I've been doing a lot of research on springs, how to calculate rates as well as suspension theory (involving levers and all sorts of geometry math). Bottom line is I wanted to know what the stock springs rates were on my 2008 FDI Sport. Once I know what's in there, then I can start looking for matching springs (or anyone here can copy/paste this into an email to a custom coil maker).
What I measured:
Coil spring wire diameter
Compressed height (vehicle sitting on level ground with no extra weight, full tank of gas)
Free length (was measured for me by another forum member)
Number of active coils at rest (see compressed height note)
How I measured:
Coil spring diameter: The coils are not perfectly round, so I took several measurements from different places and averaged them.
Compressed height: The springs sit in/on rubber isolators, so it's not possible to measure from the top to the bottom from the outside. So I measured from the bottom of the top coil to the top of the bottom coil (measured the inside), then added in the coil thickness times 2.
Free length: Measured for me by a forum member.
Number of active coils: This is just a simple count of coils not touching another coil or one of the isolators. Free to move. So from the bottom I looked where the spring was free then started counting up until only a fraction of a free coil was left. I then estimated to only about a 1/4 turn so it could be + or - an 1/8 of turn, which doesn't change the rates all that much (+/- 10 lb/ins and +/- 50lbs loaded pressure).
I plugged all of these numbers into this spring rate calculator:
http://www.pontiacracing.net/js_coil_spring_rate.htm
So here are my findings.
Front Spring
Free Height: 13.75"
Loaded (install) height: 7.305"
Wire Diameter: 19/32" (0.590")
OD of spring: 6.75" (ID: 5.57) w/Pigtail ends (4.75" OD / 3.57" ID)
Active coils: 3.25
Spring Rate: 224.31 lbs/in
Pressure @ loaded height: 1445.68 lbs
Rear Spring
Free Height: 13.25"
Loaded (install) height: 8.5"
Wire Diameter: 1/2" (0.500")
OD of spring: 4" (ID: 3") w/Pigtail ends (3.375" OD bottom / 3.75" OD Top)
Active coils: 5.5
Spring Rate: 372.71 lbs/in
Pressure @ loaded height: 1770.37 lbs
This is going to be about as close as I can get to reverse engineering the factory springs. I took a lot of time figuring this out and double checked all of my measurements on two different days. I triple checked the rears because they just seemed so high, but then the spring placement, control arm length and etc. all play into it.
I will not reverse engineer Stu's springs and I would appreciate it if no one else does either. If for some reason you already have, keep the figures to yourself and do not post them. If I had the money, I wouldn't have even started this and would have just bought Stu's springs. The guy deserves being paid for his work and knowledge.
As I stated in another post, there is some very complicated math involved in springs and what the effective "wheel rate" will ultimately be. There is a motion ratio that should be determined that includes the angle of the spring, the distance from the control arm-to-frame mount to where the spring pushes on the control arm, to where the wheel pushes on the other end. In the front, the spring is almost at the wheel so it's motion ratio is close to 1 (or like .90-.99). For the rear, the spring is mounted inboard of the wheel/body and can have a motion ratio as low as .60. There is also sprung and unsprung weight that factors in to all of this. The only thing I have done is figured out what the factory spring specifications are. These are very close approximations.
So, now how do we lift our Patriots with a spring? Well, we want to raise the loaded height. So if the fronts are 7.3" now, then we want our new spring to be 9.3" loaded for a 2" lift. But here's the thing. We need our loaded pressure to be close to the above number of 1445 lbs. We can choose a slightly stiffer rate of say 250 lbs/in which means we'll now have a shorter free length but still get approx. 1445 lbs @ 9.3". We won't be able to increase this loaded pressure simply because the weight of the Jeep is our constant (unless we add more weight to the front or rear of course). So if we find a spring that does 1600 lbs @ 9.3", well it's really just going to lift the front of the Jeep some more until the spring is at say 9.8" @ 1445 lbs.
What I measured:
Coil spring wire diameter
Compressed height (vehicle sitting on level ground with no extra weight, full tank of gas)
Free length (was measured for me by another forum member)
Number of active coils at rest (see compressed height note)
How I measured:
Coil spring diameter: The coils are not perfectly round, so I took several measurements from different places and averaged them.
Compressed height: The springs sit in/on rubber isolators, so it's not possible to measure from the top to the bottom from the outside. So I measured from the bottom of the top coil to the top of the bottom coil (measured the inside), then added in the coil thickness times 2.
Free length: Measured for me by a forum member.
Number of active coils: This is just a simple count of coils not touching another coil or one of the isolators. Free to move. So from the bottom I looked where the spring was free then started counting up until only a fraction of a free coil was left. I then estimated to only about a 1/4 turn so it could be + or - an 1/8 of turn, which doesn't change the rates all that much (+/- 10 lb/ins and +/- 50lbs loaded pressure).
I plugged all of these numbers into this spring rate calculator:
http://www.pontiacracing.net/js_coil_spring_rate.htm
So here are my findings.
Front Spring
Free Height: 13.75"
Loaded (install) height: 7.305"
Wire Diameter: 19/32" (0.590")
OD of spring: 6.75" (ID: 5.57) w/Pigtail ends (4.75" OD / 3.57" ID)
Active coils: 3.25
Spring Rate: 224.31 lbs/in
Pressure @ loaded height: 1445.68 lbs
Rear Spring
Free Height: 13.25"
Loaded (install) height: 8.5"
Wire Diameter: 1/2" (0.500")
OD of spring: 4" (ID: 3") w/Pigtail ends (3.375" OD bottom / 3.75" OD Top)
Active coils: 5.5
Spring Rate: 372.71 lbs/in
Pressure @ loaded height: 1770.37 lbs
This is going to be about as close as I can get to reverse engineering the factory springs. I took a lot of time figuring this out and double checked all of my measurements on two different days. I triple checked the rears because they just seemed so high, but then the spring placement, control arm length and etc. all play into it.
I will not reverse engineer Stu's springs and I would appreciate it if no one else does either. If for some reason you already have, keep the figures to yourself and do not post them. If I had the money, I wouldn't have even started this and would have just bought Stu's springs. The guy deserves being paid for his work and knowledge.
As I stated in another post, there is some very complicated math involved in springs and what the effective "wheel rate" will ultimately be. There is a motion ratio that should be determined that includes the angle of the spring, the distance from the control arm-to-frame mount to where the spring pushes on the control arm, to where the wheel pushes on the other end. In the front, the spring is almost at the wheel so it's motion ratio is close to 1 (or like .90-.99). For the rear, the spring is mounted inboard of the wheel/body and can have a motion ratio as low as .60. There is also sprung and unsprung weight that factors in to all of this. The only thing I have done is figured out what the factory spring specifications are. These are very close approximations.
So, now how do we lift our Patriots with a spring? Well, we want to raise the loaded height. So if the fronts are 7.3" now, then we want our new spring to be 9.3" loaded for a 2" lift. But here's the thing. We need our loaded pressure to be close to the above number of 1445 lbs. We can choose a slightly stiffer rate of say 250 lbs/in which means we'll now have a shorter free length but still get approx. 1445 lbs @ 9.3". We won't be able to increase this loaded pressure simply because the weight of the Jeep is our constant (unless we add more weight to the front or rear of course). So if we find a spring that does 1600 lbs @ 9.3", well it's really just going to lift the front of the Jeep some more until the spring is at say 9.8" @ 1445 lbs.
