The holy grail for many SL-C builders is a comfortable car on the street which can also tear up the track. While it’s impossible to optimize for both, technology can help transition between the two use cases. To that end, I’ve added cockpit-adjustable anti sway bars, driver tunable electric power-assisted steering, an active wing, and multiple engine, traction control and shift tunes. These are collectively modified via the manettino rotary switch on the steering wheel. Once a given mode is selected, the driver can dynamically tweak any of the settings.
Over the last couple of years I’ve been trying to upgrade my Penske dampers with TracTive’s semi-active dampers which are apparently used by Pagani, Donkervoort and a variety of track/race cars. Similar to the banned F1 active suspension technology the damping rates are dynamically modified by an ECU based on settings and multiple accelerometers. For example, going through the same corner with the same speed, once on the brakes and once on power, you would see totally different damper settings for all individual wheels. However, unlike the F1 tech the ride height is not changed, hence the semi-active moniker.
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The TracTive ACE dampers can transition to any desired stiffness setting in 6 milliseconds. According to the manual:
To dampen the wheel and body movements, the motion of the vehicle is monitored. The following axis are included:
- Vehicle Vertical Axis (Z-Axis)
- Vehicle Lateral Axis (X-Axis)
- Vehicle Longitudinal Axis (Y-Axis)
The following signals are used for monitoring:
- LATERAL acceleration
- LONGITUDAL acceleration
The following signals are used to calculate the current driving situation:
Control of the damping action for the vertical movement of the vehicle can be subdivided into comfort and safety. The ACE system is set to use a small amount low-speed (body) damping to enhance comfort. At the same time, the wheels may not loose contact with the road-surface. Depending on the situation, the TCU control algorithm should shift towards optimized vertical force transmission.
Lateral G-force
The ACE system is able to detect steering input by monitoring lateral G-force. (e.g. transition from driving in a straight line and into a corner) When a rapid enlargement of the lateral G-force is detected, the TCU concludes that cornering has begun. As a result, the dampers can be adjusted accordingly. This means that the ACE system can be used to reduce the rolling motion of the vehicle. This “anti-roll” behavior is programmed inside the TCU and is specific to each application.
Longitudinal G-force
Using the longitudinal G-force, the ACE system is able to determine whether the vehicle is accelerating, braking or moving at constant velocity. During (excessive) braking, the vehicle would normally see a pitching motion. While the acting G-forces are detected by the TCU, this pitching motion can be counteracted by increasing front damper stiffness. Based on the G-force-rate and/or the absolute G-force, it is determined how much increase or decrease in damping is required for optimum wheel and body control. The same goes for sudden accelerations, depending on the application, either a stiffer or softer damper is required to gain maximum grip on the driven wheels. This is done by using the longitudinal G-force together with the control algorithm in the TCU.
Combined G-forces
Combining both longitudinal and lateral G-force, actual driving situations can be extracted from the sensor data. Different damper behavior can be set for all kinds of combinations. Going through the same corner with the same speed, once on the brakes and once on power, you would see totally different damper settings for all individual wheels. This is how the ACE system increases safety, more grip, more control and better predictability.
The control panel shown above enables the driver to dynamically control the following four settings:
- FRONT damping adjustments gives you control over the damper stiffness over the whole axle and not the individual dampers.
- REAR damping adjustments gives you control over the damper stiffness over the whole axle and not the individual dampers.
- ROLL support allows you to adjust the percentage of damping that is added when G-force is detected in the LATERAL motion to support the chassis thru the dampers while entering and moving thru the corners.
- PITCH support allows you to adjust the percentage of damping that is added when G-force is detected in LONGITUDAL motion. This can be braking and or accelerating of the vehicle.
These settings can be be stored in five different tunes which can be activated via CAN bus messages.
The primary challenge to the upgrade is that it’s more complex than just getting the dampers to fit. The valving, spring rates and bump stops need to be engineered. I subsequently learned that Henry Nickless with Chiron World Motorsports is building two high-spec Lambo V10 SL-Cs with TracTive dampers. He has extensive experience building and racing LMP cars and setting up suspensions on a wide-range of cars, so I’ve been working with him.
Starting with first principles, the chassis and suspension were scanned and a 17-page vehicle dynamics report. The good news was that the front suspension geometry was spot on and there was no need for any changes (the rear didn’t fare as well, more about that in a later post). We opted for a hydraulic nose lift that’s seamlessly integrated into the top of the damper rather than the typical cups that go around the shock body. This approach has several advantages:
- The springs are standard length which provides increased suspension travel, especially after a bump stop and bump springs are added.
- It’s more compact which results in less weight.
- The hydraulic ram is located at the top of the damper, so it’s weight is sprung. The cups have a larger OD which often collides with the upper control arm, resulting in the shock being inverted and the weight becoming un-sprung.
The downside is higher cost and that the shocks are longer.
Penske front damper with with RamLiftPro cup (left), TracTive front damper with integral lift (middle), TracTive rear damper (right). The two silver plugs at the top of the middle damper are M10 1.0 mm hydraulic ports for the nose lift.
Initial observations:
- The TracTive dampers appear to have the build quality as the Penske dampers.
- Other than the thin wire exiting the bottom of the TracTive dampers, one wouldn’t know that they were active.
- The springs on the TracTive have a smaller OD whereas the Penske’s.
- The TracTive reservoir hoses have a larger OD.
- The TracTive lift has two hydraulic lines per damper whereas the RamLiftPro has one.
To accommodate the longer damper a new top bracket was CNC’d from 7075 aluminum. It uses the lower stock hole and requires two new upper holes to be drilled. There is small bump on an the backside of the damper to accommodate the hydraulic piston and a small hole must be cut into the monocoque. I laser cut a plywood jig to cut the hole with a router. That was easy. However, I had nut plates integrated into the removable side-impact bars which required a fair amount of rework.
Top front shock brackets, flanged bushings and indexing cone washers (all shown parts are custom)
Installing/removing the stock dampers is a pain-in-the-ass. You need remove one side and loosen the other side of the upper control arm and then you have four stacks of flat and cone washers for the that need to be aligned and held in place while you push the bolt through the assembly. Inevitably you wind up dropping one or more stacks on the floor… I’m constantly amazed how far stuff can roll to a find creative hiding place.
With this version there is no need to touch the upper control arm. Each stack for the top rod end is replaced with a single flanged bushing that indexes the bracket and each stack on the bottom is replaced with a single cone washer that indexes the rod end. Since everything is custom machined it slides together like butter and the parts hold them in place while being installed.
In the pictures below, the shock has been installed. You will note that it projects above the monocoque which requires the notch in the spyder’s fiberglass on to be extended. However, there is no effect on the nose. Also note that the hydraulic lines haven’t been installed yet and that reservoir hose is routed under the control arm, but the plan is to route it above the control arm.
The hydraulic system
