Tav 1970 Road Runner RM23N0G217270

[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.

[cataclysm80]

Option 3: Change the torsion bars   (rear sway bar still installed)
1.00 diameter torsion bars
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
457.56 Front Roll Couple ÷ 647.78 Total Roll Couple x 100 = 70.6% Front Roll Couple

1.06 diameter torsion bars
236.3 Torsion Bar Wheel Rate + 236.3 Other Torsion Bar Wheel Rate + 83.16 Front Sway Bar Wheel Rate = 555.76 Front Roll Couple
555.76 Front Roll Couple ÷ 745.98 Total Roll Couple x 100 = 74.5% Front Roll Couple

1.12 diameter torsion bars
294.5 Torsion Bar Wheel Rate + 294.5 Other Torsion Bar Wheel Rate + 83.16 Front Sway Bar Wheel Rate = 672.16 Front Roll Couple
672.16 Front Roll Couple ÷ 862.38 Total Roll Couple x 100 = 77.9% Front Roll Couple

1.18 diameter torsion bars
362.9 Torsion Bar Wheel Rate + 362.9 Other Torsion Bar Wheel Rate + 83.16 Front Sway Bar Wheel Rate = 808.96 Front Roll Couple
808.96 Front Roll Couple ÷ 999.18 Total Roll Couple x 100 = 81% Front Roll Couple


You can see that increasing the torsion bar size gets us closer to our goal. 
Decreasing the torsion bar size would be worse for handling.

Increasing one torsion bar size gave us the same improvement as removing the rear sway bar.  Doing both these things together, might put us pretty close to where we want to be on the handling line.

Increasing two torsion bar sizes puts us pretty close to the handling line without removing the rear sway bar.

Increasing 3 torsion bar sizes puts us past the handling line, but still pretty close to it, so this is an acceptable option.

The downside here is that super stiff bars don't have as good of a ride quality.  They'd be perfectly fine on a smooth race track, but if you're driving around town hitting bumps and dodging potholes, then they might not be the best choice for you.  Larger diameter torsion bars also require higher quality shocks to help in the ride quality department.
Increasing the Front Roll Couple is clearly what we need to do on this car, but we're probably better off making some of that increase with a larger diameter front sway bar, so that we don't have to use as large of torsion bars.
Remember the rule of thumb for the torsion bar wheel rate being roughly 10% of the front end weight.
Front end weight is 2177 on this car, so a torsion bar wheel rate around 218 would be nice.  The closest one is the 1.06 bar, with a wheel rate of 236.3.  Let's keep this in mind as we proceed.

There may also be some straight line acceleration benefit to having smaller torsion bars.  Drag racing cars are not good at cornering, but they use the smallest diameter torsion bars to promote weight transfer to the rear end for added traction during straight line acceleration.  The rear springs need to be stiff enough to handle this weight transfer though.  A car focused on handling probably won't keep up with an all out drag car in a straight line, but it could still make sense to use a large front sway bar so that you can use smaller diameter torsion bars, but in this case you should probably not use a rear sway bar, so that your leaf springs will be higher spring rate.  This might make sense for a street/strip car that's mostly street driven and only occasionally drag raced.

[cataclysm80]

Option 4: Change the front sway bar
7/8 diameter
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
457.56 Front Roll Couple ÷ 647.78 Total Roll Couple x 100 = 70.6% Front Roll Couple

15/16 diameter
187.2 Torsion Bar Wheel Rate + 187.2 Other Torsion Bar Wheel Rate + 109.59 Front Sway Bar Wheel Rate = 483.99 Front Roll Couple
483.99 Front Roll Couple ÷ 674.21 Total Roll Couple x 100 = 71.8% Front Roll Couple

1-1/8 diameter
187.2 Torsion Bar Wheel Rate + 187.2 Other Torsion Bar Wheel Rate + 227.24 Front Sway Bar Wheel Rate = 601.64 Front Roll Couple
601.64 Front Roll Couple ÷ 791.86 Total Roll Couple x 100 = 76% Front Roll Couple

1-1/4 diameter
187.2 Torsion Bar Wheel Rate + 187.2 Other Torsion Bar Wheel Rate + 346.36 Front Sway Bar Wheel Rate = 720.76 Front Roll Couple
720.76 Front Roll Couple ÷ 910.98 Total Roll Couple x 100 = 79.1% Front Roll Couple


The front sway bar is a very important part of making a car handle well.  The factory 7/8 & 15/16 sway bars usually aren't big enough for my tastes. 

The 1-1/8 front sway bar combined with one size larger torsion bars might make a nice combination with the existing leaf springs and rear sway bar.

I'd also consider using the 1-1/8 front sway bar and removing the rear sway bar.
This could be handy in a situation where you're still building the car, and don't have the exact weight yet.  It gives you the option of installing the rear sway bar or getting a larger front sway bar if you need to adjust one way or the other, after you finally get the car done and get it weighed.

The 1-1/4 front sway bar looks like it would be a great match for the other existing suspension components in our example.  That should handle nicely and ride well.  If you were going to do any occasional drag racing with it, you might consider removing the rear sway bar, and installing higher spring rate leaf springs.  The formulas in this article can be used to calculate what spring rate you'd need.

[cataclysm80]

Option 5: Change the leaf springs
(.738 Motion Ratio x .738 Motion Ratio = .544644) x 120 Spring Rate = 65.36 Wheel Rate.
65.36 Leaf Spring Wheel Rate + 65.36 Other Leaf Spring Wheel Rate + 33.64 Rear Sway Bar Wheel Rate = 164.36 Rear Roll Couple
457.56 Front Roll Couple ÷ 621.92 Total Roll Couple x 100 = 73.6% Front Roll Couple

(.738 Motion Ratio x .738 Motion Ratio = .544644) x 143.744 Spring Rate = 78.29 Wheel Rate.
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

(.738 Motion Ratio x .738 Motion Ratio = .544644) x 160 Spring Rate = 87.14 Wheel Rate.
87.14 Leaf Spring Wheel Rate + 87.14 Other Leaf Spring Wheel Rate + 33.64 Rear Sway Bar Wheel Rate = 207.92 Rear Roll Couple
457.56 Front Roll Couple ÷ 665.48 Total Roll Couple x 100 = 68.6% Front Roll Couple


Leaf springs are available in a wide variety of styles and spring rates, and can be customized by adding or subtracting leaves.  It's not to difficult to find leaf springs that are more or less original appearing, in the 120 to 160 pound range.  Higher or lower spring rates look less like original springs, but they're out there if you need them.  This variety is why you usually choose leaf springs last.  They can be adjusted to be what you need.
The leaf springs on this chart aren't sufficient to solve our suspension issues by themselves.  Maybe a super low spring rate mono leaf spring would do the trick, but at the moment, I think some of the other options that have been mentioned are better choices.

[cataclysm80]

There you go, that should give you a pretty good idea of how changing any suspension spring affects the whole system, and also how to know when you're about where you need to be for your car.


A note on drag racing suspension.  Drag racing suspension is quite a bit different from a good handling car.  Good drag race cars don't corner well.  Drag race cars are designed to go from a standstill to really fast, in a straight line, for about 10 seconds.  They use the smallest diameter torsion bars for a low spring rate in the front, and high spring rate leaf springs in the rear.  Caltrack traction bars are common, but those aren't designed for cornering either.  They don't use sway bars because that's just unnecessary weight since they're not cornering.  Drag shocks also aren't intended for handling.
A drag race suspension is pretty much the opposite of what you'd want for good handling.  Don't expect a drag car to handle well on the street or on a road course, and similarly, don't expect a good handling car to be winning drag races against serious drag cars.  You can still have some fun within your bracket / class.
Be honest with yourself about how you'll actually drive the car, and design it to fit those needs.  An autocross race track suspension designed for constant cornering also wouldn't be great for a street cruiser where you're driving straight more often than cornering.  (It would probably have excessive inside tire wear from the negative camber of the autocross alignment.)


Getting an accurate weight of the car is important if you want to have good results in the end.
I highly recommend weighing your own car. Every car is a little bit different.  If you look at the charts above, one or two percent different front weight bias can be a pretty big difference.
All to frequently though, someone is trying to select their suspension parts during a restoration, with the car completely disassembled and unable to be weighed.  My suggestion in this instance is to install your old torsion bars and leaf springs until the car is done and can be driven to a scale to be properly weighed.  Anything else is just guessing, and your final results will reflect that.
However, if you insist on guessing your weight based on someone elses car weight, this list of engine weights may be helpful if their car doesn't have the same engine as yours.

[cataclysm80]

OK, with all that suspension info explained, Here's the details for the suspension on this 1970 Road Runner. 

The car was completely disassembled for restoration, so based on the known weights of other 1970 Plymouth B bodies, I guessed the 1970 383 Road Runner weight at...
Front   2094   54%  Front Wheel Bias
Rear   1783   46%
Total   3877   100%


Rear Suspension
Leaf Springs custom built by Firm Feel Inc to my specifications
Spring Rate 160

Motion Ratio
15 x 7 Rallye wheels on a 1970 B Body, is...
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.
44 Leaf Spring 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.

Rear Wheel Rate
The formula for wheel rate is (Motion Ratio x Motion Ratio) x Spring Rate = Wheel Rate (per wheel)
(.738 Motion Ratio x .738 Motion Ratio = .544644) x 160 Spring Rate = 87.14 Rear Wheel Rate

No Rear Sway Bar

Rear Roll Couple
(87.14 Rear Wheel Rate x 2 = 174.28) + 0 Rear Sway Bar = 174.28 Rear Roll Couple

Front Suspension
Firm Feel 1-1/8 diameter Front Sway Bar  =  1.125 diameter

((1.125 diameter x 1.125 diameter x 1.125 diameter x 1.125 diameter = 1.6018066) x 500,000 = 800,903.3) ÷ ((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) = 827.07207 Sway Bar Rate

Motion Ratio
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
.524173 motion ratio x .524173 motion ratio x 827.07207 Sway Bar Rate = 227.24 Front Sway Bar Wheel Rate

Firm Feel B & E body torsion bars
1.00 diameter 187.2 pounds per inch wheel rate

Front Roll Couple
(187.2  wheel rate x 2 = 374.4) + 227.24 front sway bar wheel rate = 601.64 Front Roll Couple
601.64 front roll couple ÷ (601.64 front + 174.28 rear = 775.92 total roll couple) = 77.5% of the total roll couple is at the front

[cataclysm80]

In addition to the spring selection mentioned above, the car has...

Bilstein shocks
All of the Hemi Suspension chassis reinforcements (Resto Rick)
Lower Radiator Support reinforcement (XV Engineering)
Lower Control Arm Plates (Firm Feel)
Firm Feel Tubular Upper Control Arms for more positive caster, with a performance handling alignment
Firm Feel Stage 3 Power Steering Box
Sector Support Reinforcement
Roller Bearing Idler Arm
Power Disc Brakes with 11.75 inch diameter rotors

The car is built for someone who wants to have fun in the corners while cruising with a 4 Speed Pistol Grip and a pop up Air Grabber hood scoop. 
Drive it

[cataclysm80]

I meant to post an update sooner. 

I took pictures at the auction.
The car sold at Mecum Indy on May 18th 2023 for $ 97,000. 
https://www.mecum.com/lots/553648/

Had a great time hanging out at the auction, and checking out the RTS 'Cuda & Black Ghost. 

[cataclysm80]

Not exactly sure who bought it at Indy, but I recently heard it was a classic car dealership from Pennsylvania looking for inventory. 
The car came back to Florida for the 2024 Mecum Kissimmee auction, and sold on January 13th for $ 87,000. 
https://www.mecum.com/lots/1109604/1970-plymouth-road-runner/
It looks like they didn't put as much effort into taking nice photographs, and the car no longer had its yellow cap reproduction battery. 
I've heard that cars sell for more money at the Indy auction, which is why I hauled it up there instead of waiting for the local Kissimmee auction. 
Glad I did that!    

I hear that the car lives in Florida now, where the driving weather is nice all year.