long post, broken into three sections. car is a 70 duster, 17x8 wheels with 255 rubber at all 4 corners, aluminum headed 360, 5 speed, efi, currently with .89 torsion bars, 340 leafs, subframe connectors, biggest addco swaybars, KyB shocks, firm feel stage 3 box, etc. problems are body roll at autocross, undersprung (bottoming), and odd ride from the KYB shocks. primary use is street car (occasional multistate trips with the family) and monthly autocross.

1. what sixze and brand torsion bars do I want? im thinking 1.03, but have heard all sorts of things positive and negative about PST and other brands. im on a slim budget, so cost is an object. what size, and why?

2. the leaf springs I want for my duster are either not available, or way to expensive. so ive been looking into modifying my spring pack.

is there a way to figure out rates? like measuring individual leaves and doing some math?

what does clamping do to the rate?

some leafs have an additional half leaf on TOP of the main leaf. they state that this is to reduce brake hop. does this leaf add into the additional rate? does it actually work?

rubber or poly spring eye bushings and why?

sliders on a street car: yes/no and why?

3. i have a line on a full brand new set of spax adjustable shocks for 250 shipped.

im thinking that an adjustable will aloow me to go with softer valving for ride quality, and stiffer for autocross. other than the konis on my ACR (which I never got the opportunity to adjust) ive never had higher end or adjustable shocks. the KYBs suck. I know bilsteins and like them, but never ridden in or driven a set on a vintage mopar.

what says the hive? Spax what I want, as far as a cheaper koni type shock?



thanks guys. feel free to get into hardcore tech here. I know im not looking at bolt ons or the more normal type of things. im also considering additional unibody stiffening at this time, such as torque boxes and a lower radiator brace, but with it being a fully painted and detailed car, im not sure I can bring myself to do it. paint and body is expensive.

pretty much every question you asked has already been discussed ad nauseum in this very forum.

Except maybe spax shocks.

Care t o share? I couldn't find anything as far as custom leaf packs, etc.

Hey Duster, I follow your builds on GRM/P-T and know you're a capable guy. What type of budget are we working with?

When we made torsion bars for the A-bodies they were 1.03" so I would recommend that size from either PST or Firm Feel.

You can do pretty well mixing and matching leafs to build a custom pack. I would use as many full-length leafs as it takes to get around 130-150lbs/in when the spring is near-flat or flat. Then take another 2 (thick) leafs, cut the rear section off, mount one above and one below the pack to reduce axle wrap, these stubby leafs will need to be clamped up front. The hardest part will be getting the correct free arch; somewhere around 6" will get you close and you can fine tune with different shackles and front mount. I would drill the tips of the main leafs and use nylon pucks to reduce stiction as well. Keep rubber in the front eye and use urethane bushings top and bottom in the shackle.

I'm a little biased, but I wouldn't run any shock other than our Fox's for those spring rates. Bilsteins would be the only other consideration if budget won't allow the Fox's.

edit: The Addco bars will be OK once you get the higher spring rates front and rear. If autocross becomes more of a main function you will want to upgrade to stiffer sway bars.

I daily drive my '73 Dart with 1" torsion bars and its my believe these A-bodies should have come stock with these bars.
If I were able to have fun on the track often I wouldn't hesistate to install at least 1.06" bars.

My Dart has adjustable shocks all around. Hotchkis shocks up front and QA1 ones in the rear.
Front Hellwig swaybar.

About poly/rubber leafspring bushings;
Poly locate/maintain the leafs better under the car, specially in corners, without making the ride harser.

As for leafspring tuning; adding a leaf or two yourself isn't that big of a deal.
I've added a leafspring and a previous owner added another before.
Current;y running with 7 leafs per side, but that's also because there are 2 propane tanks in the trunk and under the packagetray.

I think the proper (number of) leafspring clamps should take care of any brake hop, if present.

How's the balance of the car? Understeer, oversteer, or neutral?

Ok, budget is under a grand. I've realistically got 800, but would much prefer to spend less if possible.

I've re-lined packs before on model a's, but never built a custom pack. Nylon buttons instead of the long rubbers was recommended. What is a source? I've also found a few calculators for spring rate online, so at least I'll have a general idea of what the rate is, but will still have to fine tune it.

I'm currently running poly bushings in the leaves, but the fronts are already worn.

Jon, thanks. It's not often a manufacturer that you don't buy from compliments your style. I will say I've used a bunch of hotchkis products on customer cars, and a few sets of your springs on my own stuff, and like them. Just not in the budget for this one.

Car is very odd handling. Takes an initial set on turn in well, but then afterwards transition to mid corner understeer. Will not oversteer without breaking the tires loose with throttle and essential drifting. I blame too little rear rate and kyb shocks more than anything else. Body roll in the front I blame on too soft of spring rate. Hence wanting to stiffen both ends.

I do have a set of truck bilstien shocks that should be easily adapted to the mopar, but have no idea what the valving is in them, and if they would be worth the effort. Original application was a one ton suburban.

if there is a spring shop near you, look at seeing what they have laying around, maybe you will find some pairs of leafs that would increase your rate. If you want the most out of your ride, look at aluminum bushings instead of rubber or poly. They make greaseable bolts to accommodate them. Bergman may have some big torsion bars for your application too, Jon from Hotchkis also gave excellent advice. Tim

I was just talking about this the other day with the burnout gif guy above. I am going to be adding an extra full leaf to my Hotchkis springs to bring in some balance with my existing 1.18" t-bars and 275/335 tire combo. I would put on the Spax shocks before changing anything else. If anything else I would guess you're under T-barred in the front. If the car is rolling over to much, just get FF 1.12" bars and it is money well spent. Yes they're more money but they are also BETTER. I think the 1.12's would be best ALL AROUND size for a dual purpose car, but my bars ride great with the non-adjustable Fox's.

For reference, I added one full leaf to my MP XHD springs (E body) which took the rate from ~142 to ~172. Measured by placing them on a smooth floor upside down, me standing on them and bouncing a little then a helper measuring height with and without me on them. The added leaf was about $100 at a spring shop. You could DIY by cutting the eyes off a spare main spring and adding it to the pack.

For the aggressive driving you do, agree for sure you need more torsion bar. The green brick ran 1". Also better shocks to go with the bigger bar. These two items are going to pretty much use up your budget I think... and should probably come before the rear springs.

What size bump stops? I'm willing to bet you're on the bump stop when you get the excessive understeer condition.

With an $800 budget, I'd suggest buying stuff used where possible. 1.04 or bigger TB's, Mopar rear oval leafs with appropriate hangers (or just redrill yours- they're 20.5" vs 20.0"), and bilstein's or better for shocks. Those are the exact components I'm running. Those 0" arch oval leafs measured at 190# for me, btw. Also needed 1.5" drop blocks for my ridge height.

The oval track leafs would benefit from Jon's suggested added front segments. I'm currently looking for free leafs to cut up and accomplish just that.

, and same for your response, other sites, like Corner carvers, would result in repeated widespread beatings for asking before researching first, I personally would like to see something in between. But its go with the herd here. And This is not directed personally at the OP.

The mid-corner understeer your experiencing is most likely from those dreaded KYB's. The car is transitioning too quickly due to being under sprung/dampened and is rebounding ans unloading the outside tire causing it too loose traction mid-corner. Throw the SPAX shocks and the 1.03-1.12 torsions on the car and see how if feels.

A car can only go around a corner as fast as the front tires will allow, so work on the front first with the budget you have and get it working right. You can band-aid the rear leafs for practically nothing by adding a leaf or 2 to your existing ones and then modifying the front hanger/shackles or adding a block to get the desired ride height.

Also, make sure you have an aggressive enough front alignment. After you've done the torsions and shocks you will need a little help since the sway bars are a little on the soft side. I would run:

-1.5* camber or more, I run -2* on my daily driver
As much positive caster as you can get keeping it the same side to side
and Zero toe

I'd call or e-mail the guys at Firm Feel about T-bars, they make them and are Mopar guys. Additional chassis stiffening is good, the stiffer the better.