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I want to dig a little deeper in to this topic we have discussed spring rate before (on the old forum) and after doing some "backyard" mechanic-ing tests I found that when compressed on to the shock Jag springs are at a rate of about 200# each.
of course that is a vertical rate and you have to consider the angle. With the springs angled that rate drops some where around 170# each which when combine to give you g a total spring rate of around 340# per side . A jag weighs in at about 4100# which is a lot more than my 2700 Mustang. If I take 40% of the curb weight of each vehicle (typical back half weight of the average car 60/40) and the 340# spring rate that a Jag has and cross multiply (all things being equal except curb weight) my Mustang needs 223# springs. Thats all good and fine, but then I was looking at a Mustang catalog and I found the spring rates for OEM and performance leaf springs and saw this.
the performance application is only calling for a 138# spring so I am thinking 223# might be a bit much. Am I missing something?? what do you think based on what I am seeing one jag coil over placed as vertical as possible should do the trick. but then we are running just one coil over per side instead of two and that is a debate for a different thread
Last edited by Daze (5/19/2011 5:32 pm)
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OEM numbers equal 40% rear weight minus the axle. Then you need to add shock travel plus my guess is that when the OEM shock was/is fully extended the leaf spring would still be loaded. I'm thinking the OEM spring would be compressed 4 to 5 inches.
4X138=552X2=1104 lbs.
5X138=690X2=1380 lbs.
1104+1380=2484 divided X 2= 1242lbs. average.
Last edited by Ralphy (5/19/2011 4:53 pm)
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I did all of the same calculations and came up with the same conclusions ... one spring should be plenty. Additionally, the QA1's I had were giving another 60 lbs of force. But when I did the mock up with one spring it clearly wasn't up to the task. I put two springs on at it sat right where I wanted it and when I jumped up and down on the rear bumper the dampening effect was not excessively stiff. Now I did use the stock springs in the stock locations plus who knows how old the springs were but I do remember thinking that the math just didn't add up to the results. I know this doesn't help with your decision making but I also know that you like all of the inputs you can get 
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for me the math almost worked. I tried it with just one OEM coil over today in the most vertical position. the shock was at 15º off of vertical which makes my 200# springs function at a rate of 186# with that rate the coil over compressed about 1" when I set the car on the suspension. which I think is about right for the 4" of travel the OEM shocks have. The car was sitting a little lower than I wanted about 1/2" to 1", BUT if I go with QA1s I they will be using 14"shock when fully extended rather than the 13" Jag shocks SOOO it should be sitting about right, also with the QA1s I will have some ride height adjustability. With the OEM shock I bounced up and down on the suspension and was not able to get it to bottom out, but it was a little softer than I like, so now I just have to figure out if I want to go with 225s or 250s, but before I can make that decision one thing has me a little concerned. when you load an OEM jag coil-over you have to compress about 1" to fit it on the shock, so how does that play in to it. I am thinking with the QA1 it should be the same because you can adjust the unit and add some preload to the spring, but I am not really sure. thoughts???
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here is another thought. I took a look at the front spring rate for a coil over front suspension designed for a classic Mustang like the once TCP sells.
350# is a performance rate for these units. Since the shock angle and wheel rate are similar to the IRS (just ball parking here) I should be able to work backwards using the 60/40 weight ratio.
350/.6 = 584#
584 * .4 = 234#
SOOOOO I am probably going to buy either a 225# coil over or a 250# coil over and leaning very heavily to buying a 250# coil over since I can adjust the upper mount and increase the angle of the shock which will decrease the rate if I need to. also I can back the adjuster off on the coil over to decrease the initial rate if need be.
To aide in my decision making I called Duane S. Carling @ and he was extremely helpful!!!!! we talked quite a wile about IRS and I invited him to our site, hopefully he stops by, any way he was going to get the info for me as to what spring rate he was running on his units. obviously the design is a little different but close enough that it will give me one more pice of info to help me get the correct spring rate.
further thoughts on spring rate?????
Last edited by Daze (5/24/2011 8:43 am)
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I'm completely confused. When I first built my hot rod I used a chart Speedway Motors had in their catalogs. They suggested the lowest rate spring for t-buckets. So I tried that, but did not find it stiff enough, so got the next level springs and took the old ones off. I'm still not sure I have the right spring rate. I need to either get new springs when I get through this re-build or do something different. I want a better ride.
Frank
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I hope people will study the effects of motion ratio when determinining spring rates. I noticed the link in previous forum topics is broken. This one seems to work:
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