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on October 1, 2010, 6:07 pm, Daze wrote:
In an earlier post I talked about finding that the OEM units were about 200# per inch. Turns out there is even more to keep in mind. Yes the OEM springs were 200# per inch at normal ride height however they must be compressed to 9" from 11" to install them on the shocks that means it takes over 400# of pressure just to compress them further. This is a huge factor!!! I finally found a company that sells aftermarket springs and for a Jag application they have sold 175#/" springs and it totally makes sense because the springs are 12" long so once they are on the shock you are looking at 525# just to begin to compress them. This got me thinking I needed to do a bunch of math to see how all the different options laid out. I set up an Excel work sheet and plugged in the numbers for an OEM spring, an OEM spring with a 1/2" spacer and 4 different rates of aftermarket springs and then graphed them. The result is very interesting. After looking at the numbers and the linear progression, I am leaning toward the OEM springs with the 1/2" spacer or a set of 175 aftermarket units, thoughts???
I believe the spacer is not a good idea. A spacer effectively increases the free length of your spring. You are already pre-loading the springs two inches or about 400 lbs. A half inch spacer will increase the pre-load to 500 lbs. 2.5 X 200. What you need is a shorter spring to decrease the preload that must be overcome by the weight of your car before any suspension travel even starts. I would think an inch of preload would be sufficient. This makes the free length 10 inches using your 9 inch pre-load dimension. As I recall, Jag springs have closed ends, so cutting coils is out. Also, never heat your springs because you will anneal the springs and ruin them. Custom springs are the only alternative.
Before you order springs, the first thing you need to do is determine the compressed spring length at ride height. This can be easily determined by mounting a shock without a spring, setting your car at the height you want, and measuring the distance between the spring perches.
Once you have this compressed length you can calculate the spring rate you need in pounds per inch. At this point 175 pounds per inch spring rate is just a guess. As you point out the free length of the charted spring causes excessive pre-load.
Again, before ordering custom springs, you need to know the maximum compression when the hubs are against the bump stops plus a safety margin. Coil bind must be avoided.
You will also have to look at motion ratio and wheel rate. You're also going to have to look at spring harmonic motion and keep the cycles per minute in a comfortable range. This rate should not be the same rate as the front suspension, by the way. Otherwise porpoising over bumps will occur. The two coilovers per side in the rear will probably insure the rates being different enough from the front that this will not be a problem.
After considering all this, I have decided to order some adjustable ride height coilovers. I can get a good idea of spring rate by gathering the data above and using the calculations in the suspension worksheet you found. The coilover manufacturers will usually exchange springs, allowing some experimentation.
Either way, for those out there that want to use stock Jag shocks, let's get some more data and see where the math leads us.
One other thing. Your graph shows spring load versus deflection. Not spring rate versus deflection. The heading of the third row should be installed preload not installed rate. Your graph correctly shows that all these springs are linear.
All the math aside (not willing to abandon the #s just more info)
when I had the system mocked up and added 100# to the trunk to simulate a full gas tank the suspension dropped a full inch which indicates to soft a spring.
"I believe the spacer is not a good idea. A spacer effectively increases the free length of your spring."
unfortunately I don't have a lot of other options. The company that make 3" aftermarket springs only sells them in 12" and 10" lengths and the rate on the 10" springs only goes up to 250# making them to soft when installed on the shock.
Aftermarket Springs
Where did you find 10 inch free length springs? Can you add them to your chart?
I'm not asking you to throw away the numbers in your chart. Your graph represents spring load versus deflection. The vertical axis should be labeled spring load. The spring load at zero deflection is the preload. It is incorrectly labeled as a rate in the chart.
The only 3" I.D. springs I could find are made by Eibach
same place I got the work sheet from. They are about $90.00 each. I pluged in there stiffest 10" spring in to my chart and the results were softer than stock all the way through
by 2" of compression they are finally starting to catch up. Idealy a 350# spring is about right, but they don't make one in a 10" length.
I'm not asking you to throw away the numbers in your chart. Your graph represents spring load versus deflection. The vertical axis should be labeled spring load. The spring load at zero deflection is the preload. It is incorrectly labeled as a rate in the chart.
My statement about not throwing out the math had nothing to do with what you had said. I was saying it in relation to my observation that my springs were to soft based on the ride height change due to simulating a full gas tank.
as far as your question about my MR. I need to figure it up exactly. when I fill out the work sheet I had most of the info but approximated on a few numbers because I hadn't had a chance to measure them. I will need to weight tell I get the system mocked up again before I know for sure.
Have you checked Hyperco?
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