The factory SPASM suspension spring rates on the 718 Cayman T were good enough for the factory tires, but my upgraded wheel and tire package proved to have too much grip. The factory springs were just too soft resulting in too much body roll and tire rub. The solution is to increase the spring rates and I did that by going with a set of TracTive Track Line coilovers. While I was in there, I swapped out the factory PADM active transmission mounts with Rennline Transmission mounts.
The big question became, what spring rates to go with? Remember, this car is a dual-purpose car and I didn’t want to sacrifice the daily street comfort. Therefore, I wanted to keep the electronically adjustable damping. Back to the topic of spring rates; for the life of me, I couldn’t find the factory spring rates for the SPASM suspension on the 718 Cayman T. I always like to know what the baseline is of anything I’m changing so that I know the percentage change. The best I could find with an internet search was somewhere around 28 N/mm front and 44 N/mm rear. The TracTive Touring line for the 718 runs 22-32 front and 50-80 rear. The 718 GT4 runs 45/80 from the factory with the popular upgrade being 80/100; recall the GT4 does run a different tire width stagger front to rear of 245/295 compared to the other 718s at 235/265. I found two other points of reference. The Ohlins Road & Track setup runs 70/80. Nitron specs 70/90 as their performance street package.
With this knowledge, I contacted the team at Inertia Lab and inquired about the Nitron setup. They didn’t have any Nitron setups in stock, but they had one set of TracTive Track Lines in stock. The Track Line setup typically comes with 100/120 spring rates, so much stiffer than I wanted and stiffer than the typical GT4 upgrade. However, Inertia Lab is more than happy to customize the spring rates to whatever you need. After considering my 245/275 tire stagger and 718 weight distribution with the 2.0L, 4-cylinder engine, I decided to go with the Ohlins rates of 70/80. 70 N/mm on the front should be at least double what the factory spring rate is, so my tire rubbing issues should be eliminated. I’ve always said the factory springs were too soft for serious track use. Hopefully, the 70/80 springs rates are not too stiff for the daily driver role. Paired with the TracTive coilovers and being around factory GT4 spring rates, those factored into my calculation in the selection of the 70/80 rates. Typically, TracTive coilovers come with Eibach springs, but Inertia Lab accommodated my request for Swift main springs; I’ve had really good experience with Swift springs not sagging over time. Like I said, the team at Inertia Lab was very easy to work with.
The TracTive setup from Inertia Lab came with rear toe-links and wire harness extensions for the wires to control the electronic dampers. I didn’t need the rear toe-links, but the wire harness extensions are definitely needed.
Inertia Lab also sent a set of covers, but I opted not to use them. The tool with the handle in the bag is to adjust the spring perch height.
The front coilovers have the house shaped adjustable camber plates. The rears have the round adjustable camber plates.
Here is a close-up of the spherical bearing from the top and the fasteners used to hold the alignment in place. Going back about twenty-five years, the first time I used adjustable camber places with spherical bearings was a set of Stillen plates on my Nissan SE-R. Then I got a set on my Evo 8. On the S2000, the KW Clubsports came with spherical bearing top hats.
5 comments
You can actually get really accurate results using the equation if you have access to the actual springs:
:Using Coil Spring Dimensions and Material Properties
If you are designing a coil or want to find the theoretical rate from its physical build, use the Spring Rate Calculator formula:
Equation: \(k = \frac{G \cdot d^4}{8 \cdot D^3 \cdot n}\)
G: Shear modulus of the wire material (approx. 11.25 × 10⁶ psi for standard steel)
d: Wire diameter (thickness of the metal wire)
D: Mean coil diameter (Outer Diameter minus the wire diameter d)
n: Number of active coils (the total coils that move and flex)
https://www.thespringstore.com/spring-rate-calculator.html
I gave it a try, but a there is a large unknown of material. That said, I feel like the numbers the online calculator generated are about 60% above any reference numbers I have found for other generations and models of Porsches. Any numbers I post are just pure speculation, so I’ll just stay quiet on them. I could bust out my old college textbook and run numbers myself, but the unknown material is still a big uncertainty. Key observations of the factory coilovers, the rear is a linear rate spring and the front is a progressive. On the assemblies, Porsche labeled that the springs are under 1000N of load.
Do you have the old springs still? Apparently you can actually measure them. I found this old forum post:
“CC, the variables you listed are what makes a spring…spring. When I was involved in IMSA and NASCAR, I tested each spring before it went on. The reason was that even a spring from a very well established company could be off by as much as 5%, which was too much for dialing in a suspension. Sometimes the error was due to the spring being bent because of a wreck. Usually it was just off. It didn’t have to be out much to be off. Evenly wound coil springs are very linear. To sum it up, you can either trust the manufacturer to be close, or test them yourself. Put the spring inside a PVC pipe slightly larger in diameter and set on an accurate bathroom scale and zero the scale. Using a push rod like a wooden dowel, push down until the spring compresses 1 inch and read the scale. These springs are rated kg/mm so there will have to be a conversion from lbs/inch.”
I also have a 718 with swift linear springs in the same rate, 70 up front and 80 in the rear. Mine are on B16s so not as exciting as your Tractives. For me the biggest improvement came when I added low speed rebound damping to my DSC to compensate for the heavier springs.
Looking forward to your extended review of these as dual duty and what changes you make to the tune long term.
What is the operating mA range of the B16s? I bet they are quieter than my TracTives. It was a bit surprising to me how ‘loud’ the car is going over bumps with the full spherical setup and the TracTives. My S2000 was still basically quiet with the full spherical control arm setup and the KW Clubsports. I think the strut suspension setup on the 718 chassis transmits more shock loads into the chassis compared to the control arm setup of the S2000. I do think the spherical bearing adjustable camber plates of the TracTives is a major contributor to the noise over bumps and they may just be due to the bearing sizing selected.
I have another significant change to the turbo and exhaust setup coming in stages. And I will need a new tune. The new setup from Raggdoll will have better response and I think the same or better power and it’s all bolt-on. I designed a new exhaust tip to replace my x-pipe too which will flow better.