I agree.

wonder what kind of torsion bar system they are talking about. Did you see these two notes under the "arm length" data field?
not sure what exactly they mean...

"Rear - torsion bar (center) to birdcage bolt center
Front - center of torsion tube to center of axle."

i think you would need to study sprint car chassis designs to figure that one out...

"Center", when viewed head-on from the front and the rear.


I just ordered their (currently) last stocked set of 1.08" A body bars to upgrade again from my 1" bars.
Figured what the heck...

Sniper isn't totally off as even Mother Mopar claims t-bar spring rate to be wheel rate as the t-bar is the pivot and not subject to the same motion ratio as a coil spring car. As he illustrated in his link, this ratio in a mopar is 1:1. If you look at the loading of a torsion bar, it will apply the same downward force along the length of that control arm because that control arm is all directly 90* from the rotation of the bar. As a simple torsion event, it is reasonable to say torque = force x 1. ie, spring rate is wheel rate. Since the mopar ball joint is on plane with the t-bar socket and the motion ratio is 1:1, I can see where it is one and the same.

Logic would also seem to dictate that altering the lever arm length would impact this force measurement. Additionally, adding any degrees of alteration of angle in this twist also changes the formula to require a vector factor which again, changes the force required. I also can see where it is logical that changing the control arm length changes the force. Cranking off that 100ft lb nut with a 12" wrench vs a 24" wrench is an obvious impact of the difference to this force. Perhaps this is why there are some widely divergent claims of rates advertised by suppliers as some are using spring rate=wheel rate while others are calculating a motion ratio into the rate based on the stock control arm length. Don't know, I can't answer for how everyone is measuring their rates when it should be a simple matter of diameter and length. In a t-bar, is that rate measurement the same at the pivot, the adjusting blade, and the ball joint? Does the fact there is an adjustment blade within the control arm effect this? Does the length of this blade and where it sits in the control matter? I don't know, I've never measured the force on any of them.

In my other lives using alternate brands, spring rate was simply the force required to compress (or twist in the case of our t-bar) the spring one inch. Wheel rate was typical the motion ratio adjusted spring rate as measured at the ball joint. Adjusted rate was including or deducting wheel offset from the ball joint point

If you really want to get into the math of it, here is a short, 13 page read for you: http://web.mit.edu/course/3/3.11/www/modules/torsion.pdf

You have to know the distance from the tire contact patch to the torsion bar pivot point in order to calculate the wheel rate. The farther out the tire is from the torsion bar then the softer the wheel rate. So even things like wheel spacers, wheel offset, etc. will change the wheel rate on a Mopar. Just like it does on a Camaro. I think that different assumptions on the lever arm length is why some charts show different spring rates for the same bar diameter. In my spreadsheet I use 13 inches for the arm length and 39 inches for the effective length of a B/E bar. The overall length is 41 inches but the ends don't count so I use 39 inches. I'm not sure if that number is correct but as long as you are consistent it doesn't really matter.

As for the original question I'd say try some bars and see what you like. Shocks are very, very important when the bar gets bigger than 1 inch. My Coronet rode like a dump track with cheap shocks on it but once I put the Koni shocks on there it felt more like a BMW. I used to wince when I'd hit a bump but with Koni shocks took care of that.

Yup.

The spring rate on a torsion bar spring is torque/degree which isn't very useful. You need to know the lever arm length to convert that spring rate into a wheel rate.

the only difference from a "mopar" is that a sprint car spindle is at a right angle to "control arm". The calc for wheel rate is identical on both and the length of the "arm" twisting the TB is critical.

If you are just comparing stiffness of one bar to another most of the calculations can be ignored and just focus on the diameter to the fourth power. If you use a 1.0 inch bar as the baseline with a fourth power of 1.00 then it makes it easy. A 0.90 bar has a fourth power of 0.66 so it is 34% softer while a 1.10 bar has a fourth power of 1.46 so it is 46% stiffer. Since the diameter is taken to the fourth power a small change in bar diameter makes a big difference in rate. A 1.19 bar has a fourth power of 2.00 so it is twice as stiff as a 1.00 bar but is only 20% thicker. So the big bars get really stiff really quickly.

The wheel rate really only is used when you're trying to balance the car front to back but that gets pretty complicated. The old DC books had some circle track formulas for comparing front to rear roll couples and spring rates and stuff like that but I never really learned that stuff. Personally I don't drive any of my stuff hard enough to make it an issue. Tim's car went 160 mph on the Portland International road course and that car used the biggest torsion bars and sway bars possible up front with 340 leaf springs out back. No idea how that car compared to what the old DC manual wanted, it was just what felt comfortable to him.

I appreciate the thought that went into your question. I have wondered if there is a formula to use as a guide. If there is one, it is a guarded secret or one that I have yet to stumble onto so far.
Hotchkis probably has it figured out in their TVS line but I never looked at their spec sheet, nor do I know if they have published the science behind one if one exists.
Me? I installed Two RH Mopar Performance 440/Hemi leaf springs in my '70 Charger in 2006. Six stage leafs with two half leaves facing forward. I used a 1.0 torsion bar for many years but a couple of years ago switched to 1.15 bars and Bilstein shocks. The ride feels great. It is about as firm as the Wife's car......2015 Challenger R/T 5.7.

There are formulas out there. None of them are uber secret, they just aren't thrown around in the press nor discussed frequently like engine topics are regularly discussed. However, nearly all of them start with total weight and front weight to begin the formula. Very, very few drivers ever scale their car to know this.

Andy also touches on it in his book as a percentage ratio of weight, which could be estimated reasonably close.

Yep, the easy way is to scale the car and then go from there. Spring rate has to be matched to the car weight. There is a bunch of science on the subject but it isn't something that gets published in Hot Rod magazines. The OEM's know this stuff inside and out. That is why you have luxury cars that float down the road or performance tuned cars. There are charts showing what CPS (cycles per second) to shoot for depending on what type of ride you are willing to live with.