Understanding Suspension and How to Make Your Car Handle Corners Better

[cataclysm80]

Hello, this is Tavis King again.  I'm usually over on our E-Bodies.org Facebook group, where I help admin the group.  You might also know me as cataclysm80 on various internet forums, Mopar and otherwise.  I wrote this article years ago, but didn't publish it until now because there's one chart that's incomplete.  Hopefully someday I'll finish that last chart, but in the meantime, I really think the information here could help a lot of people. 
If you want to know which sway bars, torsion bars, or leaf springs to get, or if you just want to make your car handle better, this article is what you need. 
Most aftermarket suspension kits are one size fits all solutions, that aren't optimized for the specific weight distribution of your car.  Spend a little time learning the information in this article, and you can probably build a better performing suspension, for less money than the one size fits all kit.

This article is meant to teach the average car guy how to select suspension parts that will improve their car's handling, making it more fun to drive.
It's a lengthy article, but if you grab a calculator and follow along, you'll have a pretty firm grasp on Mopar suspensions by the end of the article.
It also includes some information on coil spring suspension if that's your thing.  Whatever you're working on, start with the basics in this article.
Anyone can do this, even you.  Just take one step at a time.

Let's get started...       


Suspension Basics
Good handling is all about controlling how the car's center of gravity pivots around the roll center.

Center of Gravity:  The center of gravity for the whole car is the exact center of the cars weight.  It's an imaginary point in space, but if you could lift the car and support it by a single post, the center of gravity is where you would need to position the post so that the car would be balanced and not fall off the post.
However, cars aren't supported in the center like a unicycle, they're supported by a front axle and a rear axle.  If you could lift the car and support it by a post at the front axle and a second post at the rear axle, so that the car is perfectly balanced and doesn't fall off the posts, the position of those posts is the center of gravity for each axle.
If you drew a line through the front axle center of gravity, and the rear axle center of gravity, the whole car center of gravity would be between them on that line (closer to the front because the front of the car is heavier).  If you could skewer the car on this line like a shish kabob, all sides would weigh equally so that the car would not spin from one side being heavier.  Instead it would be perfectly balanced, and it could easily be turned by hand.
It can be difficult to locate the exact center of gravity, but to give you a rough idea, the front axle center of gravity is usually located about where the crankshaft is at, and the rear axle center of gravity is usually located about where the trunk floor is at.
If you want to try and calculate center of gravity more accurately, here's a website.
http://www.longacreracing.com/technical-articles.aspx?item=42586 

[cataclysm80]

Roll Center:   The roll center is the point that the car's weight pivots around.  It's an imaginary point in space, and its location is determined by the suspension geometry of the car.
To find the front roll center, draw a line through the upper and lower control arms for one front wheel, with the intersection of those lines being the instantaneous center.  Then draw a line from there back to the center of the tire contact patch (the contact patch is where a line through both ball joints meets the ground under the tire).  Now do the same for the other front wheel.  The lines between the instantaneous centers and the tire contact patches will intersect at the roll center.

[cataclysm80]

The distance between the center of gravity and the roll center, works like a lever when the car is cornering.
The roll center is the fulcrum of the lever.
The distance between the center of gravity and the roll center, is the length of the lever.
The weight of the car is the force that's being applied to the lever.

[cataclysm80]

Now that you understand how it works, you can see that there are two ways to improve handling here.
1. Reduce the cars weight.  This means you'll be applying less force to the lever during cornering.
2. Reduce the distance between the center of gravity and the roll center.  (usually by lowering the car)
     This means your weight will have less leverage during cornering.
Additionally, the car will have suspension springs, which will help to resist these cornering forces.

A few things to take care of before proceeding...
Before choosing your suspension springs, there are a few things you should do.
1. Select the wheels and tires that you want to run.
Wheel size can limit your brake selection.
Tire sidewall profile affects handling.   (low profile is more firm)

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.
If you're using 14" wheels, you're stuck with OE sized 10" rotors.

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 %     This is called Front Weight Bias.
These weight adjustments change the center of gravity for your car.  Every car is a little different, depending on what options it has and what upgrades have been performed.  Weighing your own car will give you the best results.  Copying someone else's suspension setup probably won't be a perfect match for your car, especially if they were just guessing when they put it together.

Selecting Suspension Springs: Spring Rate
The main purpose of the suspension springs is to support the weight of the car.
Springs are chosen by their spring rate.
Spring rate is 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 as 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).

[cataclysm80]

Motion ratio can be calculated by measuring the distance between the pivot point and spring, and dividing that by the distance between the pivot point and wheel.
For example, if the distance between the pivot and the spring was 4 inches, and the distance between the pivot and the wheel was 10 inches, then  4 ÷ 10 = .4 Motion Ratio.
This means that for every 1 inch that the wheel moves, the spring will only move .4 inches.
The wheel would need to move 2.5 inches in order to compress the spring 1 inch.
The Spring Rate is the force required on the spring to compress it 1 inch.
The Wheel Rate is the force required to compress the wheel 1 inch.
(Motion Ratio x Motion Ratio) x Spring Rate = Wheel Rate
Continuing the example from above, if the Spring Rate is 100 pounds, then
(.4 Motion Ratio x .4 Motion Ratio = .16) x 100 Spring Rate = 16 pound Wheel Rate.
This is the power of leverage.
16 pounds will move the wheel 1 inch.  Another 16 pounds will move it another inch.  Another 8 pounds will move it a half inch.  16 + 16 + 8 = 40 pounds to move the wheel 2.5 inches.
As we've said, 2.5 inches of wheel movement = 1 inch of spring movement.
100 pound per inch Spring Rate x .4 Motion Ratio = 40 pounds per 2.5 inches at the wheel.
You can see that it all works out, any way you look at it.
Mopars don't use coil springs though, they use torsion bars.

Torsion Bars
Torsion bars are a different type of spring.
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.
Because torsion bars ARE the pivot point, the distance between the pivot point and the spring is 0.
For this reason, we don't have to worry about motion ratio on 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.
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.
1962 to 1965 sway bars only came on police cars.  It was a different design, and poor quality.  It was better than nothing back then, but isn't really worth installing today.
Sway bars were available on some regular cars from 1966 to 1974.
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.  The 1970 and later sway bar is a much better design than the older style.  Sway bars are discussed more in depth later, in the Sway Bars section.




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.
This second measurement will be handy later when we talk about sway bars.
On the 1966 to 1969 sway bar control arms, the distance from pivot to sway bar hole is 10 1/16.
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.

[cataclysm80]

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 (listed above),
the active length of your torsion bar (listed above),
and the diameter of your torsion bar.
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.
https://swayaway.com/tech-room/torsion-bar-wheel-rate-calculator/


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.

So which torsion bars are best for your application?
That's where the vehicle weight is needed.  The spring's job is to support the weight of the car, so it makes sense that the cars weight should be considered when selecting springs. Heavier cars need higher spring rates. Every car is different.  For handling performance, the rule of thumb is that the wheel rate for each torsion bar should be about 10% of the weight supported by the front tires.

The largest diameter factory torsion bars came on cars that had torque boxes and leaf spring reinforcements to stiffen the chassis.  You should also install those components if you're using that size of torsion bar.
If your using larger than factory torsion bars, you should seriously consider additional chassis stiffening, such as subframe connectors, front shock tower bracing, and lower radiator support reinforcement.
Your chassis needs to be stiffer than your springs, or it will flex instead of the springs, which reduces the effectiveness of your suspension and eventually risks breaking body welds.
Usually you choose your torsion bars first, and then select the best leaf springs and sway bars to pair with them for your vehicles weight distribution.  It can be done the other way around if you're on a budget and trying to select the best part to pair with some of your existing suspension parts.  However, limiting yourself to parts you already have on hand, may result in suboptimal performance.


Leaf Springs
Leaf Springs are another type of spring, and they're also measured by spring rate.
To measure the spring rate of a leaf spring, place it upside down on a hard smooth relatively level surface, and measure the height from the surface to the highest part of the arch.  This is the uncompressed height.
Now put a heavy object on top, so that the leaf spring compresses at least one inch.  (This is where it's important to have a hard smooth surface, because the spring eyes need to be able to easily slide outward  when the weight is put onto the spring.  Smooth concrete is ok, a greased metal plate is better.)  Then remeasure  the height from the surface to that same highest part of the arch.  This is the compressed height.
Now weigh your heavy object, so that you know how much weight was applied.
For Example, you could use yourself as the weight, by standing on the leaf spring, and have an assistant take the measurement.  Then check your current weight on a bathroom scale.
With these numbers and some simple math, we can calculate the spring rate of the leaf spring.
First things first though, if your measurement is in fractions of an inch, you'll need to convert it to a decimal number.  This is easily done.  You take the top number (numerator), and divide it by the bottom number (denominator).   For example:    1/16th is 1 ÷ 16, which is .0625
Once your measurements are decimals instead of fractions, we can calculate the spring rate.
Uncompressed Height – Compressed Height = Travel
Applied Weight ÷ Travel = Spring Rate
If your measurements were in pounds and inches, then your spring rate will be the number of pounds required to compress the spring one inch.  I'm going to continue using pounds and inches in my examples, but the formulas still work if you're more comfortable with kilograms and centimeters.

[cataclysm80]

For example, 8.8125 Uncompressed Height – 7.25 Compressed Height = 1.5625 Travel
224.6 pounds Applied Weight ÷ 1.5625 Travel = 143.744 Spring Rate

Over time, leaf springs can lose some of their arch due to the constant pressure of the car's weight.  This changes the ride height (which moves the center of gravity), but it doesn't change the spring rate.  The spring rate only changes if the metal leafs lose mass, like if they're rusting away.  Metal loss lowers the spring rate.  Broken leafs also lower the spring rate, and should be replaced.  Old (or new) leaf springs can be re-arched to whatever ride height you want, and that's the proper way to adjust your rear ride height.


The wheel is located outboard of the spring, so Leaf Springs also have a Motion Ratio.
Center to center distance between the two leaf spring perches on the axle (Perch Width) ÷ center to center distance between the rear tires (Track Width) = Leaf Spring Motion Ratio.

If your car hasn't had its rear axle swapped for something else, here's the info you need.
This info is for 8.75 rear ends, and also Dana rear ends.  Both styles of rear end have the same width measurements, because that's what was required to fit into these vehicles.
Any size or width of factory drum brakes on these cars, can be installed onto any of these 8.75 or Dana rear ends, without affecting the Brake Drum to Drum Width.  Any difference in brake width is handled in the offset of the brake backing plate for those brakes.
Car                                    Leaf Spring Perch Width           Brake Drum to Drum Width
1962 - 1963 B Body & 64 Max Wedge  44                            58.5
1964 B Body except Max Wedge          44                            60.875
1965 - 1967 B Body                           44                            59.5
1968 - 1970 B Body                           44                            60.125
1971 - 1974 B Body                           47.3                          63
1971 - 1973 B Body Station Wagon     47.3                          64.375
1970 - 1974 E Body                           46                             61.625

The wheels mount against the brake drums, so the Brake Drum to Drum Width is the amount of space between the rear wheels.
To get the Track Width, we'll need some wheel measurements.
The wheel width that people usually talk about, is the width where the tire goes inside the rim.
What we need is the overall wheel width, which is measured from the outside edge of the rim.
We'll also need the Back Spacing measurement.
When you measure Back Spacing, make sure your straight edge is resting on the edge of the wheel, and not up on the tire.

[cataclysm80]

Overall Wheel Width ÷ 2 = Wheel Centerline.
Wheel Centerline – Back Spacing = Wheel Offset.
(Wheel Offset x 2) + Brake Drum to Drum Width = Track Width.

If you're running factory wheels, here's the info you need.
(The 15 x 7 Rallye wheel & 15 x 7 Plain steel wheel were both introduced on 1970 model cars.
The 15 x 7 Rallye was used in 1970 & 1971.  The 15 x 6.5 Rally was introduced around 1972.  The Magnum 500 was only available from Mopar in 14 inch diameter.  15 inch Magnum 500 wheels are either Ford wheels, or aftermarket creations.)
Wheel                       Wheel Width              Overall Wheel Width           Back Spacing             Offset
15 x 7 Rallye                     7                                     8                                 4.25                -.25
15 x 6.5 Rallye                  6.5                                  7.5                        (I'm still looking for this info)
14 x 5.5 Rallye                  5.5                                  6.5                               3.75                -.50
14 x 5.5 Magnum 500        5.5                                  6.5                               4.00                -.75
14 x 6 Plain steel wheel      6                                     7                          (I'm still looking for this info)
15 x 7 Plain steel wheel      7                                     8                                  4.25               -.25

For example, if you're running 15 x 7 Rallye wheels on a 1970 B Body, then
8 Overall Wheel Width ÷ 2 = 4 Wheel Centerline.
4 Wheel Centerline – 4.25 Back Spacing = -.25 Wheel Offset
(-.25 Wheel Offset x 2 = -.5 total offset for both wheels) + 60.125 Brake Drum to Brake Drum Width = 59.625 Track Width.
To continue with this example, 44 Perch Width ÷ 59.625 Track Width = .738 Leaf Spring Motion Ratio
For every inch that the rear wheel moves up or down, the leaf spring will move .738 of an inch.

As described earlier, the formula for wheel rate is
(Motion Ratio x Motion Ratio) x Spring Rate = Wheel Rate
Continuing with the example, (.738 Motion Ratio x .738 Motion Ratio = .544644) x 143.744 Spring Rate = 78.29 Wheel Rate.

As you can see, wheel offset affects your motion ratio, and thus affects your wheel rate.
Using these formulas, an offset that's a negative number, gives you a higher wheel rate than an offset that's a positive number.
If you needed to fine tune your wheel rate for handling, you could potentially change to wheels with a different offset, or add wheel spacers.  In some cases, this could be a better option than replacing or modifying the leaf springs for a different spring rate.


Torsion bars and leaf springs are required, just to hold the weight of the car.  They do their job constantly, 24 hours a day, 7 days a week, driving in a straight line, hitting a bump, cornering, or even just sitting in the garage.
However, when you turn a corner at speed, the weight of the car will shift to the side of the car that's on the outside of the turn, lifting the side of the car that's on the inside of the turn.  This is called body roll.  To improve handling, we need to reduce body roll while cornering.  This is done with a special kind of spring that runs side to side on the car, and functions by twisting similar to how a torsion bar works.  It's called a sway bar (also known as an anti-sway bar or stabilizer bar).

Sway Bars
A sway bar only functions when one wheel is trying to lift and the other wheel is not, such as during cornering, or if you hit a bump with only one wheel.
For a single wheel to lift, it would need to overcome the stiffness of the sway bar.
Like other springs, sway bars have a spring rate, which is how many pounds of force it takes to move one end of the bar up or down by 1 inch.

There are two basic shapes of factory front sway bars, the 1966 to 1969 B Body sway bar, and the improved design which fits 1970 to 1972 B Body & 1970 to 1974 E Body.
These two designs use differently positioned sway bar mounting tabs on the lower control arms, so make sure to use lower control arms that match your sway bar style.  If your lower control arms don't have sway bar mounting tabs, you should be able to purchase some that can be welded on.

The 1970 and later 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.
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.

[cataclysm80]

An original 1966 to 1969 B Body front sway bar is 15/16 diameter = .9375
Firm Feel offers it in 1-1/8 = 1.125
Firm Feel offers it in 1-1/4 = 1.25

Most original 1970 to 1972 B Body & 1970 to 1974 E Body front sway bars are 7/8 diameter = .875
The 1970 Challenger T/A & AAR 'Cuda came with a front sway bar that was 15/16 diameter = .9375
Firm Feel offers it in 1-1/8 = 1.125
Firm Feel offers it in 1-1/4 = 1.25

These are all solid bars.  Some aftermarket companies may offer hollow bars for weight savings.
Having a hollow bar reduces the stiffness of the bar, but it's usually better than having a small bar.

The formula for calculating the spring rate of a solid sway bar is...
((diameter x diameter x diameter x diameter) x 500,000) ÷ ((A x A x A x .2264) + (C x C x B x .4244)) = Sway Bar Spring Rate

I won't spend much time on hollow bars in this article, but if you need it, the formula for calculating the spring rate of a hollow sway bar is...
(((outside diameter x outside diameter x outside diameter x outside diameter) – (inside diameter x inside diameter x inside diameter x inside diameter)) x 500,000) ÷ ((A x A x A x .2264) + (C x C x B x .4244)) = Sway Bar Spring Rate

A quick note on mathematical order of operations: solve inside parentheses first, then do multiplication & division from left to right, then do addition & subtraction from left to right.

For example, an original 7/8 diameter 1970 solid sway bar is...
((.875 diameter x .875 diameter x .875 diameter x .875 diameter = .5861816) x 500,000 = 293,090.8) ÷ ((10.125 dimension A x 10.125 dimension A x 10.125 dimension A x .2264 = 234.99654) + (8 dimension C x 8 dimension C x 27 dimension B x .4244 = 733.3632) =  968.35974) = 302.66727 Sway Bar Rate

Front Sway Bar Motion Ratio
The wheel is located outboard of the sway bar end link mount, so a sway bar also has a motion ratio.
Distance from lower control arm pivot to sway bar mount ÷ distance from lower control arm pivot to ball joint = Front Sway Bar Motion Ratio

All original 1962 to 1972 B Body & 1970 to 1974 E Body lower control arms measure 12.28125 from the center of the pivot to the center of the ball joint.

A 1966 to 1969 B Body sway bar mount is located 10.0625 from the lower control arm pivot.
10.0625 lower control arm pivot to sway bar mount ÷ 12.28125 lower control arm pivot to ball joint = .8193384 Front Sway Bar Motion Ratio

A 1970 to 1972 B Body & 1970 to 1974 E Body sway bar mount is located 6.4375 from the lower control arm pivot.
6.4375 lower control arm pivot to sway bar mount ÷ 12.28125 lower control arm pivot to ball joint = .524173 Front Sway Bar Motion Ratio

Front Sway Bar Wheel Rate
As described earlier, the formula for wheel rate is
(Motion Ratio x Motion Ratio) x Spring Rate = Wheel Rate

Continuing with the example of an original 7/8 diameter 1970 solid sway bar...
.524173 motion ratio x .524173 motion ratio x 302.66727 Sway Bar Spring Rate = 83.16 Front Sway Bar Wheel Rate

I went ahead and did the math...
1966 to 1969 B Body design               Front Sway Bar Spring Rate        Front Sway Bar Wheel Rate
15/16 diameter = spring rate                           74.448564                             49.98
1-1/8 diameter = spring rate                          154.37655                             103.64
1/1/4 diameter = spring rate                          235.29425                             157.96

1970 to 1972 B Body & 1970 to 1974 E Body design
7/8 diameter = spring rate                             302.66727                               83.16
15/16 diameter = spring rate                         398.85802                              109.59
1-1/8 diameter = spring rate                         827.07207                              227.24
1-1/4 diameter = spring rate                      1,260.5884                                346.36

You can see how much better the improved sway bar design is, by comparing equal diameters of the older design to the newer design.

Rear Sway Bar
A rear sway bar was not originally available on 1962 to 1970 B Bodies.  However, Firm Feel offers a 3/4 inch diameter rear sway bar for these vehicles.

The first rear sway bar offered by the factory was on the 1970 Challenger T/A & AAR 'Cuda.  It's 3/4 inch diameter, and was later used on other E Bodies.  Firm Feel offers a reproduction of this bar.

[cataclysm80]

A rear sway bar was available on 1971 to 1972 B Bodies, but it was an unusual design.  The end links were not vertical, and were located on the center portion of the bar instead of at the end of the arms.

[cataclysm80]

Dimension A is the length of the arm.  Dimension C is the depth of the bar perpendicular to B.
On these rear bars, the arms are perpendicular to B, which makes dimensions A & C be the same.

The spring rate formula for a rear sway bar is the same formula that was used for front sway bars.
((diameter x diameter x diameter x diameter) x 500,000) ÷ ((A x A x A x .2264) + (C x C x B x .4244)) = Sway Bar Spring Rate

For example, a Firm Feel 3/4 diameter solid rear sway bar for a 1962 to 1970 B Body would be...
((.75 diameter x .75 diameter x .75 diameter x .75 diameter = .3164062) x 500,000 = 158,203.1) ÷ ((11 dimension A x 11 dimension A x 11 dimension A x .2264 = 301.3384) + (11 dimension C x 11 dimension C x 44 dimension B x .4244 = 2,259.5056) =  2,560.844) = 61.777718 Sway Bar Spring Rate




Rear Sway Bar Motion Ratio
The wheel is located outboard of the sway bar end link mount, so a sway bar also has a motion ratio.
Center to center distance between the two end link mounting points on the rear sway bar ÷ center to center distance between the rear tires (Track Width) = Rear Sway Bar Motion Ratio.

Because the bar arms are perpendicular to measurement B, the distance between the end link mounts should equal measurement B.

For information on calculating Track Width, refer to the Leaf Spring Motion Ratio section earlier in this article.  Here is the Track Width example from that section.
For example, if you're running 15 x 7 Rallye wheels on a 1970 B Body, then
8 Overall Wheel Width ÷ 2 = 4 Wheel Centerline.
4 Wheel Centerline – 4.25 Back Spacing = -.25 Wheel Offset
(-.25 Wheel Offset x 2 = -.5 total offset for both wheels) + 60.125 Brake Drum to Brake Drum Width = 59.625 Track Width.

To continue with this example,...
44 distance between end link mounts ÷ 59.625 Track Width = .7379454 Rear Sway Bar Motion Ratio.

Rear Sway Bar Wheel Rate
As described earlier, the formula for wheel rate is
(Motion Ratio x Motion Ratio) x Spring Rate = Wheel Rate

Continuing with the example of a Firm Feel 3/4 diameter solid rear sway bar on a 1968 to 1970 B Body with 15 x 7 Rallye wheels...
.7379454 motion ratio x .7379454 motion ratio x 61.777718 Sway Bar Spring Rate = 33.64 Rear Sway Bar Wheel Rate

                                                        Rear Sway Bar Spring Rate     Rear Sway Bar Wheel Rate
1962 to 1970 B Body Firm Feel 3/4 diameter        61.777718                              ?
1970 to 1974 E Body 3/4 diameter                      83.328246                              ?
This chart is complex because in order to calculate wheel rate, it needs to list all the different axle widths with their different wheel offset combinations.  I can't complete this chart until I have the wheel offset info.  (Maybe I'll also insert a track width chart for the different wheel combinations in the leaf spring section?)
This chart is an unfinished work in progress.  All the formulas are here to do the math for whatever parts your using, but for this section, the answers for every possible combination haven't been conveniently precalculated.


OK, that covers all the suspension springs and how to calculate their Wheel Rates.
Where possible, I've tried to do the math so that you can easily  grab the numbers you need.


Roll Couple
Now that we can calculate the Wheel Rates for all the springs, we can move on to Roll Couple.
Roll Couple is pretty simple, you just add up the Wheel Rate for each spring.

Torsion Bar Wheel Rate + Other Torsion Bar Wheel Rate + Front Sway Bar Wheel Rate = Front Roll Couple

Leaf Spring Wheel Rate + Other Leaf Spring Wheel Rate + Rear Sway Bar Wheel Rate = Rear Roll Couple

Front Roll Couple + Rear Roll Couple = Total Roll Couple

Using the 1970 B Body examples we've discussed earlier in this article, with 1.00 Firm Feel torsion bars, original 7/8 front sway bar, 15 x 7 Rallye wheels, 78.29 wheel rate leaf springs, & Firm Feel 3/4 rear sway bar, our Roll Couple would be...

187.2 Torsion Bar Wheel Rate + 187.2 Other Torsion Bar Wheel Rate + 83.16 Front Sway Bar Wheel Rate = 457.56 Front Roll Couple

78.29 Leaf Spring Wheel Rate + 78.29 Other Leaf Spring Wheel Rate + 33.64 Rear Sway Bar Wheel Rate = 190.22 Rear Roll Couple

457.56 Front Roll Couple + 190.22 Rear Roll Couple = 647.78 Total Roll Couple


What we really want to know here, is what percent of the Total Roll Couple is in the front of the car.
Front Roll Couple ÷ Total Roll Couple x 100 = Percentage of Front Roll Couple

Continuing our example,...
457.56 Front Roll Couple ÷ 647.78 Total Roll Couple x 100 = 70.6% Front Roll Couple

We use this Percentage of Front Roll Couple, along with the Front Weight Bias which was discussed earlier in this article.  Here's a quick review of Front Weight Bias.
You can check your cars weight 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 %     This is called Front Weight Bias.


Mark your Front Weight Bias and Percentage of Front Roll Couple on this chart, and see where they intersect.

[cataclysm80]

Ideally, you want to be on the Theoretical Handling Line, or just a little on the Understeer side of the line.
Understeer results in loss of grip on the front tires.  You turn the steering wheel, and the car wants to keep going straight.  The further you are from the handling line, the more severe it is.  When the driver feels understeer, their natural reaction is to slow down to regain grip as they go around the corner.
Oversteer results in loss of grip on the rear tires.  As the rear end slides sideways, the driver finds themselves facing a different direction than expected, and the only recourse is corrective steering.  Again, the further you are from the handling line, the more severe it is.
Understeer is safer than Oversteer.


Let's look at an example again, using all of the 70 B Body info from the previous examples.
The car is a 1970 Hemi GTX.  With the driver, it weighed...
Front   2177   54%     Front Weight Bias
Rear   1845   46%
Total   4022   100%
with 1.00 Firm Feel torsion bars, original 7/8 front sway bar, 15 x 7 Rallye wheels, 78.29 wheel rate leaf springs, & Firm Feel 3/4 rear sway bar
187.2 Torsion Bar Wheel Rate + 187.2 Other Torsion Bar Wheel Rate + 83.16 Front Sway Bar Wheel Rate = 457.56 Front Roll Couple
78.29 Leaf Spring Wheel Rate + 78.29 Other Leaf Spring Wheel Rate + 33.64 Rear Sway Bar Wheel Rate = 190.22 Rear Roll Couple
457.56 Front Roll Couple ÷ 647.78 Total Roll Couple x 100 = 70.6% Front Roll Couple

Here's what it looks like on the chart.  Not great, but now that we can see what's going on, we can figure out how to fix it.

[cataclysm80]

Interpreting the Results
There's more than one way to arrive at where you want to be.
By trying different combinations here on paper, you can get a pretty good idea of what's going to work, without having to spend the considerable amount of money, effort, and time, buying, installing, and testing with real parts.
I'll run through some more examples here, to give you an idea of how swapping parts (on paper) would affect your results.

[cataclysm80]

Option 1: Adjust the weight of the car.
The suspension in our example 70 Hemi GTX would be perfect for a car that had 58.5% Front Weight Bias.  How about we add weight to the front of the car (or remove it from the back of the car), until we hit that 58.5% number?

NO, THAT'S NOT A GOOD IDEA!
That's like making the rest of the car worse, to match your poor suspension combination.  It might handle ok for what it is, but it's not something good.
It's not good to have a car be that front end heavy.  You want your Front Weight Bias to be between 50% to 55%, and the closer it is to 50%, the better.
Get your cars weight as balanced as possible from front to rear, and then select suspension components based on that weight.
It would also be bad to reduce the front end weight (or increase the rear end weight) to lower your Front Weight Bias.  That would make the example suspension even less adequate, increasing the chance of losing control of the car.

[cataclysm80]

Option 2: Change the rear sway bar
How does it affect things if we remove the rear sway bar?
78.29 Leaf Spring Wheel Rate + 78.29 Other Leaf Spring Wheel Rate + 0 Rear Sway Bar Wheel Rate = 156.58 Rear Roll Couple
457.56 Front Roll Couple ÷ 614.14 Total Roll Couple x 100 = 74.5% Front Roll Couple

Removing the rear sway bar helps considerably.  Installing a larger diameter sway bar would be worse than the bar we already have.  Depending on which direction you're trying to move on this chart, sometimes a rear sway bar is helpful, and sometimes it's not.  In this case, the car is better without the rear sway bar, but it's not enough change to put us where we want to be on the handling line.
Removing the rear sway bar, reduced the Rear Roll Couple.  You could get a similar effect by reducing the rear roll couple through softer leaf springs.  A rear sway bar is a good idea, but you need to have soft enough leaf springs that the rear bar doesn't cause excessive rear roll couple.