Custom Coilover Suspension

After transferring my old Swift spring setup to a new set of coilovers, I encountered compatibility issues fitting 70mm inner diameter springs onto a perch designed for 62mm. To resolve this, I designed custom adapters along with a laser-cut 316 stainless steel washer to properly distribute the spring load onto the coilover lock nuts.

The adapters were 3D printed with 100% infill PLA+ after confirming through calculations that the maximum compressive force—based on an 8k spring across a ~2500 mm² surface area—remained well below 10% of the material’s compressive strength. A recessed seat with a side seal was also integrated to house a thrust bearing assembly, improving steering response and reducing spring bind compared to the manufacturer-supplied Teflon washers.

These adapters were run for approximately 6,000 miles without issue. However, they were eventually removed in favor of proper 62mm springs due to clearance constraints with wider track tires. The only design flaw observed was minor delamination on the edge of the front driver-side adapter, caused by unaccounted spring side-loading. If I were to design another set, I would include additional strain relief features or modify the print orientation to prevent side loads from acting along a layer line. 

Due to the extremely limited aftermarket support for this chassis, finding properly damped suspension setups has been nearly impossible. The coilovers I initially installed came with a spring and damping setup clearly intended for street use—not optimized for track driving. While researching the brand further, I discovered that the manufacturer offers a wide range of dampers for various vehicle platforms.

After corresponding with a company representative, I was able to confirm that the shock tube threading was standardized across their inventory. This opened the door to a temporary mix-and-match solution, allowing me to pair dampers from other applications with my existing hardware to better suit the demands of track use.

This approach served as an interim setup while I worked on a more permanent solution—re-valving the original dampers using motorsport grade aftermarket valve assemblies and tuning the shim stacks to match the spring rates and dynamic behavior required for my specific application.

After weighing the car both with and without driver weight, I was able to accurately estimate the corner weights of the vehicle. Through a combination of interior stripping and other weight reduction modifications, I successfully removed approximately 200 lbs from the stock configuration. Strategic battery relocation allowed me to fine-tune the balance, achieving a near-ideal 49/51 cross weight distribution with the driver onboard.

With this data in hand, I began the process of properly calculating spring rates for the new coilover system, ensuring they are matched to the revised weight distribution and performance goals.

Each corner’s unsprung mass was carefully measured part by part to ensure accurate data. Despite increasing rotor diameter by 50mm and tire width by 50mm, I managed to keep the front unsprung mass within 1 lb of the stock setup. This was achieved using lightweight wheels and a two-piece floating hat rotor, which offset the added mass of the larger components. A second set of rotors, planned for installation, weighs within 1 lb of the factory units—bringing the front unsprung mass precisely back to OEM levels.

In the rear, unsprung mass was reduced by 4 lbs through the use of lighter wheels and custom adjustable toe links adapted from another vehicle platform. However, the planned rear big brake kit (BBK) will introduce a slight increase in unsprung mass: the new rotors are 6 lbs heavier than stock, while the calipers are 1 lb lighter, resulting in a net gain of 1 lb over the factory rear setup.

This detailed weight tracking directly informed spring rate selection and overall suspension tuning for the updated coilover system.

The motion ratios for both the front and rear suspension were calculated using 20mm increments throughout the full range of shock travel—from full droop to bump stop compression. This incremental approach allowed for an accurate representation of how the motion ratio changes dynamically through the suspension stroke, providing critical data for precise spring rate selection and damper tuning. 

After extensive suspension research, I developed a custom Excel spreadsheet to organize key setup data and apply motorsports-specific calculations. The tool is fully editable to reflect ongoing tuning changes and includes a section for calculating natural frequencies based on various front and rear spring pairings.

Using the vehicle’s wheelbase, I determined that a 13% increase in rear natural frequency would help the car settle more effectively during compression oscillations. I calculated motion ratios in 20mm increments throughout the full suspension travel to visualize how they change from full droop to bump stop compression. Notably, the front motion ratio dropped below 1 at full droop, prompting the addition of a 2.8k helper spring to smooth the transition and reduce the wheel rate spike experienced during rebound-to-compression movement—particularly in bump recovery situations.

To address rapid chassis transitions, reduce body roll, and prepare for potential future downforce modifications, I selected a spring set with a natural frequency above 2 Hz. A 9k/7k spring pairing closely matched my performance targets. To complement this, I decoded the coilover company’s off-the-shelf damper part number schema and verified compatibility with the manufacturer’s staff. I identified a set of similar shock bodies from other vehicle platforms that featured the same M12 top thread, increased stroke length, and—most importantly—digressive valving. I also verified that these dampers were originally paired with spring rates similar to those in my setup, ensuring baseline damping compatibility.

The digressive valving provides a more balanced handling profile by decreasing damping force at high shaft velocities (above ~25 mm/s), allowing for softer bump absorption, and exponentially increasing damping during low-speed shaft movement (10–20 mm/s). This results in more direct chassis feedback during cornering and a more compliant ride over bumps and berms commonly encountered on track surfaces.

After receiving all the springs, thrust bearings, and custom radial bearing top hats, I was able to fully assemble the four custom-selected coilovers. Once installed, I’ve since driven over 10,000 miles on this setup. On smooth pavement, the system performs admirably—initial turn-in is noticeably sharper, and mild bumps are absorbed with less impact to chassis stability.

The current coilover configuration was also designed with future modifications in mind, specifically accounting for the anticipated 100 lb weight reduction at the front driver-side corner from the upcoming manual transmission swap. This forward-thinking approach ensures that spring rates and damping characteristics will remain balanced even after the drivetrain conversion.

However, after thoroughly testing the setup across various conditions, I’ve found that the current natural frequency is too aggressive for uneven road surfaces in the absence of additional aerodynamic load. On bumpier pavement, the high spring rates lead to chassis upset, which compromises mechanical grip and overall ride compliance.

This issue is now being addressed in the next phase of development: a set of custom-valved coilovers. These will be specifically tuned to maintain the existing responsiveness while improving compliance under real-world road conditions.

After extensive research, I concluded that BC Racing’s linear valving—and even their digressive options—are not ideally suited for consistent motorsport use. To better tailor the system to my needs, I disassembled the original dampers and discovered they utilize 46mm pistons with 12mm shafts, a configuration shared with some Bilstein and Öhlins shocks. This compatibility opens the door for re-valving using higher-end shim stack configurations and components.

I fully labeled and documented the existing compression and rebound shim stacks and am now working with a virtual dyno software to simulate different valve and shim configurations. This allows me to predict damper behavior and generate rebound and compression force vs. velocity curves prior to physical assembly, significantly streamlining the tuning process.

In parallel with the re-valving effort, I’ve also decided to reduce the spring rates to a 6k/5k setup, targeting a flat 2 Hz natural frequency in the front. This change prioritizes mechanical grip and significantly improves performance over rough pavement—ideal for my current use case in the absence of significant aerodynamic loading.

I’m currently studying Bilstein’s factory tuning manual, specifically their implementation of the COB (Compression-Only Bypass) valve system, to further refine the compression stack and enhance low- to mid-speed damping performance.