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Determining spring rate #2658126
05/22/19 06:19 PM
05/22/19 06:19 PM
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Granite Bay CA
Frankenduster Offline OP
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Hey there,
With torsion bars, it is easy to determine what is going to be firmer....Just compare diameters. A 1.24 is going to be stiffer than a 1.0.
With leaf springs, how do you determine the rate? Common sense tells me that leaf thickness, width and the amount of leafs in the pack are a big factor but are there different rates of the spring steel? The car I have is a '70 Charger.
In 2006 I installed Mopar Performance 440/Hemi spec XHD leaf springs even though I do not drag race the car. I wanted something firmer than a 318 car would have had. I currently have 1.15 torsion bars in the car. I was just curious as to how the leaf spring rate is determined.

Re: Determining spring rate [Re: Frankenduster] #2658135
05/22/19 06:37 PM
05/22/19 06:37 PM
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Easiest way is to put it upside down on the floor, measure the freestanding height, put a known weight on it, measure the resulting height and do the math.

Or ask the maker.

Or use this Calculator

Remember that in this case spring rate and wheel rate are not the same, unlike in the front.

Re: Determining spring rate [Re: Sniper] #2658345
05/23/19 11:06 AM
05/23/19 11:06 AM
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Pikes Peak Country
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TC@HP2 Offline
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...and these are simple free rate calculations. A spring's installed roll rate can also vary based on mounting layouts and shackle angles. For your Charger, with its parallel mounting, it is a good starting point as the splayed layouts are only found in Late B, E, F, J, and M bodies.

To figure out the wheel rate of leaf springs, you need the center to center distance of the axle housing mounting pad points and center to center of the tread width. Divide pad width by tread width, multiply by 100, that's your wheel rate.

Your rear anti-sway bar also contributes to wheel rate as well.

Re: Determining spring rate [Re: TC@HP2] #2658485
05/23/19 05:11 PM
05/23/19 05:11 PM
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Granite Bay CA
Frankenduster Offline OP
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Christ....It can't be that difficult.

Re: Determining spring rate [Re: TC@HP2] #2658542
05/23/19 08:38 PM
05/23/19 08:38 PM
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jcc Offline
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Originally Posted by TC@HP2
...and these are simple free rate calculations. A spring's installed roll rate can also vary based on mounting layouts and shackle angles. For your Charger, with its parallel mounting, it is a good starting point as the splayed layouts are only found in Late B, E, F, J, and M bodies.

To figure out the wheel rate of leaf springs, you need the center to center distance of the axle housing mounting pad points and center to center of the tread width. Divide pad width by tread width, multiply by 100, that's your wheel rate.

Your rear anti-sway bar also contributes to wheel rate as well.


We have had this discussion before, but imo, in say hitting railroad tracks head on, the pad distance is not factor, spring rate is wheel rate, as both springs move the exact same amount as the wheel moves.. Where I disagree with the pad dimension factor, whatever one loses in rate with the pad not being directly over the center tire patch on a one wheel bump, one regains by the other leaf now being closer and it then makes up equally for what little was lost with the spring pad being mounted closer to the center. I am not addressing roll rates here, they are harder to calculate with an OEM mounted leaf, which has designed in anti roll properties already.

Re: Determining spring rate [Re: Frankenduster] #2658555
05/23/19 09:05 PM
05/23/19 09:05 PM
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Originally Posted by Frankenduster
Christ....It can't be that difficult.


If it were easy anyone could do it

Re: Determining spring rate [Re: Frankenduster] #2658868
05/24/19 07:36 PM
05/24/19 07:36 PM
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central IL
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myduster360 Offline
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My 1982 Direct Connection Chassis manual lists most all Hemi and Max wedge leaf springs at 125lb/in

Here's the 2 outliers
1966-67 Street Hemi Manual trans : 300lb/in
1968 Hemi Dart/Baracuda(aka super stock): 160lb/in

Last edited by myduster360; 05/24/19 07:37 PM.

1972 Swinger 3.6L Pentastar
Diablo CMR tuner
Re: Determining spring rate [Re: myduster360] #2658884
05/24/19 08:30 PM
05/24/19 08:30 PM
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Summit has a house brand E body leafs at 130 lbs/in for a reasonable price. They have a 10% off code for all summit brand parts going on right now.

Re: Determining spring rate [Re: Frankenduster] #2658929
05/24/19 11:16 PM
05/24/19 11:16 PM
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Oregon
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I have a spring design manual and it is complicated for leaf springs. Torsion bars are fairly easy and so are coil springs. Of course, a coil spring is a type of torsion bar, just shaped different. There is a local leaf spring company who is very easy to work with. I've had them build custom stuff for me before. But for a car like yours it is probably easiest to just buy some springs off the shelf. Hotchkis should have what you need.

https://benzspring.com/

Last edited by AndyF; 05/24/19 11:26 PM.
Re: Determining spring rate [Re: Frankenduster] #2658939
05/25/19 12:03 AM
05/25/19 12:03 AM
Joined: Feb 2010
Posts: 16,097
Granite Bay CA
Frankenduster Offline OP
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Frankenduster  Offline OP
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Originally Posted by Frankenduster

In 2006 I installed Mopar Performance 440/Hemi spec XHD leaf springs even though I do not drag race the car. I wanted something firmer than a 318 car would have had.


Thanks but...I do have aftermarket leaf springs in the car. They are from Mopar Performance. The previous set of springs were probably original. The car was built with a 318 so I figured the MP springs were an upgrade. They were. The car rode firmer and sat a little bit higher. I was just curious as to the way that leaf spring rate is calculated. Obviously a pack with more leafs would have a higher rate, as would one with wider leafs. A shorter segment between the eyelets would be firmer as well. There were some early A body cars that had leafs with unique thinner sections in the middle, assuming that was to reduce the rate while keeping the overall shape the same as with V8 cars?
Anyhow...I was just thinking about this because it makes sense to me that a neutral handling car is achieved by balancing the front to rear spring rates. Of course, the sway bars are factored in to it all as well.

Re: Determining spring rate [Re: Frankenduster] #2659026
05/25/19 10:16 AM
05/25/19 10:16 AM
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Originally Posted by Frankenduster
Originally Posted by Frankenduster

In 2006 I installed Mopar Performance 440/Hemi spec XHD leaf springs even though I do not drag race the car. I wanted something firmer than a 318 car would have had.


Thanks but...I do have aftermarket leaf springs in the car. They are from Mopar Performance. The previous set of springs were probably original. The car was built with a 318 so I figured the MP springs were an upgrade. They were. The car rode firmer and sat a little bit higher. I was just curious as to the way that leaf spring rate is calculated. Obviously a pack with more leafs would have a higher rate, as would one with wider leafs. A shorter segment between the eyelets would be firmer as well. There were some early A body cars that had leafs with unique thinner sections in the middle, assuming that was to reduce the rate while keeping the overall shape the same as with V8 cars?
Anyhow...I was just thinking about this because it makes sense to me that a neutral handling car is achieved by balancing the front to rear spring rates. Of course, the sway bars are factored in to it all as well.


Except "spring rate" is only one of many many factors in obtaining "neutral" balance.

Re: Determining spring rate [Re: jcc] #2659078
05/25/19 01:25 PM
05/25/19 01:25 PM
Joined: Feb 2010
Posts: 16,097
Granite Bay CA
Frankenduster Offline OP
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Frankenduster  Offline OP
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I get that. The car is pretty neutral right now with some slight understeer. I'm fine with that. I am looking to build a similar car within a year or so and was thinking of having it a step or two softer. Again, it is easy enough to do with torsion bars and sway bars due to the diameters/lever arms but....

Re: Determining spring rate [Re: Frankenduster] #2659108
05/25/19 02:30 PM
05/25/19 02:30 PM
Joined: Feb 2010
Posts: 1,354
Phila. Pa.
Mattax Offline
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Originally Posted by Frankenduster
Hey there,
With torsion bars, it is easy to determine what is going to be firmer....Just compare diameters. A 1.24 is going to be stiffer than a 1.0.
With leaf springs, how do you determine the rate? Common sense tells me that leaf thickness, width and the amount of leafs in the pack are a big factor but are there different rates of the spring steel? The car I have is a '70 Charger.
In 2006 I installed Mopar Performance 440/Hemi spec XHD leaf springs even though I do not drag race the car. I wanted something firmer than a 318 car would have had. I currently have 1.15 torsion bars in the car. I was just curious as to how the leaf spring rate is determined.


The most straight forward method is measurement as already stated. Weight over distance moved.
We have no information on the MP springs. So pretty difficult to compare what it originally was when installed.
Yes there are different rates of spring steel as a finished product but not enough for us to be using a design factor.
Main leaf thickness is important.
How well the second leaf supports the front eye is important.
How much of the rate is controlled by the front portion vs the back portion must have an effect and make calculations more difficult. The DC books call this spring ratio.

Stanley has a pretty wide selection. Stengel has pretty good spec table. Might find a local retailer.
http://www.stengelbros.com/suspensi...-coronet-charger-super-bee-leaf-springs/

As you mentioned, and the reason I responded, there are other factors on the balance. Most closely tied to roll rates is where the weight is. DC called this the neutral handling line.,
Best I could determine was that there was no single Neutral line for all situations, but rather a line that could be established and used for guidance. I summarized this here http://www.heritech.org/cuda/mgcudah.html regarding my own car.
Some discussion here about this. https://board.moparts.org/ubbthreads/ubbthreads.php/topics/2195534/1.html

Re: Determining spring rate [Re: Frankenduster] #2659363
05/26/19 03:45 PM
05/26/19 03:45 PM
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central IL
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myduster360 Offline
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Originally Posted by Frankenduster
I get that. The car is pretty neutral right now with some slight understeer. I'm fine with that. I am looking to build a similar car within a year or so and was thinking of having it a step or two softer. Again, it is easy enough to do with torsion bars and sway bars due to the diameters/lever arms but....


I used Direct Connection's "neutral handling" methodology to find my Swinger's needed rear leaf spring rate and its a decent starting point. You can do the same using your current car as a starting point since its "pretty neutral" handling is what you want to emulate only "softer".

1) Calculate your current cars Front roll couple. That value will represent 75%-80% the cars total roll stiffness. Thus the 20%-25% remaining will be the range of your CURRENT car's rear roll couple. I'm guessing if you back calculate the leaf spring rate, your MP XHD may be in the range of 110-140 lb/in

2) Now substitute in "softer" TB redo the calcs and see what the rear leafs you'd need. Just error on the side of too soft because a rear bar can bridge the gap and bump up the rear roll couple.

Last edited by myduster360; 05/26/19 04:02 PM.

1972 Swinger 3.6L Pentastar
Diablo CMR tuner
Re: Determining spring rate [Re: Frankenduster] #2659497
05/26/19 10:51 PM
05/26/19 10:51 PM
Joined: Nov 2008
Posts: 1,456
Lethbridge, AB, Canada
dangina Offline
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do you use coilovers? It's how I determine the rates - on my race car I would run 6kg front /4kg rears on the street - 8/6kg was about as stiff as I'd like on the street and 10kg/8kg was racing pavement

right now I have 1.12 torsion bars rated at 248 and leaf springs at 200 (hotchkis is 170 for comparison) 3.5kg and 4.4kg respectively

The best way I found if you want to convert to kg rates is here: https://www.redline360.com/garage/spring-rate-conversion-chart-easily-convert-kgmm-to-lbsin

basically divide by 56 to get the rate - I find it to be pretty close ride wise - there is other factors to consider like havinga big block up front vs a small block - I'd like to go with higher rates now that I've driven it the past few years, I took peoples advice and went withthe safer 1.12" bars I should have gone with the 1.24" I

Re: Determining spring rate [Re: dangina] #2659955
05/28/19 12:06 PM
05/28/19 12:06 PM
Joined: Jan 2010
Posts: 1,108
NW Chicago suburban area
Mopar Mitch Offline
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DANGINA stated: "I took peoples advice and went with the safer 1.12" bars... I should have gone with the 1.24" "

The 1.12" TBs aren't bad, and they're a world better than the factory original sizes, but you could still improve the car's handling with larger TBs (diving, rising, leaning... overall flatness). I've always stated and suggested that... after you install larger TBs (such as 1.0, 1.10, 1.12, etc,), and you get used to them after a short while, you will only wonder and ask yourself, with realization, why you didn't go larger, considering the effort and time/expense that you went through. I've done all that in the past, and offer my suggestions... go larger (at least 1.12... better yet.. 1,18, 1.20,1.22, 1.24. the hex is 1.25", so the largest available is a 1.24 to fit through the hex... contact Firm Feel and they make you a set. (don't have them make it 1.25 or else you WILL have difficulty fitting it through the hex.... 1.24 is the largest size to fit through the factory hex. Don't be afraid of larger TBs.

From my experience, and setting up the suspension on my Challenger for AX, those stiffer 1.24 TBs are the best available... but people need to learn that 1.24" TBs aren't really that stiff. The RACE AAR/TAs that ran SCCA TransAm ... and still run today in Vintage/Historic events... have much larger TBs... ~1.4 diameter. I love my 1.24 TBs and would never give them up for something softer.

My leafs are Flex-A-Form fiberglass mono-leafs.. rated 225#/inch... having an approximate 5" uncompressed arch.... matched to the intended AX weight of the car. IF I'd ever change again, I'd go a little stiffer... maybe 250#/inch and a 4" uncompressed arch.

IF your concerned about ride comfort on the street, then: avoid rough bumpy roads/potholes, etc. Learn to adjust your tire psi within reason for that ride comfort (higher psi = rougher rides)...; consider the sidewall heights of your tires.. 30-35-series will ride stiffer, 40-45 still stiff but a touch softer, 50-55-60 smoother yet... for the STREET. Get the idea? Highway cruising typically has smoother roads. Also, consider multi-adjustable shock absorbers... 2x better than 1x for fine -tuning.

As you make compromises (street, AX, etc)... you give up the advantages for the other.


Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!
Re: Determining spring rate [Re: Mopar Mitch] #2659987
05/28/19 12:57 PM
05/28/19 12:57 PM
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"but you could still improve the car's handling with larger TBs (diving, rising, leaning... overall flatness). I've always stated and suggested that... after you install larger TBs (such as 1.0, 1.10, 1.12, etc,), and you get used to them after a short while, you will only wonder and ask yourself, with realization, why you didn't go larger, "


For more then a decade. up biggrin

Re: Determining spring rate [Re: Mopar Mitch] #2660457
05/29/19 08:26 PM
05/29/19 08:26 PM
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Aurora, CO
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Why no mention of sway bars?

Instead of running a really stiff spring rate on the street, why not run a softer spring rate and stiff sway bars?

Re: Determining spring rate [Re: jbeintherockies] #2660503
05/29/19 11:43 PM
05/29/19 11:43 PM
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Granite Bay CA
Frankenduster Offline OP
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I do have a rear sway bar. I pulled it from an '83 Imperial...similar design to a 5th Ave/Diplomat. Frame hung, 3/4". I had an Addco 7/8" but it was too stiff and made the car twitchy.

Re: Determining spring rate [Re: jbeintherockies] #2662241
06/04/19 12:54 PM
06/04/19 12:54 PM
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NW Chicago suburban area
Mopar Mitch Offline
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JB -- my sway bars are customer made, solid steel (just like the old pro-am guys used to do... a T/A driver made them for me back in the late 70s)... bent to match the OE shape front and rear... 1.25" front, 1.00" rear. The front bar is heavy... recently purchased a hollow Firm Feel front 1.25" bar.., saves about 6-7 pounds from the front end.... I'll see how I'll like it when I start driving the car in about another year (under re-construction now getting new engine, etc). Actually, the FF front bar measures almost 1-5/16" (slightly larger that 1-1/4".

The rear bar is 1" solid steel... and I truly like that setup best... the smaller 7/8 bar (hard to find) and the common 3/4" bars are too small/too weak. The balance of the 1-1/4 front and 1" bars are the ticket to best handling pending most common driving (street/hwy, AX, HPDE/HSAX) for our cars (especially E-B bodies). You can play with the end links (all rubber/ all poly, half-half etc).. to eventually find a good feel for your intentions... that's what I've done in past years. Also, playing with the adjustable shocks is part of the whole package. A 1" rear bar is not that big as some may think... sure, it helps introduce more potential oversteer, but with our front-end heavy cars its needed... the potential oversteer doesn't really happen as some may think... regardless of pylon AX or HSAX/HPDE track events... per my experience... it helps neutralize and balance much better than with a smaller 7/8 or 3/4 rear bar. Remeber, the factories wanted some understeer to be kept.. thanks to Raph Nader for his safety concerns stemming from the Corvair.

I am considering making/modifying that 1" rear bar with adjustable link ends... so to becoming stiffer yet if needed.

When I've swapped rear bars (factory 3/4" to my custom 1"), I've immediately noticed more understeer with the factory 3/4" rear bar... and both bars have poly from mounts inserted within their frame brackets. that 1" rear bar truly better helps to balance the sway bar's assistance in body roll.

Those larger TBs help reduce front end dive and lift... especially when driving in serious AX/HPDE/HSAX events. In my opinion, the factory should have made ~1.12" as their minimum TBs for BB cars and the T/A-AAR cars.... but that was way back in time when softer/cushion rides has some say... a compromise in ride/handling.


Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!
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