Matching Your Spring Rate to Dampening Rate

Prokit post from last year.........

I decided to go ahead and install the pro-kit springs this weekend to see how they perform—that way I can evaluate each suspension upgrade individually (springs, shocks, tires, rear strut tower bar). I went to Home Depot to pick up 15-40lb bags of sand, for 600 lbs total for my experiment. The general idea to the experiment is to measure spring rate starting from a normal resting position on the stock and pro-kit springs. The goal was to place additional weight to on the car and measure spring compression in order to approximate spring rate under real progressive force. Placing varying weights on the car, I measured total compression first on the rear, then on the front. Here are the results:

-all inches were converted to decimals to do the calculations
-measurements do not account for possible tire compression
-measurements do not account for minimal weight transition to front/rear of car as weight was added, for example of the 600 lbs. Of weight added to the rear, it is impossible to say exactly how much was transitioned to the front springs.
-my assumption is that the bags of dirt/sand from Home Depot actually weighed 40lbs each. I was not inclined to check it.
-measurements were taken at the wheel well, centered vertically on the center of the rims.

Rear STOCK Measurements:
0 lb. L/R average height = 27.34 inches
400 lbs. L/R average height = 25.79 inches
600 lbs. L/R average height = 25.07 inches
Rear Calculations
From 0lb.- 400lbs., k=f/x = 400/1.55 = 258 lb/in or 129.03 lb/inch per spring
From 0lb – 600lbs., k=f/x = 600/2.27 = 264 lb/in or 132.15 lb/inch per spring
From 400lb – 600lbs, k=f/x = 200/.72 = 277.8 lb/in or 138.8 lb/inch per spring
Conclusion:
Spring rate for STOCK REAR SPRINGS are about 130-140 lb/inch and seem slightly progressive—but unsure.

Front STOCK Measurements:
0.0 lbs. L/R average height = 27.41 inches
320 lbs. L/R average height = 26.44 inches
480 lbs. L/R average height = 25.94 inches
Front Calculations
From 0.0lbs. - 320lbs., k=f/x = 320/0.97 = 329.90 lbs/in or 164.95 lbs/inch per spring
From 0.0lbs. – 480lbs., k=f/x = 480/1.47 = 326.53 lbs/in or 163.26 lbs/inch per spring
From 320lbs. – 480lbs., k=f/x = 160/0.5 = 320.0 lbs/in or 160.0 lbs/inch per spring
Conclusion:
Spring rate for STOCK FRONT SPRINGS are about 165 lbs/inch and are not progressive

Eibach Prokit Install:
Total drop was about 1.2 inches in the front and rear. I drove the car hard to settle it in, and the car had been on the springs for a couple of weeks anyway..

REAR Eibach:
Placed 200 then 400 lbs in the rear and came out with around 135 lb/in for both weights, so the REAR Pro-Kit is NOT PROGRESSIVE and about the same stiffness as stock (probably about 140 lb/in).

FRONT Eibach
I placed 200, then 400, then 480 on the rear and was AMAZED by the results. Even with 480 lbs on the front end, measured compression was only .87 (less than an inch on both sides). The ratio was constant, so I do not believe they are progressive. But do the math – 480lb/.87 inch = 552 lbs/inch or about 275 lb/inch per side. If you doubt it, try it. I did it a couple of times and it came out in about the same range. That’s a pretty huge difference. I did all measurements exactly the same, so I don’t know what to say. Could it be 275? That would make the fronts 65% stiffer than stock. I have no problem believing that, because that’s the way it feels. What do you think?????

From 0lb – 400lbs., k=f/x = 400/.74 = 541 lb/in or 270 lb/inch per spring
From 400lb – 480lbs, k=f/x = 80/.13 = 615 lb/in or 308 lb/inch per spring
From 0lb – 480lbs, k=f/x = 480/.87 = 552 lb/in or .276 lb/inch per spring

I have been wondering the same thing.

When I was running the ProKit with KYBs, it seemed like the shocks set at 2-3 would limit the initial compession when I hit a bigger bump or rut, but that the spring was too week to take it from there, and it would compress too easily.

So I put on Tein S-tech that up the rates 30% front and back over the stock rates.

Have not had to worry about scraping front end due to compression since.

Now I have been fiddling with tire pressure and KYB setting. After many months of different combinations I am running 36 PSI on setting 1 front, and 30 PSI on setting 3 in back.

This feels tight, the least bouncy, and the least punishing on Portlands rough inner city roads. On the smoother freeways the settings don't matter as much.

As far as KYB rates, "firm-firmer-damn firm-f**king firm"...all agreed ?

I remember last year when you did this test. Again, great work BTW.

I've driven one car with Pro-Kits and Koni's, with P-Zero's, and it felt... different than stock. Slightly stiffer, but not by much. The struts, even though they were just Koni's (relatively soft on the compression side) felt like they were overkill for the Pro-Kit, it was that soft. Also at the time I had 350 lb/in springs (GC's) on my Eclipse, and it felt subjectively 10x stiffer (on the same struts) than that car with the Pro-Kit.

I don't doubt you're results, I'm just still surprised by them.

I've currently got the Tein S-tech springs, and again as an opinion, they are much stiffer (with a similar Koni setup) than the Pro-Kit. Tein claims them to be approx 240 in front.

My opinion (see how I use that word and "subjective" and such a lot ?) on the KYB's is that they were fairly stiff on compression, where many guys complained about it actually, and if also gave people a sense of higher handling capabilities (maybe it did, who knows). But perhaps a little soft on rebound. Your bouncing near the bridges may be a symptom of the spring compressing very quickly and then launching the car, the KYB not controlling it well enough. The Koni's on the other hand are somewhat the opposite. They were somewhat soft on compression, but could be set stiff for rebound. Some guys would bottom out over rough pavement, because each bump would compress the suspension a bit more and the Koni's were too slow to give it back after ~ some number of hits. This may make some springs feel softer - where they control the body roll and absorb the bumps and not the struts. But the high rebound adjustability controlled my GC's without a problem.

I had thought for a long time that the perfect strut might have no comrpession resistance at all, and be very high rebound. It would be matched to some very stiff springs. My GC setup was something like that, and I was always happy with it. I swapped to Tein's because I moved to a section of town that was ridiculous for potholes and crappy pavement, the GC's were just beating me up too much. nowadays I wish I had a setup where I could adjust springs, and strut compression and rebound settings (independantly) at the touch of a button from the dash. Or just pick apart say a new M5, and see what the Germans do.

More info than you wanted probably, but i just love these types of conversations

Just heard back from KYB. This should definitely be a sticky....

From: KYB
Date: 2004/08/03 Tue PM 03:24:50 EDT
To: Cameron
Subject: RE: [KYB.COM Contact Form] Dampening Rate

The AGX products have the following approximate damping force values.
When adjusted, both rebound and compression damping forces change.

Setting#:
Relative Value
1 - Same as Factory (100%)
2 - Same as our GR2 products (10~15% firmer than Factory --115%)
3 - 50% firmer than #2 (172%)
4 - 30% firmer than #3 (223%)

For an eight position adjustable product, the damping force values are
approx. the following: With #2 being Factory at a relative value of 100,

#1 = 95 (5% softer than Factory)
#2 = 100 (Factory damping force)
#3 = 107.5 (7.5% firmer than Factory)
#4 = 115 (15% firmer than Factory)(GR-2)
#5 = 143.5 (43.5% firmer than Factory)
#6 = 172 (72% firmer than Factory)
#7 = 197.5 (97.5 firmer than Factory)
#8 = 223 (123 % firmerthan Factory)

When using Eibach Prokit springs recommended setting on a 1~4 is no
lower than 2, 1~8 no lower than 4.