Update on new PST 1.03 Torsion bars.
^^^ Any reason why you would just not back-compute the rate on the smaller bar from the larger ot get in the ballpark? I think it is safe to assume the spring steel properties are the same, bar to bar, as well as the shape at the ends. A simple re-clock should not effect it either.
Works out to 191 lbs/in for 1.00", 213 lbs/in for 1.03", and 240 lb/in for the 1.06" (starting from 300 lb/in for the 1.12"). If all other things are equal, the change in spring rate should be the 4th power of the ratio of the diameters.
The properties of the spring steel can in fact be different enough to create some variation if you're going from one manufacturer to another. Yes, its fairly small, usually within about 10 lb/in or so, but since PST is the manufacturer they should be able to get the spring constant for the actual steel they used, and not just a generic number.
For example, the rates you calculated don't match Firm Feel's advertised rates of 195 lb/in for 1" and 252 lb/in for their 1.06" bars, and that's starting from
their advertised wheel rate for the 1.12" bars.
It's a small difference, but if the 1.03" bars are 215 lb/in I'd definitely use FFI's 1.06's. If they're 230 lb/in I would seriously consider the 1.03" bars because they're so much less expensive. It's enough of a difference to make it an issue. The 1" Just Suspension bars I bought are on the "light" side for 1" bars, I doubt they're above 190 lb/in, which is why I'm not running them on my Duster, and won't even be running them on my Dart. I'm pretty sure I can count on the PST bars to be between 215 and 230 lb/in, but that's enough of a difference (7% of the wheel rate) for me to change my mind on using them.
Also, if you look at the Mopar Performance chart below, you can see even bigger differences, especially for the larger bars. The MP 1.09" A body torsion bar is listed at 300 lb/in, which is the same as the FFI 1.12" bars I run. That's a pretty big difference given the change in diameter.
72blunblu states 'wheel rate' in lbs per inch; an actual t-bar rate by itself has to be lbs per degree (of rotation) with a given leverage arm ( ft-lbs per degree really). Any spring will have it's actual rate by itself, and then there is the 'wheel rate', which is how many pounds to make the wheel move 1 inch when the spring is installed in the suspension.
The relation between the 2 is a mechanical calculation; for a typcial coil spring set up, it is based on the the wheel leverage on the spring based on the ratio of arm length to spring position on the arm. You can find the calculations plenty of places on the internet and in suspnsion and mechnical engineering books.
Typically the torsional spring constant for the bars is given as in-lb/degree, which is why the wheel rate is typically given in lb/in. Torsion bars specs are usually listed by wheel rate, even if that's not made clear. Mopar Performance for example just lists their torsion bar specs by "rate", but it's in lb/in, so whether it says it or not it is in fact the wheel rate, and that's what most of us are used to seeing. You can find the equations on the 'net or in books like "Engineer to Win" or "Tune to Win" by Carrol Smith.
