To control the motion of the sprung and unsprung masses, keep the tire in contact with the road, and hold the aerodynamic platform at the ride height it was designed for.
Ohlins TTX 25 MkII twin-tube hydraulic dampers, 200 mm overall length, 57 mm stroke, 394 g without spring, with four-way adjustability: low speed compression and rebound in 25 discrete clicks of 1/6 turn each, high speed compression and rebound over 4.25 turns. They are mounted indirect, from the bellcrank to the frame with the damper body on the frame, which keeps actuating and unsprung weight down. Hyper-Co springs sit over them: 18FS150 at 150 lb/in front and 18FS250 at 250 lb/in rear, both 4.4 in free length, on a mounting shackle with adjustable preload jam nuts giving up to 0.75 in of preload. Bump stops set the minimum ride height, which is what protects the aerodynamic devices from ground contact and what makes the LLTD swing between the no-bump-stop and bump-stop-only cases. Spacers set the damper's position in the mounting.
Maintain the damper rather than replace it. The TTX 25s are rebuilt biennially with fluids replaced annually, and axles are chosen by similarity from dyno results. CR26I's dampers were selected from CR22's dampers after being dyno'd, so a known unit goes on the car rather than an assumed one. The bump stop is a design element, not a crash stop. It sets the aerodynamic ride height floor, and its effect on lateral load transfer is calculated and reported alongside the ARB settings. 1. Kinematics sets wheel rates; spring rate follows from wheel rate and motion ratio. 2. Dyno the available dampers and select matched units by axle. 3. Set bump stop height from the aerodynamic minimum ride height and the worst-case heave and roll. 4. Tune on track using damper position data from the bellcrank rotary hall-effect sensors. 5. Read the traces for asymmetry between bump and rebound travel time, which is the practical indicator of over- or under-damping.
- Keep the platform predictable so the aerodynamic package delivers what CFD predicted. - Maintain damper performance over the service life through scheduled rebuild. - Provide enough adjustability to tune at a drive day without changing hardware.
- Damper position data quality, which is the bellcrank sensor issue and gates all damper tuning. CR26I's data is good enough to use; earlier cars' was not. - Damper tuning asymmetry. Comparing outing 4 against outing 5 on CR26I, the rears were spending more time in rebound travel than bump travel, indicating too much rebound damping; outing 5's low-speed damping was more symmetric. - On-track damper position data was still described as poor at the time of the CR26I transient modelling work, with new data yet to be post-processed.
Damper dynamometer for characterising and matching units. Amphenol Piher PST360G2 rotary hall-effect damper position sensors on the bellcranks. Simulink four-wheel transient quarter-car model for observing transient ride height for front wing, undertray and bump stop development. MoTeC i2 Pro for damper position traces.