex ends to active length relationship.

IMO, differences under 10# aren't easily seen on a stop watch, never mind the butt dyno.