What's The Deal With Coil Spring Rates??

[lanky 44]

I have been searching everywhere trying to find consistent (key word) spring rates listed for all the different types of coil springs for our cars. As far as I can tell, the only consistent thing I have found is that the "620" springs everyone has talked about are called that because the diameter of the spring metal is .620", but even with regard to the "620's" I have read inconsistent information about what the spring rate actually is. I have seen everything from 560 in/lbs to 600 in/lbs. So many sellers of various springs might tell you the application they are for, but few to none say what the actual spring rate is.

 

The reason I am interested in this is because I have been looking for a new set of springs that are roughly 20-30% softer than what I have :http://opentrackerracing.com/product/performance-coil-springs-1967-1973/ . I say 20-30% because I plan on doing the spring perch relocation, and because my cars front end is already at the tolerable limit of stiffness I will not do the relocation without softer springs. Those familiar with this mod know that it allows you to run a lighter spring but the effective spring rate can be the same. I don't know the exact % difference that would be ideal, but the threads I have seen estimate around 20% or so...I know I can't be exact as far as finding the perfect stiffness, but if the springs above are 600 in/lbs that would mean I am looking at a range between  420-480in lbs.

[1969_Mach1 333]

I'm starting to believe some vendors have no idea what the specs are for the springs they are selling.  I purchased a pair of 620's from Mustang plus that are supposed to be rated at 560 lb./inch to replace 720's rated at 600 lb./inch that Mustangs Plus use to sell.  Well, the wire diameter on these new 620's are about 0.740 including the powder coating and are identical in every dimension to the old 720's.  What did I purchase?  I called Mustangs plus twice and got different answers.  I found a coil spring rate calculator online at Engineers Edge, entered in the parameters for my new springs and the rate was approximately 620 lb./inch.  I'm saying approximately because it's a little bit of an estimate for the number of active coils which affects the spring rate.

For your situation, look at Global West's coil springs.  That have a spring rated at 540 lb./inch, part number S-26.  You can get it direct or through Summit Racing.

Curious, what shocks are you using?  I still using old Gabriel Striders.  The ride isn't bad on the softest or middle of the three settings.  I wish Gabriel still made them.  If you are running a strong spring you don't want to use a high pressure gas shock.

 

[mwye0627 44]

6 hours ago, lanky said:

I have been searching everywhere trying to find consistent (key word) spring rates listed for all the different types of coil springs for our cars. As far as I can tell, the only consistent thing I have found is that the "620" springs everyone has talked about are called that because the diameter of the spring metal is .620", but even with regard to the "620's" I have read inconsistent information about what the spring rate actually is. I have seen everything from 560 in/lbs to 600 in/lbs. So many sellers of various springs might tell you the application they are for, but few to none say what the actual spring rate is.

 

The reason I am interested in this is because I have been looking for a new set of springs that are roughly 20-30% softer than what I have :http://opentrackerracing.com/product/performance-coil-springs-1967-1973/ . I say 20-30% because I plan on doing the spring perch relocation, and because my cars front end is already at the tolerable limit of stiffness I will not do the relocation without softer springs. Those familiar with this mod know that it allows you to run a lighter spring but the effective spring rate can be the same. I don't know the exact % difference that would be ideal, but the threads I have seen estimate around 20% or so...I know I can't be exact as far as finding the perfect stiffness, but if the springs above are 600 in/lbs that would mean I am looking at a range between  420-480in lbs.

When you refer to the spring perch relocation are you referring to the Arning (Shelby) drop where you move the Upper Control Arm mounting bolts down?

Other than that, I have not heard of another way to relocate the spring perches...   Maybe I have been missing out on some information here?

Thanks,

Mike

[Mike65 475]

Lanky, the springs you linked to from OTR are the same ones I put in my 69 Coupe when I rebuilt the f/e last year, but I have not driven the car yet since it is still apart so I can not give you a review on then one way or another.

[lanky 44]

 

I have been running the springs in that link. I have a 351w w/fmx right now and I live in CA where roads suck, and like I said these springs are at the limit of stiffness I would want for any street car. I have bilstein shocks at all four corners. 

No I am not talking about the Shelby drop. You can drill new holes on the UCA further outboard which improves motion ratio and ride quality supposedly. However if you use the same set of springs it will ride stiffer. That is out of the question for me. The only limiting factors for how far out you can go is interference with the spring/shock covers and room on the UCA for holes. I have read that 1" is doable and some cars can have more.

As far as the GW springs, 540 in/lbs X 1.2 (b/c they would act ~20% stiffer after the relocation)=648 in/lbs which is even stiffer than what I'm running now. I might end up having Detroit Spring make me springs. Doing this mod isn't at the top of my list right now, but if I found OTS useable springs I'm sure I would do it soon.

[1969_Mach1 333]

I would just install a softer spring and not relocate the lower spring saddle.  Regardless of much improvement they claim there might be from doing that.  But that's me.  I'm skeptical about those types of modifications.  There are usually pro's and con's to it.  So often the end result is simply different and not entirely better.

I'm in California as well.  I agree, the roads are horrible.  Some ask why don't I drive my Mach 1 more often.  A big reason is there really isn't anywhere in this area.  The roads are either severely congested with traffic and/or beat up with pot holes, cracks, etc.

[lanky 44]

Cost of springs aside, you get nice geometry improvement from doing this. Almost all modern cars use strut systems or a spring perch of some kind that is located as far outward in the suspension design as possible, because it is beneficial. Or a cantilever type suspensions use a link goes to the outward edges of the suspension for the same reason. Race cars have been doing one or the other for a long time. It helps ride quality by allowing the shock to control the wheel movement better (motion ratio), it helps the chassis resist body roll during turning, decreases stress in the suspension components and in our case the unibody because the spring is on the longer end of the lever now (lever meaning UCA), and although insignificant it does make the front end very slightly lighter haha.

Yes I could just get lighter springs, but if I am buying new ones anyway I think its definitely worth my time even if it only helped a little. I just wish there was actual official inch pound ratings for all the offerings.

[1969_Mach1 333]

I understand changing the geometry to move the spring and shock out closer to the wheel helps.  It would be nice if the top of the spring and shock were moved outward the same amount.  as well.  I'm probably just getting too old for modifications like that to interest me.

[RPM 1,190]

I solved my issue on my Mach 1 and couldn't be happier with the results.

[lanky 44]

If I could go back in time, I would have definitely gone with a coilover system. But my parts are nice and I sure as hell don't want to change it now.