Sounds like I jumped the gun on the rear sway bar. I can just send it back though. The weight distribution should be decent on my car, I have aluminum heads/intake, manual box, battery in the trunk and a fiberglass hood. I did order the RCD Bilsteins and the XHD lead springs, and I am not against going with big front t-bars, as long as the ride isn't ridiculously harsh. I may give Firm Feel a call and see what they recommend.

remember if you don't like it - you can always swap it out for a bigger/smaller bar later!

Kzinge - Since the car has a lighter front, let's take this relationship a bit further. Reducing mass (weight) on one end of the car has the same effect as increasing spring rate. Also, street tires and particularly unpaved roads are better suited to softer spring. Let me give an example in the other direction.

My car was using 200 #/in t-bars and a 1.125 sway bar which were working reasonably well in autocross on A008 tires. After installing a set of R1 tires, traction increased but the car rolled much more. Stickyier tires needed more roll resistance (or less weight, lower cg and roll centers). But if we put the super stiff setup on loose surface or even a normal dirty road with street tires and it might skate instead of bite.

That said, to pinpoint exactly the right t-bar for any vehicle requires much more specific information. Shifting as little as 50 pounds from the front the rear can effect the balance when driving. Also, on a B-body, my recollection is that the t-bars are longer than an A-body. Therefore a 1.03" B-body spring is softer than a 1.03" A-body t-bar (about 200 #/in). I*think* you will be in the ball park with anything in the 1" range. Yes, it worth discussing with the venders. And the business about ride height has to do with hex offsets.

If you want to baseline your car, my suggestion is to start collecting information. When you get the XHDs, measure the rate and free arches. When you remove the SS springs, measure them too. Next time the opporunity comes, weigh the car and if you can weigh the front. When you do, note how much fuel and other material is in the car (100 lbs of tools?) and its best if you sit in it. There's a page on my website discussing neutral line and how I applied it to my car but that is probably a step more than most people want to do or than makes sense on street car.

Dangina - As far as just trying the rear sway bar, if you put the car into a tree or telephone pole sideways, you may not get a chance to just take it off. That's what Rick E, FrankenDuster and the rest of us are at pains to point out. You need to sneak up on the rear roll from the safe side, especially on a street car. Please read what Rick wrote about oversteer and speed in that other thread.

Thanks again Matt, for the good information. I'm learning a lot in this thread. I'm most likely going to go with the 1" bars from Just Suspension. Seems like everyone's had good luck with them. That's a good point about the weight reduction making the spring effectively have more rate.

I don't know if I'm going to go as far as weighing the car and measuring the springs rates at this point in time. That may be my next step if I decide to up the ante and go with better tires, a bigger front bar and the rear sway bar. If I can get in the ballpark of good handling it will be a major improvement over what I had. I think you guys have me on the right track now. I should start getting parts in today so I'll keep you updated on my progress.

Hell, I'm working on a 72 Duster with an inclination more towards street performance rather than Drag strip. The car was a 225 slanty with a 3 speed on the column, but with lots of help, it is now a 360 4 speed. Much of the car is a mix and match of parts from many different cars. I even have an 8 3/4 axle from a 68 Coronet!
The slant six torsion bars look barely thicker than the fuel line, so I bought some 1" torsion bars at a swap meet. I'm going to swap those in after I pull the engine to paint the engine bay. Is there a recommended leaf spring rate to compliment the added stiffness of these torsion bars? I'm not looking for anyone to do all the thinking here, I was just looking for some direction. Is it a matter of calculating a spring rate increase of the torsion bars and then adding some additional leafs to the rear? Thanks for any help.
Jeff

I went with 6-leaf rear springs from Espos on my Swinger to match the .890 torsion bars. I don't know the spring rates, but they seem a good match.

Just don't forget GOOD shocks. My KYB's were a waste of money with my set-up.

vm

Like mattax said, there are a number of somewhat precise but actually kind of simple calculations that need to be known to fully dial in the needed spring rates a car need.

It all starts with total weight and front/rear weight distribution. Without at least these two, everything else is just guesses. With just these two, you can at least dial it in to where you are not just in the ball park, but you're in the right section at the stadium.

If you know your front/rear weight distribution, you can find the roll couple percentage required on the neuteral handling line. Once you know what roll couple you need, then you can find out what the required wheel rates need to be. Once you know that, then you can divide that rate into the available t-bars and s-bars to achieve balance. None of this is rocket science, it all is simple calculations actually, but if you don't have access to all the info to give you this information and know how to put it all together, then I guess it might just as well be rocket science.

In simplest terms without doing any math, you could always look up the old Direct Connection Oval Track Kit Car specifications. These tables are all published in the associated mopar oval track book. They show a variety of t-bar, s-bar and leaf spring specifications in combinations that will put you within certain ranges of roll couple distribution. You'll still need to know weight and distribution, but it is better than guessing.

IMO, the biggest issue with handling in classic mopars is the limited choice of front torsion bars. With only 5 choices in large rates ( maybe 7 if you find some of the out of production large sizes on ebay) you can only do so much with actual spring rates.

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The weight distribution should be decent on my car, I have aluminum heads/intake, manual box, battery in the trunk and a fiberglass hood.

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Assuming you have a 56-58% front weight bias, and that is being generous for a big block, even with the lightening you've done, you may need around 180-200# worth of rear spring to match the 1.03 torsion bars and 1.125 sway bar. So if swap out those SS springs for something considerably softer, you may be able to get away with a rear sway bar.

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I'm not looking for anyone to do all the thinking here, I was just looking for some direction. Is it a matter of calculating a spring rate increase of the torsion bars and then adding some additional leafs to the rear? Thanks for any help.
Jeff

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If your happy with the overall balance now, then yes, you can calculate the percentage of step up on your t-bars, leave your sway-bar in place, and step up the leafs a corresponding amount. However, I'm talking about the feel of balance front to rear, not whether it is a wallowing pig that rolls over for a belly scratch when you turn. If you change your front s-bar at the same time or add a rear s-bar, or it plows and slides too easily now, then the percentages have changed and that type of straight calcualtion won't work.

Kzinge1 - You'll be less likely to have a a hieght adjustment issue if you talk with Just Suspension and Firm Feel (and any other vender you wish) about hex clocking before you purchase. I know Firmfeel has worked out clocking for stock weight vehicles. Just Suspension I don't know so ask.

Even better, before calling, crawl under or drive up on some 2x10 boards and measure the Chrysler ride hieght. At the same time, note how far in the torsion bar adjuster bolt is. Stock ride height is availble from any shop manual. The 1970 Chiltons's says 1968-70 Plymouth [Belvidere] spec is 1 3/8". Now you'll have some points of reference when talk to the vendors.

JjS - What Tony said. You can go to the tech section on my website to see how I've used the neutral line concept. Several of the books mentioned in the Where to Start Reading thread will help with all the calcs. The B-body axles may slightly change the track and therefore the motion ratio.

I actually find rear leafs more problematic because its hard to get good specific info from many of the vendors. There's quite a bit to leaf spring design. Besides rate, there is arch, numbers and thickness of leaves (especially main leaf), interleaf, leaf end shape and placement, front to rear spring ratio, and eye placement. If you want to discuss more about rear leafs or more about your own set up its probably best to start a new thread.

Someone needs to buy some A-body bars from Dick so we can speed up the need for him to make another batch. Looks like I may have about a 2 month wait for the 1.18s I want for mine. And I just sold my .920 bars yesterday!

Thanks again guys for the responses. You guys clearly know what you are talking about! As far as weighing the car goes and getting the front/rear bias, I don't have the scales or access to them. So my combo will not be optimized until I can get it weighed. Keep in mind though this will be a street car. My ultimate goal is to have a car that drives like a modern vehicle but has 600 HP and 620 ft-lbs.

I just got my XHD Mopar leaf springs from Summit. Do you guys happen to know their spring rate? I plan on calling Firm Feel in the morning to see what they recommend for t-bars. I'll let you know FF recommends.

While a four corner weight review is the ideal way to do it, you can get a good idea of front to rear bias with a simple truck scale. Landscaping suppliers, scrap metal dealers, drywall supply, junkyards, trucking companies, even some drag strips and oval tracks have basic drive on/off scales that can give you a rough idea of where you are at.

I've seen data on the XHD saying they are around 130-140# rate. SS springs range from 130-160#. MP Oval track springs are 120#. Used up stock springs are maybe 50-120#, depending on how abused they were.

FWIW, I believe the baseline Kit Car set up was 1" t-bars, 1" front s-bar and 120# rear leafs. XV Motorsports seems to have found that page in the Chassis manual as most of their level 1 set ups use t-bars in the 1.1 range, front sway bar in the 1.1 range, much lighter (but they wont say how much) leaf springs and a 7/8-1" axle hung rear sway bar.
http://www.xvmotorsports.com/products/detail/index.cfm?nPID=279&cid=2&cdesc=Suspension

Place the spring on a level suface, arch up.
Measure the distance from surface. Lets call it Free Height.
Weigh yourself. This will be Weight.
Holding on to something, carefully load the spring with your full weight.
Remeasure from the surface to the same place on the spring pad as before. Call this Loaded Height.
Carefully get off the spring while holding on to something (or you may get tossed).

Now do the math.
Distance displaced = Free Height - Loaded Height
Spring Rate = Weight / Distance Displaced

Weight is in pounds force, Distance in inches, therefore Spring rate will be in lbs/in.
This is crude but approximately correct.

If you're a bit more ambitious, pull a string from eye center to eye center and measure the distance from the spring to the mounting pad. This is the free arch, or darn close to it.

After doing this, please share along with the exact part number of the springs. Other details you may wish to note is spring thickness (if you have a caliper or micrometer) number of full and partial leaves and bushing diameters. This will be good for your own reference but will also help others in the future.

Later you can do the same on your SS springs. In that case, you'll have to check both the left and right asthey may be different in free arch.

XHD leaf spring specs: Part# 4452982, 4452983

D/S: 155 lbs/in. 6 leafs
P/S: 175 lbs/in. 7 leafs (2 half)

That's measuring the way described above, so it's not exact.


I also talked to Dick at Firm Feel today, very nice guy. I ordered the 1.06 torsion bars from him and they should be sent out next week. He said that I should have a pretty well balanced setup and that I should experiment with the rear bar. He also said the RCD shocks were very good. I bolted the new leaf springs in today and started swapping out my brakes. Should have it all done tomorrow.might be able to test drive it sometime next week.

Got my XHD leaf springs and Bilstein shocks installed out back. Also installed the 11" brakes and put new shoes on them. I'm leaving the rear sway bar off for now. I'm going to replace the rear brake hose because it looks a little dry-rotted. Also need to plumb in my disc/drum distribution block and my adjustable proportioning valve. I'll install the new 1.06 torsion bars as soon as they come in, hopefully before next weekend, and install the front shocks at that time too.

Did you change the U-bolts?

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Got my XHD leaf springs and Bilstein shocks installed out back. Also installed the 11" brakes and put new shoes on them. I'm leaving the rear sway bar off for now. I'm going to replace the rear brake hose because it looks a little dry-rotted. Also need to plumb in my disc/drum distribution block and my adjustable proportioning valve. I'll install the new 1.06 torsion bars as soon as they come in, hopefully before next weekend, and install the front shocks at that time too.

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Yeah I put new u-bolts on. The ones that were on it weren't very old but the nuts kept getting stuck, so I cut them off.

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This is not enough front sway bar and front torsion bar to justify a 7/8 rear bar. I would consider staying away from the rear bar for now IMHO

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I have to agree with this........based upon my conversation with Dick at Firm Feel. He didn't think it was necessary for my street only Challenger. He recommended his tubular upper control arms instead of the rear sway bar. The UCA's will allow for much improved handling by allowing for for more positive caster adjustment. I haven't bought them yet.
I did buy his stage 2 box, Bilstein RCD shocks and sector shaft support bearing. Great stuff!
The Bilsteins were a huge improvement for sure but I think the improvement I experienced won't be the same for the next guy. It depends on what you currently have on your car that you are replacing. Drag shocks, air shocks, cheap Monroes etc. Also if you do any other changes at the same time you install the Bilsteins, like I did.

I recommend you call Dick and talk to him, he will take his time and answer your questions based upon his vast experience!