Torsion bar spring rate ?

[72bluNblu]

So I've looked around a bit and can't seem to find anything that would give the actual spring rate in lbs for a torsion bar. I know that they're indexed by diameter and that makes sense, but pretty much everything I've ever dealt with has had coil springs which have rates given in lbs. Is there some kind of conversion? It wouldn't have to be exact, but at least a ballpark number would be nice to pick out my new torsion bars.

[cudazappa]

here's a good chart to give you an idea.

http://www.firmfeel.com/torsionb_b.htm

[HP2]

Remember that torsion bars are a 1:1 ratio so their spring rate is the actual wheel rate, unlike a coild spring that has a motion ratio associated with it. A 1000 pound coil spring is only around 270# worth of wheel rate, where as a 1.12 torsion bar is the same 270# wheel rate.

[72bluNblu]

Remember that torsion bars are a 1:1 ratio so their spring rate is the actual wheel rate, unlike a coild spring that has a motion ratio associated with it. A 1000 pound coil spring is only around 270# worth of wheel rate, where as a 1.12 torsion bar is the same 270# wheel rate.

Hmmm. Ok, so maybe I should explain what I'm trying to do. I'm in the process of trying to spec out the suspension for my '72 Challenger, and have been looking at different handling "kits" as well as just individual parts. While I'm sure the kits out there work great, they also seem to cost a lot more than just buying individual parts, some more than others of course. So I was going to install a rebuild kit (all new bushings and bearings) in addition to new torsion bars, sway bars, and upper control arms (for adjustability). I've done this stuff before, my "first" car was a vintage race prepped 1956 Austin Healey (that I drove daily), and my most recent car was a 2004 Dodge SRT4 with mopar stage 3 coilovers and upgraded springs (342 lb/in front, 247lb/in rear), as well as an upgrade to a 1" front sway bar and 3/4" rear bar.

I was looking at the 1.22 in "oval track" torsion bars over at Moparts, but have heard these are more for cars that spend more time on the track than the street. Of course, the same was said about the previous set-ups i've run, so I was looking for a more direct comparison. I liked how the SRT4 was set up as far as stiffness, so those kind of wheel rates I would be fine with. The SRT4 had a Macpherson strut type suspension; the actual wheel rate is pretty hard to calculate since it changes throughout the travel of the suspension, but if you figure that the angle correction is fairly small the wheel rates are pretty close to the spring rate...figure over 300 lb/in for my SRT4 in the front, and it was a good 300 lbs lighter than the Chally...

So will 1.22" torsion bars, 1 1/8 " front sway bar, XHD rear springs and 1" rear sway make my kidneys bleed on the street? And more importantly, is it a good setup to make my Chally go around corners?

[Aussie Challenger]

  You have to factor in the shockers also as everything needs to work together, a soft shocker might work better with the above set for the street. However that can defeat what you are trying to do.
  The better named suppliers spend a bit on reseach and development to put their packages together. Some cheaper parts may also give a short term result but loose their spring rate after a short time.   

[HP2]

Motion ratio on strut cars is calculated a little differently than traditional coil spring cars. I'll admit having never done calculations for struts. However, if you have had cars with 300# wheel rates, then you'll have a pretty good idea what you are in for ride wise. Wheel rate is wheel rate regardless of how it is achieved.

XV took a page right out of the Mopar oval track catalog and use 1.12 to 1.18 torsion bars, 1.125 front sway bars, 110-120-# rear leafs and a .75 rear sway bar. This is the starting base line that mopar suggested for its kit car race cars. XV says this is the best set up for ride comfort combined with exceptional handling. What a difference 30 years makes, eh?

The thing with going with a 1.22 torsion bar and a 1.125 sway bar is that you now have almost 850# wheel rate. Pretty stout. However, the XV set up is coming in around 760-825# up front with their set up, which means the step to 850# doesn't seem so big any more.

In the back, sway bar selection will also be important as there are the stock style frame hung bars and under axle after market bars. All range in size from .75 to 1.0 but depending on the mounting application will produce different rates. The under axle bars tend to produce higher figures than the frame hung bars.

When doing calcs for my Challenger for heavy duty street and autocross use, I came up with 1.22 torsion bars, 7/8 or 15/16 front sway bar, and 175-200# rear springs with no rear sway bar. A slight alteration would be changing to a  1 1/8 to 1 1/4 front sway bar and adding a 3/4 to 7/8 rear sway bar. Changing sway bars is a quick and easy way to alter the wheel rate.

Like Aussie said, shocks are also of paramount importance. QA1 adjustables for $150 each would be the starting point for this type of  set up with Afco, Pro, or Koni being the next step up

[72bluNblu]

Whoops! Yeah, the SRT4 would have had a 300+ lb/in wheel rate just based on the coilover, plus the sway bar on top of that! Finding the actual motion rate is a little difficult for the strut type since the angle is always changing, but my understanding is that the rate is very close to the rate on the spring for the most part.

I think I'll just go with the 1.22's, I'm perfectly happy sacrificing a nice cushy ride for actual handling. Mopar suggested 110-120 # for the rear? I guess the XHD's are 140 lb or so, with the sway bar maybe I should go down to 120# springs?

I know all of the suspension outfits spend time on their setups, but I notice most of them use pretty close to the same stuff as far as shocks (QA1's) and bushings and all that go. So really the difference is just the spring rates and size of the bars. My guess is that they aren't reinventing the wheel on that stuff either, and for the price of some of those "kits" I could trial and error with several different springs (or torsion bars) if I wanted to. I appreciate that they're out there doing the work and testing, but I think a big chunk of that change is going to having the "name brand" stuff. Not that "cheap" stuff is the way to go, obviously there are quality issues out there, but $2400 for XV's kit? Ok, the going rate on adjustable shocks is 5-600 bucks, but that leaves $1,800 for torsion bars, struts, rear springs and sway bars? Even avoiding mopar performance stuff you can get all of the above for less than $1k...

So think I'll give it a rip with the 1.22's, a 1 1/8" front sway, maybe go with the 120# rear springs with a 3/4" or 1" rear sway and try to stick to factory style mounting. That'll also leave me the money to go polygraphite on all the bushings and with adjustable UCA's to get some more modern alignment figures...

Then I'll just need to go with some frame connectors and upgrade to some adjustable shocks...

[HP2]

I did happen across the motion ratio formula for strut cars. Measurement D2 is from the lower arm pivot point to the lower attachment point of the strut. Measurement D1 is from the lower arm pivot point to the point on the strut that creates a 90 degree angle to the strut.

Coil overs are calculated the same why, BTW.