Torsion Bars... Best Diameter for a Lowered Street / Autocross car?

Suspension, Steering and Chassis

  1. JWRICH

    JWRICH Active Member

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    Looking for feedback / suggestions on what diameter of torsion bar I should plan to purchase for my 71 Demon. The car will be sporting a 408 stroker and A833, and I plan to run either 17" or 18" wheel/tire combo for both street and track. I consider myself to be an "autocross hobbiest" vs an "enthusiast"... so probably 2-3 track events a year. Otherwise, it will be a fun street car that I want to sit low and handle well. I'm running SPC fully adjustable UCAs, QA1 adjustable strut rods, and factory (reinforced) LCAs. For what it's worth, I likely add on the Hotchkis front & rear sway bars also. I haven't decided on shocks yet...but will probably be either Hotchkis or QA1.

    I'm looking at the torsion bar options through Firm Feel. Considering the 1.06, 1.12, and 1.18. I'm okay with a somewhat rough ride on the street considering it will be hard to avoid while being lowered... but not sure which of those 3 options to choose from.

    So, let me know your recommendations. If you have photos of your ride with a specific setup, that's even better! And of course, if anyone has a used set they'd be interested in selling... let me know!

    Thanks
     
  2. JWRICH

    JWRICH Active Member

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    Firmfeel, BAC, Sway-a-way.... all options. Thanks

    @72bluNblu
    @BergmanAutoCraft
    I know these guys have some experience... hoping they'll be able to chime in.
     
  3. #67barracudabob

    #67barracudabob Member

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    Talk to firm feel and take notes tell them what you got and what you wanta do there great people to deal with. They helped me out a lot on my 67 cuda build, there a wealth of information and knowledge
     
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    • HemiDenny

      HemiDenny HDK Suspension FABO Gold Member

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      Better yet, instead of waiting for a chime in...why don't you call / contact BergmanAutoCraft?

      He not only can give you great advice ( pretty much the OEM suspension professor) but can SELL exactly what you need. It is his business....and passion.
       
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      • Syleng1

        Syleng1 Karma is real and Life is short... FABO Gold Member

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        Just ask a lot of questions on Mopar suspension geometry. Just lowering the torsion bars puts all the other parts out of alignment. Factory mounting points were designed for a certain height and operating range. Heavy or lighter torsion bars are great but also designed to work under a certain amount of preload. If you don’t twist them enough or too much they do funky things. I’m no expert but it is why coil over parts are popular with MoPars doing what you seem to be working toward. IMHO.
         
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        • autoxcuda

          autoxcuda Well-Known Member

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          I run 1.14” with about the same setup as you with 245/50 sidewall tires.

          So I’d go 1.12”

          If you are more aggressive/committed autocross with autocross slicks you would go 1.18 or more.

          They do not do funky things without “preloading” them. You’re not doing wheel stands, completely taking all the weight off them, with bars like these.
           
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          • tanis4457

            tanis4457 Well-Known Member

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            I run the firm feel 1.06 bar with 120lb leaf springs and hellwig sway bars front/rear on 17's. It feels very balanced to me.
             
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            • AJ/FormS

              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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              On the street, my 1.03 bars are not too big; maybe just right.
              Going into a corner hot, the shocks drop the outboard corner onto the bumpstop, and yeah it can be a tad rough, but it flies around the turns pretty sweetly. The problem is that the 235/60-14 front street tires give up, and the car just skates over to the outside of the turn. But that's Ok, a lil gas and the other end skates too.
              I learned to pick better lines, with lots of run-off space, and I get the back end out first, so it can push the front back to the chosen line,lol.
              Street is all I know.

              The 1.03 bars are not really stiff, but the shocks are old school 3-way Munroes set on hard, so they hammer pretty hard over speed-bumps that I rarely slow down much for. I mean, my seats have cushions so I gotta give them a workout too.
               
            • 72bluNblu

              72bluNblu FABO Gold Member FABO Gold Member

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              I run 1.12” Firm Feel torsion bars on my Duster with Hellwig front and rear sway bars, 120 lb/in rear springs, Hotchkis Fox shocks and 275/35/18’s in the front and 295/35/18’s our back. I also have SPC UCA’s, QA1 LCA’s, adjustable strut rods and BAC’s delrin bushings.

              Based on your description, I think you’d be fine with 1.12” torsion bars. Especially if you’re lowered any significant amount. On my car I would need to use even stiffer torsion bars to lower further without hitting the bump stops all the time, and I already have the short button style bump stops on my car as well as the earlier QA1’s that add some suspension travel back in. And my car isn’t really super low, it’s about 4” from my Doug’s D453’s to the ground.

              The ride with that combination of parts isn’t bad at all, it’s pretty close to a modern performance car. I wouldn’t describe it as harsh at all. Good shocks are the key to controlling the larger torsion bars. If I wasn’t also using my car as my daily driver I would probably go up to 1.14” or 1.18” torsion bars and some 200 tread wear tires.

              Sorry, but this is pretty much all inaccurate.

              Lowering the suspension does change the geometry, that’s true. But you seem to be forgetting that these cars were not designed for radial tires. Lowering these cars IMPROVES the suspension geometry for radial tire use- you get better camber curves and a much improved roll center, both of which improve handling.

              Yes, if you lower the car you need a new alignment done, but again, that doesn’t mean anything by itself. With the OP’s selection of parts, especially the SPC UCA’s, dialing in modern alignment specs is easy. My car is set up with -1.1* of camber, +6.5* of caster and 1/16” toe at its current ride height. None of those specs were hard to get, there’s plenty of adjustment left in either direction.

              And, this “preload” nonsense. Sorry, but it’s nonsense. Preload on a Mopar torsion bar suspension is nothing like a preload on a coil spring suspension. The spring rates of the bars are linear. My 1.12” torsion have a spring rate of 300 lb/in, that’s true no matter how much they’re compressed. With the car on it’s wheels and at ride height the bars have a load equivalent to the weight of the car unless it’s sitting on the bump stops.
               
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              • Syleng1

                Syleng1 Karma is real and Life is short... FABO Gold Member

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                If you don't agree with what I have to say- that's fine move along. My interpretation of the question and my answer is based on my information from 40 years in this hobby and having been a professional mechanic for 20 years- yes even back when these cars were modern. Torsion bars do need pre-load otherwise they twist backwards and snap. There is a left and right side for a reason. I have seen them break when cranked up too high or driving around lowered. Driving on the bump stops are not what these front ends were designed for.

                The unequal a-arms on these cars from the factory were designed to operate within a certain range. End of story. If you operate them off the bump stops and you will damage the parts, the ball joints are also bottoming out...they are designed to operate with the shaft at a fixed center starting point and not at the edge of the seal. They need as much movement left of center as they do right of center. The factory has a ride height measurement for a reason. Modifying a car for a particular form of racing is not factory settings so I understand you may not agree with what I am saying.
                If you dont like the information that's okay- we can agree to disagree.
                 
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                • autoxcuda

                  autoxcuda Well-Known Member

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                  I think there’s confusion here due to terminology.

                  You are right, bottoming out the suspension where the LCA hit the frame is not good.

                  Raising the ride height will prevent that. I believe that is what you are calling preloading.

                  What also prevents that is stiffing Springs. The 1.12” is 3 times stiffer that stock T-bars that resist bottoming out.


                  I have bottomed out and broke a k-member stud hole weld out. I loaded my Barracuda to 5600 lbs total of stuff coming home from collage. I had to add air shocks in the rear cause the leaf were reverse arching. BUT I didn’t turn up/adjust up/pre-load up the Torsion Bars.

                  Well, in 1600 miles I cracked that weld.
                   
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                  • 72bluNblu

                    72bluNblu FABO Gold Member FABO Gold Member

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                    Oh boy. If you think you can “bend the torsion bars backward” then you have a fundamental misunderstanding of how the suspension on these cars work. What you’re describing is physically impossible.

                    If you fully unload the torsion bars, the adjuster bolt comes off of the lever on the hex end of the LCA. Once that happens, moving the LCA further in that direction doesn’t apply any force on the bar at all. The adjuster, and therefore the LCA, can only put load on the torsion bar in one direction. I don’t care if you’ve done this for 200 years, it doesn’t change that.

                    My car doesn’t ride around on the bump stops. It is lowered, but the short bumpstops and QA1 LCA’s provide plenty of travel. And, because the torsion bars on my car are 3x stiffer, they don’t need as much travel. I almost never hit the bump stops on my car. Which is better than these cars do with factory components. They’re frequently contacting the bump stops with stock bars and at the stock height. Put a dab of grease on the bump stop if you don’t believe me.

                    The factory geometry was based on bias ply tires. The best geometry for bias plys and the best geometry for radials is different. And don’t give me that Rick Ehrenburg “over-angling” the ball joints nonsense. I checked the range of travel on my suspension, it’s not an issue, the ball joints are fine, they operate well within their capabilities and are not bottoming on anything.

                    You have to consider why the factory put the settings where they did. When you change things, a lot of what the factory did is no longer relevant. The more parts you change out, the less important the factory settings are. The OP’s suspension is highly modified from stock. What those components can do is far above and beyond the factory settings. You have to understand the changes and the capabilities of the components. Not just parrot the factory manual.
                     
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                    • AJ/FormS

                      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                      Same here. I even removed the shocks and the T-bars and the lower bumpstops,and cycled the steering, cuz I wanted to know. The Moogs were happy.
                      In the end, I raised my K, a half an inch higher off the tarmac, cuz with street tires and on the street it's no big deal, the car is just as much fun, because the tires are by far the limiting factor.
                      And because of potholes. Our city streets have potholes; lots and lots of them, and I occasionally hit one, and it's nice to have the extra suspension travel.
                      Plus I am 20 years older than when I first started ,lol, and them RX-7 seats ain't as cushiony as they once were,.
                       
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                      • 72bluNblu

                        72bluNblu FABO Gold Member FABO Gold Member

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                        The diameter of the torsion bar ultimately determines the minimum ride height you can run, you need to have enough travel left so the suspension doesn’t bottom out. The larger the bar the less travel you need to avoid bottoming.

                        Of course that’s not everything, suspension is always a compromise and you have to go with torsion bars that match the use of the car, roads it will be used on, the compounds of the tires etc. If you’ve got smooth roads and sticky tires you can go way bigger than if you’re running all seasons on rough terrain. And being tire limited is fine, the car is usually nice and predictable even if it’s a little “loose”. And if you get better tires it’ll rip with no other changes. Plus a little loose is more entertaining anyway :D.
                         
                        Last edited: Jul 6, 2020
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                        • Cool Hand

                          Cool Hand Well-Known Member

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                          If you add more braking to the front you wont push the corners. If the fronts hit harder you add weight up front. More weight better traction
                           
                        • 273

                          273 Well-Known Member

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                          To me it depends on what tires your using and do you plan on pushing them to them limits.
                           
                        • GMachineDartGT

                          GMachineDartGT Senior Member

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                          I’ll agree with both of you here.
                          The suspension is designed to operate in a range. Lowering too much without using shorter bump stops is not good. That said, the bigger issue with lowering too much even with short bump stops is the fact you are inducing body roll by lengthening the moment arm. What this means is while lowering drops the center of gravity (good) you are giving the suspension more leverage. This means more roll as you lower. This is why people compensate with large bars and sway bars. You have to get just the right combo of tire height and body height to find the happy medium. The high roll center of the FMJ spindles can help. While the roll center may be higher, the moment arm is short. This prevents body roll and inspires confident handling when combined with a good alignment and tires.
                          The mopar performance books eluded to this very fact. They suggested minor lowering and the addition of additional roll couple. Mopar performance had their own line of big t bars for this.
                          Mike Martin’s book from the 80’s explains this well too.
                           
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                          • JWRICH

                            JWRICH Active Member

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                            My plan is almost identical to your setup on the Duster. Knowing what you do about your car's current handling, would you step down to 1.08, or up to 1.14 bars? As I look at options, those two sizes seem to be at a better price point than the 1.12 from Firm Feel. (I'm looking at BAC 1.08 and Sway Away 1.14). I'll be running the Hotchkis Fox shocks as well. Thanks
                             
                          • AndyF

                            AndyF Well-Known Member

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                            You'll just have to take a stab at it and then work from there. There is no perfect torsion bar rate, it all depends on what you are trying to do as well as the rest of the combination. Once you have the car running and driving you can start to work on tuning the combination. At that point you might decide to add more rate up front or add rate to the rear or add more bar or reduce the bar or revalve the shocks or change air pressure in the tires or add tire or whatever.
                            cornering.jpg
                             
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                            • 273

                              273 Well-Known Member

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                              I got a book on how to setup A bodies for autocross, gives you formulas need find center of gravity and roll centers etc.. But still it only gonna give you ballpark and if you Plan on pushing car and tires to the limit your gonna have to play with different bars and springs and shocks tires etc... like AndyF said.
                              Generally you want the rear the give slightly before the front, but at top level that can very from course to course or even corner to corner, everything is a series of compromises. If your not pushing to the limit as long your not bottoming out and keep the car somewhat flat through the corners it's probably fine.
                               
                            • 72bluNblu

                              72bluNblu FABO Gold Member FABO Gold Member

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                              On my car I would be more I inclined to go up to 1.14” than down to 1.08”. At the ride height my car sits at I’m basically at the minimum wheel rate, if I lowered the wheel rate I would need to raise the car to keep it from hitting the bump stops.

                              I haven’t had any issues with the 1.12’s being too stiff. My 275’s are 340 treadwear tires too, so with stickier tires larger bars would be fine (and possibly necessary). I wouldn’t want to go a lot stiffer on my car just because I use it as a daily driver. Autoxcuda runs 1.14’s on his barracuda. The key is running good shocks.

                              It’s all set up dependent- tire choice, ride height, alignment settings and drivers style all come into play. But with how my Duster is set up I wouldn’t go any smaller than 1.12”. I like the 1.12’s a lot, they are more expensive coming from FFI but it’s not like you have to buy them every year. They’re probably a one time purchase so I for one would rather spend a little more to get exactly what I want.
                               
                              Last edited: Jul 20, 2020
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                              • BergmanAutoCraft

                                BergmanAutoCraft FABO Vendor FABO Vendor

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                                It’s a fine line. We did 4 post testing at Multimatic with XV years ago. 250-300 wheel rate seems like a happy medium. The problem comes when you lower too much. The moment arm becomes so long we compensate with more t bar. We’re lowering the C of G, but increasing roll at the same time. I found keeping the tire height at 26 helps.
                                 
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                                • kursplat

                                  kursplat FABO Gold Member FABO Gold Member

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                                  just what i was thinking while going through this thread. you can perfectly copy someone else's setup and make the car completely undriveable, to you.
                                   
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                                  • JWRICH

                                    JWRICH Active Member

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                                    I appreciate everyone's feedback. I decided on a set of 1.14 bars from Sway-a-way.
                                     
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                                    • autoxcuda

                                      autoxcuda Well-Known Member

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                                      For all the info you put out in this thread, that seems like a good starting point for what your are after and your background.

                                      I could see you even going/trying/testing more bar.
                                       
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